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Protease inhibitors are largely used for the treatment of HIV infection in combination with other antiretroviral drugs. Their improved pharmacokinetic profiles can be achieved through the concomitant administration of low doses of ritonavir (RTV), a protease inhibitor currently used as a booster, increasing the exposure of companion drugs. Since ritonavir-boosted regimens are associated with long-term adverse events, cobicistat, a CYP3A4 inhibitor without antiviral activity, has been developed. Recently, high intracellular concentrations of ritonavir in lymphocytes and monocytes were reported even when ritonavir was administered at low doses, so we aimed to compare its theoretical antiviral activity with those of the associated protease inhibitors. Intracellular concentrations of ritonavir and different protease inhibitors were determined through the same method. Inhibitory constants were obtained from the literature. The study enrolled 103 patients receiving different boosted protease inhibitors, darunavir-ritonavir 600 and 100 mg twice daily and 800 and 100 mg once daily (n = 22 and 4, respectively), atazanavir-ritonavir 300 and 100 mg once daily (n = 40), lopinavir-ritonavir 400 and 100 mg twice daily (n = 21), or tipranavir-ritonavir 500 and 200 mg twice daily (n = 16). According to the observed concentrations, we calculated the ratios between the intracellular concentrations of ritonavir and those of the companion protease inhibitor and between the theoretical viral protease reaction speeds with each drug, with and without ritonavir. The median ratios were 4.04 and 0.63 for darunavir-ritonavir twice daily, 2.49 and 0.74 for darunavir-ritonavir once daily, 0.42 and 0.74 for atazanavir-ritonavir, 0.57 and 0.95 for lopinavir-ritonavir, and 0.19 and 0.84 for tipranavir-ritonavir, respectively. Therefore, the antiviral effect of ritonavir was less than that of the concomitant protease inhibitors but, importantly, mostly with darunavir. Thus, further in vitro and in vivo studies of the RTV antiviral effect are warranted.

Intracellular Antiviral Activity of Low-Dose Ritonavir in Boosted Protease Inhibitor Regimens