Alnylam Reports Continued Progress in Clinical Development of ALN-RSV01 for the Treatment of Respiratory Syncytial Virus (RSV) Infection.

- Company Reports Phase I Clinical Safety Results with Inhalation Dosing of ALN-RSV01 and New Pre-clinical Multi-Dose Data -

- Accrual Complete for Phase II Experimental Infection Study; On Track to Announce Data Early First Quarter 2008 -

CAMBRIDGE, Mass. -- Alnylam Pharmaceuticals, Inc. (Nasdaq: ALNY), a leading RNAi therapeutics company, today announced the presentation of results from its Phase I trial with ALN-RSV01 delivered via inhalation at the 18th Annual Drug Delivery to the Lungs meeting being held in Edinburgh, U.K. These data represent an important milestone in the company's efforts to advance ALN-RSV01 as an anti-viral drug for the treatment of RSV infection in patients. The Phase I study represents the first-ever clinical study of an RNAi therapeutic administered via inhalation.

This Phase I study was a randomized, double-blind, placebo-controlled trial in healthy adult volunteers to assess the safety, tolerability, and pharmacokinetics of inhaled ALN-RSV01 administered via nebulizer. All major objectives of the trial were met, including definition of a safe and well-tolerated dose and regimen for advancement of ALN-RSV01 into further Phase II development.

"These results put us firmly into Phase II development with the ongoing experimental infection study, and now with a planned multi-dose inhalational Phase II strategy in naturally infected adults," said Akshay Vaishnaw, M.D., Ph.D., Vice President, Clinical Research of Alnylam. "With these data in hand, and the upcoming Phase II data expected in the first quarter of 2008, we are excited about the continued development of this novel RNAi therapeutic."

In total, 109 subjects were enrolled in the Phase I trial; 71 were exposed to drug and 38 to placebo. Both single and multiple doses were evaluated, ranging from the equivalent of 0.1 mg/kg to 3 mg/kg in the single dose arm, and 0.01 mg/kg to 0.6 mg/kg, administered once daily for 3 days in the multi-dose arm. Data showed that the efficiency of delivery of ALN-RSV01 delivered via inhalation, as measured by plasma levels, was significantly greater in humans than observed pre-clinically. Adverse events reported for both drug and placebo were predominantly mild; there were no severe or serious adverse events. In the single dose arm, a mild to moderate flu-like adverse event was observed at the higher doses. This was found to be transient and was potentially linked to the enhanced efficiency of delivery. In the multi-dose arm, daily doses up to 0.6 mg/kg for 3 days were found to be safe and well tolerated, pointing to a dose and regimen of ALN-RSV01 for further Phase II evaluation.

Other updates from the presentation in Edinburgh today included the following.

* New Pharmacology Data. Previous pre-clinical data had shown the ability of a single dose of ALN-RSV01 to potently reduce virus load in a mouse RSV model. Additional pre-clinical data reported today now significantly extend these observations. Comparison of multi-dose and single dose ALN-RSV01 treatment paradigms demonstrated that a multi-dose regimen (once daily for 3 days) is significantly more efficacious than a single dose, for the same total amount of drug.

* Ongoing Phase II Experimental Infection Study with ALN-RSV01. The company announced today that the Phase II study in experimentally infected adult volunteers is now fully enrolled with 88 subjects and top-line data are expected early in the first quarter 2008.

"In addition to our Phase I inhalational data, we are particularly encouraged by the new complementary pre-clinical data demonstrating enhanced anti-viral activity for ALN-RSV01 administered in multiple doses," said Sara Nochur, Ph.D., Vice President, Regulatory Affairs of Alnylam and RSV Program Leader. "Indeed, our current Phase II study of ALN-RSV01 in experimentally infected subjects, that has now completed enrollment and remains blinded, has incorporated the use of a multi-dose treatment both prior to and after viral inoculation."

"I am excited by the rapid progress that has been made with ALN-RSV01, as there is significant need for a novel therapy to treat RSV, a serious viral infection that hospitalizes over 320,000 pediatric and adult patients in the U.S. annually," said John P. DeVincenzo, M.D., Associate Professor of Pediatrics and Infectious Diseases at the University of Tennessee Health Science Center. "These new inhalational tolerability data greatly extend our understanding of ALN-RSV01 and add to the existing encouraging safety profile via the intranasal route. In aggregate, the significant safety database supports continued Phase II development of ALN-RSV01 in RSV-infected children and adults."

About RNA Interference (RNAi)

RNAi (RNA interference) is a revolution in biology, representing a breakthrough in understanding how genes are turned on and off in cells, and a completely new approach to drug discovery and development. Its discovery has been heralded as "a major scientific breakthrough that happens once every decade or so," and represents one of the most promising and rapidly advancing frontiers in biology and drug discovery today which was awarded the 2006 Nobel Prize for Physiology or Medicine. RNAi is a natural process of gene silencing that occurs in organisms ranging from plants to mammals. By harnessing the natural biological process of RNAi occurring in our cells, the creation of a major new class of medicines, known as RNAi therapeutics, is on the horizon. RNAi therapeutics target the cause of diseases by potently silencing specific messenger RNAs (mRNAs), thereby preventing disease-causing proteins from being made. RNAi therapeutics have the potential to treat disease and help patients in a fundamentally new way.

About Alnylam Pharmaceuticals

Alnylam is a biopharmaceutical company developing novel therapeutics based on RNA interference, or RNAi. The company is applying its therapeutic expertise in RNAi to address significant medical needs, many of which cannot effectively be addressed with small molecules or antibodies, the current major classes of drugs. Alnylam is leading the translation of RNAi as a new class of innovative medicines with peer-reviewed research efforts published in the world's top scientific journals including Nature, Nature Medicine, and Cell. The company is leveraging these capabilities to build a broad pipeline of RNAi therapeutics; its most advanced program is in Phase II human clinical trials for the treatment of respiratory syncytial virus (RSV) infection. In addition, the company is developing RNAi therapeutics for the treatment of influenza, hypercholesterolemia, and liver cancers, among other diseases. The company's leadership position in fundamental patents, technology, and know-how relating to RNAi has enabled it to form major alliances with leading companies including Medtronic, Novartis, Biogen Idec, and Roche. The company, founded in 2002, maintains headquarters in Cambridge, Massachusetts. For more information, visit www.alnylam.com.

Alnylam Forward-Looking Statements

Various statements in this release concerning Alnylam's future expectations, plans and prospects, including without limitation statements concerning the commencement of clinical trials and studies, the availability of results of clinical trials and studies, the need for novel RSV therapeutics, the expected formulation for an RSV therapeutic, and our views with respect to the potential for RNAi therapeutics, including ALN-RSV01, constitute forward-looking statements for the purposes of the safe harbor provisions under The Private Securities Litigation Reform Act of 1995. Actual results may differ materially from those indicated by these forward-looking statements as a result of various important factors, including risks related to: Alnylam's approach to discover and develop novel drugs, which is unproven and may never lead to marketable products; obtaining, maintaining and protecting intellectual property; Alnylam's ability to enforce its patents against infringers and to defend its patent portfolio against challenges from third parties; Alnylam's ability to obtain additional funding to support its business activities; Alnylam's dependence on third parties for development, manufacture, marketing, sales and distribution of products; obtaining regulatory approval for products; competition from others using technology similar to Alnylam's and others developing products for similar uses; Alnylam's dependence on collaborators; and Alnylam's short operating history; as well as those risks more fully discussed in the "Risk Factors" section of its most recent quarterly report on Form 10-Q on file with the Securities and Exchange Commission. In addition, any forward-looking statements represent Alnylam's views only as of today and should not be relied upon as representing its views as of any subsequent date. Alnylam does not assume any obligation to update any forward-looking statements.