Assembly Biosciences

Company overview

• Industry: Biotechnology focused on antiviral therapeutics.
• Focus: Discover and develop next-generation antivirals for serious viral diseases with unmet medical needs.
• Status: All product candidates are in clinical or nonclinical development; no approved products as of December 31, 2025.
• Location: Principal office in South San Francisco, California.

Pipeline and programs

• Recurrent genital herpes (HSV-1 and HSV-2)
  • Long-acting helicase-primase inhibitors (HPIs): ABI-5366 (5366) and ABI-1179 (1179).
  • 5366 and 1179 advanced to clinical development; these HPI programs were licensed to Gilead.
• HDV and HBV
  • 6250: Oral HDV entry inhibitor (NTCP-targeted).
  • 4334: Next-generation capsid assembly modulator (CAM) targeting HBV.
  • 6250 and 4334 are in early clinical/regulatory-readiness stages; 6250 completed Phase 1a and Phase 2 initiation is anticipated in 2026.
• Transplant-associated herpesviruses
  • 7272: Oral broad-spectrum non-nucleoside polymerase inhibitor (NNPI) targeting transplant-associated herpesviruses.
  • 7423: Prodrug precursor to 7272; transition to 7272 completed in October 2025 and regulatory filing-enabling studies for 7423 were completed.
• Research and discovery
  • Ongoing identification and nomination of new viral targets and compounds.

Gilead collaboration and equity financing

• Agreement: In October 2023 Assembly entered an Option, License and Collaboration agreement with Gilead Sciences, Inc.
  • Gilead obtained exclusive licensing for the HSV HPI program and the NNPI program and retains opt-in rights to other programs.
  • Gilead holds option rights to exclusive licenses on a program-by-program basis during the collaboration term.
• Financial terms and payments
  • Upfront payment: $84.8 million from Gilead.
  • December 2024 amendment: restructured timing and fees; $10 million accelerated funding credited toward future payments.
  • December 2025 option exercise: Gilead paid $35 million to license the HPI program (5366 and 1179).
  • Milestones and royalties: For exercised options, Assembly may receive up to $330 million in regulatory and commercial milestones per program, plus royalties in the high single-digits to high-teens on net sales.
  • Profit-share option: After development plans are provided, Assembly may opt to share 40% of U.S. costs and profits for an optioned program in lieu of milestones/royalties in the United States.
  • Gilead ownership and governance: Gilead acquired approximately 28% of Assembly’s outstanding common stock, has the right to designate two directors (or observers) under the Investor Rights Agreement, and may acquire up to 35% total ownership through subsequent purchases.
• Equity financing and related actions
  • June 2024: Gilead purchased 1,089,472 shares at $13.92 per share.
  • December 2024: Gilead exercised additional equity rights and acquired more shares.
  • August 2025: Gilead participated in a private placement (2,295,920 shares at $19.60 per share) and received corresponding warrants.
  • 2025: Form S-3 registration statement filed and declared effective to register shares and warrants issued to Gilead.

Clinical progress highlights

• 5366 (HPI)
  • Phase 1a interim results (February 2025) and Phase 1b topline results (June 2025) with weekly dosing showed strong antiviral activity and favorable safety.
  • December 2025: Phase 1b interim results from a monthly oral dosing cohort supported ongoing development.
  • Phase 1b data demonstrated reductions in HSV-2 shedding and lesion rates; pharmacokinetic data supported weekly and potential monthly dosing.
  • Completion of 5366 and 1179 Phase 1b cohorts triggered an option under the Gilead collaboration.
• 1179 (HPI)
  • Phase 1a results reported in February 2025; Phase 1b progression announced in July 2025 with cohorts showing strong antiviral activity and favorable safety.
  • Phase 1b data supported progression to Phase 2-enabling studies, subject to chronic toxicology results.
  • Gilead exercised its option in December 2025 to exclusively license the HPI program (5366 and 1179).
• 6250 (HDV entry inhibitor)
  • Phase 1a completed enrollment; interim PK and biomarker data (August 2025) showed dose-proportional exposure and NTCP target engagement via bile acid biomarkers.
  • Phase 2 initiation is expected in Q4 2026 after completion of chronic toxicology studies.
• 4334 (HBV CAM)
  • Phase 1a/1b results reported (initial cohort Dec 2024; topline Jun 2025); final Phase 1b data presented at the Liver Meeting 2025.
  • Gilead declined its option for 4334 in March 2026; Assembly retained full control and is evaluating partnering opportunities.
• 7272/7423 (NNPI)
  • 7423 transitioned to parent 7272; regulatory filing-enabling nonclinical studies completed and 7272 is in filing-enabling studies.
• Near-term expectations
  • Gilead is expected to initiate Phase 2 for the HPI program in 2026.
  • Continued nonclinical/toxicology work and potential Phase 2-enabling studies for other programs, subject to partnerships or funding.

Intellectual property

• Patent portfolio includes published and nationalized PCT applications covering:
  • 5366 and derivatives/analogs (HSV-related) with expected patent life through 2043–2045.
  • 6250 (HDV/HBV) with anticipated protection into the mid-2040s.
  • 4334 (HBV) with expected protection into the early 2040s.
  • Broad-spectrum NNPI technologies for transplant-related herpesviruses.
• Protection strategy: composition-of-matter, use, and formulation patents, supplemented by trade secrets and confidential disclosures.

Regulatory and legal environment

• The company operates under extensive U.S. and international regulatory requirements, including FDA processes (INDs, NDAs, cGMP, GCP, post-approval commitments), privacy and data security obligations (HIPAA, GDPR/UK GDPR, CCPA/CPRA), and pricing, reimbursement, and anti-kickback laws.
• Intellectual property enforcement varies by jurisdiction.
• The company identifies risks from regulatory delays, approval timing, and changes in law that could affect development timelines or commercialization.

People, facilities, and operations

• Employees: 73 total as of December 31, 2025 (plus contractors, CROs, and CMOs).
• Facilities: Leased office, administrative, and laboratory space in South San Francisco under a sublease ending September 2029.
• Manufacturing: Relies on third-party contract manufacturers and CROs for nonclinical and clinical work; no internal production facilities planned.

Financials and funding

• Key collaboration and equity funding from Gilead:
  • Upfront: $84.8 million.
  • December 2024 amendment: $10 million credited toward future payments.
  • December 2025: $35 million payment on Gilead’s exercise of the HPI option.
  • June 2024 equity purchase: 1,089,472 shares at $13.92 per share.
  • August 2025 private placement: 2,295,920 shares at $19.60 per share plus warrants.
  • 2025: Registration statement filed and declared effective to register shares and warrants issued to Gilead.
• The company states it is not profitable and expects to require additional financing. Assembly plans to rely on collaboration funding, milestone payments, royalties, the profit-share option, and other financing sources.

Forward-looking statements and risks

• The company makes forward-looking statements concerning regulatory approvals, clinical study timing (including Gilead-related timelines), and potential collaboration outcomes. These plans and timelines are subject to clinical, regulatory, and commercial risks.