Breaking: FDA Approves Novel Antiviral Small Molecule — First New-Mechanism HSV Therapy in ~30 Years

Asahi Kasei Therapeutics' oral antiviral Hovilpri (pritelivir) won FDA approval for acyclovir-resistant HSV mucosal-skin infections in immunocompromised adults — the first new-mechanism HSV therapy approved by the FDA in nearly three decades. The pivotal Phase 3 PRIOH-1 met its primary endpoint (63% vs 34% complete healing at day 28).

Herpes simplex virus (HSV-1/HSV-2) establishes lifelong latency and can reactivate, causing painful oral or genital lesions. In immunocompromised populations — transplant recipients, people living with HIV, cancer patients — infections can become frequent, severe and refractory.

Asahi Kasei Therapeutics announced today that the US FDA approved its oral antiviral Hovilpri (pritelivir) 100 mg tablets for mucocutaneous HSV infections in immunocompromised adults who are unresponsive to acyclovir, valacyclovir or famciclovir. Per the company, this is the first FDA-approved HSV therapy with a new mechanism of action in nearly 30 years. Hovilpri was originally developed by Aicuris, which Asahi Kasei acquired in April this year.

A novel mechanism that bypasses resistant strains

Nucleoside analogs such as acyclovir, valacyclovir and famciclovir remain mainstays, but their antiviral effect depends on specific viral enzymes; mutations can erode susceptibility and drive resistance. Unlike them, Hovilpri directly targets the HSV helicase–primase complex, interfering with the key steps of viral genome replication — retaining activity against some nucleoside-analog-resistant strains. As an oral agent, it also avoids the need for intravenous administration.

Pivotal Phase 3 PRIOH-1 met its primary endpoint

Approval was based primarily on Part C of the pivotal Phase 3 PRIOH-1 study, a randomized, open-label, controlled superiority trial in 101 immunocompromised adults randomized to oral Hovilpri or investigator's choice, for up to 28 days (extendable to 42 days for improving-but-unhealed lesions). The primary endpoint was met: by day 28, 63% of Hovilpri patients achieved complete healing of all lesions versus 34% in the control arm, an adjusted treatment difference of 28.4 percentage points (95% CI 9.6–47.3; p=0.0047).

With extension to 42 days, complete-healing rates were 82% for Hovilpri versus 42% for investigator's choice (adjusted difference 40 percentage points). Safety: treatment-emergent adverse events were reported by 22% of Hovilpri patients versus 54% of controls, and discontinuations due to adverse events were 2% versus 20%; the most common Hovilpri event was headache (6%). Given the open-label design and limited sample size, safety interpretations should account for event types and patient baselines.