Pritelivir vs. foscarnet: an oral pill for drug-resistant herpes

Acyclovir and valacyclovir work well for most people. But in people with weakened immune systems, such as transplant recipients or people with advanced HIV, herpes outbreaks can become severe and stop responding to these drugs. The main fallback, intravenous foscarnet, can damage the kidneys and often requires a hospital stay.

Pritelivir, developed by AiCuris, works differently: it blocks the virus’s helicase-primase enzyme, so it remains active against strains resistant to acyclovir and foscarnet. Results of its Phase 2 trial were published in Open Forum Infectious Diseases.

How the trial worked

• Part A: 22 immunocompromised adults with acyclovir-resistant or refractory HSV were randomized 2:1 to oral pritelivir once a day or IV foscarnet, for up to 28 days. About 40% were immunocompromised due to HIV and 47% due to a transplant.
• Part B: 8 people whose infection also failed on, or couldn’t tolerate, foscarnet received pritelivir.

What they found

• Lesions healed in 93% of people on pritelivir vs 57% on foscarnet. The 36-point difference was not statistically significant (p = 0.077).
• Median time to healing was 26 days with pritelivir. For foscarnet it couldn’t be estimated, because too few people healed.
• Drug-related side effects affected about 9% of people taking pritelivir vs 71% on foscarnet.
• In Part B, 63% healed, with a median time of 19 days.

What to keep in mind

With 22 randomized patients, chance can’t be ruled out. The larger Phase 3 trial, PRIOH‑1 with 101 patients, has since reported statistically significant results: complete healing in 63% with pritelivir vs 34% with other treatments within 28 days. Pritelivir treats outbreaks; it doesn’t remove the dormant virus, so it isn’t a cure. It has been under FDA priority review for this group of patients, as covered in our earlier update.