The oral drug, called pritelivir, was developed by German drugmaker Aicuris, which Asahi Kasei acquired in April Pritelivir blocks proteins the herpes virus needs to copy its genetic material, stopping it from multiplying and allowing the drug to work against some infections resistant to older treatments Older first-line drugs must be activated by a viral enzyme before blocking replication, and mutations that disrupt this activation can cause resistance The US Food and Drug Administration's approval was based on a late-stage trial involving 101 participants, in which sores healed completely within 28 days in 62.7% of patients who were given pritelivir, compared to 34% receiving other treatments The trial compared pritelivir with doctors' choice of intravenous foscarnet, the main approved alternative, or cidofovir and imiquimod — older medicines used off-label The commonly reported side effects in the trial included headache, diarrhea, nausea, reduced appetite, vomiting and dizziness Asahi Kasei had previously estimated pritelivir could serve about 15,000 US patients with weakened immune systems, and generate peak annual sales of more than $400 million in the mid-to-late 2030s