Assembly Biosciences Reports Positive Interim Results from Phase 1B Clinical Studies of Long-Acting Helicase-Primase Inhibitor Candidates Abi-1179 and Abi-5366 Showing Reductions in Viral Shedding Rate and Virologically Confirms Genital Herpes

Assembly Biosciences, Inc. announced positive interim results from two Phase 1b studies of its investigational long-acting herpes simplex virus (HSV) enzyme-primase inhibitors in participants seropositive for HSV type 2 (HSV-2) with recurrent genital herpes. These interim results include the first reported Phase 1b data for ABI-1179, evaluating weekly oral dosing. The ABI-5366 monthly oral dosing results are also encouraging and show significant reductions in viral shedding and virologically confirmed genital lesion rate, supporting the continued optimization of exposure to evaluate its potential for monthly oral dosing.

In the ABI-1179 study, highly potent antiviral activity was observed with a 98% reduction in HSV-2 shedding rate compared to placebo There have been no serious adverse events reported to date. ABI-1179-101 Study Overview: ABI-1179- 101 is a randomized, blinded, placebo-controlled Phase 1a/b clinical study. Positive interim data have been previously reported for Part A (Phase 1a), evaluating the safety, tolerability and PK of ABI-1179 following single dose administration in healthy participants.

Part B also evaluates antiviral activity by measuring changes in viral parameters including shedding rate, quantification of HSV-2 DNA levels obtained from anogenital swab samples, and clinical parameters including genital lesion rate and duration. Due to the observed half-life of ABI-1179, the safety follow-up period for participants extends for 29 days after dosing (Day 57), with safety data available as of the data cutoff date through Day 57 for all participants from these cohorts that completed the evaluation period. The study uses pooled data from placebo recipients across cohorts as a control.

As additional placebo recipients are enrolled in later cohorts, the sample size for the pooled placebo will change, which is expected to result in adjustments to both the observed effect sizes compared to placebo and the tests of statistical significance for those observed effects. Additional information about the Phase 1a/b trial is available at clinicaltrials.gov using the identifier NCT06698575. ABI-5366-101 Study Overview: A BI-5366-101 is a randomized, blinded, randomized, placebo-controlled Phase 1 a/b clinical study.

Positive Interim data have been previously reported for part A (Phase 1a), evaluate the safety, tolerability andPK of ABI-5366 following single dose administration in healthy Participants. These risks and uncertainties include: Assembly Bio's ability to realize the potential benefits of its collaboration with Gilead, including all financial aspects of the collaboration and equity investments; Assembly Bio's ability to initiate and complete clinical studies involving its therapeutic product candidates, including studies contemplated by Assembly Bio's collaboration with Gilead, in the currently anticipated timeframes or at all; safety and efficacy data from clinical or nonclinical studies may not warrant further development of Assembly Bio's product candidates; clinical and nonclinical data may not differentiate Assembly Bio's product candidates;clinical and nonclinical data may not differentiated Assembly Bio's product candidates; regulatory and nonclinical data may not differentiation Assembly Bio's product candidates; Company to hold conference call on December 8, 2025 at 6 p.m. ET.