FDA Approves HOVILPRI (Pritelivir), First New-Class Herpes Drug in Nearly 30 Years, for Immunocompromised Patients

The United States Food and Drug Administration (FDA) has approved HOVILPRI (pritelivir) tablets for immunocompromised adults with mucocutaneous lesions caused by herpes simplex virus (HSV) that no longer respond to standard antiviral medicines, granting patients with weakened immune systems the first new type of treatment for stubborn herpes infections in nearly three decades.

The approval, announced on Friday 9 October 2026, covers adults with weakened immune systems whose skin or mucosal lesions — on the lips, nostrils, eyes or genitals — are refractory to treatment with acyclovir, valacyclovir or famciclovir, with or without documented resistance to those drugs, according to the FDA. Pritelivir is a helicase-primase inhibitor, a mechanism of action distinct from every herpes medicine approved before it.

What the pivotal trial showed

The approval rests on Part C of the PRIOH-1 study (NCT03073967), a randomised, open-label, comparator-controlled phase 3 superiority trial in 101 immunocompromised adults with mucocutaneous HSV lesions that had not responded to standard therapy, according to Contagion Live, which reported the trial details.

Patients in the pritelivir group received a 400 mg dose on day one, followed by 100 mg once daily for up to 28 days, extended to 42 days if their lesions were still improving. The comparison group received the investigator’s choice of existing fallback treatments: intravenous foscarnet, intravenous or topical cidofovir, or 5 per cent topical imiquimod — in other words, the messy mix of harsh drugs that doctors currently reach for when standard pills fail.

The primary endpoint was complete healing of all lesions by day 28. According to Contagion Live, 63 per cent of patients receiving pritelivir achieved that result, compared with 34 per cent on the investigator’s choice — an adjusted treatment difference of 28.4 percentage points (95 per cent confidence interval 9.6 to 47.3; p = 0.0047). By day 42, lesion responses reached 82 per cent with pritelivir versus 42 per cent with the comparator, a difference of 40.0 percentage points (95 per cent CI 22.7 to 57.7).

The safety profile also favoured the new drug. Adverse reactions were reported in 22 per cent of the pritelivir group compared with 54 per cent of the comparator group, according to Contagion Live. Discontinuations due to adverse reactions were 2 per cent versus 20 per cent. The most common adverse reaction was headache, occurring in 6 per cent of pritelivir patients and 4 per cent of those on the comparator.

That gap in side effects is not a footnote. The current fallback options for refractory HSV are unpleasant: foscarnet must be given intravenously and is notorious for kidney toxicity, while cidofovir carries similar risks. An oral tablet that outperforms those regimens while causing markedly fewer adverse events is precisely why infectious-disease specialists have been watching this programme.

A different way to stop the virus

Pritelivir is described as a first-in-class helicase-primase inhibitor, developed by the German anti-infectives company AiCuris, which Japan’s Asahi Kasei acquired in April. Asahi Kasei’s specialty pharmaceutical arm, Asahi Kasei Therapeutics, holds the product.

The mechanism matters because it explains the drug’s activity against resistant strains. Standard herpes medicines — acyclovir, valacyclovir and famciclovir — are nucleoside analogues: they must first be activated by the virus’s own thymidine kinase enzyme before they can block viral DNA copying. When the virus mutates that enzyme, the drugs stop working. That is the single most common route by which herpes becomes resistant to acyclovir-family medicines.

Pritelivir does not need thymidine kinase at all. Instead, it jams the viral helicase-primase complex, the molecular machine that unwinds the virus’s double-stranded DNA so it can be replicated. According to the company, the drug is active against HSV-1 and HSV-2, including strains resistant to nucleoside analogues and to foscarnet.

Asahi Kasei Therapeutics called the approval “the first FDA-approved treatment for refractory mucocutaneous HSV lesions with a novel mechanism of action in nearly 30 years,” a statement made in the company’s announcement. Contagion Live independently described the agent as a mechanism distinct from nucleoside analogues.

Genovefa Papanicolaou, clinical director of the infectious disease service at Memorial Sloan Kettering Cancer Center and a PRIOH-1 investigator, said in the company’s announcement that pritelivir showed superior lesion healing through day 28, noting that in patients with blood cancers or after stem cell transplantation, refractory HSV can be prolonged and painful, while the available intravenous treatments carry substantial risks.

Who needs it

Herpes simplex virus infects most of humanity. The World Health Organization estimates that about 3.8 billion people under the age of 50 — 64 per cent of that age group — carry herpes simplex virus type 1 (HSV-1), the main cause of oral herpes, while roughly 520 million people aged 15 to 49 carry HSV-2, the main cause of genital herpes. Most infections are mild or silent, and the virus cannot be cured: it retreats along sensory nerves and hides in nerve ganglia, reactivating periodically for life.

For healthy people, standard antivirals control outbreaks well. The problem this approval addresses sits at the other end of the spectrum. People with weakened immune systems — organ and stem cell transplant recipients, people living with HIV, patients with cancer or autoimmune and inflammatory conditions — can develop more frequent, more severe and more persistent infections, according to the company. When standard drugs fail, either because the virus has mutated or because the patient cannot tolerate them, lesions can persist for weeks or months, causing pain, preventing eating or speaking, and opening the door to secondary infections.

The FDA’s indication is narrow by design: immunocompromised adults whose mucocutaneous lesions have not responded to the standard three antivirals. This is the population with the greatest unmet need and the clearest risk-benefit case, and it is where the trial evidence was generated.

The regulatory fast lane and the business behind it

The FDA granted pritelivir Fast Track and Breakthrough Therapy designations, and its new drug application received Priority Review, according to the company — a set of designations reserved for medicines that address serious conditions with unmet needs and that show meaningful advantages over existing options.

Asahi Kasei Therapeutics said HOVILPRI is expected to become available in the United States before the end of 2026. “HOVILPRI offers a new oral treatment option for adult patients with a persistent, difficult-to-treat disease,” Stacy Wheeler, chief executive officer of Asahi Kasei Therapeutics in the US and Europe, said in the announcement. Carl Kraus, the company’s chief medical officer, described refractory HSV as a serious clinical challenge for immunocompromised adults with limited treatment options.

There are practical details doctors will need to mind. Acid-reducing medicines can lower pritelivir exposure and reduce its effectiveness, according to the company announcement: clinicians should avoid esomeprazole and rabeprazole, limit omeprazole or lansoprazole to 20 mg daily or less and pantoprazole to 40 mg daily or less, and patients should take pritelivir at least two hours before or twelve hours after H2-receptor antagonists, and at least two hours before or after antacids. Pritelivir also inhibits the breast cancer resistance protein (BCRP), which may raise blood levels of other drugs that use that transporter — a detail that will matter in transplant clinics, where patients typically take long lists of medicines.

A busy Friday at the FDA

The pritelivir approval was one of several health decisions on Friday. According to a Reuters health news summary, the FDA also approved Teva’s Weltruza, a once-monthly injection for adults with schizophrenia — the second US-approved product from Teva’s partnership with France’s Medincell — and the Pfizer-BioNTech XFG-adapted COVID-19 vaccine for the 2026–2027 season. The Reuters summary also reported that US measles cases had risen to 4,080 as of 8 October, and that the World Health Organization said it still does not know what caused the pneumonia that killed a Russian lab worker at an anti-plague research centre in Siberia.

Analysis: Why It Matters

The striking thing about the HOVILPRI approval is not just what it does — it is how long patients waited. Herpes medicines are among the oldest modern antivirals: acyclovir dates to the early 1980s, and its successors valacyclovir and famciclovir are reformulations of the same basic idea. For roughly four decades, drug development in herpes has been incremental. A genuinely new mechanism clearing phase 3 and winning FDA approval is therefore a genuine event, and the company’s “nearly 30 years” framing — referring to the wait for a new class of treatment — is not marketing puff.

Why did it take so long? Part of the answer is commercial. Herpes in healthy people is a solved-enough problem: cheap generic acyclovir manages it, and there is little financial incentive to develop a new drug for a market dominated by pennies-a-pill generics. The resistant niche — immunocompromised patients — is medically urgent but commercially small. Pritelivir’s long development history reflects that reality: it spent years in the hands of AiCuris, a specialist anti-infectives firm, before Asahi Kasei saw enough value to acquire the whole company in April.

Part of the answer is also scientific difficulty. Running a randomised trial in severely immunocompromised patients — people with blood cancers, recent stem cell transplants, advanced HIV — is hard. Patients are heterogeneous, some are dying of other causes, and the comparator arm cannot be a placebo: it has to be the toxic real-world fallback. PRIOH-1’s design, with the investigator choosing among foscarnet, cidofovir or imiquimod, was a pragmatic concession to that reality. The result — a clean superiority win against exactly the drugs doctors actually use — makes the data more credible, not less, because it mirrors clinical practice rather than an artificial ideal.

The oral-versus-intravenous contrast deserves emphasis. For a transplant patient already spending half their life in infusion centres, the difference between swallowing a daily tablet and being hooked up to intravenous foscarnet — with its kidney damage, electrolyte chaos and hospital stays — is the difference between treating the infection and reorganising the patient’s life around it. The 2 per cent versus 20 per cent discontinuation figures tell that story in numbers.

Three cautions belong in any honest assessment. First, the trial was small: 101 randomised patients. That is normal for an orphan-adjacent indication, but it means rare side effects and long-term resistance patterns are not yet visible. Second, the drug-interaction burden is real. Transplant recipients take calcineurin inhibitors, antifungals and a pharmacy shelf of other drugs; the acid-reducer restrictions and the BCRP inhibition note mean pritelivir’s rollout will require careful medication reviews, and real-world effectiveness may be lower than trial efficacy in patients who cannot follow the timing rules. Third, nothing is known about the price. A novel oral antiviral with breakthrough designation will not be cheap, and access for the uninsured immunocompromised patients who need it most is an open question Asahi Kasei has not yet answered.

There is also a broader scientific significance. The helicase-primase complex exists across the herpesvirus family, which includes not only HSV-1 and HSV-2 but also varicella-zoster virus (chickenpox and shingles) and cytomegalovirus — the latter a constant threat to transplant patients. Success with pritelivir validates the target class, and it would not be surprising to see follow-on programmes aimed at those cousins. The WHO’s staggering prevalence figures — billions carrying the virus — mean that even a niche approval can have outsized ripple effects in research.

Finally, the approval is a reminder of how the FDA’s expedited pathways are supposed to work: Fast Track, Breakthrough Therapy and Priority Review all converged on a drug for a small, suffering population with no good options, and the trial delivered a superiority result against the standard of care. Whatever one thinks of the agency’s recent controversies, this is the system functioning as designed.

What to watch next

The questions that will define HOVILPRI’s impact are now practical ones. First: pricing and access. Asahi Kasei has not disclosed a list price; the speed with which insurers and hospital formularies adopt the drug will determine whether the patients in the trial population actually get it. Second: regulatory reach beyond the United States. European regulators have historically been receptive to novel anti-infectives for immunocompromised patients, but no timeline has been announced. Third: indication expansion. The current label is limited to refractory mucocutaneous lesions in immunocompromised adults — but the mechanism could in principle help patients with severe genital herpes, neonatal herpes or even shingles, and the company’s development plans will be watched. Fourth: resistance surveillance. Every antiviral eventually meets resistant mutants; how quickly HSV adapts to helicase-primase inhibition in real-world use is unknown. And fifth: real-world effectiveness in the polypharmacy-heavy transplant population, where drug interactions and adherence to strict timing rules will be tested outside the controlled trial setting.

For now, though, the headline stands: after decades without a new weapon, doctors treating drug-resistant herpes in their most vulnerable patients finally have one — and it comes in a bottle, not an IV bag.

Sources

• US Food and Drug Administration: FDA Approves Drug to Treat Lesions Caused by Herpes Simplex Virus
• Contagion Live: FDA Approves Pritelivir for Refractory HSV in Immunocompromised Adults
• Asahi Kasei Therapeutics (Business Wire, 9 Oct 2026): FDA Approval of HOVILPRI (pritelivir) Tablets
• dpa-AFX: Asahi Kasei Wins FDA Approval For HOVILPRI To Treat Drug-Resistant HSV In Immunocompromised Adults
• World Health Organization: Herpes simplex virus factsheet