Avidity Biosciences’ Del-zota Demonstrated Reversal of Disease Progression Across Key Functional Endpoints in EXPLORE44® and EXPLORE44-OLE™ Phase 1/2 Trial in People Living with DMD44

— Unprecedented improvement compared to baseline and natural history in multiple functional measures including Time to Rise from Floor (TTR), 4-Stair Climb (4SC), Performance of Upper Limb (PUL) and 10-Meter Walk/Run Test (10mWRT) at approximately one year —

— Unprecedented rapid reduction in creatine kinase (CK) to near normal levels maintained over 16 months of follow-up and 25% increase of normal in dystrophin production, reflecting sustained muscle fiber protection —

— Avidity remains on track to submit Biologics License Application (BLA) at year end 2025 for accelerated approval —

— Investor and analyst webcast event today at 8:00 a.m. ET —

SAN DIEGO, CA, Sept. 10, 2025 — Avidity Biosciences, Inc. (Nasdaq: RNA), a biopharmaceutical company developing Antibody Oligonucleotide Conjugates (AOCs™), today announced positive new data from participants treated continuously with del-zota for approximately one year in the EXPLORE44® and EXPLORE44-OLE™ trials. These data demonstrated reversal of disease progression and unprecedented improvement compared to baseline and natural history across multiple functional measures. Additional data from the EXPLORE44 program will be presented at upcoming scientific congresses.

Duchenne muscular dystrophy (DMD) is a rare genetic condition characterized by progressive muscle damage and weakness beginning at a very young age due to the absence of dystrophin protein from birth. Del-zota is designed to deliver phosphorodiamidate morpholino oligomers (PMOs) to skeletal muscle and heart tissue to specifically skip exon 44 of the dystrophin mRNA and enable production of functional, near-full length dystrophin. Near-full length dystrophin retains key functional domains and may offer improved muscle protection for people living with DMD44.

“For the first time, we have data showing that sustained muscle protection leads to meaningful improvements across multiple key functional measures in DMD,” said Sarah Boyce, President and Chief Executive Officer of Avidity. “These unprecedented data underscore the impact of our targeted approach to deliver RNA directly to muscle. We are acting with urgency to rapidly advance the del-zota development program and remain on track to submit a Biologics License Application (BLA) to FDA at year end 2025 for accelerated approval. We extend our deepest appreciation for the continued dedication of the investigators and their teams and, most importantly, the participants in our clinical trials and their families as we pursue a new treatment option for this relentless and devastating disease.”

Data from EXPLORE44® Clinical Development Program

Trial participants treated with del-zota demonstrated statistically significant increases of approximately 25% of normal in dystrophin production and restored total dystrophin up to 58% of normal. Creatine kinase (CK) levels rapidly reduced by greater than 80% compared to baseline and were sustained at near normal levels with participants followed for up to 16 months. Additionally, 50% of participants had CK levels within the normal range at one year of treatment.

A total of 17 participants (12 ambulatory and 5 non-ambulatory) who began on the del-zota treated arm of EXPLORE44® and continued into the EXPLORE44-OLE™ have been followed for approximately one year. Given the study design, some participants received 5 mg/kg once every six weeks (Q6W) and some received 10 mg/kg once every eight weeks during EXPLORE44. All participants were transitioned to the 5 mg/kg (Q6W) dosing schedule during EXPLORE44-OLE. Functional data from these pooled dosing cohorts for del-zota treated participants, compared to DMD44 natural history (PRO-DMD-01), demonstrated improvement:

• 4-Stair Climb (4SC): Improved from baseline by 2.1 seconds. In contrast, the natural history group declined from baseline by 2.7 seconds (DMD44 Nat Hx N=22; del-zota N=10).
• 10-Meter Walk/Run Test (10mWRT): Improved from baseline by 0.7 seconds. In contrast, the natural history group declined from baseline by 1.5 seconds (DMD44 Nat Hx N=22; del-zota N=10).
• Time to Rise from Floor (TTR): Improved from baseline by 3.2 seconds. In contrast, the natural history group declined from baseline by 1.6 seconds (DMD Nat Hx N=19; del-zota N=6).
• North Star Ambulatory Assessment (NSAA): Remained stable. In contrast, the natural history group declined from baseline by 2.4 points (DMD44 Nat Hx N=20; del-zota N=10).
• Performance of Upper Limb (PUL2): Improved from baseline by 1.5 points. In contrast, the natural history group declined from baseline by 0.7 points. Similar PUL improvements were seen in both ambulatory and non-ambulatory participants (DMD44 Nat Hx N=27; del-zota N=17).

Safety was assessed in all participants in the EXPLORE44-OLE trial, and del-zota continued to demonstrate a favorable long-term safety and tolerability profile. Most treatment emergent adverse events (TEAEs) were mild or moderate with the most common TEAEs (occurring in greater than 3 participants) being upper respiratory tract symptoms, diarrhea, fall, back pain and headache. One participant discontinued from EXPLORE44-OLE following an event of hypersensitivity.

Avidity remains on track to submit a BLA to the U.S. Food and Drug Administration (FDA) at year end 2025. This will be the company’s first of three planned BLA submissions over a 12-month period. Avidity continues to prepare for a confirmatory study to support full global approval.

Video Webcast Information

The company hosted an investor and analyst event on Wednesday, September 10, 2025, at 8:00 a.m. ET. The virtual event was available via live webcast and a replay will be archived on Avidity’s website under Events and Presentations.

About the EXPLORE44® Phase 1/2 Trial

The EXPLORE44® trial was a randomized, placebo-controlled, double-blind, Phase 1/2 clinical trial that enrolled 26 participants with Duchenne muscular dystrophy mutations amenable to exon 44 skipping (DMD44). The study evaluated safety, tolerability, pharmacokinetics, and pharmacodynamic effects of single and multiple ascending doses of del-zota administered intravenously. Participants with DMD44 could enroll in EXPLORE44-OLE™, an open-label extension. For more information search NCT05670730 on clinicaltrials.gov or visit the EXPLORE44 study website.

About EXPLORE44-OLE™

EXPLORE44-OLE™ is an open-label, multi-center trial designed to evaluate the long-term safety, tolerability, pharmacokinetics, pharmacodynamic effects and efficacy of del-zota in participants with DMD44. Enrollment completed with participants receiving 5 mg/kg every six weeks. Total active treatment in the OLE is approximately 24 months, followed by a three-month safety follow-up.

About the PRO-DMD-01 Natural History Study

PRO-DMD-01 was an observational, prospective natural history study (N=269) intended to study progression of subjects with DMD. In collaboration with Analysis Group®, a matched subset of DMD44 participants (N=22) was used to analyze progression on key ambulatory functional endpoints at 12 months (see clinicaltrials.gov NCT01753804).

About Duchenne muscular dystrophy (DMD)

DMD causes lack of functional dystrophin, leading to muscle cell damage and progressive loss of function. Over time, people with DMD develop problems walking and breathing and face reduced life expectancy. DMD is X-linked and primarily affects males, with an incidence of about 1 in 3,500 to 5,000 boys born worldwide. Despite approved treatments, significant unmet need remains.

About del-zota

Del-zota is designed to deliver PMOs to skeletal muscle and heart tissue to skip exon 44 of the dystrophin gene and enable dystrophin production in people with mutations amenable to exon 44 skipping. Del-zota consists of a monoclonal antibody that binds transferrin receptor 1 (TfR1) conjugated to a PMO targeting exon 44. The Phase 1/2 EXPLORE44® trial demonstrated robust increases in dystrophin production, significant exon 44 skipping, and sustained reductions in CK to near normal. Del-zota has received Rare Pediatric Disease, Orphan Drug, Fast Track and Breakthrough Therapy designations from FDA and Orphan designation from EMA.

About Avidity

Avidity Biosciences, Inc. seeks to improve lives by delivering AOCs™, combining monoclonal antibody specificity with oligonucleotide precision to address targets previously unreachable with RNA therapies. Avidity is advancing clinical programs for rare muscle diseases (DM1, DMD, FSHD) and cardiology candidates, and is headquartered in San Diego, CA. For more information visit www.aviditybiosciences.com and follow the company on LinkedIn and X.

Forward-Looking Statements

This press release contains forward-looking statements about del-zota, clinical development plans, regulatory filings (including the planned BLA submission at year-end 2025), the meaning of clinical data, and Avidity’s future operations. Actual results may differ due to risks and uncertainties, including preliminary data not reflecting final results, potential adverse effects or inadequate efficacy, regulatory outcomes, dependence on third parties, funding and other risks described in Avidity’s SEC filings. Readers are cautioned not to place undue reliance on forward-looking statements, which speak only as of the date hereof.

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Source: Avidity Biosciences, Inc.