– Vutrisiran Met All 18-Month Secondary Endpoints, Including Statistically Significant Improvements in Progression of Neuropathy, Quality of Life (QOL), Gait Speed, Nutritional Status and Overall Disability, Relative to External Placebo –

– At Month 18, Vutrisiran Also Showed Improvements in Exploratory Endpoints, Including Technetium Uptake Relative to Baseline in a Planned Cohort, Providing Potential Evidence for Reduced Cardiac Amyloid Burden –

– In Addition, Vutrisiran Continued to Demonstrate an Encouraging Safety and Tolerability Profile –

– Alnylam Intends to Present Full HELIOS-A 18-Month Results at a Medical Congress in Early 2022 –

– Alnylam to Discuss Data as Part of Third Quarter 2021 Financial Results Conference Call to be Held Tomorrow, October 28 at 8:30 am ET –

Alnylam Pharmaceuticals, Inc. (Nasdaq: ALNY), the leading RNAi therapeutics company, today announced that the HELIOS-A Phase 3 study of vutrisiran, an investigational RNAi therapeutic in development for the treatment of the polyneuropathy associated with hereditary transthyretin-mediated (hATTR) amyloidosis, met all secondary endpoints measured at 18 months, including statistically significant improvements in neuropathy as measured by the modified Neuropathy Impairment Score (mNIS+7), quality of life (QOL), gait speed, nutritional status and overall disability, relative to external placebo data from the APOLLO Phase 3 study of patisiran. The final secondary endpoint, reduction in serum TTR levels with vutrisiran, demonstrated non-inferiority relative to the within-study patisiran arm, as expected. In addition, patients treated with vutrisiran showed improvements in exploratory endpoints, including the biomarker NT-proBNP and certain echocardiographic parameters, relative to placebo, and an improvement in technetium uptake relative to baseline in a majority of patients in a planned cohort, providing potential evidence for reduced cardiac amyloid burden. Vutrisiran continued to demonstrate an encouraging safety and tolerability profile. Alnylam previously announced that HELIOS-A met its primary and secondary endpoints at nine months and study results were presented at the 2021 American Academy of Neurology (AAN) Virtual Annual Meeting.

“These results build on the positive vutrisiran data shared earlier this year and suggest that the reduction of neurologic impairment and improvement in quality of life in patients with hATTR amyloidosis with polyneuropathy observed as early as nine months are maintained at 18 months. We are also encouraged by the hypothesis-supporting, exploratory endpoints and cardiac amyloid imaging results,” said Rena N. Denoncourt, Vice President, TTR Franchise Lead. “Vutrisiran is currently under review by multiple regulatory authorities around the world, bringing this low-dose, once-quarterly, subcutaneously administered investigational therapy with a highly-attractive profile one step closer to being a potentially available therapeutic option for patients living with this progressive, life-threatening, multi-system disease. We look forward to presenting full 18-month results of the HELIOS-A study at a medical conference in early 2022, and progressing our efforts to build an industry-leading franchise of medicines for the treatment of the polyneuropathy of hATTR amyloidosis and potentially additional indications for the future.”

At 18 months, patients treated with vutrisiran showed quantitative improvement across a number of exploratory endpoints. Compared to placebo, patients in the vutrisiran arm demonstrated improvement in the cardiac biomarker endpoint, NT-proBNP, a measure of cardiac stress. In addition, patients treated with vutrisiran also demonstrated improvement in certain echocardiographic parameters, relative to placebo. Finally, in a planned cohort of 48 patients, treatment with vutrisiran was associated with an improvement in technetium uptake in the heart in a majority of patients, providing potential evidence for reduced cardiac amyloid burden.

Vutrisiran demonstrated an encouraging safety profile. There were three study discontinuations (2.5 percent) due to adverse events in the vutrisiran arm during the 18 Month treatment period; the single new discontinuation since Month 9 was an event of cardiac failure considered unrelated to study drug by the investigator. During the 18 Month treatment period, there were two deaths (neither of which was considered related to study drug) and two serious adverse events (SAEs) deemed related to vutrisiran by the study investigator; these deaths and related SAEs all occurred by Month 9 and were previously reported. Treatment emergent adverse events (AEs) occurring in 10 percent or more patients included fall, pain in extremity, diarrhea, peripheral edema, urinary tract infection, arthralgia and dizziness; with the exception of pain in extremity and arthralgia, each of these events occurred at a similar or lower rate as compared with external placebo. Injection site reactions (ISRs) were reported in 5 patients (4.1 percent) and were all mild and transient. There were no hepatic safety concerns.

Vutrisiran is under review by the U.S. Food and Drug Administration (FDA), the European Medicines Agency (EMA), and the Brazilian Health Regulatory Agency (ANVISA). Vutrisiran has been granted Orphan Drug Designation in the U.S. and the European Union (EU) for the treatment of ATTR amyloidosis. Vutrisiran has also been granted a Fast Track designation in the U.S. for the treatment of the polyneuropathy of hATTR amyloidosis in adults. In the U.S., vutrisiran has received an action date under the Prescription Drug User Fee Act (PDUFA) of April 14, 2022. The Company received Orphan Drug Designation in Japan for transthyretin type familial amyloidosis with polyneuropathy.

Conference Call

Alnylam Management will discuss the HELIOS-A 18-month topline during the third quarter 2021 results conference call on Thursday, October 28, 2021 at 8:30 am ET. To access the call, please dial 877-312-7507 (domestic) or +1-631-813-4828 (international) five minutes prior to the start time and refer to conference ID 1428538. A replay of the call will be available beginning at 11:30 am ET on the day of the call. To access the replay, please dial 855-859-2056 (domestic) or +1-404-537-3406 (international) and refer to conference ID 1428538.

A live audio webcast of the call will be available on the Investors section of the Company’s website at www.alnylam.com/events. An archived webcast will be available on the Alnylam website approximately two hours after the event.

About hATTR Amyloidosis

Hereditary transthyretin-mediated (hATTR) amyloidosis is an inherited, progressively debilitating, and fatal disease caused by variants (i.e., mutations) in the TTR gene. TTR protein is primarily produced in the liver and is normally a carrier of vitamin A. Variants in the TTR gene cause abnormal amyloid proteins to accumulate and damage body organs and tissue, such as the peripheral nerves and heart, resulting in intractable peripheral sensory-motor neuropathy, autonomic neuropathy, and/or cardiomyopathy, as well as other disease manifestations. hATTR amyloidosis, represents a major unmet medical need with significant morbidity and mortality affecting approximately 50,000 people worldwide. The median survival is 4.7 years following diagnosis, with a reduced survival (3.4 years) for patients presenting with cardiomyopathy.

About Vutrisiran

Vutrisiran is an investigational, subcutaneously administered RNAi therapeutic in development for the treatment of ATTR amyloidosis, which encompasses both hATTR and wild-type ATTR (wtATTR) amyloidosis. It is designed to target and silence specific messenger RNA, potentially blocking the production of wild-type and variant transthyretin (TTR) protein before it is made. Quarterly, and potentially biannual, administration of vutrisiran may help to reduce deposition and facilitate the clearance of TTR amyloid deposits in tissues and potentially restore function to these tissues. Vutrisiran utilizes Alnylam’s Enhanced Stabilization Chemistry (ESC)-GalNAc-conjugate delivery platform, designed for increased potency and high metabolic stability that may allow for infrequent subcutaneous injections. The safety and efficacy of vutrisiran have not been evaluated by the U.S. Food and Drug Administration, European Medicines Agency, or any other health authority.

About HELIOS-A Phase 3 Study

HELIOS-A (NCT03759379) is a Phase 3 global, randomized, open-label study to evaluate the efficacy and safety of vutrisiran. The study enrolled 164 patients with hATTR amyloidosis with polyneuropathy at 57 sites in 22 countries. Patients were randomized 3:1 to receive either 25mg of vutrisiran (N=122) via subcutaneous injection once every three months or 0.3 mg/kg of patisiran (N=42) via intravenous infusion once every three weeks (as a reference comparator) for 18 months. The efficacy results of vutrisiran in HELIOS-A are compared to the external placebo group from the landmark APOLLO Phase 3 study, which evaluated the efficacy and safety of patisiran in a patient population similar to that studied in HELIOS-A. The primary endpoint was the change from baseline in mNIS+7 at nine months. Secondary endpoints at nine months were the change from baseline in the Norfolk QoL-DN score and the timed 10-MWT. Additional secondary endpoints at 18 months evaluated in the HELIOS-A study included change from baseline in mNIS+7, Norfolk QoL-DN, 10-MWT, modified body mass index (mBMI), Rasch-built Overall Disability Scale (R-ODS), and serum transthyretin (TTR) levels. Additional exploratory endpoint data at 18-months, included NT-proBNP, echocardiographic measures and cardiac amyloid burden as measured by technetium scintigraphy imaging. Following the 18-month treatment period, all patients are eligible to receive vutrisiran for an additional 18 months as part of the randomized treatment extension where they will receive either 25mg vutrisiran once quarterly or 50mg vutrisiran once every six months.

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 silencing messenger RNA (mRNA) – the genetic precursors – that encode for disease-causing 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 expectations, plans, aspirations, and goals, including those related to vutrisiran and its potential as a low-dose, once quarterly, subcutaneously administered treatment option with an encouraging safety profile for patients with hATTR amyloidosis with polyneuropathy, with data suggesting that the reduction of neurologic impairment and improvement in quality of life in patients with hATTR amyloidosis with polyneuropathy observed as early as nine months are maintained at 18 months, encouraging exploratory cardiac endpoint results, the expected timing of the presentation of full 18-month results from HELIOS-A, the status and timing for regulatory review of filings for vutrisiran by regulatory authorities around the world, the progress of its efforts to build an industry-leading franchise in medicines for the treatment of the polyneuropathy of hATTR amyloidosis and potentially additional indications for the future, 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; 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 vutrisiran; 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, including vutrisiran, 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 ONPATTRO (and potentially vutrisiran) 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, Regeneron and Vir; the outcome of litigation; the potential impact of a current government investigation 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.

This release discusses investigational RNAi therapeutics and uses of previously approved RNAi therapeutics in development and is not intended to convey conclusions about efficacy or safety as to any investigational RNAi therapeutics or investigational uses of previously approved therapeutics. There is no guarantee that any investigational therapeutics or expanded uses of commercial products will successfully complete clinical development or gain health authority approval.

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