Intracellular accumulation of ritonavir combined with different protease inhibitors and correlations between concentrations in plasma and peripheral blood mononuclear cells.

Objectives: Ritonavir, used at low doses as a boosting agent of other protease inhibitors (PIs), is known to be associated with metabolic complications and gastrointestinal disturbances. The rate of accumulation of ritonavir within cells is still debated due to scarce data and methodological limitations. Therefore, our aim was to evaluate intracellular ritonavir penetration when used with different boosted PIs in the clinical setting.
Methods: Patients administered with atazanavir/ritonavir (300/100 mg, once daily), darunavir/ritonavir [600/100 mg, twice daily (darunavir-600) and 800/100 mg, once daily (darunavir-800)], lopinavir/ritonavir (400/100 mg, twice daily) and tipranavir/ritonavir (500/200 mg, twice daily) were considered. Blood sampling at the end of the dosing interval (Ctrough) was performed. Peripheral blood mononuclear cell (PBMC)-associated and plasma ritonavir and PI concentrations were measured by validated HPLC methods. PBMC count and individual mean cell volume (MCV) were measured using a Coulter Counter instrument.
Results: One hundred patients were enrolled. Frequencies of ritonavir-boosted PIs were atazanavir, 37%; darunavir-600, 23%; lopinavir, 19%; tipranavir, 13%; and darunavir-800, 8%. The median intracellular and plasma concentrations of ritonavir were 1279 ng/mL (IQR 727-2087) and 170 ng/mL (IQR 82-384), respectively, accounting for a cellular accumulation ratio of 7.69 (5.7-10.9). Significant differences in ritonavir intracellular concentrations emerged among different PIs (P<0.001): specifically between darunavir-600 and atazanavir (P<0.001), between darunavir-600 and tipranavir (P=0.009), between atazanavir and lopinavir (P<0.001) and between lopinavir and tipranavir (P=0.027).
Conclusions: Our study showed a higher rate of ritonavir intracellular accumulation than previously reported, possibly due to the more accurate calculation of intracellular concentrations by MCV. The ratio varied according to concomitantly administered PIs, suggesting their influence on the rate of ritonavir intracellular penetration.