Bioanalysis and clinical pharmacology of antiviral drugs -focus on HIV and Influenza

Abstract

Adequate treatment of HIV and influenza is of the utmost importance considering the high mortality of both diseases. This thesis describes the development of bioanalytical methods for antiviral drug measurement. Furthermore, these methods were used in clinical studies to increase knowledge of the clinical pharmacology of antiviral drugs with the ultimately goal to improve current treatment strategies of HIV and influenza.

Firstly, the implementation of dried blood spots (DBS) for antiretroviral therapeutic drug monitoring (TDM) was described. This is a patient friendly and cost effective alternative to plasma sampling. Before implementation of DBS in clinical practice, the relationship between plasma and DBS concentrations was established. A theoretical plasma concentration could be calculated from DBS concentrations using the formula (DBS concentration/[1-hematocrit]) x fraction bound to plasma proteins = plasma concentration. The theoretical plasma concentrations of nevirapine and efavirenz were similar to measured plasma concentrations. Furthermore, a feasibility study showed that antiretroviral drug concentrations could be measured in 87.5% of the collected DBS by 50 HIV-infected adults at home. Most patients preferred DBS over plasma sampling. In an HIV infected pediatric population TDM with DBS was compared to TDM with plasma sampling. Of the 86 dosing advices, only 3 showed a discrepancy. Also, the DBS sampling method showed to be a useful tool for monitoring nevirapine exposure and virological efficacy in a resource limited setting.

Secondly, the clinical pharmacology of antiretroviral nucleoside reverse transcriptase inhibitors (NRTI) was described. NRTIs require phosphorylation to their active triphosphate metabolite and therefore, the intracellular NRTI triphosphate levels are considered to be the best predictor of antiviral activity. Two bioanalytical methods using liquid chromatography coupled to tandem mass spectrometry (HPLC-MS/MS) were developed; one allowed the quantification of NRTI plasma concentrations, whereas the other quantifies the intracellular anabolites of NRTIs. Furthermore, a small clinical study showed that tenofovir diphosphate proved to be the predominant intracellular anabolite of tenofovir, while no relationship was observed between tenofovir plasma concentrations and the intracellular concentrations.

Thirdly, the pharmacokinetics of oseltamivir and oseltamivir carboxylate were described in specific patient populations with the use of an assay for the simultaneous quantification of both analytes in plasma using HPLC-MS/MS. In critically ill patients, a highly variable oseltamivir carboxylate exposure was observed which showed no relationship with the administered dose due to alterations in organ functions. Therefore, in critically ill patients with an increased renal function or distribution volume an increased oseltamivir dose should be administered. In infants also highly variable oseltamivir and oseltamivir carboxylate concentrations were observed, most likely due to ontogeny. Furthermore, a case report showed that oseltamivir and oseltamivir carboxylate cross the placenta at significant levels, which may be beneficial for the fetus.

In conclusion, this thesis shows the successful validation of bioanalytical methods for the quantification of antiviral drugs. Moreover, these methods have been implemented in a clinical setting thereby improving currently available TDM practices for antiretroviral drugs, increasing knowledge of the pharmacology of intracellular antiretroviral drug concentrations and optimizing oseltamivir therapy.