RISK FACTORS

Investing in our common stock involves a high degree of risk. You should carefully consider the following risk factors, as well as the other information in this prospectus, including our financial statements and related notes, before deciding whether to invest in shares of our common stock. The occurrence of any of the following adverse developments described in the following risk factors could materially and adversely harm our business, financial condition, results of operations or prospects. In that case, the trading price of our common stock could decline, and you may lose all or part of your investment.

Risks Relating Our Financial Position and Need for Additional Capital

We have never been profitable. Currently, we have no products approved for commercial sale, and to date we have not generated any revenue from product sales. As a result, our ability to reduce our losses and reach profitability is unproven, and we may never achieve or sustain profitability.

We have never been profitable and do not expect to be profitable in the foreseeable future. We have incurred net losses in each year since our inception, including net losses of $28.2 million, $26.8 million and $53.5 million for 2009, 2010 and 2011, respectively. As of December 31, 2011, we had an accumulated deficit of $244.3 million. We have devoted most of our financial resources to research and development, including our preclinical development activities and clinical trials. We have not completed development of any product candidate and we have therefore not generated any revenues from product sales. We expect to incur increased expenses if and as we commence Phase 3 development of delafloxacin, satisfy our obligations under our agreement with Sanofi, advance our other product candidates and expand our research and development programs. We also expect an increase in our expenses associated with seeking regulatory approvals and preparing for commercialization of our product candidates, and adding infrastructure and personnel to support our product development efforts and operations as a public company. As a result of the foregoing, we expect to continue to experience net losses and negative cash flows for the foreseeable future. These net losses and negative cash flows have had, and will continue to have, an adverse effect on our stockholders’ equity and working capital.

Because of the numerous risks and uncertainties associated with pharmaceutical product development, we are unable to accurately predict the timing or amount of increased expenses or when, or if, we will be able to achieve profitability. In addition, our expenses could increase if we are required by the United States Food and Drug Administration, or FDA, to perform studies in addition to those currently expected, or if there are any delays in completing our clinical trials or the development of any of our product candidates. The amount of future net losses will depend, in part, on the rate of future growth of our expenses and our ability to generate revenues. To date, our only source of revenue has been our collaboration and license agreement with Sanofi. Future payments from Sanofi under this collaboration are uncertain because Sanofi may choose not to continue research or development of activities for one or more potential RX-04 product candidates under the collaboration, we may not achieve milestones under the agreement with Sanofi, Sanofi may not exercise its option to license any RX-04 product candidates, and RX-04 product candidates may not be approved or, if they are approved, may not be accepted in the market. If we are unable to develop and commercialize one or more of our product candidates, either alone or with collaborators, or if revenues from any such collaboration product candidate that receives marketing approval are insufficient, we will not achieve profitability. Even if we do achieve profitability, we may not be able to sustain or increase profitability.

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If we are unable to raise capital when needed, we would be forced to delay, reduce or eliminate our product development programs.

Developing pharmaceutical products, including conducting preclinical studies and clinical trials, is expensive. We expect to incur increased expenses as we commence Phase 3 development of delafloxacin, satisfy our obligations under our agreement with Sanofi, advance our other product candidates and expand our research and development programs. Moreover, proceeds from this offering will not be sufficient to complete the development and commercialization of our lead product candidate, delafloxacin, or to continue the development of radezolid. Accordingly, we will need to obtain additional funding beyond the proceeds of this contemplated offering to complete the development and commercialization of delafloxacin as well as to continue to advance the development of radezolid and our other clinical and preclinical candidates. In order to complete Phase 3 development of delafloxacin, we estimate that our first planned ABSSSI Phase 3 study will cost approximately $             million and our second planned ABSSSI Phase 3 study will cost approximately $             million. If the FDA requires that we perform additional studies beyond those that we currently believe will be required, our expenses would further increase beyond what we currently anticipate and the anticipated timing of any potential product approvals may be delayed. Under our collaboration and license agreement with Sanofi, we are required to use personnel and other resources in the conduct of a joint development plan directed toward identifying and optimizing product candidates thereunder meeting mutually agreed target product profiles. We currently have no commitments or arrangements to fund our research and development programs other than future contingent milestone or royalty payments from Sanofi, which require the successful development, regulatory approval and commercialization of one or more product candidates thereunder and may not be received for several years. We believe that the net proceeds from this offering, together with amounts we anticipate receiving under our collaboration with Sanofi and existing cash and cash equivalents and interest thereon, will be sufficient to fund our projected operating requirements through the first quarter of 2014.

Our future funding requirements, both short-term and long-term, will depend on many factors, including, but not limited to:

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Unless and until we can generate a sufficient amount of revenue from our product candidates, we expect to finance future cash needs through public or private equity offerings, debt financings or regional collaborations and licensing arrangements. Additional funds may not be available when we need them on terms that are acceptable to us, or at all. If adequate funds are not available, we may be required to delay, reduce the scope of or eliminate one or more of our research or development programs or our commercialization efforts. To the extent that we raise additional funds by issuing equity securities, our stockholders may experience additional dilution, and debt financing, if available, may involve restrictive covenants. To the extent that we raise additional funds through collaborations and licensing arrangements, it may be necessary to relinquish some rights to our technologies or our product candidates or grant licenses on terms that may not be favorable to us. We may be required to access the public or private capital markets from time to time when conditions are unfavorable, or we may seek to access them when conditions are favorable even if we do not have an immediate need for additional capital.

We have a limited operating history and we expect a number of factors to cause our operating results to fluctuate on a quarterly and annual basis, which may make it difficult to predict our future performance.

Our operations to date have been primarily limited to developing our technology and undertaking preclinical studies and clinical trials of our product candidates. We have not yet obtained regulatory approvals for any of our product candidates. Consequently, any predictions made about our future success or viability may not be as accurate as they could be if we had a longer operating history or approved products on the market. Our financial condition and operating results have varied significantly in the past and are expected to continue to significantly fluctuate from quarter-to-quarter or year-to-year due to a variety of factors, many of which are beyond our control. Factors relating to our business that may contribute to these fluctuations include:

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Accordingly, the results of any quarterly or annual periods should not be relied upon as indications of future operating performance.

The timing of milestone, royalty and other payments we are required to make under our license agreements are uncertain and could adversely affect our cash flows and results of operations.

We are obligated, including pursuant to an exclusive license and supply agreement with CyDex Pharmaceuticals, Inc. (now a wholly owned subsidiary of Ligand Pharmaceuticals Incorporated, both hereafter referred to as Ligand), an exclusive license agreement with Wakunaga Pharmaceutical Co., Ltd., or Wakunaga, and an exclusive license agreement with Yale University, to make milestone payments and pay royalties and other fees in connection with the development and commercialization of our product candidates. The timing of our achievement of these milestones and the corresponding milestone payments is subject to factors relating to the clinical and regulatory development and commercialization of our product candidates, which are difficult to predict and for which many are beyond our control. We may become obligated to make a milestone or other payment at a time when we do not have sufficient funds to make such payment, which could result in the loss of required intellectual property rights to further develop or commercialize one or more of our product candidates, or at a time that would otherwise require us to use funds needed to continue to operate our business, which could delay our clinical trials, curtail our operations, scale back our commercialization and marketing efforts or seek funds to meet these obligations on terms unfavorable to us. In addition, disputes with a licensor regarding compliance with the requirements of our agreements could result in our making milestone, royalty or other payments when we do not believe they are due to avoid potentially expensive litigation. If we are unable to make any payment when due or if we fail to use commercially reasonable efforts to achieve certain development and commercialization milestones within the timeframes required by these agreements, the other party may have the right to terminate the agreement and all of our rights to develop and commercialize product candidates using the applicable technology.

Our independent registered public accounting firm has expressed doubt about our ability to continue as a going concern.

Based on our cash balances, recurring losses, net capital deficiency, and significant debt outstanding as of December 31, 2011 and our projected spending in 2012, which raise

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substantial doubt about our ability to continue as a going concern, our independent registered public accounting firm has included an explanatory paragraph in its report on our financial statements as of and for the year ended December 31, 2011 regarding this uncertainty. We believe that the net proceeds from this offering, together with proceeds of $15.0 million from a loan agreement entered into in February 2012 and amounts we have received and anticipate receiving under our collaboration with Sanofi and existing cash and cash equivalents and interest thereon, will be sufficient to fund our projected operating requirements through the first quarter of 2014. However, if we are unable to continue as a going concern, we might have to liquidate our assets and the values we receive for our assets in liquidation or dissolution could be significantly lower than the values reflected in our financial statements. Amounts due under the February 2012 loan agreement may become immediately due and payable upon the occurrence of a material adverse change, as defined under the loan agreement. Under the terms of the loan agreement, we are subject to operational covenants, including limitations on our ability to incur liens or additional debt, pay dividends, redeem stock, make specified investments and engage in merger, consolidation or asset sale transactions, among other restrictions. In addition, the inclusion of a going concern statement by our auditors, our lack of cash resources and our potential inability to continue as a going concern may materially adversely affect our share price and our ability to raise new capital or to enter into critical contractual relations with third parties.

Risks Relating to Regulatory Review and Approval of Our Product Candidates

We cannot be certain that delafloxacin, radezolid, our RX-04 product candidates or any of our other product candidates will receive regulatory approval, and without regulatory approval we will not be able to market our product candidates.

We have invested a significant portion of our efforts and financial resources in the development of our most advanced product candidates, especially delafloxacin. Our ability to generate revenue related to product sales, if ever, will depend on the successful development and regulatory approval of these product candidates.

While it is not required, we plan to request a special protocol assessment, or SPA, for our Phase 3 clinical protocol for delafloxacin from the FDA. An SPA is intended to provide assurance that if the agreed upon clinical trial protocols are followed, the clinical trial endpoints are achieved, and there is a favorable risk-benefit profile, the data may serve as the primary basis for an efficacy claim in support of an NDA. However, SPA agreements are not a guarantee of an approval of a product candidate or any permissible claims about the product candidate. In particular, SPAs are not binding on the FDA if previously unrecognized public health concerns arise during the performance of the clinical trial, other new scientific concerns regarding product candidate safety or efficacy arise or if the sponsoring company fails to comply with the agreed upon clinical trial protocols. We cannot predict whether we will be able to reach agreement with the FDA on an SPA or, if we do reach agreement, whether any issues will arise during the clinical trial that would negate that agreement. In addition, we do not know how the FDA will interpret the commitments under the agreed upon SPA and how it will interpret the data and results.

We currently have no products approved for sale and we cannot guarantee that we will ever have marketable products. The development of a product candidate and issues relating to its approval and sale are subject to extensive regulation by the FDA in the United States and regulatory authorities in other countries, with regulations differing from country to country. We are not permitted to market our product candidates in the United States until we receive approval of an NDA from the FDA. We have not submitted an NDA for any of our product

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candidates. An NDA must include extensive preclinical and clinical data and supporting information to establish the product candidate’s safety and effectiveness for each desired indication. The NDA must also include significant information regarding the chemistry, manufacturing and controls for the product. Obtaining approval of an NDA is a lengthy, expensive and uncertain process, and may not be obtained. The FDA review process typically takes years to complete and approval is never guaranteed. If we submit an NDA to the FDA, the FDA must decide whether to accept or reject the submission for filing. We cannot be certain that any submissions will be accepted for filing and review by the FDA. Even if a product is approved, the FDA may limit the indications for which the product may be marketed, include extensive warnings on the product labeling or require expensive and time-consuming post-approval clinical trials or reporting as conditions of approval. Foreign regulatory authorities also have requirements for approval of drug candidates with which we must comply prior to marketing. Obtaining regulatory approval for marketing of a product candidate in one country does not ensure that we will be able to obtain regulatory approval in other countries. In addition, delays in approvals or rejections of marketing applications in the United States or foreign countries may be based upon many factors, including regulatory requests for additional analyses, reports, data and studies, regulatory questions regarding or different interpretations of data and results, changes in regulatory policy during the period of product development and the emergence of new information regarding our product candidates or other products. Also, regulatory approval for any of our product candidates may be withdrawn. If delafloxacin, radezolid or any of our other product candidates do not receive regulatory approval, we may not be able to generate sufficient revenue to become profitable or to continue our operations.

Delays in the commencement, enrollment and completion of clinical trials could result in increased costs to us and delay or limit our ability to obtain regulatory approval for our product candidates.

Delays in the commencement, enrollment and completion of clinical trials could increase our product development costs or limit the regulatory approval of our product candidates. We plan to commence the first of two planned Phase 3 trials of delafloxacin for the treatment of ABSSSI in the second half of 2012. However, the timing of the second planned Phase 3 clinical trial will depend upon obtaining additional funding beyond the proceeds of this offering. We may be unable to initiate or complete such development on schedule, if at all. In addition, we do not know whether any future trials or studies of our other product candidates will begin on time or will be completed on schedule, if at all. The commencement, enrollment and completion of clinical trials can be delayed for a variety of reasons, including:

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Changes in regulatory requirements and guidance may also occur and we or any of our partners may need to amend clinical trial protocols to reflect these changes with appropriate regulatory authorities. Amendments may require us or any of our partners to resubmit clinical trial protocols to IRBs for re-examination, which may impact the costs, timing or successful completion of a clinical trial. In addition, a clinical trial may be suspended or terminated at any time by us, our current or future partners, the FDA or other regulatory authorities due to a number of factors, including:

In addition, if we or any of our partners are required to conduct additional clinical trials or other testing of our product candidates beyond those contemplated, our ability to obtain regulatory approval of these product candidates and generate revenue from their sales would be similarly harmed.

Clinical failure can occur at any stage of clinical development and we have never conducted a Phase 3 trial or submitted an NDA before. The results of earlier clinical trials are not necessarily predictive of future results and any product candidate we, Sanofi or our potential future partners advance through clinical trials may not have favorable results in later clinical trials or receive regulatory approval.

Clinical failure can occur at any stage of our clinical development. Clinical trials may produce negative or inconclusive results, and we or our partners may decide, or regulators may require us, to conduct additional clinical or preclinical testing. In addition, data obtained from tests are susceptible to varying interpretations, and regulators may not interpret our data as favorably as we do, which may delay, limit or prevent regulatory approval. Success in preclinical testing and early clinical trials does not ensure that subsequent clinical trials will generate the same or similar results or otherwise provide adequate data to demonstrate the efficacy and safety of a product candidate. Frequently, product candidates that have shown promising results in early clinical trials have suffered significant setbacks in subsequent clinical trials. In addition, the design of a clinical trial can determine whether its results will support approval of a product and flaws in the design of a clinical trial may not become apparent until the clinical trial is well advanced. We have limited experience in designing clinical trials and may be unable to design and execute a clinical trial to support regulatory approval. Further, clinical trials of potential products often reveal that it is not practical or feasible to continue development efforts. If delafloxacin, radezolid or our other product candidates are found to be unsafe or lack efficacy, we will not be able to obtain regulatory approval for them and our

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business would be harmed. For example, if the results of our planned Phase 3 clinical trials of delafloxacin do not achieve the primary efficacy endpoints or demonstrate expected safety, the prospects for approval of delafloxacin would be materially and adversely affected. A number of companies in the pharmaceutical industry, including those with greater resources and experience than us, have suffered significant setbacks in Phase 3 clinical trials, even after seeing promising results in earlier clinical trials.

In some instances, there can be significant variability in safety and/or efficacy results between different trials of the same product candidate due to numerous factors, including changes in trial protocols, differences in size and type of the patient populations, adherence to the dosing regimen and other trial protocols and the rate of dropout among clinical trial participants. We do not know whether any Phase 2, Phase 3 or other clinical trials we or any of our partners may conduct will demonstrate consistent or adequate efficacy and safety to obtain regulatory approval to market our product candidates.

Our product candidates may have undesirable side effects which may delay or prevent marketing approval, or, if approval is received, require them to be taken off the market, require them to include safety warnings or otherwise limit their sales.

We refer to those adverse events observed in our clinical trials with an incidence rate equal to or greater than 5% of the subjects in a clinical trial as common adverse events. The common adverse events observed in clinical trials of delafloxacin were nausea, diarrhea, vomiting, pruritus, fatigue, headache, dizziness, infusion site pain, insomnia, constipation, rhinitis and dry mouth. The common adverse events observed in clinical trials of radezolid were nausea, diarrhea, headache, dizziness and fungal infection. Three patients receiving delafloxacin in our clinical trials have had serious adverse events that were thought by the investigator to be possibly related to delafloxacin therapy. One patient with a previously non-disclosed recent onset seizure disorder had a further seizure on delafloxacin. One patient with a complicated medical history was hospitalized with abdominal pain and diarrhea. A third patient had a single episode of mouth swelling and shortness of breath. Two patients receiving radezolid in our clinical trials have had serious adverse events that were thought by the investigator to be possibly related to radezolid therapy. One patient with lung cancer had a pneumonia that did not respond to radezolid therapy. A second patient with prior peptic ulcer disease discontinued ulcer therapy prior to enrolling in a radezolid trial and had a recurrent ulcer with perforation. Additional or unforeseen side effects from these or any of our other product candidates could arise either during clinical development or, if approved, after the approved product has been marketed. Our product candidates are being developed for the systemic treatment of multi-drug resistant and extremely-drug resistant infections caused by Gram-positive and Gram-negative bacteria and are still in the early stages of clinical development. The range and potential severity of possible side effects from systemic therapies is significant. The results of future clinical trials may show that our product candidates cause undesirable or unacceptable side effects, which could interrupt, delay or halt clinical trials, and result in delay of, or failure to obtain, marketing approval from the FDA and other regulatory authorities, or result in marketing approval from the FDA and other regulatory authorities with restrictive label warnings.

If any of our product candidates receives marketing approval and we or others later identify undesirable or unacceptable side effects caused by such products:

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Any of these events could prevent us, Sanofi or our potential future partners from achieving or maintaining market acceptance of the affected product or could substantially increase commercialization costs and expenses, which in turn could delay or prevent us from generating significant revenues from the sale of our products.

Reimbursement decisions by third-party payors may have an adverse effect on pricing and market acceptance. If there is not sufficient reimbursement for our products, it is less likely that our products will be widely used.

Market acceptance and sales of delafloxacin, radezolid, or any other product candidates that we develop will depend on reimbursement policies and may be affected by future healthcare reform measures. Government authorities and third-party payors, such as private health insurers and health maintenance organizations, decide which drugs they will cover and establish payment levels. We cannot be certain that reimbursement will be available for delafloxacin, radezolid, or any other product candidates that we develop. Also, we cannot be certain that reimbursement policies will not reduce the demand for, or the price paid for, our products. If reimbursement is not available or is available on a limited basis, we may not be able to successfully commercialize delafloxacin, radezolid, or any other product candidates that we develop.

In the United States, the Medicare Prescription Drug, Improvement, and Modernization Act of 2003, also called the Medicare Modernization Act, or MMA, changed the way Medicare covers and pays for pharmaceutical products. The legislation established Medicare Part D, which expanded Medicare coverage for outpatient prescription drug purchases by the elderly but provided authority for limiting the number of drugs that will be covered in any therapeutic class. The MMA also introduced a new reimbursement methodology based on average sales prices for physician-administered drugs.

The United States and several foreign jurisdictions are considering, or have already enacted, a number of legislative and regulatory proposals to change the healthcare system in ways that could affect our ability to sell our products profitably. Among policy makers and payors in the United States and elsewhere, there is significant interest in promoting changes in healthcare systems with the stated goals of containing healthcare costs, improving quality and/or expanding access to healthcare. In the United States, the pharmaceutical industry has been a particular focus of these efforts and has been significantly affected by major legislative initiatives. We expect to experience pricing pressures in connection with the sale of delafloxacin and radezolid and any other products that we develop, due to the trend toward managed healthcare, the increasing influence of health maintenance organizations and additional legislative proposals.

In March 2010, the Patient Protection and Affordable Care Act, as amended by the Health Care and Education Affordability Reconciliation Act, or collectively, ACA, became law in the U.S. The goal of ACA is to reduce the cost of health care and substantially change the way health care is financed by both governmental and private insurers. While we cannot predict what impact on federal reimbursement policies this legislation will have in general or on our business specifically, the ACA may result in downward pressure on pharmaceutical reimbursement,

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which could negatively affect market acceptance of delafloxacin, radezolid or any future product candidates. Members of the U.S. Congress and some state legislatures are seeking to overturn at least portions of the legislation and we expect they will continue to review and assess this legislation and possibly alternative health care reform proposals. We cannot predict whether new proposals will be made or adopted, when they may be adopted or what impact they may have on us if they are adopted.

Proposed legislation before Congress specific to antibiotics may have a material impact on antibiotic drug development.

In the past few months, identical bills intended to encourage development of antibiotics were introduced in the U.S. House and Senate. The Generating Antibiotic Incentives Now Act, or GAIN, would provide incentives for the development of infectious disease products to address the growing epidemic of antibiotic resistant infections. The bill recommends that qualified infectious disease products receive both Fast Track designation and Priority Review from the FDA and an additional five year period of market protection at the end of existing exclusivity periods. While GAIN may have a positive impact on our business if enacted, there is no guarantee that it will be enacted in its current form or that we would benefit from it for delafloxacin, radezolid, or any of our other product candidates. Furthermore, GAIN may have a disproportionately favorable effect for our competitors’ products compared to delafloxacin, radezolid, and our other product candidates which will make it harder for our product candidates to compete in the antibiotic market.

If we do not obtain protection under the Hatch-Waxman Amendments and similar foreign legislation by extending the patent terms and obtaining data exclusivity for our product candidates, our business may be materially harmed.

Depending upon the timing, duration and specifics of FDA marketing approval of delafloxacin, radezolid, and our RX-04 and other product candidates, one or more of our U.S. patents may be eligible for limited patent term restoration under the Drug Price Competition and Patent Term Restoration Act of 1984, referred to as the Hatch-Waxman Amendments. The Hatch-Waxman Amendments permit a patent restoration term of up to five years as compensation for patent term lost during product development and the FDA regulatory review process. However, we may not be granted an extension because of, for example, failing to apply within applicable deadlines, failing to apply prior to expiration of relevant patents or otherwise failing to satisfy applicable requirements. Moreover, the applicable time period or the scope of patent protection afforded could be less than we request. If we are unable to obtain patent term extension or restoration or the term of any such extension is less than we request, the period during which we will have the right to exclusively market our product will be shortened and our competitors may obtain approval of competing products following our patent expiration, and our revenue could be reduced, possibly materially. In the event that we are unable to obtain any patent term extensions, the issued composition of matter patents for delafloxacin, delafloxacin meglumine and radezolid are expected to expire in 2016, 2027 and 2024, respectively, and, if issued, the pending composition of matter patents for our RX-04 compounds would be expected to expire in 2030, in each case assuming the appropriate maintenance, renewal, annuity or other governmental fees are paid.

If we market products in a manner that violates healthcare fraud and abuse laws, or if we violate government price reporting laws, we may be subject to civil or criminal penalties.

In addition to FDA restrictions on marketing of pharmaceutical products, several other types of state and federal healthcare laws commonly referred to as “fraud and abuse” laws have been applied in recent years to restrict certain marketing practices in the pharmaceutical industry.

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These laws include false claims and anti-kickback statutes. At such time as we market our products and our products are paid for by governmental programs, it is possible that some of our business activities could be subject to challenge under one or more of these laws.

Federal false claims laws prohibit any person from knowingly presenting, or causing to be presented, a false claim for payment to the federal government or knowingly making, or causing to be made, a false statement to get a false claim paid. The federal healthcare program anti-kickback statute prohibits, among other things, knowingly and willfully offering, paying, soliciting or receiving remuneration to induce, or in return for, purchasing, leasing, ordering or arranging for the purchase, lease or order of any healthcare item or service reimbursable under Medicare, Medicaid or other federally financed healthcare programs. This statute has been interpreted to apply to arrangements between pharmaceutical manufacturers on the one hand and prescribers, purchasers and formulary managers on the other. Although there are several statutory exemptions and regulatory safe harbors protecting certain common activities from prosecution, the exemptions and safe harbors are drawn narrowly, and practices that involve remuneration intended to induce prescribing, purchasing or recommending may be subject to scrutiny if they do not qualify for an exemption or safe harbor. Most states also have statutes or regulations similar to the federal anti-kickback law and federal false claims laws, which apply to items and services reimbursed under Medicaid and other state programs, or, in several states, apply regardless of the payor. Administrative, civil and criminal sanctions may be imposed under these federal and state laws.

Over the past few years, a number of pharmaceutical and other healthcare companies have been prosecuted under these laws for a variety of promotional and marketing activities, such as: providing free trips, free goods, sham consulting fees and grants and other monetary benefits to prescribers; reporting to pricing services inflated average wholesale prices that were then used by federal programs to set reimbursement rates; engaging in off-label promotion; and submitting inflated best price information to the Medicaid Rebate Program to reduce liability for Medicaid rebates.

If the FDA does not approve the manufacturing facilities of any future manufacturing partners for commercial production, we may not be able to commercialize any of our product candidates.

We do not intend to manufacture the pharmaceutical products that we plan to sell. We may not be able to identify and reach arrangement with a contract manufacturer to manufacture delafloxacin, radezolid or any of our other product candidates. Additionally, the facilities used by any contract manufacturer to manufacture delafloxacin, radezolid or any of our other product candidates must be the subject of a satisfactory inspection before the FDA approves an NDA for the product candidate manufactured at that facility. We are completely dependent on these third-party manufacturing partners for compliance with the FDA’s requirements for manufacture of our finished products. If our manufacturers cannot successfully manufacture material that conforms to our specifications and the FDA’s current good manufacturing practice requirements, our product candidates will not be approved or, if already approved, may be subject to recalls. Reliance on third-party manufacturers entails risks to which we would not be subject if we manufactured the product candidates, including:

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Any of these factors could cause the delay of approval or commercialization of our products, cause us to incur higher costs or prevent us from commercializing our product candidates successfully. Furthermore, if any of our product candidates are approved and contract manufacturers fail to deliver the required commercial quantities of finished product on a timely basis and at commercially reasonable prices and we are unable to find one or more replacement manufacturers capable of production at a substantially equivalent cost, in substantially equivalent volumes and quality and on a timely basis, we would likely be unable to meet demand for our products and could lose potential revenue. It may take several years to establish an alternative source of supply for our product candidates and to have any such new source approved by the FDA.

Even if our product candidates receive regulatory approval, we may still face future development and regulatory difficulties.

Our product candidates will also be subject to ongoing regulatory requirements for the labeling, packaging, storage, advertising, promotion, record-keeping and submission of safety and other post-market information on the drug. In addition, approved products, manufacturers and manufacturers’ facilities are required to comply with extensive FDA requirements, including ensuring that quality control and manufacturing procedures conform to current Good Manufacturing Practices, or cGMPs. As such, we and our contract manufacturers are subject to continual review and periodic inspections to assess compliance with cGMPs. Accordingly, we and others with whom we work must continue to expend time, money, and effort in all areas of regulatory compliance, including manufacturing, production, and quality control. We will also be required to report certain adverse reactions and production problems, if any, to the FDA and to comply with certain requirements concerning advertising and promotion for our products. Promotional communications with respect to prescription drugs are subject to a variety of legal and regulatory restrictions and must be consistent with the information in the product’s approved label. Accordingly, we may not promote our products for indications or uses for which they are not approved.

If a regulatory agency discovers previously unknown problems with a product, such as adverse events of unanticipated severity or frequency, or problems with the facility where the product is manufactured, or disagrees with the promotion, marketing, or labeling of a product, it may impose restrictions on that product or us, including requiring withdrawal of the product from the market. If our product candidates fail to comply with applicable regulatory requirements, a regulatory agency may:

We will need to obtain FDA approval of any proposed product names, and any failure or delay associated with such approval may adversely affect our business.

Any name we intend to use for our product candidates will require approval from the FDA regardless of whether we have secured a formal trademark registration from the United States

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Patent and Trademark Office, or USPTO. The FDA typically conducts a review of proposed product names, including an evaluation of the potential for confusion with other product names. The FDA may also object to a product name if it believes the name inappropriately implies medical claims. If the FDA objects to any of our proposed product names, we may be required to adopt an alternative name for our product candidates. If we adopt an alternative name, we would lose the benefit of our existing trademark applications for such product candidate and may be required to expend significant additional resources in an effort to identify a suitable product name that would qualify under applicable trademark laws, not infringe the existing rights of third parties and be acceptable to the FDA. We may be unable to build a successful brand identity for a new trademark in a timely manner or at all, which would limit our ability to commercialize our product candidates.

Risks Relating to Our Business

Even if approved, if any of our product candidates do not achieve broad market acceptance among physicians, patients and the medical community, our revenues generated from their sales will be limited.

The commercial success of delafloxacin, radezolid, our RX-04 product candidates and our other product candidates will depend upon their acceptance among physicians, patients and the medical community. The degree of market acceptance of our product candidates will depend on a number of factors, including:

If our product candidates are approved, but do not achieve an adequate level of acceptance by physicians, patients and the medical community, sufficient revenue may not be generated from these products, and we may not become or remain profitable. In addition, efforts to educate the medical community and third-party payors on the benefits of our product candidates may require significant resources and may never be successful.

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Bacteria might develop resistance to any of our product candidates, which would decrease the efficacy and commercial viability of those product candidates.

Drug resistance is primarily caused by the genetic mutation of bacteria resulting from sub-optimal exposure to antibiotics where the drug does not kill all of the bacteria. While antibiotics have been developed to treat many of the most common infections, the extent and duration of their use worldwide has resulted in new mutated strains of bacteria resistant to current treatments. We are developing product candidates to treat patients infected with drug-resistant bacteria. If physicians, rightly or wrongly, associate the resistance issues of other products of the same class as our product candidates, physicians might not prescribe our product candidates for treating a broad range of infections. If our product candidates are improperly dosed, bacteria might develop resistance to those product candidates causing the efficacy of these product candidates to decline, which would negatively affect our potential to generate revenues from those product candidates.

We currently have no sales and marketing infrastructure and have no experience in marketing drug products, and if we are unable to establish an effective sales force and marketing infrastructure, or enter into acceptable third-party sales and marketing or licensing arrangements, we may not be able to commercialize our product candidates successfully.

We plan to develop a sales and marketing infrastructure to market and sell our products in the United States. We currently do not have any sales, distribution and marketing capabilities, the development of which will require substantial resources and will be time consuming. These costs may be incurred in advance of any approval of our product candidates. In addition, we may not be able to hire a sales force in the United States that is sufficient in size or has adequate expertise in the medical markets that we intend to target. If we are unable to establish our sales force and marketing capability, our operating results may be adversely affected. In addition, we plan to enter into sales and marketing or licensing arrangements with third parties for international sales of any approved products. If we are unable to enter into any such arrangements on acceptable terms, or at all, we may be unable to market and sell our products in these markets.

We face significant competition from other biotechnology and pharmaceutical companies and our operating results will suffer if we fail to compete effectively.

The biotechnology and pharmaceutical industries are intensely competitive and subject to rapid and significant technological change. We have competitors both in the United States and internationally, including major multinational pharmaceutical companies, established biotechnology companies, specialty pharmaceutical and generic drug companies and universities and other research institutions. Many of our competitors have greater financial and other resources, such as larger research and development staff and more experienced marketing and manufacturing organizations. As a result, these companies may obtain regulatory approval more rapidly than we are able to and may be more effective in selling and marketing their products as well. Smaller or early-stage companies may also prove to be significant competitors, particularly through collaborative arrangements with large, established companies. Our competitors may succeed in developing, acquiring or licensing on an exclusive basis, technologies and drug products that are more effective or less costly than delafloxacin, radezolid, or any other product candidates that we are currently developing or that we may develop, which could render our products obsolete and noncompetitive.

The competition in the market for antibiotics is intense. If approved, our product candidates will face competition from commercially available antibiotics such as vancomycin, marketed as

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a generic by Abbott Laboratories and others; daptomycin, marketed by Cubist Pharmaceuticals, Inc. as Cubicin; linezolid, marketed by Pfizer Inc. as Zyvox; ceftaroline, marketed by Forest Laboratories, Inc. as Teflaro; tigecycline, marketed as Tygacil by Pfizer; and telavancin, marketed by Theravance, Inc. and Astellas Pharma, Inc. as Vibativ. Vancomycin has been a widely used and well known antibiotic for over 40 years and is sold in a relatively inexpensive generic IV form. Vancomycin, daptomycin, ceftaroline, tigecycline, linezolid and telavancin are all approved treatments for serious gram-positive infections such as ABSSSI. Additionally, daptomycin is an approved treatment for bacteremia, tigecycline is an approved treatment for cIAI and CABP, linezolid is an approved treatment for pneumonia and vancomycin is an approved treatment for both bacteremia and pneumonia. If we are unable to obtain regulatory approval of our product candidates for some or all of the indications for which our competitors are approved, we may not be able to compete effectively with such antibiotics.

In addition, if approved, our product candidates may face additional competition from antibiotics currently in clinical development. Other antibiotics currently in development include ceftobiprole, under development by Basilea Pharmaceutica AG and approved in Canada and Switzerland, CEM-102, under development by Cempra, Inc., dalbavancin, under development by Durata Therapeutics, Inc., tedizolid, under development by Trius Therapeutics, Inc., NXL-103, under development by AstraZeneca PLC, oritavancin, under development by The Medicines Company, and PTK 0796, previously under development by Paratek Pharmaceuticals, Inc. and Novartis AG, which, if approved, would compete in the antibiotic market. In addition, our product candidates may each face competition from product candidates currently in clinical development and product candidates that could receive regulatory approval before our product candidates in countries outside the United States and the European Union. If we are unable to demonstrate the advantages of our product candidates over competing products and product candidates, we will not be able to successfully commercialize our product candidates and our results of operations will suffer.

Established pharmaceutical companies may also invest heavily to accelerate discovery and development of novel compounds or to in-license novel compounds that could make delafloxacin, radezolid or any other product candidates that we develop obsolete. As a result of all of these factors, our competitors may succeed in obtaining patent protection and/or FDA approval or discovering, developing and commercializing antibiotics before we do.

If approved, delafloxacin and radezolid will face competition from less expensive generic versions of branded antibiotics of competitors, and if we are unable to differentiate the benefits of delafloxacin or radezolid over these less expensive alternatives, we may never generate meaningful product revenues.

Generic antibiotic therapies are typically sold at lower prices than branded antibiotics and are generally preferred by insurers and other third party payors. We anticipate that, if approved, delafloxacin and radezolid will face increasing competition in the form of generic versions of branded products of competitors that have lost or will lose their patent exclusivity. For example, both delafloxacin and radezolid, if approved, will initially face competition from the inexpensive generic forms of vancomycin that are currently available and, in the future, may face additional competition from generic forms of other antibiotics and from generic versions of our product after any applicable marketing exclusivity periods expire. If we are unable to demonstrate to physicians and payors that the key differentiating features of delafloxacin and radezolid translate to overall clinical benefit or lower cost of care, we may not be able to compete with generic antibiotics.

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We may not be successful in establishing and maintaining development and commercialization collaborations, which could adversely affect our ability to develop certain of our product candidates and our financial condition and operating results.

Developing pharmaceutical products, conducting clinical trials, obtaining regulatory approval, establishing manufacturing capabilities and marketing approved products is expensive. For example, we have entered into a research collaboration with Sanofi with respect to product candidates developed in our RX-04 program. We plan to establish additional collaborations for development and commercialization of product candidates and research programs, including to fund the continued development of delafloxacin and radezolid. Additionally, if delafloxacin, radezolid or any of our other product candidates receives marketing approval, we intend to enter into sales and marketing arrangements with third parties for international sales, and to develop our own sales force in the United States. If we are unable to maintain our existing arrangements or enter into any new such arrangements on acceptable terms, if at all, we may be unable to effectively market and sell our products in our target markets. We expect to face competition in seeking appropriate collaborators. Moreover, collaboration arrangements are complex and time consuming to negotiate, document and implement and they may require substantial resources to maintain. We may not be successful in our efforts to establish and implement collaborations or other alternative arrangements for the development of our product candidates. When we partner with a third party for development and commercialization of a product candidate, we can expect to relinquish some or all of the control over the future success of that product candidate to the third party. Our collaboration partner may not devote sufficient resources to the commercialization of our product candidates or may otherwise fail in their commercialization. The terms of any collaboration or other arrangement that we establish may not be favorable to us. In addition, any collaboration that we enter into, including our collaboration with Sanofi, may be unsuccessful in the development and commercialization of our product candidates. In some cases, we may be responsible for continuing preclinical and initial clinical development of a partnered product candidate or research program, and the payment we receive from our collaboration partner may be insufficient to cover the cost of this development. If we are unable to reach agreements with suitable collaborators for our product candidates, we would face increased costs, we may be forced to limit the number of our product candidates we can commercially develop or the territories in which we commercialize them and we might fail to commercialize products or programs for which a suitable collaborator cannot be found. If we fail to achieve successful collaborations, our operating results and financial condition will be materially and adversely affected.

If we fail to develop delafloxacin and radezolid for additional indications, our commercial opportunity will be limited.

To date, we have focused primarily on the development of delafloxacin for the treatment of ABSSSI. A key element of our strategy is to pursue clinical development of delafloxacin for other indications, including CABP and cIAI. Although we believe there is substantial commercial opportunity for the treatment of ABSSSI alone, our ability to generate and grow revenues will be highly dependent on our ability to successfully develop and commercialize delafloxacin for the treatment of these additional indications. The development of delafloxacin for these additional indications will require substantial additional funding beyond that needed to commercialize delafloxacin for the treatment of ABSSSI and is prone to the risks of failure inherent in drug development and we cannot provide you any assurance that we will able to successfully advance any of these programs through the development process. Even if we receive FDA approval to market delafloxacin for the treatment of any of these additional indications, we cannot assure you that any such additional indications will be successfully commercialized, widely accepted in the marketplace or more effective than other commercially

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available alternatives. If we are unable to successfully develop and commercialize delafloxacin for these additional indications, our commercial opportunity will be limited and our business prospects will suffer.

We currently plan to develop radezolid initially for the treatment of ABSSSI. A key element of our strategy is to pursue clinical development of radezolid for other indications, including severe CABP, and long-term treatment of serious infections, such as osteomyelitis and prosthetic and joint infections. Although we believe there is substantial commercial opportunity for the treatment of ABSSSI alone, our ability to generate and grow revenues will be highly dependent on our ability to successfully develop and commercialize radezolid for the treatment of these additional indications. The development of radezolid for these additional indications is prone to the risks of failure inherent in drug development and we cannot provide you any assurance that we will be able to successfully advance any of these programs through the development process. Even if we receive FDA approval to market radezolid for the treatment of any of these additional indications, we cannot assure you that any such additional indications will be successfully commercialized, widely accepted in the marketplace or more effective than other commercially available alternatives. If we are unable to successfully develop and commercialize radezolid for these additional indications, our commercial opportunity will be limited and our business prospects will suffer.

We depend on third-party contractors for a substantial portion of our operations and may not be able to control their work as effectively as if we performed these functions ourselves.

We outsource substantial portions of our operations to third-party service providers, including the conduct of preclinical studies and clinical trials, chemical synthesis, biological screening and manufacturing. Our agreements with third-party service providers and clinical research organizations are on a study-by-study basis and are typically short-term. In all cases, we may terminate the agreements with notice and are responsible for the supplier’s previously incurred costs. In addition, any contract research organization that we retain will be subject to the FDA’s regulatory requirements and similar foreign standards and we do not have control over compliance with these regulations by these providers. Consequently, if these providers do not adhere to the governing practices and standards, the development and commercialization of our product candidates could be delayed, which could severely harm our business and financial condition.

Because we have relied on third parties, our internal capacity to perform these functions is limited. Outsourcing these functions involves the risk that third parties may not perform to our standards, may not produce results in a timely manner or may fail to perform at all. In addition, the use of third-party service providers requires us to disclose our proprietary information to these parties, which could increase the risk that this information will be misappropriated. There is a limited number of third-party service providers that specialize or have the expertise required to achieve our business objectives. Identifying, qualifying and managing performance of third-party service providers can be difficult, time consuming and cause delays in our development programs. We currently have a small number of employees, which limits the internal resources we have available to identify and monitor our third-party providers. To the extent we are unable to identify, retain and successfully manage the performance of third-party service providers in the future, our business may be adversely affected.

We will need to expand our operations and increase the size of our company, and we may experience difficulties in managing growth.

As we increase the number of ongoing product development programs and advance our product candidates through preclinical studies and clinical trials, we will need to increase our product development, scientific and administrative headcount to manage these programs. In addition, to meet our obligations as a public company, we will need to increase our general and

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administrative headcount. Our management, personnel and systems currently in place may not be adequate to support this future growth. Our need to effectively manage our operations, growth and various projects requires that we:

If we are unable to successfully manage this growth, our business may be adversely affected.

We may not be able to manage our business effectively if we are unable to attract and retain key personnel.

We may not be able to attract or retain qualified management, finance, scientific and clinical personnel in the future due to the intense competition for qualified personnel among biotechnology, pharmaceutical and other businesses. If we are not able to attract and retain necessary personnel to accomplish our business objectives, we may experience constraints that will significantly impede the achievement of our development objectives, our ability to raise additional capital and our ability to implement our business strategy.

Our industry has experienced a high rate of turnover of management personnel in recent years. We are highly dependent on the development, regulatory, commercialization and business development expertise of our executive officers and key employees identified in the “Management” section of this prospectus. If we lose one or more of our executive officers or key employees, our ability to implement our business strategy successfully could be seriously harmed. We intend to enter into change of control and severance agreements with each of our officers as part of our retention efforts. The terms of these agreements are described in the “Executive Compensation—Potential Payments upon Termination or Change in Control” section of this prospectus. However, any of our executive officers or key employees may terminate their employment at any time. Replacing executive officers and key employees may be difficult and may take an extended period of time because of the limited number of individuals in our industry with the breadth of skills and experience required to develop, gain regulatory approval of and commercialize products successfully. Competition to hire from this limited pool is intense, and we may be unable to hire, train, retain or motivate these additional key personnel. Our failure to retain key personnel could materially harm our business.

Failure to build our finance infrastructure and improve our accounting systems and controls could impair our ability to comply with the financial reporting and internal controls requirements for publicly traded companies.

As a public company, we will operate in an increasingly demanding regulatory environment, which requires us to comply with the Sarbanes-Oxley Act of 2002, or the Sarbanes-Oxley Act, and the related rules and regulations of the Securities and Exchange Commission, expanded disclosure requirements, accelerated reporting requirements and more complex accounting rules. Company responsibilities required by the Sarbanes-Oxley Act include

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establishing corporate oversight and adequate internal control over financial reporting and disclosure controls and procedures. Effective internal controls are necessary for us to produce reliable financial reports and are important to help prevent financial fraud.

We have begun implementing our system of internal controls over financial reporting and preparing the documentation necessary to perform the evaluation needed to comply with Section 404 of the Sarbanes-Oxley Act. Although we will need to hire additional finance personnel and build our financial infrastructure as we transition to operating as a public company, including complying with the requirements of Section 404 of the Sarbanes-Oxley Act, we have recently taken actions to improve our financial infrastructure, including the hiring of a corporate controller and an accounting staff person. Following this offering as we begin operating as a public company, we will continue improving our financial infrastructure with the hiring of additional financial and accounting staff, the enhancement of internal controls, and additional training for our financial and accounting staff.

We will be required to comply with Section 404 of the Sarbanes-Oxley Act in connection with our Annual Report on Form 10-K for the year ending December 31, 2013. We may be unable to do so on a timely basis. Until we are able to expand our finance and administrative capabilities and establish necessary financial reporting infrastructure, we may not be able to prepare and disclose, in a timely manner, our financial statements and other required disclosures or comply with the Sarbanes-Oxley Act or existing or new reporting requirements. If we cannot provide reliable financial reports or prevent fraud, our business and results of operations could be harmed and investors could lose confidence in our reported financial information.

Our employees may engage in misconduct or other improper activities, including noncompliance with regulatory standards and requirements and insider trading.

We are exposed to the risk of employee fraud or other misconduct. Misconduct by employees could include intentional failures to comply with FDA regulations, provide accurate information to the FDA, comply with federal and state health care fraud and abuse laws and regulations, report financial information or data accurately or disclose unauthorized activities to us. In particular, sales, marketing and business arrangements in the health care industry are subject to extensive laws and regulations intended to prevent fraud, misconduct, kickbacks, self-dealing and other abusive practices. These laws and regulations may restrict or prohibit a wide range of pricing, discounting, marketing and promotion, sales commission, customer incentive programs and other business arrangements. Employee misconduct could also involve the improper use of information obtained in the course of clinical trials, which could result in regulatory sanctions and serious harm to our reputation. We intend to adopt a code of conduct prior to the completion of this offering, but it is not always possible to identify and deter employee misconduct, and the precautions we take to detect and prevent this activity may not be effective in controlling unknown or unmanaged risks or losses or in protecting us from governmental investigations or other actions or lawsuits stemming from a failure to comply with these laws or regulations. If any such actions are instituted against us, and we are not successful in defending ourselves or asserting our rights, those actions could have a significant impact on our business, including the imposition of significant fines or other sanctions.

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We face potential product liability exposure, and if successful claims are brought against us, we may incur substantial liability for a product candidate and may have to limit its commercialization.

The use of our product candidates in clinical trials and the sale of any products for which we may obtain marketing approval expose us to the risk of product liability claims. Product liability claims may be brought against us or our partners by participants enrolled in our clinical trials, patients, health care providers or others using, administering or selling our products. If we cannot successfully defend ourselves against any such claims, we would incur substantial liabilities. Regardless of merit or eventual outcome, product liability claims may result in:

We have obtained limited product liability insurance coverage for our clinical trials domestically and in selected foreign countries where we are conducting clinical trials. Our product liability insurance coverage is currently limited to $5 million per occurrence with an annual aggregate limit of $5 million. As such, our insurance coverage may not reimburse us or may not be sufficient to reimburse us for any expenses or losses we may suffer. Moreover, insurance coverage is becoming increasingly expensive, and, in the future, we may not be able to maintain insurance coverage at a reasonable cost or in sufficient amounts to protect us against losses due to product liability. We intend to expand our insurance coverage for products to include the sale of commercial products if we obtain marketing approval for our product candidates in development, but we may be unable to obtain commercially reasonable product liability insurance for any products approved for marketing. Large judgments have been awarded in class action lawsuits based on drugs that had unanticipated side effects. A successful product liability claim or series of claims brought against us, particularly if judgments exceed our insurance coverage, could decrease our cash resources and adversely affect our business.

Our insurance policies are expensive and protect us only from some business risks, which will leave us exposed to significant uninsured liabilities.

We do not carry insurance for all categories of risk that our business may encounter. For example, we do not carry earthquake insurance. In the event of a major earthquake in our region, our business could suffer significant and uninsured damage and loss. Some of the policies we currently maintain include general liability, employment practices liability, property, auto, workers’ compensation, products liability and directors’ and officers’ insurance. We do not know, however, if we will be able to maintain existing insurance with adequate levels of coverage. Any significant uninsured liability may require us to pay substantial amounts, which would adversely affect our cash position and results of operations.

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Our operations involve hazardous materials, which could subject us to significant liabilities.

Our research and development processes involve the controlled use of hazardous materials, including chemicals. Our operations produce hazardous waste products. We cannot eliminate the risk of accidental contamination or discharge or injury from these materials. Federal, state and local laws and regulations govern the use, manufacture, storage, handling and disposal of these materials. We could be subject to civil damages in the event of exposure of individuals to hazardous materials. In addition, claimants may sue us for injury or contamination that results from our use of these materials and our liability may exceed our total assets. We have general liability insurance of up to $2 million per occurrence with an annual aggregate limit of $2 million, which excludes pollution liability. We also have umbrella liability insurance of up to $5 million per occurrence with an annual aggregate limit of $5 million, which excludes product liability. This coverage may not be adequate to cover all claims related to our biological or hazardous materials. Furthermore, if we were to be held liable for a claim involving our biological or hazardous materials, this liability could exceed our insurance coverage, if any, and our other financial resources. Compliance with environmental and other laws and regulations may be expensive and current or future regulations may impair our research, development or production efforts.

Risks Relating to Our Intellectual Property

Our ability to pursue the development and commercialization of delafloxacin depends upon the continuation of our license from Wakunaga.

Our license agreement with Wakunaga provides us with a worldwide exclusive license to develop and sell delafloxacin. In particular, we obtained an exclusive license to certain patents, patent applications and proprietary information covering the composition of matter to delafloxacin, and rights to other patents and applications, which license requires us to make certain payments to Wakunaga. If we are unable to make the required milestone and royalty payments under the license agreement, or if we do not use commercially reasonable efforts to achieve certain development and commercialization milestones for delafloxacin within the timeframes required by the license agreement, our rights to develop and commercialize delafloxacin could be terminated and would revert to Wakunaga. In addition, either we or Wakunaga may terminate the license agreement upon a material breach of the license agreement not cured within 90 days from notice of breach. If our license agreement with Wakunaga were terminated, we would lose our rights to develop and commercialize delafloxacin, which would materially and adversely affect our business, results of operations and future prospects.

It is difficult and costly to protect our proprietary rights, and we may not be able to ensure their protection. If our patent position does not adequately protect our product candidates, others could compete against us more directly, which would harm our business, possibly materially.

Our commercial success will depend in part on obtaining and maintaining patent protection and trade secret protection of our ribosome-based discovery platform and of our current and future product candidates and the methods used to manufacture them, as well as successfully defending these patents against third-party challenges. Our ability to stop third parties from making, using, selling, offering to sell or importing our products and ribosome-based discovery platform is dependent upon the extent to which we have rights under valid and enforceable patents or trade secrets that cover these activities.

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The patent positions of pharmaceutical companies can be highly uncertain and involve complex legal and factual questions for which important legal principles remain unresolved. No consistent policy regarding the breadth of claims allowed in pharmaceutical patents has emerged to date in the United States or in many foreign jurisdictions. Changes in either the patent laws or interpretations of patent laws in the United States and foreign jurisdictions may diminish the value of our intellectual property. Accordingly, we cannot predict the breadth of claims that may be enforced in the patents that may be issued from the applications we currently or may in the future own or license from third parties. Further, if any patents we obtain or license are deemed invalid and unenforceable, our ability to commercialize or license our technology could be adversely affected.

The degree of future protection for our proprietary rights is uncertain because legal means afford only limited protection and may not adequately protect our rights or permit us to gain or keep our competitive advantage. For example:

As of March 31, 2012, we are the owner of record of over 15 issued or granted U.S. and foreign patents with claims directed to pharmaceutical compounds, pharmaceutical compositions, methods of making these compounds, and methods of using these compounds in various indications, and also to ribosome-based technology platforms and drug discovery methods. We are also the owner of record of over 100 pending U.S. and foreign patent applications in these areas.

As of March 31, 2012, we are the licensee of over 40 issued or granted U.S. and foreign patents and over 20 pending U.S. and foreign patent applications, with claims directed to pharmaceutical compounds, pharmaceutical compositions, methods of making these compounds, methods of using these compounds in various indications, and also to ribosome-based technology platforms and drug discovery methods.

Due to the patent laws of a country, or the decisions of a patent examiner in a country, or our own filing strategies, we may not obtain patent coverage for all of our product candidates or methods involving these candidates or for our ribosome-based technology platform in the parent patent application. We plan to pursue divisional patent applications or continuation patent applications in the United States and many other countries to obtain claim coverage for inventions which were disclosed but not claimed in the parent patent application.

We may also rely on trade secrets to protect our technology, especially where we do not believe patent protection is appropriate or feasible. However, trade secrets are difficult to

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protect. Although we use reasonable efforts to protect our trade secrets, our employees, consultants, contractors, outside scientific collaborators and other advisors may unintentionally or willfully disclose our information to competitors. Enforcing a claim that a third party illegally obtained and is using any of our trade secrets is expensive and time consuming, and the outcome is unpredictable. In addition, courts outside the United States are sometimes less willing to protect trade secrets. Moreover, our competitors may independently develop equivalent knowledge, methods and know-how.

We may incur substantial costs as a result of litigation or other proceedings relating to patent and other intellectual property rights.

If we choose to go to court to stop another party from using the inventions claimed in any patents we obtain, that individual or company has the right to ask the court to rule that such patents are invalid or should not be enforced against that third party. These lawsuits are expensive and would consume time and resources and divert the attention of managerial and scientific personnel even if we were successful in stopping the infringement of such patents. In addition, there is a risk that the court will decide that such patents are not valid and that we do not have the right to stop the other party from using the inventions. There is also the risk that, even if the validity of such patents is upheld, the court will refuse to stop the other party on the ground that such other party’s activities do not infringe our rights to such patents. In addition, the U.S. Supreme Court has recently modified some tests used by the USPTO in granting patents over the past 20 years, which may decrease the likelihood that we will be able to obtain patents and increase the likelihood of challenge of any patents we obtain or license.

We may infringe the intellectual property rights of others, which may prevent or delay our product development efforts and stop us from commercializing or increase the costs of commercializing our products.

Our success will depend in part on our ability to operate without infringing the proprietary rights of third parties. Patents of which we are not aware, and that our products infringe, may be issued. Additionally, patents that we believe we do not infringe, but that we may ultimately be found to infringe, could be issued. Furthermore, a third party may claim that we or our manufacturing or commercialization partners are using inventions covered by the third party’s patent rights and may go to court to stop us from engaging in our normal operations and activities, including making or selling our product candidates. These lawsuits are costly and could affect our results of operations and divert the attention of managerial and scientific personnel. There is a risk that a court would decide that we or our commercialization partners are infringing the third party’s patents and would order us or our partners to stop the activities covered by the patents. In that event, we or our commercialization partners may not have a viable way around the patent and may need to halt commercialization of the relevant product with it. In addition, there is a risk that a court will order us or our partners to pay the other party damages for having violated the other party’s patents. In the future, we may agree to indemnify our commercial partners against certain intellectual property infringement claims brought by third parties. The pharmaceutical and biotechnology industries have produced a proliferation of patents, and it is not always clear to industry participants, including us, which patents cover various types of products or methods of use. The coverage of patents is subject to interpretation by the courts, and the interpretation is not always uniform. If we are sued for patent infringement, we would need to demonstrate that our products or methods either do not infringe the patent claims of the relevant patent or that the patent claims are invalid, and we may not be able to do this. Proving invalidity is difficult. For example, in the United States, proving invalidity requires a showing of clear and convincing evidence to overcome the presumption of validity enjoyed by issued patents. Even if we are successful in these

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proceedings, we may incur substantial costs and divert management’s time and attention in pursuing these proceedings, which could have a material adverse effect on us. If we are unable to avoid infringing the patent rights of others, we may be required to seek a license, defend an infringement action or challenge the validity of the patents in court. Patent litigation is costly and time consuming. We may not have sufficient resources to bring these actions to a successful conclusion. In addition, if we do not obtain a license, develop or obtain non-infringing technology, fail to defend an infringement action successfully or have infringed patents declared invalid, we may incur substantial monetary damages, encounter significant delays in bringing our product candidates to market and be precluded from manufacturing or selling our product candidates.

Because some patent applications in the United States may be maintained in secrecy until the patents are issued, because patent applications in the United States and many foreign jurisdictions are typically not published until eighteen months after the priority date, and because publications in the scientific literature often lag behind actual discoveries, we cannot be certain that others have not filed patent applications for technology covered by our pending applications, or that we were the first to invent the technology. Our competitors may have filed, and may in the future file, patent applications covering technology similar to ours. Any such patent application may have priority over our patent applications, which could further require us to obtain rights to issued patents covering such technologies. If another party has filed a U.S. patent application on inventions similar to ours, we may have to participate in an interference proceeding declared by the USPTO to determine priority of invention in the United States. The costs of these proceedings could be substantial, and it is possible that such efforts would be unsuccessful if, unbeknownst to us, the other party had independently arrived at the same or similar invention prior to our own invention, resulting in a loss of our U.S. patent position with respect to such inventions.

Patents covering the composition of matter of delafloxacin expire in 2016, excluding any additional term for patent term adjustments or patent term extensions. We expect that the other patents and patent applications in the delafloxacin portfolio, if issued, and if the appropriate maintenance, renewal, annuity or other governmental fees are paid, would expire between 2025 and 2029. Patents covering the composition of matter of radezolid expire in 2024, excluding any additional term for patent term adjustments or patent term extensions. We expect the other patents and patent applications in the radezolid portfolio, if issued, and if the appropriate maintenance, renewal, annuity or other governmental fees are paid, to expire between 2024 and 2031. Our patent applications and patents include or support claims on other aspects of delafloxacin and radezolid such as pharmaceutical formulations containing delafloxacin and radezolid, methods of using delafloxacin and radezolid to treat disease and methods of manufacturing delafloxacin and radezolid. Without patent protection on the composition of matter of delafloxacin or radezolid, our ability to assert our patents to stop others from using or selling delafloxacin or radezolid in a non-pharmaceutically acceptable formulation may be limited.

Some of our competitors may be able to sustain the costs of complex patent litigation more effectively than we can because they have substantially greater resources. In addition, any uncertainties resulting from the initiation and continuation of any litigation could have a material adverse effect on our ability to raise the funds necessary to continue our operations.

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Obtaining and maintaining our patent protection depends on compliance with various procedural, document submission, fee payment and other requirements imposed by governmental patent agencies, and our patent protection could be reduced or eliminated for non-compliance with these requirements.

Periodic maintenance fees, renewal fees, annuity fees and various other governmental fees on patents and/or applications will be due to be paid to the USPTO and various foreign governmental patent agencies in several stages over the lifetime of the patents and/or applications. We have systems in place to remind us to pay these fees, and we employ an outside firm, CPA Global Limited, and rely on our outside counsel and Yale University, to pay these fees due to foreign patent agencies. The USPTO and various foreign governmental patent agencies require compliance with a number of procedural, documentary, fee payment and other similar provisions during the patent application process. We employ reputable law firms and other professionals to help us comply, and in many cases, an inadvertent lapse can be cured by payment of a late fee or by other means in accordance with the applicable rules. However, there are situations in which noncompliance can result in abandonment or lapse of the patent or patent application, resulting in partial or complete loss of patent rights in the relevant jurisdiction. In such an event, our competitors might be able to enter the market and this circumstance would have a material adverse effect on our business.

We have not yet registered our trademarks in all of our potential markets, and failure to secure those registrations could adversely affect our business.

We have filed trademark applications with the USPTO for our marks, “Rib-X” and “Rib-X Pharmaceuticals Antibiotics in Three Dimensions” for use in connection with our services and anticipate also filing with respect to these marks at the appropriate time in conjunction with our goods. We also anticipate filing foreign trademark applications for the same marks for goods and services outside the United States. The “Rib-X” mark has been approved for publication by the USPTO, but is subject to a 30-day public opposition period, which can be extended by an additional 90 days upon the request of an interested party. It is possible that the marks could be opposed or cancelled after registration. The registrations will be subject to use and maintenance requirements. We have not yet registered all of our trademarks in all of our potential markets, and it is also possible that there are names or symbols other than “Rib-X” and “Rib-X Pharmaceuticals Antibiotics in Three Dimensions” that may be protectable marks for which we have not sought registration, and failure to secure those registrations could adversely affect our business. We cannot assure you that opposition or cancellation proceedings will not be filed against our trademarks or that our trademarks would survive such proceedings.

We have not yet registered trademarks for any of our product candidates in any jurisdiction. When we file trademark applications for our product candidates in the U.S., our trademark applications in the U.S. and any other jurisdictions where we may file may not be allowed for registration, and registered trademarks may not be obtained, maintained or enforced. During trademark registration proceedings, we may receive rejections. Although we are given an opportunity to respond to those rejections, we may be unable to overcome such rejections. In addition, in the USPTO and in comparable agencies in many foreign jurisdictions, third parties are given an opportunity to oppose pending trademark applications and to seek to cancel registered trademarks. Opposition or cancellation proceedings may be filed against our trademarks, and our trademarks may not survive such proceedings.

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We may be subject to claims that our employees have wrongfully used or disclosed alleged trade secrets of their former employers. If we are not able to adequately prevent disclosure of trade secrets and other proprietary information, the value of our technology and products could be significantly diminished.

As is common in the biotechnology and pharmaceutical industries, we employ individuals who were previously employed at other biotechnology or pharmaceutical companies, including our competitors or potential competitors. We may be subject to claims that these employees, or we, have inadvertently or otherwise used or disclosed trade secrets or other proprietary information of their former employers. Litigation may be necessary to defend against these claims. Even if we are successful in defending against these claims, litigation could result in substantial costs and be a distraction to management.

We rely on trade secrets to protect our proprietary technologies, especially where we do not believe patent protection is appropriate or obtainable. However, trade secrets are difficult to protect. We rely in part on confidentiality agreements with our employees, consultants, outside scientific collaborators, sponsored researchers and other advisors to protect our trade secrets and other proprietary information. These agreements may not effectively prevent disclosure of confidential information and may not provide an adequate remedy in the event of unauthorized disclosure of confidential information. In addition, others may independently discover our trade secrets and proprietary information. For example, the FDA, as part of its Transparency Initiative, is currently considering whether to make additional information publicly available on a routine basis, including information that we may consider to be trade secrets or other proprietary information, and it is not clear at the present time how the FDA’s disclosure policies may change in the future, if at all. Costly and time-consuming litigation could be necessary to enforce and determine the scope of our proprietary rights, and failure to obtain or maintain trade secret protection could adversely affect our competitive business position.

Risks Relating to Owning Our Common Stock

No public market for our common stock currently exists and an active trading market may not develop or be sustained following this offering.

Prior to this offering, there has been no public market for our common stock. An active trading market may not develop following the completion of this offering or, if developed, may not be sustained. The lack of an active market may impair your ability to sell your shares at the time you wish to sell them or at a price that you consider reasonable. The lack of an active market may also reduce the fair market value of your shares. An inactive market may also impair our ability to raise capital to continue to fund operations by selling shares and may impair our ability to acquire other companies or technologies by using our shares as consideration.

Our share price may be volatile, which could subject us to securities class action litigation and prevent you from being able to sell your shares at or above the offering price.

The initial public offering price for our shares will be determined by negotiations between us and the representative of the underwriters and may not be indicative of prices that will prevail in the trading market. The market price of shares of our common stock could be subject to wide fluctuations in response to many risk factors listed in this section, and others beyond our control, including:

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Furthermore, the stock markets have experienced extreme price and volume fluctuations that have affected and continue to affect the market prices of equity securities of many companies. These fluctuations often have been unrelated or disproportionate to the operating performance of those companies. These broad market and industry fluctuations, as well as general economic, political and market conditions such as recessions, interest rate changes or international currency fluctuations, may negatively impact the market price of shares of our common stock. In addition, such fluctuations could subject us to securities class action litigation, which could result in substantial costs and divert our management’s attention from other business concerns, which could seriously harm our business. If the market price of shares of our common stock after this offering does not exceed the initial public offering price, you may not realize any return on your investment in us and may lose some or all of your investment.

After this offering, affiliates of Warburg Pincus will have the ability to control all matters submitted to our stockholders for approval.

When this offering is completed, affiliates of Warburg Pincus LLC, or Warburg Pincus, will beneficially own shares representing approximately     % of our common stock, assuming that the closing of the offering made hereby occurs with an initial public offering price per share of $                , the mid-point of the price range set forth on the cover page of this prospectus, and that the closing occurs on                 , 2012. As a result, Warburg Pincus will be able to control all matters submitted to our stockholders for approval, as well as our management and affairs. For

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example, Warburg Pincus will control the election of directors and approval of any merger, consolidation, sale of all or substantially all of our assets or other business combination or reorganization. This concentration of voting power could delay or prevent an acquisition of us on terms that other stockholders may desire. The interests of Warburg Pincus may not always coincide with your interests or the interests of other stockholders and Warburg Pincus may act in a manner that advances their best interests and not necessarily those of other stockholders, including seeking a premium value for their common stock, and might affect the prevailing market price for our common stock. Pursuant to our fourth amended and restated securityholders agreement, dated January 10, 2011, we are required to nominate and use our best efforts to elect to our board of directors up to three individuals designated by an affiliate of Warburg Pincus, as more specifically described in “Description of Capital Stock – Voting Rights.” Our board of directors, which currently consists of six directors and one vacancy, has the power to set the number of directors on our board from time to time.

We have broad discretion in the use of net proceeds from this offering and may not use them effectively.

Although we currently intend to use the net proceeds from this offering in the manner described in “Use of Proceeds” elsewhere in this prospectus, we will have broad discretion in the application of the net proceeds. Our failure to apply these funds effectively could affect our ability to continue to develop and eventually to manufacture and sell our products.

Being a public company will increase our expenses and administrative burden.

As a public company, we will incur significant legal, accounting and other expenses that we did not incur as a private company. In addition, our administrative staff will be required to perform additional tasks. For example, in anticipation of becoming a public company, we will need to adopt additional internal controls and disclosure controls and procedures, retain a transfer agent, adopt an insider trading policy and bear all of the internal and external costs of preparing and distributing periodic public reports in compliance with our obligations under the securities laws.

In addition, changing laws, regulations and standards relating to corporate governance and public disclosure, including the Sarbanes-Oxley Act and related regulations implemented by the Securities and Exchange Commission and the NASDAQ Global Market, are creating uncertainty for public companies, increasing legal and financial compliance costs and making some activities more time consuming. We are currently evaluating and monitoring these rules and proposed changes to rules, and cannot predict or estimate the amount of additional costs we may incur or the timing of such costs. These laws, regulations and standards are subject to varying interpretations, in many cases due to their lack of specificity, and, as a result, their application in practice may evolve over time as new guidance is provided by regulatory and governing bodies. This could result in continuing uncertainty regarding compliance matters and higher costs necessitated by ongoing revisions to disclosure and governance practices. We intend to invest resources to comply with evolving laws, regulations and standards, and this investment will result in increased general and administrative expenses and may divert management’s time and attention from product development activities. If our efforts to comply with new laws, regulations and standards differ from the activities intended by regulatory or governing bodies due to ambiguities related to practice, regulatory authorities may initiate legal proceedings against us and our business may be harmed. In connection with this offering, we are increasing our directors’ and officers’ insurance coverage which will increase our insurance cost. In the future, it may be more expensive for us to obtain director and officer liability insurance, and we may be required to accept reduced coverage or incur substantially higher

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costs to obtain coverage. These factors could also make it more difficult for us to attract and retain qualified members of our board of directors, particularly to serve on our audit committee and compensation committee, and qualified executive officers.

Purchasers in this offering will experience immediate and substantial dilution in the book value of their investment.

The initial public offering price will be substantially higher than the net tangible book value per share of shares of our common stock based on the total value of our tangible assets less our total liabilities immediately following this offering. Therefore, if you purchase shares of our common stock in this offering, you will experience immediate and substantial dilution of approximately $             per share in the price you pay for shares of our common stock as compared to its net tangible book value, assuming an initial public offering price of $             per share, the mid-point of the price range set forth on the cover page of this prospectus, and that the closing occurs on             , 2012. To the extent outstanding options to purchase shares of common stock are exercised, there will be further dilution. For further information on this calculation, see “Dilution” elsewhere in this prospectus.

A significant portion of our total outstanding shares of common stock is restricted from immediate resale but may be sold into the market in the near future. This could cause the market price of our common stock to drop significantly, even if our business is doing well.

Sales of a substantial number of shares of our common stock in the public market could occur in the future. These sales, or the perception in the market that the holders of a large number of shares of common stock intend to sell shares, could reduce the market price of our common stock. After this offering, we will have              outstanding shares of common stock based on the number of shares outstanding as of March 31, 2012, assuming an initial public offering price of $             per share, the mid-point of the price range set forth on the cover page of this prospectus, and that the closing occurs on             , 2012. Of these shares,              shares other than those shares purchased by our affiliates may be resold in the public market immediately and the remaining              shares are currently restricted under securities laws or as a result of lock-up agreements but will be able to be resold after the offering as described in the “Shares Eligible for Future Sale” section of this prospectus. Moreover, after this offering, holders of an aggregate of              shares of our common stock will have rights, subject to certain conditions, to require us to file registration statements covering their shares or to include their shares in registration statements that we may file for ourselves or other stockholders. We also intend to register all              shares of common stock that we may issue under our equity compensation plans. Once we register these shares, they can be freely sold in the public market upon issuance and once vested, subject to the 180 day lock-up periods under the lock-up agreements described in the “Underwriting” section of this prospectus.

Future sales and issuances of our common stock or rights to purchase common stock pursuant to our equity incentive plans could result in additional dilution of the percentage ownership of our stockholders and could cause our share price to fall.

We expect that significant additional capital will be needed in the future to continue our planned operations. To the extent we raise additional capital by issuing equity securities, our stockholders may experience substantial dilution. We may sell common stock, convertible securities or other equity securities in one or more transactions at prices and in a manner we determine from time to time. If we sell common stock, convertible securities or other equity securities in more than one transaction, investors may be materially diluted by subsequent

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sales. Such sales may also result in material dilution to our existing stockholders, and new investors could gain rights superior to our existing stockholders.

As of March 31, 2012, we have options to purchase 21,869,572 shares outstanding under our 2001 Stock Option and Incentive Plan, or 2001 Stock Plan, and options to purchase 150,000 shares outstanding under our 2011 Equity Incentive Plan. In addition, pursuant to the terms of our Management Bonus Plan and our Non-Employee Director Bonus Plan, we expect to grant restricted stock units for              additional shares and              additional shares, respectively, of our common stock under our 2011 Equity Incentive Plan in connection with this offering. We are also authorized to grant equity awards, including stock options, to our employees, directors and consultants, covering up to              shares of our common stock, pursuant to our 2011 Equity Incentive Plan. We plan to register the number of shares available for issuance under our 2001 Stock Plan and 2011 Equity Incentive Plan. Sales of such shares may result in material dilution to our existing stockholders, which could cause our share price to fall.

If securities or industry analysts do not publish research or publish inaccurate or unfavorable research about our business, our share price and trading volume could decline.

The trading market for our common stock will depend on the research and reports that securities or industry analysts publish about us or our business. We do not have any control over these analysts. There can be no assurance that analysts will cover us or provide favorable coverage. If one or more of the analysts who cover us downgrade our stock or change their opinion of our stock, our share price would likely decline. If one or more of these analysts cease coverage of our company or fail to regularly publish reports on us, we could lose visibility in the financial markets, which could cause our share price or trading volume to decline.

Some provisions of our charter documents and Delaware law may have anti-takeover effects that could discourage an acquisition of us by others, even if an acquisition would be beneficial to our stockholders, and may prevent attempts by our stockholders to replace or remove our current management.

Provisions in our amended and restated certificate of incorporation and bylaws, as well as provisions of Delaware law, could make it more difficult for a third party to acquire us or increase the cost of acquiring us, even if doing so would benefit our stockholders. These provisions include:

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These provisions may also frustrate or prevent any attempts by our stockholders to replace or remove our current management or members of our board of directors. In addition, we are subject to Section 203 of the Delaware General Corporation Law, which generally prohibits a Delaware corporation from engaging in any of a broad range of business combinations with an interested stockholder for a period of three years following the date on which the stockholder became an interested stockholder, unless such transactions are approved by our board of directors. This provision could have the effect of delaying or preventing a change of control, whether or not it is desired by or beneficial to our stockholders. Further, other provisions of Delaware law may also discourage, delay or prevent someone from acquiring us or merging with us.

We do not anticipate paying cash dividends, and accordingly, stockholders must rely on stock appreciation for any return on their investment.

We do not anticipate paying cash dividends in the future. As a result, only appreciation of the market price of our common stock, which may never occur, will provide a return to stockholders. Investors seeking cash dividends should not invest in our common stock.

Our ability to use our net operating loss carryforwards and certain other tax attributes may be limited.

As of December 31, 2011, we had federal net operating loss carryforwards of $203.8 million which will begin to expire in 2021 and federal research and development tax credit carryforwards of $7.4 million which will begin to expire in 2021. Our ability to utilize our federal net operating losses and federal tax credits may be limited under Sections 382 and 383 of the Internal Revenue Code. The limitations apply if an ownership change, as defined by Section 382, occurs. Generally, an ownership change occurs when certain shareholders increase their aggregated ownership by more than 50 percentage points over their lowest ownership percentage in a testing period (typically three years). We may already be subject to Section 382 limitations due to previous ownership changes. In addition, future changes in stock ownership may also trigger an ownership change and, consequently, a Section 382 limitation. Due to the significant complexity and cost associated with a change in control study, and the expectation of continuing to incur losses whereby the net operating losses and federal tax credits are not anticipated to be used in the foreseeable future, we have not assessed whether there have been changes in control since our formation. If we have experienced changes in control at any time since our formation, utilization of its net operating losses or research and development credit carryforwards would be subject to annual limitations under Section 382. Any limitation may result in expiration of a portion of the net operating loss or research and development credit carryforwards before utilization which would reduce our gross deferred tax assets and corresponding valuation allowance. As a result, if we earn net taxable income, our ability to use our pre-change net operating loss carryforwards to offset United States federal taxable income may be subject to significant limitations, which could potentially result in increased future tax liability to us.

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