WASHINGTON — An experimental treatment for genital herpes suppresses the viral infection better than the primary drug now used, a study shows. But whether the new compound will reach the market may depend on the resolution of a recent disconcerting finding of anemia in animals getting the drug candidate in separate tests.
About 17 percent of the population is infected with genital herpes, but more than two-thirds of those people don’t know it: Their symptoms fall short of the blisterlike lesions that strike some patients. Even without symptoms, carriers of the herpesvirus can spread it.
The drug candidate, called pritelivir, has now cleared two hurdles. In a study of 156 people with genital herpes, those getting pritelivir had fewer lesions and were less infectious than those getting a placebo, researchers reported in January in the New England Journal of Medicine.
In the new study, scientists pitted pritelivir against the first-line herpes drug valacyclovir, marketed as Valtrex, in 91 patients. The volunteers were people beset by genital herpes symptoms, averaging four to nine outbreaks per year, said chemist Helga Rübsamen-Schaeff, the chief executive officer at AiCuris, a pharmaceutical company in Wuppertal, Germany, which is developing the compound.
Patients got valacyclovir or pritelivir for four weeks, neither compound for four weeks and then the other drug for another four weeks. Each day, participants swabbed their genitalia for lab analysis.
During the study, volunteers had a genital herpes outbreak on 1.9 percent of the days they were taking pritelivir and on 3.9 percent of days while valacyclovir, Rübsamen-Schaeff reported September 6 at the Interscience Conference on Antimicrobial Agents and Chemotherapy. The analysis revealed virus in 2.4 percent of swabs from people taking pritelivir and 5.2 percent of swabs from people on valacyclovir. That indicates that patients taking pritelivir are less likely to spread the virus, she said.
If approved by regulators, pritelivir might serve as a daily preventive for people who experience frequent herpes outbreaks, Rübsamen-Schaeff said. “For people who only have it once a year, you would wait, and when you start to feel a tingling, then treat that episode” with pritelivir, she said.
Pritelivir should work equally well on oral herpes, which causes cold sores or fever blisters, because the compound targets a viral enzyme that is integral to both kinds of herpes infections, she said. Valacyclovir targets a different enzyme and works only in infected cells, whereas pritelivir has the advantage of protecting healthy cells while thwarting virus replication in infected cells, Rübsamen-Schaeff said.
“Having a drug with a different mechanism of action is long overdue,” says Khalil Ghanem, an infectious disease physician at Johns Hopkins University. “This is really what everyone was waiting for.”
But the study was halted after only 56 of the 91 participants had completed the 12-week regimen. A separate unpublished test in monkeys showed that some of the animals getting pritelivir had developed anemia. The researchers made those data available to the U.S. Food and Drug Administration, which considers serious adverse effects with caution. The monkey data are being analyzed now, Rübsamen-Schaeff says. No such serious side effects showed up in either study of pritelivir in people. Meanwhile, the company is working on a topical version of the drug for use on cold sores, she says.
That’s welcome news, says Ghanem. “The topical antivirals we have don’t work very well. This is an exciting paper.”
Editor’s Note: This article was updated October 2, 2014 to clarify that 1.9 percent and 3.9 percent were the fractions of days in which volunteers had lesions (not fractions of volunteers with lesions) while taking pritelivir and valacyclovir, respectively.