Tokyo, Japan, October 9, 2026
Asahi Kasei Therapeutics, Inc. has announced that the U.S. Food and Drug Administration (FDA) has approved HOVILPRI® (pritelivir) 100 mg tablets for treating mucocutaneous lesions caused by herpes simplex virus (HSV) infection that is refractory, with or without documented resistance, to standard antiviral treatments in immunocompromised adults. The approval marks a significant development in infectious disease treatment, introducing an oral antiviral medicine with a novel mechanism of action for patients whose infections do not respond adequately to acyclovir, valacyclovir or famciclovir. HOVILPRI was developed by AiCuris Anti-infective Cures AG, which is now part of Asahi Kasei Therapeutics. The FDA decision is supported by results from the pivotal Phase 3 PRIOH-1 clinical trial, in which pritelivir demonstrated statistically superior complete lesion healing compared with investigator-selected alternative treatments. The approval provides an additional treatment option for immunocompromised adults facing persistent and difficult-to-treat HSV infections, an important unmet medical need in transplantation, oncology and other clinical settings. The company expects HOVILPRI to become available in the United States before the end of 2026.
Phase 3 PRIOH-1 Trial Demonstrates Improved Lesion Healing
The FDA approval is based on Part C of the Phase 3 PRIOH-1 trial, a randomized, open-label, comparator-controlled study involving 101 immunocompromised adults with mucocutaneous HSV lesions that were refractory to standard antiviral treatment, with or without documented resistance. Participants received oral pritelivir or investigator-selected treatment, which could include intravenous foscarnet, intravenous or topical cidofovir, or topical imiquimod. The primary endpoint assessed complete lesion healing through Day 28. By Day 28, 63% of patients treated with HOVILPRI achieved complete lesion healing, compared with 34% of those receiving investigator-selected treatment. The adjusted treatment difference was 28.4 percentage points, with a 95% confidence interval of 9.6 to 47.3 and a p-value of 0.0047. At Day 42, observed healing rates reached 82% in the HOVILPRI group and 42% in the comparator group. These results demonstrate a statistically significant improvement in lesion healing for the investigational treatment in the studied population. The findings are particularly relevant for patients who may have prolonged infections because their immune systems are weakened by transplantation, cancer, HIV or immunosuppressive treatment. The Phase 3 results provide the principal clinical evidence supporting the FDA approval.
Pritelivir Introduces a Different Antiviral Mechanism
HOVILPRI contains pritelivir, an oral herpes simplex virus helicase-primase inhibitor. This mechanism differs from that of commonly used nucleoside analogue antivirals, including acyclovir, valacyclovir and famciclovir. Those medicines depend on viral thymidine kinase activation, and certain HSV mutations affecting this enzyme can reduce susceptibility to treatment. Pritelivir targets the viral helicase-primase complex, which is involved in viral DNA replication. Its distinct mechanism offers a treatment approach for eligible patients with infections that remain difficult to control with standard antiviral therapy, including some infections associated with antiviral resistance. The medicine is indicated for immunocompromised adults with refractory mucocutaneous HSV lesions, whether or not resistance to standard antiviral treatment has been documented. In the pivotal trial, adverse reactions were reported in 22% of patients receiving HOVILPRI, compared with 54% receiving investigator-selected treatment. Discontinuations due to adverse drug reactions occurred in 2% and 20% of participants, respectively. Headache was the most common reported adverse reaction, occurring in 6% of patients treated with HOVILPRI. As with any prescription medicine, treatment suitability and potential drug interactions must be considered by healthcare professionals.
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FDA Approval Addresses an Unmet Need in Immunocompromised Patients
HSV infections can cause recurrent oral, genital and other mucocutaneous lesions. In immunocompromised patients, infections may be more persistent, severe and challenging to treat than in people with normally functioning immune systems. Patients undergoing hematopoietic stem cell or solid organ transplantation, those receiving cancer treatment and people living with certain immune-related conditions may face increased risks from prolonged HSV infection. Before this approval, treatment options for refractory disease could involve intravenous therapies that require additional clinical monitoring and may present practical treatment challenges. HOVILPRI introduces an oral option for eligible patients with difficult-to-treat HSV lesions, potentially reducing reliance on certain alternative treatment approaches. The FDA had previously granted pritelivir Fast Track and Breakthrough Therapy designations, and the application received Priority Review. These regulatory pathways supported the development and review process but did not replace the requirement to demonstrate clinical evidence. The approval is an important milestone for Asahi Kasei Therapeutics and the broader antiviral research field. Continued post-approval safety monitoring and clinical experience will help establish how the medicine performs in routine practice. HOVILPRI does not cure HSV infection, but its approval expands treatment options for a specific population with persistent disease and limited therapeutic alternatives.
Source: Asahi Kasei Therapeutics press release
