At IDWeek 2025, held on 19-22 October, Xue Meng from ST Pharm presented results from a proof of concept clinical trial for ST Pharm’s investigational human immunodeficiency virus-1 (HIV-1) allosteric integrase inhibitor (ALLINI), pirmitegravir. ALLINIs are a novel drug class that target noncatalytic sites of the HIV-1 integrase enzyme. Pirmitegravir acts by binding to the lens epithelium-derived growth factor/p75 allosteric site on the HIV-1 integrase enzyme, disrupting normal multimerisation and mislocalising viral ribonucleic acid. This prevents proper viral assembly and results in the production of noninfectious viral particles. This mechanism differentiates the drug from integrase strand transfer inhibitors such as dolutegravir and bictegravir and could provide advantages in resistance management. Pirmitegravir is the first ALLINI to demonstrate clinical proof of concept in adults with HIV-1.
The ongoing Phase IIa randomised, double-blind, placebo-controlled trial is assessing pirmitegravir’s antiviral efficacy, safety, and pharmacokinetics (PKs) in antiretroviral therapy (ART)-naive adults with HIV-1 infection. The once-daily oral agent achieved significant viral load reductions after ten days of treatment and was well tolerated, marking a milestone for a mechanism that has shown longstanding preclinical promise.
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Results presented at the conference were for participants in the study who received either 200mg or 400mg pirmitegravir once daily for ten days, or placebo. The results showed strong, significant viral load reductions of -1.55 log₁₀ and -1.19 log₁₀ copies/ml for the 200mg and 400mg doses, respectively, compared with -0.25 log₁₀ copies/ml for placebo. Treatment was well tolerated, with mild gastrointestinal adverse events such as nausea and diarrhea and no serious events or discontinuations. PK findings demonstrated rapid absorption within one hour, a half-life of 12-14 hours, and minimal accumulation, supporting once-daily dosing.
Earlier Phase I studies in healthy volunteers also established favourable tolerability and predictable PKs with less-than-dose proportional exposure, supporting the current dose-ranging study.
The findings confirm that allosteric inhibition of integrase is clinically viable, validating a new antiviral drug class with potential to complement current ART regimens. If efficacy and safety are sustained in an ongoing 600mg cohort and later-phase studies, pirmitegravir could represent the first new HIV mechanism in more than a decade, signalling renewed momentum in next-generation HIV therapeutics. While the HIV therapeutics market is large and highly competitive, comprising a range of drug classes, there is still a need for continued investment in therapeutics with novel mechanisms of action in order to combat drug resistance.
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