– Company Expects to Initiate a Phase 1/2 Study for ALN-XDH in Gout in Early 2022, with Initial Human Data in Late 2022 –

Alnylam Pharmaceuticals, Inc. (Nasdaq: ALNY), the leading RNAi therapeutics company, announced today that the Company has submitted a clinical trial authorization (CTA) application to The Medicines and Healthcare Products Regulatory Agency (MHRA) in the United Kingdom to initiate a Phase 1/2 study of ALN-XDH, an investigational RNAi therapeutic targeting xanthine dehydrogenase (XDH) for the treatment of gout. Upon obtaining MHRA approval, the Company plans to initiate a Phase 1/2 study in healthy volunteers and in patients with gout in early 2022, with initial human data expected in late 2022.

“We are excited to advance our ALN-XDH program to the clinic in light of the unmet need in gout, a chronic debilitating condition accompanied by acute inflammatory attacks. We believe the existing treatment landscape for gout has substantial limitations attributed to an incomplete response to standard of care, tolerability issues, and poor patient convenience. As a result, the majority of patients are untreated, cannot adhere to prescribed therapy, or do not reach target uric acid levels,” said Neil Wallace, Program Leader for the ALN-XDH program at Alnylam. “We believe ALN-XDH has the potential to address key unmet needs for gout patients, through potent urate-lowering effects achieved with infrequent dosing, which could potentially lead to a reduction of flares and other disease manifestations.”

ALN-XDH is a wholly owned, prevalent disease asset in the Alnylam clinical development portfolio.

About ALN-XDH

ALN-XDH is an investigational, subcutaneously administered RNAi therapeutic targeting xanthine dehydrogenase (XDH) in development for the treatment of gout. Reducing XDH with an RNAi therapeutic is expected to result in potent urate lowering, essential in preventing gout-associated flares and managing the disease. ALN-XDH utilizes Alnylam's Enhanced Stabilization Chemistry Plus (ESC+) GalNAc-conjugate technology, which enables subcutaneous dosing with increased selectivity and a wide therapeutic index. The safety and efficacy of ALN-XDH have not been evaluated by the FDA, EMA, or any other health authority.

About Gout

Gout is the most common form of inflammatory arthritis, impacting 14-18 million individuals across the US, Europe, and Japan. It is caused by the accumulation of uric acid crystals in joints and is accompanied by a number of debilitating clinical manifestations including edema, inflammatory arthropathy, pain, tophi, joint destruction and, in some patients, chronic kidney disease. The current standard of care for gout has substantial limitations including side effects, inadequate reduction of uric acid levels, increased risk of kidney stones, immunogenicity, and potentially increased cardiovascular risk. Many treatments require daily oral therapy even during asymptomatic periods. As a result, the majority of patients are untreated, cannot adhere to prescribed therapy, or do not reach target uric acid levels.

About RNAi

RNAi (RNA interference) is a natural cellular process of gene silencing that represents one of the most promising and rapidly advancing frontiers in biology and drug development today. Its discovery has been heralded as "a major scientific breakthrough that happens once every decade or so," and was recognized with the award of the 2006 Nobel Prize for Physiology or Medicine. By harnessing the natural biological process of RNAi occurring in our cells, a new class of medicines, known as RNAi therapeutics, is now a reality. Small interfering RNA (siRNA), the molecules that mediate RNAi and comprise Alnylam's RNAi therapeutic platform, function upstream of today’s medicines by potently silencing messenger RNA (mRNA) – the genetic precursors – that encode for disease-causing or disease pathway proteins, thus preventing them from being made. This is a revolutionary approach with the potential to transform the care of patients with genetic and other diseases.

About Alnylam Pharmaceuticals

Alnylam (Nasdaq: ALNY) is leading the translation of RNA interference (RNAi) into a whole new class of innovative medicines with the potential to transform the lives of people afflicted with rare genetic, cardio-metabolic, hepatic infectious, and central nervous system (CNS)/ocular diseases. Based on Nobel Prize-winning science, RNAi therapeutics represent a powerful, clinically validated approach for the treatment of a wide range of severe and debilitating diseases. Founded in 2002, Alnylam is delivering on a bold vision to turn scientific possibility into reality, with a robust RNAi therapeutics platform. Alnylam’s commercial RNAi therapeutic products are ONPATTRO^® (patisiran), GIVLAARI^® (givosiran), OXLUMO^® (lumasiran), and Leqvio^® (inclisiran) being developed and commercialized by Alnylam’s partner Novartis. Alnylam has a deep pipeline of investigational medicines, including six product candidates that are in late-stage development. Alnylam is executing on its “Alnylam P⁵x25” strategy to deliver transformative medicines in both rare and common diseases benefiting patients around the world through sustainable innovation and exceptional financial performance, resulting in a leading biotech profile. Alnylam is headquartered in Cambridge, MA. For more information about our people, science and pipeline, please visit www.alnylam.com and engage with us on Twitter at @Alnylam, on LinkedIn, or on Instagram.

Alnylam Forward Looking Statements

Various statements in this release concerning Alnylam's future expectations, plans and prospects, including, without limitation, Alnylam’s views with respect to the initiation of a Phase 1/2 study of ALN-XDH in healthy volunteers and in patients with gout in early 2022, timing of initial human data, the potential of ALN-XDH to address key unmet needs for gout patients, Alnylam’s aspiration to become a leading biotech company, and the planned achievement of its “Alnylam P⁵x25” strategy, constitute forward-looking statements for the purposes of the safe harbor provisions under The Private Securities Litigation Reform Act of 1995. Actual results and future plans may differ materially from those indicated by these forward-looking statements as a result of various important risks, uncertainties and other factors, including, without limitation: the direct or indirect impact of the COVID-19 global pandemic or any future pandemic on Alnylam’s business, results of operations and financial condition and the effectiveness or timeliness of Alnylam’s efforts to mitigate the impact of the pandemic; the potential impact of the planned leadership transition at year end on Alnylam’s ability to attract and retain talent and to successfully execute on its “Alnylam P⁵x25” strategy; Alnylam's ability to discover and develop novel drug candidates and delivery approaches and successfully demonstrate the efficacy and safety of its product candidates, including ALN-XDH; the pre-clinical and clinical results for its product candidates; actions or advice of regulatory agencies and Alnylam’s ability to obtain and maintain regulatory approval for its product candidates, as well as favorable pricing and reimbursement; successfully launching, marketing and selling its approved products globally; delays, interruptions or failures in the manufacture and supply of its product candidates or its marketed products; obtaining, maintaining and protecting intellectual property; Alnylam’s ability to successfully expand the indication for OXLUMO and ONPATTRO (and vutrisiran, if approved) in the future; Alnylam's ability to manage its growth and operating expenses through disciplined investment in operations and its ability to achieve a self-sustainable financial profile in the future without the need for future equity financing; Alnylam’s ability to maintain strategic business collaborations; Alnylam's dependence on third parties for the development and commercialization of certain products, including Novartis, Sanofi, Regeneron and Vir; the outcome of litigation; the potential impact of current and the risk of future government investigations; and unexpected expenditures; as well as those risks more fully discussed in the “Risk Factors” filed with Alnylam's most recent Quarterly Report on Form 10-Q filed with the Securities and Exchange Commission (SEC) and in its other SEC filings. 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 explicitly disclaims any obligation, except to the extent required by law, to update any forward-looking statements.

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