Enanta Pharmaceuticals Announces First Patient Dosed in the RESOLVE Phase 2b/3 Study of Zelicapavir, a Potential First-in-Disease Treatment for Respiratory Syncytial Virus (RSV)

Enanta expects adult RESOLVE and pediatric LOTUS Phase 2b data readouts in 2027.

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• Global Phase 2b/3 Study Will Evaluate Symptom Duration and Hospitalization Rates in Adult Outpatients at High Risk of Severe Outcomes from RSV
• Topline Phase 2b Data Expected in 2027

WATERTOWN, Mass.--(BUSINESS WIRE)-- Enanta Pharmaceuticals, Inc. (NASDAQ:ENTA), a clinical-stage biotechnology company dedicated to creating small molecule drugs for viral infections and immunological diseases, today announced that the first patient has been dosed in RESOLVE, its global Phase 2b/3 clinical trial of zelicapavir in adults at high risk of severe outcomes from RSV infection. Topline data from the Phase 2b portion of the study are expected in 2027.

“We are excited to advance our zelicapavir program into a Phase 2b/3 study with the dosing of the first patient in RESOLVE,” said Scott T. Rottinghaus, M.D., Chief Medical Officer of Enanta Pharmaceuticals. “Based on learnings from our Phase 2 RSVHR study, the RESOLVE trial is enrolling adults at high risk for severe outcomes from RSV. These patients with congestive heart failure, chronic obstructive pulmonary disease, or age of 75 or older have substantial disease burden and the potential to benefit from an RSV treatment. The Phase 2b portion is designed to confirm the treatment effect size and generate an additional dataset with the goal of strengthening the clinical evidence supporting zelicapavir. Together with LOTUS, our pediatric Phase 2b trial, this progress positions us to deliver two important Phase 2b data readouts in 2027 and marks a significant step forward in the development of zelicapavir. With no approved RSV antiviral therapies currently available, these advancements underscore the potential for zelicapavir to become a first-in-disease antiviral treatment.”

"RSV remains a significant cause of illness, particularly among older adults and individuals with underlying health conditions who are at heightened risk of severe disease. Despite RSV’s burden, there are no treatment options available,” said Juan Antonio Carmona Herrera, M.D., a Principal Investigator in the RESOLVE study and President of C&A Clinical Trials Corp. “An effective antiviral has the potential to change the way RSV is managed by directly targeting the virus, with the goal of reducing disease severity, preventing complications, and improving patient outcomes. The development of a treatment specifically for RSV would represent an important advancement for physicians and the millions of patients at risk of serious illness each year."

The Phase 2b/3 randomized, double-blind, placebo-controlled, multicenter global trial will evaluate the efficacy and safety of zelicapavir in adult outpatients who test positive for RSV and have had respiratory tract infection symptoms for no more than 3 days. Patients will have at least one of the following risk factors: 75 years of age or older, chronic obstructive pulmonary disease, or congestive heart failure. These risk factors are the same as the HR3 population in the Company’s previous Phase 2 RSVHR trial where a one-week reduction in time to complete resolution of symptoms, as well as a reduction in hospitalization, were observed. Patients will receive an oral dose of 800mg of zelicapavir or placebo once daily for 7 days and be evaluated for 28 days thereafter. The primary endpoint is the time to complete resolution of all 13 RSV symptoms as measured by the RiiQ™ tool. A key secondary endpoint will evaluate hospitalization rate, with other secondary endpoints of additional clinical efficacy measures, antiviral activity, and safety of zelicapavir. The Phase 2b portion of the study will include a minimum of 200 patients and is designed to confirm the treatment effect size on the primary and key secondary endpoints in the targeted population, while the Phase 3 portion of the study is expected to enroll approximately 660 patients.

About Zelicapavir

Zelicapavir is being developed for the treatment of RSV infection and has been granted Fast Track designation by the U.S. Food and Drug Administration. Zelicapavir inhibits the RSV N-protein with nanomolar activity against both RSV-A and RSV-B. This mechanism is differentiated from RSV fusion inhibitors as targeting the N-protein inhibits viral replication versus viral entry and has a high barrier to resistance. In a Phase 2 trial of hospitalized and non-hospitalized pediatric RSV patients aged 28 days to 3 years old, zelicapavir demonstrated an antiviral effect and shortened time to complete resolution of RSV symptoms. In a Phase 2 trial of high-risk adults, a clinically meaningful improvement in time to complete resolution of all 13 RSV symptoms was observed for zelicapavir compared to placebo, with a benefit of 6.7 days for patients with congestive heart failure, chronic obstructive pulmonary disease, or age 75 years or older, termed the HR3 population. Importantly, zelicapavir treatment resulted in a lower hospitalization rate, with an RSV-related hospitalization rate of 5% in placebo, compared to 0% in patients who received zelicapavir. Throughout its clinical development program, zelicapavir has demonstrated a favorable safety profile and has been well-tolerated in more than 700 people to date.

About RSV

RSV is the most common cause of bronchiolitis (inflammation of the small airways in the lung) and pneumonia in children under one year of age in the United States and a significant cause of respiratory illness in older adults and immunocompromised individuals.^1,2 According to the Centers for Disease Control and Prevention, virtually all children in the United States get an RSV infection by the time they are two years old.³ RSV represents a significant health threat for adults older than 50 years of age, with up to 180,000 hospitalizations associated with RSV infections annually in the United States.² Overall, in the United States, RSV accounts for up to 6.8 million outpatient visits and approximately 370,000 hospitalizations, with 24,000 deaths.⁴

About Enanta Pharmaceuticals, Inc.

Enanta is using its robust, chemistry-driven approach and drug discovery capabilities to become a leader in the discovery and development of small molecule drugs for viral infections and immunological diseases. In virology, Enanta’s clinical programs are focused on the development of first-in-disease and best-in-disease treatments for respiratory syncytial virus (RSV). The Company’s immunology pipeline aims to develop treatments for inflammatory diseases by targeting key drivers of the type 2 immune response, with KIT, STAT6 and MRGPRX2 inhibition.

Glecaprevir, a protease inhibitor discovered by Enanta, is part of the only treatment approved for curing both acute and chronic hepatitis C virus (HCV) infection and is sold by AbbVie in numerous countries under the tradenames MAVYRET® (U.S.) and MAVIRET® (ex-U.S.) (glecaprevir/pibrentasvir). A portion of Enanta’s royalties from HCV products developed under its collaboration with AbbVie contribute ongoing funding to Enanta’s operations. Please visit www.enanta.com for more information.

Forward Looking Statements

This press release contains forward-looking statements, including statements with respect to the Phase 2b/3 study of zelicapavir and the timeline and prospects for advancement of zelicapavir for the treatment of RSV. Statements that are not historical facts are based on management’s current expectations, estimates, forecasts and projections about Enanta’s business and the industry in which it operates and management’s beliefs and assumptions. The statements contained in this release are not guarantees of future performance and involve certain risks, uncertainties and assumptions, which are difficult to predict. Therefore, actual outcomes and results may differ materially from what is expressed in such forward-looking statements. Important factors and risks that may affect actual results include: clinical drug development involves a lengthy and expensive process with uncertain timelines and uncertain outcomes, the Phase 2b portion of Enanta’s Phase 2b/3 trial of zelicapavir for RSV may not support the Phase 3 portion, Enanta’s clinical trials, including its Phase 2b/3 trial, may fail to demonstrate sufficient safety and efficacy and, if that occurs, it may be unable to commercialize its product candidates on a timely basis or ever, the impact of development, regulatory and marketing efforts of others with respect to vaccines and competitive treatments for RSV; the discovery and development risks of Enanta’s programs in virology and immunology; Enanta’s limited clinical development experience; Enanta’s ability to partner its RSV or other programs; Enanta’s need to attract and retain senior management and key research and development personnel; Enanta’s need to obtain and maintain patent protection for its product candidates and avoid potential infringement of the intellectual property rights of others; and other risk factors described or referred to in “Risk Factors” in Enanta’s Form 10-K for the fiscal year-ended September 30, 2025, as updated by periodic reports subsequently filed with the Securities and Exchange Commission. Enanta cautions investors not to place undue reliance on the forward-looking statements contained in this release. These statements speak only as of the date of this release, and Enanta undertakes no obligation to update or revise these statements, except as may be required by law.

References
1. Centers for Disease Control & Prevention – About RSV. Last accessed: October 2026.
2. Centers for Disease Control & Prevention – RSV In Adults. Last accessed: October 2026.
3. Centers for Disease Control & Prevention – RSV in Infants and Young Children. Last accessed: October 2026.
4. CDC Preliminary Estimates of RSV Burden for 2024-2025; for period: 9/29/24-9/27/25. Last accessed October 2026.

View source version on businesswire.com: https://www.businesswire.com/news/home/20261026346472/en/

Media and Investor
Jennifer Viera
617-744-3848
jviera@enanta.com

Source: Enanta Pharmaceuticals, Inc.

Key Terms

fast track designation regulatory

Fast track designation is a status the U.S. Food and Drug Administration grants to drugs intended to treat serious conditions and address an unmet medical need. It gives the developer more frequent communication with the FDA and can allow parts of the application to be reviewed on a rolling basis, and it may pave the way to priority review or accelerated approval. It can shorten development timelines, though it does not guarantee approval.

double-blind medical

A double-blind process means that neither the people conducting an activity nor the people involved know certain key details, such as who is receiving a treatment or a placebo. This approach helps prevent bias from influencing the results, making the outcome more trustworthy. For investors, it ensures that decisions or judgments are based on unbiased information rather than preconceived opinions or expectations.

placebo-controlled medical

"Placebo-controlled" describes a testing method where one group receives the actual treatment or intervention, while another group receives a harmless, inactive version called a placebo. This approach helps determine whether the real treatment has genuine effects beyond psychological expectations. For investors, understanding this ensures confidence that reported benefits are real and not influenced by bias or false perceptions.

primary endpoint medical

The primary endpoint is the single main result a clinical study is designed to measure to decide if a treatment works, like the finish line in a race that tells you who won. Investors care because meeting or missing this goal drives regulatory decisions, future sales expectations and stock value — it turns trial data into a clear yes-or-no signal about a drug’s commercial prospects.

viral replication medical

The process by which a virus produces new copies of itself inside a host organism by entering susceptible cells and using the cell’s molecular machinery to copy its genome, make viral proteins, assemble new virus particles (virions), and release those progeny to infect other cells. Viral replication only occurs within living host cells and the specific steps and enzymes involved differ by virus type (for example DNA versus RNA viruses or viruses that use reverse transcription).