Integrase is essential for HIV-1 replication; however, potent inhibition of the isolated enzyme in biochemical assays has not readily translated into antiviral activity in a manner consistent with inhibition of integration. Raltegravir is a novel HIV-1 integrase strand transfer inhibitor with potent in vitro activity against wild-type and multi-class resistant HIV-1 virus (in vitro IC(95) for HIV-1 in 50% normal human serum = 33 nM). Inhibition of integrase prevents insertion of HIV DNA into the human DNA genome, thus blocking the ability of HIV to replicate. Raltegravir is administered orally every 12 h and does not require boosting with low-dose ritonavir (RTV) to achieve therapeutic concentrations. Raltegravir is not a potent inhibitor or inducer of cytochrome P450 3A4, and it is predominantly metabolized by glucuronidation, specifically by the enzyme UDP-glucuronosyltransferase 1A1.