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ABI-5366 HSV Trial: Results, Limits and Current Status

ABI-5366 showed short-term Phase 1b activity but is not a cure or approved treatment. See the limits and why GS-1179 was selected for Phase 2.

NEWS & RESEARCH

Jordan

9/7/20264 min read

Researchers developing investigational HSV antiviral treatments

What is ABI-5366?

ABI-5366 is an investigational oral antiviral developed for recurrent genital herpes caused by HSV-2. It is a helicase-primase inhibitor, which blocks a viral enzyme complex involved in copying HSV DNA.

It is not gene therapy, a vaccine or a cure. It does not remove latent HSV from nerve cells, and it has not been approved by the MHRA, FDA or any other medicines regulator.

Current Status: GS-1179 Selected for Further Study

Assembly Biosciences completed the Phase 1b study of ABI-5366 in 2026. Gilead licensed the ABI-5366 and ABI-1179 programmes in December 2025 and took responsibility for further development.

Assembly’s September 2026 development update said Gilead had selected GS-1179, formerly ABI-1179, for a planned Phase 2 trial in recurrent genital herpes. The announced plan expected a start by the end of 2026; it did not establish that the trial had begun. ABI-5366 was not the candidate named for this next study, although that alone does not confirm that its development has ended.

GS-1179 is also investigational. Its safety and efficacy have not been established, and a planned trial can be delayed, changed or stopped.

How the ABI-5366 Phase 1b study worked

The registered Phase 1a/1b study was designed to assess safety, drug levels and antiviral activity. The Phase 1b portion enrolled adults who were HSV-2 seropositive and had recurrent genital herpes.

A 2026 conference poster reported 76 participants across three cohorts: 61 received ABI-5366 and 15 received placebo. Each active-dose cohort had about 20 evaluable participants, while placebo results were pooled across five participants per cohort. The antiviral evaluation lasted 29 days, with longer safety follow-up because the drug remains in the body for a long time.

This was an early, small and short study. It was useful for selecting doses and deciding whether further research was justified; it was not designed to establish long-term effectiveness, transmission prevention or routine clinical use.

What the reported results showed

At 350 mg once weekly, the company reported an HSV-2 shedding rate of 0.9% of sampled days, compared with 14.9% in the pooled placebo group. The modelled relative reduction was 94%. The overall genital-lesion rate was 1.1% of assessed days versus 18.3% with placebo, also reported as a 94% relative reduction.

In the alternative monthly-dose cohort, the reported reductions were smaller: 76% for overall shedding and 65% for the overall genital-lesion rate compared with pooled placebo. The reduction in subclinical shedding for this monthly regimen was not statistically significant.

These percentages describe rates of positive swabs or days with lesions during a short evaluation. They do not mean that 94% of participants were cured, that each person improved by 94%, or that transmission fell by 94%.

What the study did not prove

That ABI-5366 prevents HSV-2 transmission between partners.
That it is effective or safe when taken for months or years.
That monthly dosing provides reliable long-term suppression.
That it treats genital HSV-1, oral HSV-1 or people outside the study population.
That it works better than licensed antivirals in a direct comparative trial.
That it improves quality of life, sexual wellbeing or relationship outcomes.
That it removes latent HSV or produces a functional or sterilising cure.

“High viral load” was studied as a possible surrogate marker. A surrogate is not the same as directly measuring new infections in partners.

What is known about safety?

Company reports described similar overall rates of treatment-emergent adverse events between ABI-5366 and placebo in the Phase 1b cohorts, mostly grade 1 or 2, with no serious adverse events reported at the data cut-off. One participant discontinued after grade 3 hypertriglyceridaemia that investigators did not consider treatment-related; other grade 3 laboratory abnormalities were also reported as unrelated.

Those early findings are reassuring enough for further research, but the study was too small and short to define uncommon harms, interactions or long-term safety. A long half-life may make convenient dosing possible, but it also means exposure continues after a dose and cannot simply be stopped immediately if a problem occurs.

Why GS-1179 was selected

GS-1179 is a separate helicase-primase inhibitor from the same licensed programme. Assembly reported strong Phase 1b antiviral activity with weekly dosing, and Gilead’s September 2026 update selected it for a planned Phase 2 study.

Companies choose development candidates using efficacy, safety, pharmacokinetics, formulation, manufacturing and commercial factors. The public update does not establish that GS-1179 is clinically superior to ABI-5366. Only later, adequately controlled trials can determine whether it provides a useful balance of benefits and risks.

What Phase 2 still needs to answer

Whether antiviral and lesion effects persist over a longer treatment period.
Which dose and schedule provide consistent exposure.
How often adverse events and laboratory abnormalities occur in a larger group.
Whether benefits extend across relevant demographic and clinical subgroups.
How outcomes compare with an appropriate control and, eventually, existing treatment.
Whether later studies should directly measure transmission and patient-reported outcomes.

Phase 2 success would still not guarantee approval. Confirmatory studies and regulatory review would normally be required, and no reliable approval date can be predicted now.

What this means for people with HSV today

Neither ABI-5366 nor GS-1179 is available as a prescribed HSV treatment. Do not buy products claiming to contain them, attempt to copy study doses or change a current antiviral plan because of these results.

Aciclovir, valaciclovir and famciclovir remain established treatments. The CDC states that suppressive therapy can reduce recurrences and that daily valaciclovir can reduce HSV-2 transmission risk in a specific population of discordant heterosexual couples, without eliminating risk. Treatment choice and dose should be discussed with a qualified clinician.

Continue to avoid sexual contact during lesions or prodromal symptoms and remember that condoms reduce, but do not eliminate, transmission risk.

How to follow the research without the hype

Check the official trial registry for study status, eligibility and locations.
Use Gilead and Assembly Biosciences releases for programme decisions, while recognising that company statements are not independent clinical guidelines.
Look for peer-reviewed publications and complete trial results, not only headlines or relative percentages.
Separate shedding, lesion, recurrence, transmission and cure outcomes; they are not interchangeable.
Treat approval-date forecasts as speculation until a regulator accepts and reviews an application.

Bottom line

ABI-5366 produced encouraging short-term antiviral signals in a small Phase 1b study, especially with the weekly 350 mg regimen. It did not cure HSV, prove transmission prevention or establish long-term safety. As of September 2026, Gilead’s stated development plan selects GS-1179—not ABI-5366—for the next Phase 2 trial. The programme is worth following, but it does not change current HSV care today.