This quarterly report on Form 10-Q contains forward-looking statements within the meaning of Section 21E of the Securities Exchange Act of 1934, as amended, that involve risks and uncertainties. All statements other than statements relating to historical matters including statements to the effect that we "believe," "expect," "anticipate," "plan," "target," "intend" and similar expressions should be considered forward-looking statements. Our actual results could differ materially from those discussed in the forward-looking statements as a result of a number of important factors, including factors discussed in this section and elsewhere in this quarterly report on Form 10-Q, including those discussed in Item 1A of this report under the heading "Risk Factors," and the risks discussed in our other filings with the Securities and Exchange Commission. Readers are cautioned not to place undue reliance on these forward-looking statements, which reflect management's analysis, judgment, belief or expectation only as the date hereof. We assume no obligation to update these forward-looking statements to reflect events or circumstances that arise after the date hereof.

Overview

We are a biopharmaceutical company focused on the discovery, development and commercialization of innovative treatments for infectious diseases. Within the anti-infective market, we are currently concentrating on the development of antivirals and antibacterials. We are targeting our antiviral development efforts on treatments for chronic hepatitis C and HIV infection, and we are directing our antibacterial development efforts toward treatments for serious hospital-based bacterial infections.

We have devoted and are continuing to devote substantially all of our efforts toward product research and development. We have incurred losses of $150.8 million from inception through June 30, 2008 and had an accumulated deficit of $164.6 million through June 30, 2008. Our net losses were $12.8 million and $15.3 million for the six months ended June 30, 2008 and 2007, respectively. We have funded our operations primarily through:

• proceeds of $161.2 million from the sale of equity securities, including our initial public offering in October 2006;

• borrowings of $22.1 million from debt facilities; and

• receipts of $10.0 million from up-front and milestone payments, as well as $8.8 million in cost-sharing receipts from our collaboration partner, Gilead Sciences.

We expect to incur substantial and increasing losses for at least the next several years as we seek to:

• complete the open-label extension phases of our phase II clinical trials for elvucitabine;

• complete IND-enabling preclinical testing of ACH-1095 and begin clinical testing;

• complete IND-enabling preclinical testing of ACH-1625 and begin clinical testing;

• complete our assessment of ACH-702 preclinical data and prepare for clinical testing;

• advance our carbapenem compounds to identify a clinical lead candidate; and

• progress additional drug candidates.

We will need substantial additional financing to obtain regulatory approvals, fund operating losses, and, if deemed appropriate, establish manufacturing and sales and marketing capabilities, which we will seek to raise through public or private equity or debt financings, collaborative or other arrangements with third parties or through other sources of financing. There can be no assurance that such funds will be available on terms favorable to us, if at all. In addition to the normal risks associated with early-stage companies, there can be no assurance that we will successfully complete our research and development, obtain adequate patent protection for our technology, obtain necessary government regulatory approval for drug candidates we develop or that any approved drug candidates will be commercially viable. In addition, we may not be profitable even if we succeed in commercializing any of our drug candidates.

On August 12, 2008, we issued 10,714,655 units at a price of $2.9049 per unit, resulting in gross proceeds of $31.1 million, or $29.5 million net of offering expenses. Each unit consists of one share of our common stock and a warrant to purchase 0.25 shares of common stock at an exercise price of $3.53 per share. The investors may have the option to purchase an additional 3,679,078 units between February 2009 and August 2009. We have agreed to seek stockholder approval for the securities in accordance with NASDAQ Marketplace Rules.

Financial Operations Overview

Revenue

To date, we have not generated revenue from the sale of any drugs. The majority of our revenue recognized to date has been derived from our collaboration with Gilead Sciences to develop compounds for use in treating chronic hepatitis C infection. During the six months ended June 30, 2008 and 2007, we recognized $1.0 million and $2.7 million, respectively, under this collaboration arrangement.

Upon initiating our collaboration with Gilead Sciences, we received a payment of $10.0 million, which included an equity investment by Gilead Sciences determined to be worth approximately $2.0 million. The remaining $8.0 million is being accounted for as a nonrefundable up-front fee recognized under the proportionate performance model. Revenue under the proportionate performance model is recognized as our effort under the collaboration is incurred. When our performance obligation is complete, we will recognize milestone payments, if any, when the corresponding milestone is achieved. We will recognize royalty payments, if any, upon product sales.

Effective April 1, 2007, each party provides for the costs of their own full-time equivalents. External research costs continue to be shared equally by both parties. Through March 31, 2007 research and development expenses under our collaboration with Gilead Sciences, including internal full-time equivalent costs and external research costs, incurred by both companies prior to proof-of-concept, were borne equally by both parties. As we were providing the majority of those services and were incurring the majority of those expenses, we were the net recipient of funds under this cost-sharing portion of the arrangement and therefore recognized the reimbursed costs as revenue rather than research expense. Payments made by us to Gilead Sciences in connection with this collaboration are being recognized as a reduction of revenue.

Research and Development

Our research and development expenses reflect costs incurred for our proprietary research and development projects as well as costs for research and development projects conducted as part of collaborative arrangements. These costs consist primarily of salaries and benefits for our research and development personnel, costs of services by clinical research organizations, other outsourced research, materials used during research and development activities, facility-related costs such as rent and utilities associated with our laboratory and clinical development space and operating supplies. We expect that our research and development expenses will remain substantially unchanged for the remainder of the year.

We have established our drug candidate pipeline through our internal discovery capabilities and through the in-licensing of attractive drug candidates. Through these efforts we have identified and are developing candidates in the following areas:

• Elvucitabine for HIV Infection. Elvucitabine is an antiviral we are developing for the treatment of HIV infection. We are currently evaluating elvucitabine in phase II clinical trials to further explore its safety and efficacy in HIV-infected patients. We currently retain full development and marketing rights to elvucitabine. However, we are currently in discussions with potential collaboration partners for elvucitabine and do not plan to advance elvucitabine into Phase III clinical trials without a collaboration partner.

• ACH-1095, an NS4A Antagonist for Chronic Hepatitis C Infection. We are evaluating ACH-1095 for the treatment of chronic hepatitis C in collaboration with Gilead Sciences. ACH-1095 is currently in late-stage pre-clinical studies and we anticipate filing an investigational new drug application, or IND, for this compound during 2008.

• ACH-1625, a Protease Inhibitor for Chronic Hepatitis C Infection. In a proprietary research program targeting HCV protease, we are also evaluating ACH-1625, which was discovered by our internal research team. ACH-1625 has demonstrated strong in vitro potency and a satisfactory early safety profile. We have recently begun preparations for late-stage pre-clinical studies and we anticipate filing an IND for this compound in the first half of 2009.

All costs associated with internal research and development, and research and development services for which we have externally contracted, are expensed as incurred. The costs of obtaining patents for our candidates are expensed as incurred as indirect costs.

                                                        Six Months Ended June 30,
                                                          2008             2007
  Direct external costs:
  Elvucitabine                                        $       1,966    $       6,220
  NS4A antagonists (including ACH-1095 and ACH-806)             539            1,193
  ACH-1625                                                      740               -
  ACH-702                                                       138            2,392

                                                              3,383            9,805
  Direct internal personnel costs                             3,891            3,712

  Sub-total direct costs                                      7,274           13,517
  Indirect costs and overhead                                 3,207            2,568

  Total research and development                      $      10,481    $      16,085

Currently, we are completing the open-label extension phases of two phase II clinical trials for elvucitabine and conducting preclinical studies for ACH-1095 and ACH-1625. From inception through June 30, 2008, we incurred approximately $47.5 million in total costs for elvucitabine, approximately $28.0 million in total costs for our NS4A antagonist program, including both ACH-1095 and ACH-806, approximately $5.8 million for ACH-1625 and approximately $17.4 million in total costs for ACH-702. These figures include our internal research and development personnel costs and related facilities overhead. We currently estimate that the clinical trial costs for two phase III clinical trials of elvucitabine in different HIV populations will be approximately $50.0 million, exclusive of the internal personnel costs associated with conducting these trials; however, we currently do not plan to undertake these trials or incur these costs without entering a collaboration agreement. We estimate that the costs associated with completing phase I clinical trials for ACH-702 will be approximately $2.0 million, exclusive of the internal personnel costs associated with conducting these studies and trials. We anticipate that the costs associated with preclinical and early clinical development through proof-of-concept of ACH-1095, our next generation NS4A antagonist, will be approximately $2.7 million, exclusive of internal personnel costs. This amount represents one-half of the external costs associated with those activities, as we share such external costs with Gilead Sciences. We estimate that the costs associated with preclinical and early clinical development of ACH-1625, our HCV protease inhibitor, will be approximately $3.1 million.

General and Administrative

Our general and administrative expenses consist primarily of salaries and benefits for management and administrative personnel, professional fees for legal, accounting and other services, travel costs and facility-related costs such as rent, utilities and other general office expenses. We expect that general and administrative expenses will remain substantially unchanged over the next twelve months.

Critical Accounting Policies and Estimates

The discussion and analysis of our financial condition and results of operations set forth below are based on our financial statements, which have been prepared in accordance with accounting principles generally accepted in the United

States, or U.S. GAAP. The preparation of these financial statements requires us to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent liabilities at the date of the financial statements and the reported amounts of revenues and expenses during the reporting period. On an ongoing basis, we evaluate our estimates and assumptions, including those described below. We base our estimates on historical experience and on various other assumptions that we believe to be reasonable under the circumstances. These estimates and assumptions form the basis for making judgments about the carrying values of assets and liabilities that are not readily apparent from other sources. Management makes estimates and exercises judgment in revenue recognition, research and development costs, stock-based compensation, accrued expenses and income taxes. Actual results may differ from these estimates under different assumptions or conditions.

We believe the following critical accounting policies affect management's more significant judgments and estimates used in the preparation of our financial statements:

Revenue Recognition

We recognize revenue from contract research and development and research progress payments in accordance with Staff Accounting Bulletin No. 104, Revenue Recognition, or SAB No. 104, and Financial Accounting Standards Board, or FASB, Emerging Issue Task Force, or EITF, Issue No. 00-21, Accounting for Revenue Arrangements with Multiple Deliverables, or EITF No. 00-21. Revenue-generating research and development collaborations are often multiple element arrangements, providing for a license as well as research and development services. Such arrangements are analyzed to determine whether the deliverables, including research and development services, can be separated or whether they must be accounted for as a single unit of accounting in accordance with EITF No. 00-21. We recognize upfront license payments as revenue upon delivery of the license only if the license has standalone value and the fair value of the undelivered performance obligations can be determined. If the fair value of the undelivered performance obligations can be determined, such obligations would then be accounted for separately as performed. If the license is considered to either
(i) not have standalone value or (ii) have standalone value but the fair value of any of the undelivered performance obligations cannot be determined, the arrangement would then be accounted for as a single unit of accounting and the upfront license payments are recognized as revenue over the estimated period of when our performance obligations are performed.

When we determine that an arrangement should be accounted for as a single unit of accounting, we must determine the period over which the performance obligations will be performed and revenue related to upfront license payments will be recognized. Revenue will be recognized using either a proportionate performance or straight-line method. We recognize revenue using the proportionate performance method provided that we can reasonably estimate the level of effort required to complete our performance obligations under an arrangement and such performance obligations are provided on a best-efforts basis. Under the proportionate performance method, periodic revenue related to up-front license payments is recognized as the percentage of actual effort expended in that period to total effort expected for all of our performance obligations under the arrangement. Actual effort is generally determined based upon actual direct labor hours or full-time equivalents incurred and include research and development activities performed by internal scientists. Total expected effort is generally based upon the total direct labor hours of full-time equivalents incorporated into the detailed budget and project plan that is agreed to by both parties to the collaboration. Significant management judgment is required in determining the level of effort required under an arrangement and the period over which we expect to complete the related performance obligations. For example, under our arrangement with Gilead Sciences, the joint research committee periodically reviews and updates the project plan. In the event that a change in estimate occurs, the change will be accounted for using the cumulative catch-up method which provides for an adjustment to revenue in the current period. Estimates of our level of effort may change in the future, resulting in a material change in the amount of revenue recognized in future periods. We revised our joint research program with Gilead Sciences in the first quarter of 2007 to focus on next-generation NS4A antagonists. At that time, we extended the period over which our remaining obligations under the arrangement would be completed. In addition, we and Gilead Sciences agreed to continue to equally share external costs, but effective April 1, 2007, internal full-time equivalents would no longer be subject to a cost sharing arrangement. Instead, each party bears the costs of their respective full-time equivalents.

Generally under collaboration arrangements, payments received during the period of performance may include up-front payments, time-or performance-based milestones and reimbursement of internal and external costs. The proportion of actual performance to total expected performance is applied to these payments in determining periodic revenue, but will be limited by the aggregate cash received or receivable to date by us.

Substantive milestone payments are considered to be performance bonuses that are recognized upon achievement of the milestone only if all of the following conditions are met: (1) the milestone payments are non-refundable,
(2) achievement of the milestone involves a degree of risk and was not reasonably assured at the inception of the arrangement, (3) substantive

effort is involved in achieving the milestone, (4) the amount of the milestone payment is reasonable in relation to the effort expended or the risk associated with achievement of the milestone and (5) a reasonable amount of time passes between the upfront license payment and the first milestone payment as well as between each subsequent milestone payment.

Reimbursement of costs is recognized as revenue provided the provisions of EITF Issue No. 99-19, Reporting Revenue Gross as Principal Versus Net as an Agent, are met, the amounts are determinable and collection of the related receivable is reasonably assured.

Stock-Based Compensation - Employee Stock-Based Awards

We apply the Statement of Financial Accounting Standards No. 123 as revised in 2004, Share-Based Payment, or SFAS No. 123R, which requires measurement and recognition of compensation expense for all stock-based awards made to employees and directors, including employee stock options and employee stock purchases under our 2006 ESPP Plan based on estimated fair values. In December 2007, the SEC issued Staff Accounting Bulletin No. 110, or SAB No. 110, which extends the use of the simplified method in developing an estimate of the expected term of "plain vanilla" share options beyond December 31, 2007. We utilize the provisions of SAB No. 110 in our application of SFAS No. 123R.

We primarily grant qualified stock options for a fixed number of shares to employees with an exercise price equal to the market value of the shares at the date of grant. To the extent that the amount of the aggregate fair market value of qualified stock options that become exercisable for an individual exceeds $100,000 during any tax year, those stock options are treated as non qualified stock options. Under the fair value recognition provisions of SFAS No. 123R, stock-based compensation cost is based on the value of the portion of stock-based awards that is ultimately expected to vest during the period. Stock-based compensation expense recognized during the six months ended June 30, 2008 and 2007 includes compensation expense for stock-based awards granted prior to, but not yet vested as of December 31, 2005, based on the fair value on the grant date estimated in accordance with the pro forma provisions of SFAS No. 123. Compensation expense also includes amounts related to the stock-based awards granted subsequent to December 31, 2005, based on the fair value on the grant date, estimated in accordance with the provisions of SFAS No. 123R.

We utilize the Black-Scholes option pricing model for determining the estimated fair value for stock-based awards. The Black-Scholes model requires the use of assumptions which determine the fair value of the stock-based awards. Determining the fair value of stock-based awards at the grant date requires judgment, including estimating the expected term of stock options, the expected volatility of our stock and expected dividends. In addition, we previously accounted for forfeitures as they occurred. In accordance with SFAS No. 123R, we are required to estimate forfeitures at the grant date and recognize compensation costs for only those awards that are expected to vest. Judgment is required in estimating the amount of stock-based awards that are expected to be forfeited.

If factors change and we employ different assumptions in the application of SFAS No. 123R in future periods, the compensation expense that we record under SFAS No. 123R may differ significantly from what we have recorded in the current period. Therefore, we believe it is important for investors to be aware of the degree of subjectivity involved when using option pricing models to estimate share-based compensation under SFAS No. 123R. There is risk that our estimates of the fair values of our share-based compensation awards on the grant dates may differ from the actual values realized upon the exercise, expiration, early termination or forfeiture of those share-based payments in the future. Certain share-based payments, such as employee stock options, may expire worthless or otherwise result in zero intrinsic value as compared to the fair values originally estimated on the grant date and reported in our financial statements. Alternatively, value may be realized from these instruments that is significantly in excess of the fair values originally estimated on the grant date and reported in our financial statements. Although the fair value of employee share-based awards is determined in accordance with SFAS No. 123R and SAB No. 110 using an option pricing model, that value may not be indicative of the fair value observed in a willing buyer/willing seller market transaction.

Accrued Expenses

As part of the process of preparing financial statements, we are required to estimate accrued expenses. This process involves identifying services which have been performed on our behalf and estimating the level of service performed and the associated cost incurred for such service as of each balance sheet date in our financial statements.

In accruing service fees, we estimate the time period over which services will be provided and the level of effort in each period. If the actual timing of the provision of services or the level of effort varies from the estimate, we will adjust the

accrual accordingly. The majority of our service providers invoice us monthly in arrears for services performed. Some of our service providers require upfront or milestone payments. If our estimate of services performed is less than the upfront or milestone payments, the difference is accounted for as a prepaid expense. In the event that we do not identify costs that have been incurred or we underestimate or overestimate the level of services performed or the costs of such services, our actual expenses could differ from such estimates. The date on which some services commence, the level of services performed on or before a given date and the cost of such services are often subjective determinations. We make judgments based upon facts and circumstances known to us in accordance with U.S. GAAP.

Income Taxes

We use an asset and liability approach for financial accounting and reporting of income taxes. Deferred tax assets and liabilities are determined based on temporary differences between financial reporting and tax basis assets and liabilities and are measured by applying enacted rates and laws to taxable years in which differences are expected to be recovered or settled. Further, the effect on deferred tax assets and liabilities of a change in tax rates is recognized in income in the period that the rate changes.

Effective January 1, 2007, we adopted Financial Accounting Standards Board ("FASB") Interpretation No. 48, Accounting for Uncertainty in Income Taxes-an interpretation of FASB Statement No. 109, or FIN 48. FIN 48 prescribes a comprehensive model for how a company should recognize, measure, present, and disclose in its financial statements uncertain tax positions that the company has taken or expects to take on a tax return, including a decision whether to file or not file a return in a particular jurisdiction. Under FIN 48, the financial statements reflect expected future tax consequences of such positions presuming the taxing authorities' full knowledge of the position and all relevant facts.

We do not have any unrecognized tax benefits as of the date of adoption or June 30, 2008. We review all tax positions to ensure the tax treatment selected is sustainable based on its technical merits and that the position would be sustained if challenged.

Results of Operations

Results of operations may vary from period to period depending on numerous factors, including the timing of payments received under existing or future strategic alliances, joint ventures or financings, if any, the progress of our research and development projects, technological advances and determinations as to the commercial potential of proposed products.

Comparison of Three and Six Months Ended June 30, 2008 and 2007

Revenue. Revenue was $398,000 and $1.2 million for the three months ended June 30, 2008 and 2007, and $1.0 million and $2.7 million for the six months ended June 30, 2008 and 2007, respectively. The decrease in revenue in 2008 is primarily due to a decrease in full time equivalent, or FTE, hours we expended under the collaboration arrangement with Gilead in 2008, which is the basis of our proportionate performance calculation. Under the proportionate performance method, periodic revenue related to up-front license payments is recognized as the percentage of actual effort expended in that period to total effort expected for all of our performance obligations under the arrangement. This decrease is . . .