MK-8591 or EFdA is a novel and exceptionally long-lasting and potent HIV drug, being developed by Merck. It is a nucleoside reverse transcriptase translocation inhibitor (NRTTI). Its potency and longevity are due to the fact that it acts at two stages of the HIV lifecycle: as well as blocking HIV from making a DNA copy of its genes that can be inserted into a human cell, it also blocks integrated HIV DNA inside cells from being turned back into viruses.

It also does not get degraded easily by the bodily enzymes that digest molecules: this means its half-life in humans (the time taken for half the drug to leave the body) is 50-60 hours and 120 hours (five days) inside cells. This means it is suitable for weekly dosing.

At last year’s Conference on Retroviruses and Opportunistic Infections (CROI), studies showed that a single dose of MK-8591 of as little as 0.5mg produced HIV viral load drops of at least tenfold in HIV-positive human volunteers (For comparison the daily dose of rilpivirine, one of the most potent current HIV drugs, is 25mg.) In another study, when used as pre-exposure prophyalxis (PrEP), a weekly dose of 4.3 milligrams per kilogram (mg/kg) fully protected a group of eight rhesus macaque monkeys from infection. Read more via AIDSmap