People exposed to the AIDS virus have only one drug to try to ward off infection - and it seldom works. Now scientists have an exciting new candidate, an experimental medicine that completely protected monkeys in the same situation.
One of the nation's top AIDS experts called the discovery "unprecedented," and the drug's manufacturer is preparing to test the medicine in people next year.It's a long way from a monkey drug to one that will help people, cautioned chief researcher Dr. Che-Chung Tsai of the University of Washington Regional Primate Center. Still, in today's edition of the journal Science, Tsai calls PMPA the most promising candidate yet to help people ward off the AIDS virus.
"Such complete protection with no toxicity is unprecedented in the monkey model of AIDS and suggests a potential role for PMPA in health-care workers or others accidentally exposed to the virus," agreed AIDS expert Dr. Anthony Fauci of the National Institutes of Health.
Health-care workers are at risk of catching HIV, the AIDS virus, from accidental needle-sticks or tainted blood. Now when these people realize they were exposed to HIV, they take the drug AZT - but it often fails to protect them.
PMPA is a chemical very similar to AZT. Both attack the AIDS virus by blocking a key protein it needs to reproduce. But while AZT must wait to be turned on to begin fighting the virus, PMPA can start waging war immediately, and it is 100 times less toxic than AZT.
To see how well it worked, Tsai gave 35 monkeys 100 percent infectious doses of SIV, the primate version of HIV. Fifteen monkeys got injections of PMPA 48 hours before they were exposed to AIDS, five got PMPA shots four hours after they were exposed and another five were treated 24 hours later. They all received PMPA intravenously for four weeks.
Eight months later, none of the treated animals - not even the ones exposed for 24 hours - is infected with SIV and none has experienced any side effects.
The 10 monkeys that didn't get PMPA shots became infected.
The findings were "too good to believe," Tsai said. Yet researchers did test after test that confirmed the results.
"It's definitely an exciting finding," said Norbert Bischofberger of Gilead Sciences Inc., which manufactures PMPA. "But what this ultimately means to AIDS patients in the clinic remains to be seen."
Gilead hopes to begin testing PMPA in people next year.
Whether PMPA could help already infected people is a more difficult question. Few drugs have proved effective, zipping into infected cells to fight HIV a little while and then leaking back out. PMPA appears to stay inside cells longer, meaning it might fight HIV longer, Bischofberger said.