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Background:  Saquinavir/ritonavir (SQV/r) is currently licensed as a BD regimen. Reducing pill burden with OD dosing may enhance adherence. Clinical response to protease inhibitors (PI) must mainly be a function of intracellular drug concentrations. Moreover, PI are substrates for the multidrug resistance transporter P-glycoprotein (P-gp), which may lower intracellular concentrations via drug efflux, thus contributing to sanctuary sites. Although previous studies have measured plasma and intracellular SQV concentrations there are no data on OD regimens. Study aims were to a) determine intracellular (cell associated) and plasma concentrations of OD SQV/r and determine the half-life in the 2 compartments and b) examine relationships of drug accumulation with lymphocyte subset P-gp expression.

Methods:  Venous blood samples from 12 HIV-infected patients receiving SQV/r (1600/100 mg OD) were collected at 2, 6, 12, 24 hours. PBMC were separated by density gradient centrifugation. Plasma and intracellular (cell-associated) drug concentrations were measured by HPLC-MS/MS. Drug exposure in plasma and cells was expressed as the AUC_0-24h derived from non-compartmental modeling. The ratio of the intracellular AUC_0-24h /plasma AUC_0-24h was calculated to determine cellular drug accumulation. P-gp expression on lymphocytes subsets was determined by dual colour flow cytometry. Statistical analyses were performed using simpler linear regression analysis and Wilcoxon’s signed ranks test.

Results:  The median (range) AUC_0-24h of SQV in plasma was 24.2µM.h^-1 (8.4 to 58.6) and in cells was 69.0µM.h^-1 (36.8 to 170.7). Corresponding RTV values were 10.3µM.h^-1 (2.1 to 20.2) and 14.5µM.h^-1 (4.4 to 19.0). The median cellular accumulation of SQV and RTV was 3.31 (1.50 to 6.69) and 1.46 (0.83 to 4.15). There was a significant difference between SQV plasma and intracellular half-life (median values 4.5 hours and 5.9 hours, p = 0.034) and between RTV plasma and intracellular half-life (4.1 hours and 6.2 hours, p = 0.033), so that the intracellular /plasma ratio of SQV and RTV increased over time. Despite SQV and RTV being substrates for P-gp there was no relationship between drug accumulation and P-gp expression on CD4⁺, CD8⁺, CD56⁺, or total lymphocytes.

Conclusions:  Intracellular (cell-associated) SQV and RTV concentrations were higher than plasma, suggesting lymphocyte accumulation. The intracellular half-life of SQV and RTV was longer than plasma, indicating that intracellular drug may be available at a time when plasma concentrations are below the minimum effective concentration.

Keywords: intracellular; p-glycoprotein; pharmacokinetics