Pharmacokinetics of Low Dose Ritonavir

This study will focus on the combination of Saquinavir (SQV) and RTV. Firstly, this is because SQV is widely used in Thailand and has shown to be an adequate first and second line antiretroviral agent. Secondly, reduced dose of SQV (1600 mg OD) has proven to generate adequate efficacy and good pharmacokinetic parameters in the Thai population[5, 6]. Now 500 mg tablets of SQV are becoming more and more available, therefore a 1500 mg OD dose will be of more interest for our study. A TDM comparison study between SQV 1600 OD mg and SQV1500 mg OD has been done (Ananworanich et al, unpublished data) and did not show any difference in terms of drug concentrations . The efficacy of this regimen has shown to be adequate as described in the STACCATO cohort[7]. So we consider it safe to use the SQV 1500 mg OD in our study.

It is not clear what RTV concentrations are necessary to obtain good levels for SQV. In the study of Autar et al [5] no relation between RTV levels and boosting effect was described. Besides that it might well be that even the boosting levels of RTV contribute to the toxicity of the antiretroviral treatment.For instance, a study performed in healthy volunteers on RTV 100 mg BID monotherapy, showed a significant increase in total cholesterol, low-density lipoprotein (LDL) cholesterol and triglycerides and a marked reduction of high-density lipoprotein (HDL)[8]. This last finding was confirmed in the study of Boyd et al (submitted) in Thai HIV-infected patients.

Preliminary data from HIVNAT 019 [9] suggests that lower boosting dosage still give adequate levels of the other PIs. In this open label 4 arm study one arm contains Lopinavir/Ritonavir 266/66 mg bid together with SQV 600 mg bid. Despite the dose reduction in all PIs, adequate PK levels were obtained. Two arms (23 patients) in this study used normal dose of RTV (100 mg bid) in combination with Lopinavir and Saquinavir. Five of them had maximal concentrations (Cmax) above the therapeutic level RTV (>2.1 mg/l). This finding is supported by clinical experience.

Therefore, looking into lower doses of RTV in the Thai population is of great interest A liquid formulation for RTV 50 mg will be used, as a capsule is not available yet. The liquid formulation has the same bioequivalence as the capsule and was used extensively during the time of manufactory problems of the Norvir capsule. Because of the bad taste of this formulation PK samples will be collected after one week, thereafter patients can return to their old regimen. A dose reduction of RTV can be of great importance for the Thai population as it prevents unnecessary toxicity and costs. If this PK study turns out to be adequate, an efficacy study can be performed and the boosting of other commonly used PI's, like IDV and Lopinavir, can be studied.