Pritelivir, sold as Hovilpri, won U.S. approval on October 9, 2026 for one narrow job: clearing mucocutaneous herpes simplex lesions in immunocompromised adults when standard antiviral treatment has failed. In the pivotal trial of 101 patients, 63% had every lesion fully healed by day 28, against 34% for the treatments doctors would otherwise reach for. The drug is a once-daily tablet from Asahi Kasei Therapeutics, developed by AiCuris.
The label is deliberately tight. It does not make Hovilpri a general herpes medicine, and it does not replace acyclovir, valacyclovir or famciclovir for the ordinary cold sores and genital outbreaks that those drugs handle well.
A helicase-primase inhibitor that skips the step nucleoside drugs depend on
Specialist outlet Contagion Live reported that pritelivir belongs to a different class from acyclovir and its relatives. It is a helicase-primase inhibitor, a drug that blocks the viral machinery for unwinding and copying herpes DNA. Nucleoside analogues such as acyclovir work another way: they must first be activated by an enzyme the virus itself makes, viral thymidine kinase, before they can interfere with replication.
That dependence on activation is the weak point the new drug avoids. Because pritelivir does not need the viral thymidine kinase step, it is active against both HSV-1 and HSV-2, including strains resistant to nucleoside analogues and to foscarnet, according to the same report. The approval covers lesions that are refractory with or without documented resistance to acyclovir, valacyclovir or famciclovir, so a patient does not need a resistance test result on file to qualify; failure of standard treatment is the entry point.
PRIOH-1 part C: 101 patients against investigator’s choice
The approval rests on part C of PRIOH-1 (NCT03073967), a randomized, open-label phase 3 superiority trial. It enrolled and treated 101 immunocompromised adults whose mucocutaneous HSV lesions had not responded to standard therapy. Patients on pritelivir took 400 mg on day one and then 100 mg once daily for up to 28 days, with treatment extended to 42 days if lesions were still improving.
The comparator arm was not a placebo. Investigators chose among intravenous foscarnet, intravenous or topical cidofovir, and 5% topical imiquimod, which are the harsher, less convenient options that refractory patients are left with today. The AiCuris account of the trial data, released alongside the FDA priority review of the application in April 2026, preceded the approval by about six months.
The primary endpoint was complete healing of all lesions by day 28. Pritelivir reached 63% and investigator’s choice 34%, an adjusted difference of 28.4 percentage points (95% confidence interval 9.6 to 47.3; P = .0047). By day 42, lesion response was 82% on pritelivir and 42% on the comparators. An earlier Contagion Live report on the phase 3 results, from February 2026, covered the same trial before the regulatory decision.
Fewer side effects than the drugs it displaces, with acid-reducer limits
Tolerability favored the new tablet. Adverse reactions occurred in 22% of patients on pritelivir and 54% on investigator’s choice, and 2% stopped pritelivir because of them versus 20% on the comparators. Headache was the most common reaction, at 6% against 4%.
The label carries practical restrictions. Acid-reducing drugs can lower pritelivir exposure and blunt its effect: esomeprazole and rabeprazole are to be avoided, omeprazole or lansoprazole kept to 20 mg a day or less, and pantoprazole to 40 mg or less. Pritelivir should be taken at least two hours before or 12 hours after an H2-receptor antagonist, and at least two hours before or after an antacid. It also inhibits breast cancer resistance protein, so levels of drugs cleared by that transporter may rise.
Genovefa Papanicolaou, a Memorial Sloan Kettering Cancer Center physician and PRIOH-1 investigator, said pritelivir showed superior lesion healing to investigator’s choice through day 28, and described the problem it targets plainly: “refractory HSV infections can be prolonged, painful, and difficult to treat.”
A narrow label for a small, hard-to-count population
Neither the approval announcement nor the trial reports offer an estimate of how many people have refractory herpes, so the only hard number is the 101 patients in the pivotal study. The group in question is immunocompromised adults, typically people whose immune defenses are suppressed by disease or treatment, for whom lesions persist despite acyclovir-class drugs. The remaining options are intravenous or topical agents with their own burdens.
Stacy Wheeler, chief executive of Asahi Kasei Therapeutics for the United States and Europe, said in the company’s approval announcement that “HOVILPRI offers a new oral treatment option for adult patients with a persistent, difficult-to-treat disease.” The company expects the drug to be available in the United States before the end of 2026. The approval date is recorded on the FDA’s 2026 novel drug approvals table.
This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.
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