Ritonavir-d6 is intended for use as an internal standard for the quantification of ritonavir by GC- or LC-MS. Ritonavir is an HIV protease inhibitor. It inhibits recombinant HIV-1 protease by 79% when used at a concentration of 0.5 nM. It inhibits HIV-13B-induced cell death in MT-4 human T cell leukemia cells (EC50 = 25 nM) as well as cell death induced by HIV-1LAI, HIV-2ROD, and HIV-2EHO in human MT-2 cells (IC50s = 0.045, 0.13, and 0.24 μM, respectively). Ritonavir also inhibits the cytochrome P450 (CYP) isoform CYP3A (IC50 = 0.14 μM). It inhibits CYP-mediated oxidative metabolism of the HIV protease inhibitors saquinavir , indinavir , nelfinavir , and amprenavir in rat and human liver microsomes in a concentration-dependent manner. Ritonavir (10 mg/kg) also prevents decreases in plasma levels of these four compounds in rats. Formulations containing ritonavir have been used in the treatment of HIV-1 infection.