Zelicapavir RSV drug moves into registrational trials at Enanta (ENTA)
Rhea-AI Filing Summary
Enanta Pharmaceuticals is advancing its RSV antiviral zelicapavir into a single registrational Phase 2b/3 trial in high-risk adult outpatients after a successful End-of-Phase 2 FDA meeting. The Phase 2b portion will enroll at least 200 patients, with approximately 660 more in Phase 3, and topline Phase 2b data expected in 2027.
The company also plans a Phase 2b pediatric trial in children 28 days to 36 months old, starting in the third quarter of 2026, with topline data also expected in 2027. Earlier Phase 2 data showed clinically meaningful improvements, including a 6.7-day faster symptom resolution and lower RSV-related hospitalizations in high-risk adults, and antiviral and symptom benefits in children.
Zelicapavir, an RSV N-protein inhibitor with FDA Fast Track designation, has been well tolerated in more than 700 subjects. Enanta estimates an oral RSV antiviral for children and high-risk adults represents a global opportunity of over $2 billion, with a potential U.S. addressable population greater than 3 million patients.
Positive
- Zelicapavir progresses to a single registrational Phase 2b/3 RSV trial in high-risk adults, with supportive Phase 2 data and FDA Fast Track status, alongside a planned pediatric Phase 2b study and an estimated RSV antiviral market opportunity exceeding $2 billion globally.
Negative
- None.
Insights
Enanta gains a clear registrational path for zelicapavir in RSV with sizable market potential.
Enanta is moving zelicapavir into a single registrational Phase 2b/3 trial in high-risk adult RSV patients following a successful End-of-Phase 2 FDA meeting. This design, with a minimum of 200 patients in Phase 2b and about 660 in Phase 3, provides a unified path toward potential approval using the RiiQ™ symptom scale as the primary endpoint.
Parallel pediatric development adds a second Phase 2b dataset in 2027, in collaboration with established infectious disease groups in Thailand. Prior trials showed faster symptom resolution and reduced RSV-related hospitalizations, which, combined with Fast Track status, strengthen the program’s regulatory and clinical rationale.
Commercially, Enanta cites a potential addressable U.S. population above 3 million patients and an RSV antiviral market opportunity above $2 billion, rising to as much as $3.5B in internal modeling. With $227M in cash as of March 31, 2026 and ongoing HCV royalties, the company appears funded to pursue these late-stage RSV studies, though ultimate outcomes still depend on Phase 2b/3 results and competitive dynamics.
8-K Event Classification
Key Figures
Key Terms
registrational Phase 2b/3 clinical trial regulatory
Fast Track designation regulatory
HR3 population clinical
RiiQ™ tool clinical
RESOLVE-P clinical scoring scale clinical
N-protein inhibitor medical
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| Item 7.01 | Regulation FD Disclosure. |
On June 18, 2026, Enanta Pharmaceuticals, Inc. (“Enanta” or the “Company”) issued a press release announcing the advancement of its zelicapavir clinical development program for the treatment of respiratory syncytial virus (“RSV”), including plans for a registrational Phase 2b/3 clinical trial in high-risk adults and a Phase 2b clinical trial in pediatric patients. A copy of the press release is furnished as Exhibit 99.1 hereto and is incorporated herein by this reference. In connection with this announcement, the Company also compiled a presentation entitled “Zelicapavir Update” (the “Presentation”) that provides an overview of its RSV program and development plans. A copy of the Presentation is furnished as Exhibit 99.2 hereto and is incorporated herein by this reference.
The information in Item 7.01 of this Current Report on Form 8-K, including Exhibits 99.1 and 99.2 attached hereto, is intended to be furnished and shall not be deemed “filed” for purposes of Section 18 of the Securities Exchange Act of 1934, as amended (the “Exchange Act”), or otherwise subject to the liabilities of that section, nor shall it be deemed incorporated by reference in any filing under the Securities Act of 1933, as amended, or the Exchange Act, except as expressly set forth by specific reference in any such filing.
| Item 9.01 | Financial Statements and Exhibits. |
(d) Exhibits
| Exhibit Number |
Exhibit Description | |
| 99.1 | Press release dated June 18, 2026 | |
| 99.2 | Slide presentation entitled “Zelicapavir Update” | |
| 104 | Cover Page Interactive Data File (embedded within the Inline XBRL document) | |
SIGNATURES
Pursuant to the requirements of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by the undersigned hereunto duly authorized.
| Date: June 18, 2026 | ENANTA PHARMACEUTICALS, INC. | |||||
| By: | /s/ Jay R. Luly, Ph.D. | |||||
| Jay R. Luly, Ph.D. | ||||||
| President and Chief Executive Officer | ||||||
Exhibit 99.1
Enanta Pharmaceuticals Announces Advancement of its Zelicapavir Clinical Development Program for the Treatment of Respiratory Syncytial Virus (RSV)
| • | Registrational Ph2b/3 Trial of Zelicapavir to Initiate in High-Risk Adults with RSV in 4Q 2026, with Topline Phase 2b Data Expected in 2027 |
| • | Phase 2b Trial of Zelicapavir to Initiate in Pediatric Patients with RSV in 3Q 2026, with Topline Data Expected in 2027 |
WATERTOWN, Mass., June 18, 2026 – Enanta Pharmaceuticals, Inc. (NASDAQ:ENTA), a clinical-stage biotechnology company dedicated to creating small molecule drugs for viral infections and immunological diseases, today announced it is advancing zelicapavir into a registrational Phase 2b/3 clinical trial in adults at high risk of severe outcomes from RSV infection after a successful End-of-Phase 2 meeting with the U.S. Food and Drug Administration (FDA). The Phase 2b portion of the trial will confirm the treatment effect with topline data expected in 2027. Enanta also announced plans to initiate a Phase 2b clinical trial of zelicapavir in pediatric patients with RSV, with topline data expected in 2027.
“We are thrilled to move zelicapavir into a single Phase 2b/3 registrational clinical trial in adults with RSV at high risk of progressing to severe disease, following a productive and collaborative End-of-Phase 2 meeting with the FDA. This progress, alongside the planned initiation of our pediatric Phase 2b trial, provides the opportunity for two key Phase 2b datasets next year in two important patient populations and underscores zelicapavir’s potential to become the first antiviral therapy for RSV infection,” said Jay R. Luly, Ph.D., President and Chief Executive Officer of Enanta Pharmaceuticals. “RSV results in a substantial global disease burden, with no available treatment options. We estimate an oral RSV antiviral for children and all high-risk adults represents a global market opportunity of over $2 billion, with a potential total addressable population of greater than 3 million patients in the United States alone. The recent availability of at-home RSV diagnostics is expected to further drive disease awareness and increase earlier diagnosis. As we advance into late-stage development, we are well-positioned to continue our leadership in RSV and remain urgently focused on delivering zelicapavir to patients in need of treatment.”
“We now have a clear and efficient pathway to advance zelicapavir into a registrational program through a single Phase 2b/3 trial. The Phase 2b part is designed to confirm the primary endpoint and treatment effect size in the targeted population and will provide a key dataset to further strengthen the body of evidence supporting the Phase 3 portion of the study. We look forward to beginning the trial in the fourth quarter of 2026, with data from the Phase 2b portion expected in 2027,” said Scott T. Rottinghaus, M.D., Chief Medical Officer of Enanta Pharmaceuticals. “We are also excited to advance zelicapavir development in children, with a Phase 2b trial to be conducted in collaboration with the Penta Foundation, a leading organization dedicated to pediatric infectious disease research, and the internationally recognized AMS-PHPT Research Unit at Chiang Mai University. This trial will build on the encouraging findings from our first-in-pediatrics study and allow us to assess zelicapavir’s treatment effect on symptom resolution using RESOLVE-P, our proprietary tool being developed for use as a registrational endpoint. We plan to initiate this pediatric study in the third quarter of 2026, with topline data expected in 2027.”
Adult Registration Study
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 (COPD), or congestive heart failure (CHF). 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 and a reduction in hospitalization was 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 primary endpoint and treatment effect size in the targeted population with the optimized dosing duration and will further support the Phase 3 portion of approximately 660 patients.
Pediatric Phase 2b Study
The Phase 2b, double-blind, placebo-controlled, multicenter trial in pediatric patients is designed to evaluate the efficacy and safety of zelicapavir in hospitalized and non-hospitalized children with up to 72 hours of respiratory tract infection symptoms who test positive for RSV. Approximately 150 participants, 28 days to 36 months of age, will receive 5mg/kg (<12 months old) or 7.5 mg/kg (≥12 months old) 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 clinical signs of RSV as measured by the RESOLVE-P clinical scoring scale. Secondary endpoints will evaluate additional clinical efficacy measures, antiviral activity, and safety of zelicapavir. The trial is being conducted in Thailand in collaboration with the Penta Foundation, and the AMS-PHPT Research Unit at Chiang Mai University, which has deep expertise and decades of experience in conducting infectious disease trials. In addition, its network is comprised of clinical trial sites in geographies where the RSV season begins in the summer, making them an ideal partner to execute a timely start for this trial. Enanta plans to initiate this Phase 2b study in the third quarter of 2026 with topline data expected in 2027.
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 vitro. 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 good safety profile and has been well-tolerated in more than 700 subjects 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 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 one of the leading treatment regimens for curing 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 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, 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 our 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, and any other periodic reports filed more recently 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: June 2026. |
| 2. | Centers for Disease Control & Prevention – RSV In Adults. Last accessed: June 2026. |
| 3. | Centers for Disease Control & Prevention – RSV in Infants and Young Children. Last accessed: June 2026. |
| 4. | CDC Preliminary Estimates of RSV Burden for 2024-2025; for period: 9/29/24-9/27/25. Last accessed June 2026. |
Media and Investor Contact
Jennifer Viera
617-744-3848
jviera@enanta.com
Exhibit 99.2 Zelicapavir Update June 18, 2026 1
Forward Looking Statements Disclaimer This presentation contains forward-looking statements concerning our business, operations and financial performance and condition, as well as our plans, objectives and expectations for our research and development programs, our business and the industry in which we operate. Any statements contained herein that are not statements of historical facts may be deemed to be forward-looking statements. In some cases, you can identify forward-looking statements by terminology such as “aim,” “anticipate,” “assume,” “believe,” “contemplate,” “continue,” “could,” “due,” “estimate,” “expect,” “goal,” “intend,” “may,” “objective,” “plan,” “predict,” “potential,” “positioned,” “seek,” “should,” “target,” “will,” “would,” and other similar expressions that are predictions of or indicate future events and future trends, as well as other comparable terminology. These forward-looking statements include, but are not limited to, statements about overall trends, royalty revenue trends, research and clinical development plans and prospects, liquidity and capital needs and other statements of expectations, beliefs, future plans and strategies, anticipated events or trends, and similar expressions. These forward-looking statements are based on our management’s current expectations, estimates, forecasts and projections about our business and the industry in which we operate and our management’s beliefs and assumptions. These forward-looking statements are not guarantees of future performance or results and involve known and unknown risks, uncertainties and other factors that are in some cases beyond our control. As a result, any or all of our forward-looking statements in this presentation may turn out to be inaccurate. Please refer to the risk factors described or referred to in “Risk Factors” in Enanta’s most recent Annual Report on Form 10-K, and other periodic reports filed with the Securities and Exchange Commission. Enanta cautions investors not to place undue reliance on the forward-looking statements contained in this presentation. These statements speak only as of the date of this presentation, and Enanta undertakes no obligation to update or revise these statements, except as may be required by law. © 2026 Enanta Pharmaceuticals, Inc. | 2
Enanta Overview Clinical-stage All compounds biotechnology company discovered in dedicated to creating house, leveraging small molecule deep expertise in drugs for virology & medicinal chemistry, drug immunology indications 2 products approved with discovery & development Founded: 1995 STRONG CASH Public: 2013 POSITION CURED >1 million • Ongoing HCV royalties WHOLLY patients • $227M in cash at 4 Clinical-stage with OWNED March 31, 2026 2 Preclinical Hepatitis C PROGRAMS Virus © 2026 Enanta Pharmaceuticals, Inc. | 3
Enanta Pipeline DISEASE TARGET DISCOVERY PRECLINICAL PHASE 1 PHASE 2 PHASE 3 MARKET Virology: Hepatitis C Virus Protease Glecaprevir* Liver Zelicapavir High-Risk Adults N-Protein Respiratory Zelicapavir Pediatrics Syncytial Virus Virology: Respiratory L-Protein EDP-323 (challenge study) COVID-19 3CL Protease EDP-235** SPRINT Chronic Spontaneous KIT EDP-978 Urticaria (CSU) Immunology: Type 2 Immune Atopic Dermatitis STAT6 EPS-3903 (AD) Diseases*** CSU/AD MRGPRX2 ® ® *Fixed-dose antiviral combination contains glecaprevir and AbbVie's NS5A inhibitor, pibrentasvir. Marketed by AbbVie as MAVYRET (U.S.) and MAVIRET (ex-U.S.). **Continued development dependent on a future collaboration. ***Initial indications. Potential future indications include Asthma, Chronic Inducible Urticaria (CIndU), Eosinophilic Esophagitis (EoE); Prurigo Nodularis (PN), Migraine and others. © 2026 Enanta Pharmaceuticals, Inc. | 4
Zelicapavir RSV Treatment Opportunity Summary • Enanta pioneering development with zelicapavir, a potential first-in-disease RSV treatment First-in-Disease • First direct-acting therapeutic to demonstrate efficacy in adults at high risk for severe RSV Asset • Resolved symptoms ~7 days faster and reduced hospitalization by 66% (Phase 2b) • Single Phase 2b/3 is a registrational path in high-risk adults Path to • Primary endpoint: Complete resolution of symptoms as measured by RiiQ Approval • Registrational development to start 4Q 2026 with Phase 2b results expected 2027 – Additional expansion opportunities in pediatrics and other high risk adult populations • Despite preventatives, treatments needed as RSV is a major disease burden causing 1 Addresses 3.6m – 6.8m outpatient visits and 190k – 370k hospitalizations each year in the US Significant • Potential addressable population of >3 million patients in the US; with an RSV market Unmet Need 2 potential of up to ~$3.5B 1. During the 10/2024-9/2025 season per CDC https://www.cdc.gov/rsv/php/surveillance/burden-estimates.html 2. Assumes patient numbers and pricing in the year 2026 © 2026 Enanta Pharmaceuticals, Inc. | 5
Respiratory Syncytial Virus: Disease Overview & Market Opportunity 6
Respiratory Syncytial Virus (RSV) Causes severe lung infections, including bronchiolitis (infection of small airways in the lungs) and pneumonia. No safe and effective treatments are currently approved. 1 RSV Burden Estimates (2024 - 2025 U.S.) Significant Unmet Need for Antivirals • Adult vaccines have sub-optimal adoption – Not recommended for all FDA-approved patient groups 2 3 – Vaccine adoption for elderly: ~35% (shingles ) to <50% (flu ) • Pediatric prophylaxis only provides passive immunity; up to 6.8M up to 370K up to 24K will shift first infection to next season – Antibody approach has a low barrier to resistance Populations at higher risk for severe illness • Pediatrics (infants and young children) • Breakthrough infections can occur despite prophylaxis • High-risk adults (e.g.; >65 yrs, COPD, asthma, CHF) • Immunocompromised (e.g.; HIV, transplant) 1. CDC Preliminary Estimates of RSV Burden for 2024-2025; for period: 9/29/24-9/27/25 2. Terlizzi EP et al. NCHS Data Brief. 2020; Age 60+ 3. CDC Influenza Vaccination Coverage, Adults 65+ © 2026 Enanta Pharmaceuticals, Inc. | 7
Physicians Report High Unmet Need for RSV, Driven by Lack of Treatment Inpatient Unmet Need Outpatient Unmet Need Average Unmet Need 1 7 4.6 5.4 (Average rating on a 7-point scale where 1= no Low High unmet need and 7= extremely high unmet need) Unmet Need • Physicians desire an efficacious and indicated treatment that curtails the progression of disease, decreases symptom severity, and minimizes duration of illness ® ® ‒ An ideal treatment would be given early, similar to PAXLOVID and TAMIFLU • ER and hospital physicians particularly note desire for a treatment to decrease the length of stay, yet note minimal side effects is an important trade-off consideration • Currently available prevention lowers rating down from extremely high A lot of patients respond with It would be nice if there was something I’m pretty satisfied with supportive care but There really is no anti-viral treatment for RSV. symptomatic management but there's not like Paxlovid or Tamiflu. Something that can its not a treatment. We don't have antiviral It may require a visit to the ER or hospital, much else for those who don’t. We're kind be taken early and reduce risk of symptoms” medication for RSV – that is the greatest that's really frustrating. ” of stuck and don’t have true satisfaction for “ “ “ “ unmet need.” RSV treatment.” – Hospital Internal Medicine / – Non-hospital Pediatrician – Non-hospital Pediatrician – Hospital Pediatrician Pulmonologist Source: Trinity Primary US Market Research 2024 (n=27) © 2026 Enanta Pharmaceuticals, Inc. | 8
Epidemiology: RSV Infections in the US Adult Population At Least 1.2 Million “HR3” Adults at High Risk for Severe RSV Most prevalent and highest risk for hospitalization ~1.2M addressable high-risk “HR3” adult RSV outpatients Additional ~0.6M CKD and IC patients 1600 HR3 Type 2 Diabetes Chronic Kidney CHF Asthma 1400 COPD 700K Disease 1200 Most common & highest risk 1000 Immuno- compromised 460K CKD / IC 800 ≥75 Years Cancer 600 295K 270K 400 Pulmonary Fibrosis 200 Cystic Fibrosis 0 5.0% 6.0% 7.0% 8.0% 9.0% 10.0% 11.0% 12.0% Hospitalization Rate ≥75 COPD / CHF < 75 CKD < 75 IC < 75 More risk 2 3.25M total adult RSV outpatient & emergency department visits ; estimate of adults <75 yrs: 26% with COPD / 1 1 3 Data from ~67,000 outpatient RSV infections that led to hospitalizations across the US CHF, 11% with CKD, 10% with IC adjusted for 35% co-morbidity overlap . Assumes impact of vaccines higher in age 75+ (10%) than COPD / CHF / CKD / IC < 75 (2%) CHF: Congestive Heart Failure; COPD: Chronic Obstructive Pulmonary Disease; HR3: Patients with CHF, COPD, or age ≥75; IC: Immunocompromised CKD: Chronic Kidney Disease 1. Landi, SN et al. JAMA Netw Open. 2024 2. McLaughlin JM et al. Open Forum Infect Dis. 2022 3. Horn EK et al. Influenza Other Respir Viruses. 2025 © 2026 Enanta Pharmaceuticals, Inc. | 9 More common No. of Hospitalizations
Epidemiology: RSV Infections in the US Pediatric Population ~1.5 Million Pediatrics at High Risk for Severe RSV 1 Despite RSV prophylaxis : Hospitalizations still occur in eligible population (0-7 mo) ~1.5M at-risk addressable pediatric RSV outpatients Passive protection may shift events to subsequent seasons (8-60 mo) After Before prophylaxis prophylaxis (2024-25) (2018-20) ~70% protected 15.0 against LRTD due 1040K to prophylaxis 0 – 7 months 8.5 6.7 8 – 19 months 5.0 470K 590K At-risk 470K 2.5 19 - 60 months 1.5 0-11 months 12-23 months 24-36 months 2 Based on ~2.1m RSV outpatient and emergency department visits for ages 0 - 2 years before prophylaxis . US RSV-NET ~15,000 hospitalizations Assumptions: incidence age 24-36 mo 20% less than age 12-23 mo; 90% adoption of infant mAb/maternal 3,4 vaccine with protection rates against RSV LRTD higher for mAb (~90%) than vaccine (~60%) LRTD: Lower Respiratory Tract Disease. RSV-NET: Respiratory Syncytial Virus Hospitalization Surveillance Network 1. Patton ME et al. MMWR. 2025. 2. Lively JY et al. J of the Pediatric Infect Dis Soci 2019 3. Hsiao et al. Pediatrics. 2025 4. ABRYSVO USPI © 2026 Enanta Pharmaceuticals, Inc. | 10 Hospitalization rate per 1000
Rapid At-Home RSV Tests Now Widely Available: Increase RSV Diagnosis & Enable a Test-to-Treat Model Commercially available tests Combination Respiratory Viral Tests Now Available in major markets st • Self-test for RSV, FluA/B and COVID (1 FDA approval in 2025) • Quick results ~15 mins, appropriate for pediatrics and adults Dual Benefit • Potential to accelerate patient enrollment in clinical trials • Increase in RSV diagnosis drives expanded commercial opportunity – 70% of US adults would test at home if they suspected COVID-19 in post- 1 pandemic era – >2-times more likely to test if they have risk factors (increasing age, declining health status) 1. Fisher et al, JAMA Network Open, 2025 © 2026 Enanta Pharmaceuticals, Inc. | 11
Payers Recognize High RSV Disease Burden and Unmet Need, with Limited Management for Antivirals Zelicapavir Viewed Positively by Payers, Limited Pricing Pressure on Antivirals Addressing RSV Disease Burden • Declining payer focus on respiratory viral • Payers recognize high disease burden and infections: post-pandemic & availability of unmet need for high-risk RSV patients prophylactics • Surveyed payers believe that zelicapavir’s data package would justify pharmacy • Respiratory antivirals have limited coverage management as a “rapid-access” category given strong public benefit & potential • Restrictions, if any, are dependent on downstream infection complications pricing Typically we manage respiratory therapies with quantity limits… Among adults, the elderly and those with chronic Due to the acute nature of the treatment and needed to start in conditions such as COPD or congestive heart failure face a a time-sensitive manner, you're not seeing a lot of PAs in “ higher RSV disease burden than healthier adults, with “ place. Paxlovid is covered preferred brand here with the more severe symptoms and a greater risk of hospitalization.” quantity limits. Tamiflu preferred generic with the quantity limit. – PBM – IDN COPD: Chronic Obstructive Pulmonary Disorder; IDN: Integrated Delivery Network; PBM: Pharmacy Benefit Manager; PA: Prior Authorization Source: Trinity Primary US Market Research 2026 (n=15) © 2026 Enanta Pharmaceuticals, Inc. | 12
Multi-Billion Dollar Global Market Opportunity for an RSV Antiviral 1 Up to 4M Addressable Patients with RSV in the United States ~4M Additional High-Risk Adults: 2 ~1.3M Multi-Billion $ Global Revenue Potential CKD, IC, T2DM, ONC ~$2.6B–3.5B HR3 Adults: • Additional High-Risk Adults ~1.2M CHF, COPD, Age ≥ 75 yrs • HR3 Adults ~$1.8B–2.4B • Pediatrics Pediatrics: ~1.5M Age < 3 yrs US Patients with RSV CHF: Congestive Heart Failure, CKD: Chronic Kidney Disease, COPD: Chronic Obstructive Pulmonary Disease, HR: High Risk; IC: Immunocompromised, ONC: Oncology, T2DM: Type 2 Diabetes Mellitus 1. Addressable patients based on outpatient visits adjusted for: anticipated adoption & efficacy of RSV vaccination/prophylaxis; presence of selected comorbidities for adults. Pediatrics: Lively JY et al. J Pediatr Infect Dis Soc 2019; Hsiao A et al. Pediatrics. 2025; ABRYSVO USPI Adults: Landi SN et al. JAMA Netw Open. 2024; Horn EK et al. Influenza Other Respir Viruses. 2025; McLaughlin JM et al. Open Forum Infect Dis. 2022 2. Peak sales forecast (2035); Based on primary market research and Enanta internal modeling that accounts for diagnosis, treatment and prescription fill rates. © 2026 Enanta Pharmaceuticals, Inc. | 13
Zelicapavir: Clinical Overview & Development Path 14
Zelicapavir (EDP-938): N-Protein Inhibitor for RSV • Only N-inhibitor in clinical development for RSV RSV – Replication inhibitor: shuts down the production of new virions (vs. fusion inhibitors which block viral entry) • Granted Fast Track designation by the FDA • Strong preclinical profile – Nanomolar potency against RSV-A and RSV-B – Antiviral potency across all clinical isolates tested – High barrier to resistance – Synergistic activity and no cross-resistance with other drug mechanisms (e.g. L-inhibitors) • Favorable safety and efficacy profile in clinical studies – Challenge study showed statistically significant (p<0.001) reduction in viral load and clinical symptoms – High barrier to the development of clinical resistance – Well-tolerated in more than 700 people dosed © 2026 Enanta Pharmaceuticals, Inc. | 15
RSV Development Goal: Treatment for Patients at High-Risk for Severe RSV Infection Leading portfolio of RSV replication inhibitors with potential for first-in-disease (zelicapavir) and best-in-disease (EDP-323) treatments and ability for combination Age ≥65 years Zelicapavir High-Risk Adult Positive Phase 2b Results Chronic heart or lung disease Phase 2b Study (e.g. COPD, CHF, asthma) Zelicapavir Pediatric Infants and young children Positive Phase 2 Results Phase 2 Study High-risk populations have reduced RSV immunity, resulting in a higher and longer duration of viral load and greater disease severity © 2026 Enanta Pharmaceuticals, Inc. | 16
Zelicapavir Pediatric Program: First-in-Pediatric Phase 2 Study Design Primary Objective Double-Blind Inclusion Criteria Treatment Phase • Hospitalized or outpatient pediatric Overall: Part 1: patients with RSV Antiviral activity MAD vs Placebo of zelicapavir • Symptom onset within 3 23 Day N=96 across all patients days Follow-up Part 2: • Age: Part 1: Selected dose from Safety and PK ‒ ≥28 days – <6 months Part 1 vs Placebo Part 2: ‒ ≥6 months – ≤36 Antiviral activity months Day 1 Day 5 Day 28 First zelicapavir pediatric study: safety, dose selection, and virology © 2026 Enanta Pharmaceuticals, Inc. | 17 Trial Design
Zelicapavir Pediatric Program: First-in-Pediatric Phase 2 Study Summary • Well-tolerated, with favorable safety profile – No adverse events leading to treatment discontinuation or study withdrawal Primary Objectives of Study • Antiviral effect observed for the primary and secondary virology endpoints in overall population üOverall: Antiviral activity of zelicapavir across all patients • Viral load decline of 1.4 log at the end of treatment in Part 2 • Viral load decline of 1.2 log at Day 5 observed in prespecified üPart 1: Safety and PK subset of patients randomized within 3 days of symptom onset üPart 2: Antiviral activity • RSV Signs/Symptoms – ReSViNET: Reduced the time to complete symptom resolution by 1.6 days and 3.7 days – RESOLVE-P: Trend toward greater sign/symptom reduction with zelicapavir in a small dataset Data support further clinical development of zelicapavir in pediatrics RESOLVE-P: RESpiratory ObservabLE Reported Outcome-Pediatric; Enanta’s proprietary ObsRO © 2026 Enanta Pharmaceuticals, Inc. All rights reserved. © 2026 Enanta Pharmaceuticals, Inc. | 18
Zelicapavir Pediatric Program: Phase 2b Study Design Primary Objective Double-Blind Treatment Phase Randomized 1:1 Time to complete symptom Inclusion Criteria resolution as measured by RESOLVE-P Placebo (QD) • Pediatric outpatients 28 Day with RSV N=150 Follow-up • Symptom onset within 3 Secondary Objective days Zelicapavir (QD)* • Age 28 days - 36 months Safety, virology, additional clinical efficacy measures Day 1 Day 7 Day 35 Study to start in 3Q 2026; Phase 2b topline data in 2027 *Doses are 5mg/kg for <12 months old; 7.5mg/kg for ≥12 months old QD: once-daily; RESOLVE-P: RESpiratory ObservabLE Reported Outcome-Pediatric; Enanta’s proprietary ObsRO © 2026 Enanta Pharmaceuticals, Inc. All rights reserved. © 2026 Enanta Pharmaceuticals, Inc. | 19 Trial Design
Zelicapavir High-Risk Adult Program: Phase 2b Study Design Primary Objective Double-Blind Inclusion Criteria Treatment Phase ‡ Time to resolution of RSV LRTD • Adult outpatients with Randomized 2:1 symptoms (RiiQ symptom scale) RSV Shortness of breath, wheezing, • Symptom onset within 3 Placebo (QD) coughing, coughing up phlegm days 28 Day • At least one of N=186 the following:* Follow-up Secondary Objective † ‒ COPD 800mg (QD) PROs, MAVs, virology, antibiotic ‒ Congestive use, bronchodilator use, heart failure (CHF) corticosteroid use, hospitalization, ‒ Asthma ICU, mechanical ventilation, all Day 1 Day 5 Day 33 ‒ Age ≥65 cause mortality, PK & safety • HR3 = ~80% of the population with CHF, COPD, or age >75 First proof-of-concept Phase 2 high-risk adult outpatient study with positive clinical signal COPD: Chronic Obstructive Pulmonary Disease; LRTD: Lower respiratory Tract Disease; PROs: Patient Reported Outcomes; MAVs: Medically Attended Visits; ICU: Intensive Care Unit; PK: Pharmacokinetics; QD: Once-daily † ‡ *Proportion of patients aged 65-74 years or those with asthma capped at 20% of the total population; Equivalent to 600mg suspension dosage form used in challenge study; Resolution: all symptoms mild or absent © 2026 Enanta Pharmaceuticals, Inc. | 20 Trial Design
Zelicapavir Phase 2b High-Risk Adult Study Faster Time to Complete Symptom Resolution by RiiQ Industry’s first proof-of-concept demonstrated in high-risk adult outpatients Efficacy Population HR3 Population* 7.2d 20 (34%) 6.7d 3.6d Placebo 18 (35%) (19%) Zelicapavir 16 2.2d (14%) 3.0d 14 (20%) 0.5d 12 (4%) 10 8 6 4 2 13.0 12.5 16.0 13.8 18.9 15.3 15.0 12.0 18.9 12.2 21.0 13.8 0 LRTD All RSV Total RiiQ LRTD All RSV Total RiiQ 4 Symptoms 13 Symptoms 29 Parameters 4 Symptoms 13 Symptoms 29 Parameters *HR3 Population = Patients with CHF, COPD, or age >75 ; LRTD = lower respiratory tract disease © 2026 Enanta Pharmaceuticals, Inc. | 21 Median days to alleviation of symptoms
Zelicapavir Phase 2b High-Risk Adult Study Faster Time to Symptom Resolution by PGI-S • Statistically significant 2-day faster symptom resolution by PGI-S compared to placebo Efficacy Population (p=0.0446) H R3 Population (p=0.0465) Time Since First Dose (days) PGI-S: Patient Global Impression of Severity: “In the past 24 hours, what was the severity of your overall RSV-related symptoms at their worst?” HR3 Population: Patients with CHF, COPD, or age >75 © 2026 Enanta Pharmaceuticals, Inc. | 22 Survival Probability of RSV Symptoms Being Unresolved
Zelicapavir Phase 2b High-Risk Adult Study Hospitalization and Death Endpoints • Lower hospitalization rate for patients treated with zelicapavir Placebo Zelicapavir All-cause hospitalizations 5.0% (3/60) 1.7% (2/115) RSV-associated hospitalizations 5.0% (3/60) 0% (0/115) (blinded investigator attribution) • One death on placebo; no deaths on zelicapavir © 2026 Enanta Pharmaceuticals, Inc. | 23
Zelicapavir Phase 2b High-Risk Adult Study Conclusions • Zelicapavir demonstrated compelling results on multiple clinically meaningful & potential registrational endpoints measuring different aspects of RSV disease ü Up to one week improvement in complete RiiQ symptom resolution ü Statistically significant improvement in PGI-S ü Lower hospitalization rate • Robust antiviral effect • Well tolerated, with a favorable safety profile Data support advancing zelicapavir into registrational development RiiQ: Respiratory Infection Intensity and Impact Questionnaire; PGI-S: Patient Global Impression of Severity © 2026 Enanta Pharmaceuticals, Inc. | 24
Zelicapavir Phase 2b/3 Study Elements Element Outcome Registrational Path Single Phase 2b/3 Study TM Primary endpoint Time to complete resolution of all 13 RSV symptoms as measured RiiQ Study population Adult outpatients with CHF, COPD or 75 years or older (HR3) MITT of unvaccinated patients, vaccinated patients (10%) would be Primary analysis set included in a separate supportive ITT analysis Sample size 860 Dosing duration 800 mg of zelicapavir per day for 7 days MITT: Modified Intent to Treat © 2026 Enanta Pharmaceuticals, Inc. | 25
Zelicapavir High-Risk Adult (HR3) Program: Phase 2b/3 Global Registrational Study Design Primary Objective* Double-Blind Inclusion Criteria Treatment Phase Randomization 1:1 Time to complete resolution of • Adult outpatients with 13 RSV symptoms as RSV* measured by RiiQ • Symptom onset within 3 Placebo (QD) Ph2b/3 days 28 Day N= 200/ • At least one of Follow-up Secondary Objective the following: 660 800mg (QD) ‒ COPD Reduction in hospitalization rate due to RSV ‒ Congestive heart failure Other Secondary Objectives: ‒ Age ≥75 Additional clinical efficacy Day 1 Day 7 Day 35 measures, virology, safety Study to start in 4Q 2026; Phase 2b data in 2027 to confirm endpoints and sizing * Patients who have received an RSV vaccine are capped at 10%; Primary endpoint is on MITT of unvaccinated patients (vaccinated patients included in a separate supportive ITT analysis) COPD: Chronic Obstructive Pulmonary Disease; PGI-S: Patient Global Impression of Severity; QD: Once-daily; RiiQ: Respiratory Infection Intensity and Impact Questionnaire © 2026 Enanta Pharmaceuticals, Inc. | 26 Trial Design
Zelicapavir RSV Treatment Opportunity Summary • Enanta pioneering development with zelicapavir, a potential first-in-disease RSV treatment First-in-Disease • First direct-acting therapeutic to demonstrate efficacy in adults at high risk for severe RSV Asset • Resolved symptoms ~7 days faster and reduced hospitalization by 66% (Phase 2b) • Single Phase 2b/3 is a registrational path in high-risk adults Path to • Primary endpoint: Complete resolution of symptoms as measured by RiiQ Approval • Registrational development to start 4Q 2026 with Phase 2b results expected 2027 – Additional expansion opportunities in pediatrics and other high risk adult populations • Despite preventatives, treatments needed as RSV is a major disease burden causing 1 Addresses 3.6m – 6.8m outpatient visits and 190k – 370k hospitalizations each year in the US Significant • Potential addressable population of >3 million patients in the US; with an RSV market Unmet Need 2 potential of up to ~$3.5B 1. During the 10/2024-9/2025 season per CDC https://www.cdc.gov/rsv/php/surveillance/burden-estimates.html 2. Assumes patient numbers and pricing in the year 2026 © 2026 Enanta Pharmaceuticals, Inc. | 27
Enanta RSV Portfolio Enables Sustained Leadership Position Designed to Address Broad Unmet Medical Needs First-in-Disease Best-in-Disease / Expansion Zelicapavir EDP-323 • Goal is to treat all high-risk patient populations (in/out-patients) – Pediatric patients (28 days – 3 years) – HR3 Adults: ≥75 years old, COPD, CHF – Chronic kidney disease, immunocompromised, diabetes, cancer • Possibility to develop as a preventative – Post-exposure prophylaxis for all populations – Pre-exposure prophylaxis for immunocompromised • Potential to provide additional benefit in harder to treat patients (e.g. severely immunocompromised, etc) with a combination treatment COPD: Chronic Obstructive Pulmonary Disease; CHF: Congestive Heart Failure © 2026 Enanta Pharmaceuticals, Inc. | 28
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