Highlights from the First International Workshop on Clinical Pharmacology of HIV Therapy, Noordwijk, The Netherlands, March 30-31, 2000.
Therapeutic drug monitoring has largely focussed on protease inhibitors, partly because of large observed variations between individuals, but also because of the short half-lives of the PIs. However, several studies have called attention to the potential therapeutic implications of individual variations in the ability to phosphorylate (process) the nucleoside analogues, notably the Altiphar and Retrophar studies. However, until recently it was very difficult to measure how well nucleoside analogues were penetrating into cells, and it was difficult to do large scale studies on the impact of variations in intracellular levels on virological outcomes.
At the Noordwijk meeting, the University of Liverpool HIV Pharmacology team reported on the development of a simple assay which requires a small amount of blood and which appears to produce consistent results (Hoggard). In the past, studies which have reported on differences in phosphorylation rates have been dogged by controversy because researchers cannot agree on the best way to measure intracellular levels of nucleoside analogues.
Professor David Back of Liverpool University, one of the organisers of the conference, told aidsmap why the development of such assays is so important.
"In the past assays were taking two and a half days to do, [because of the need to separate lymphocytes from a blood sample] and were not as sensitive as we would like, so we have tried to simplify the process using a molecular biology approach which detects the triphosphates within lymphocytes. We have an assay up and running which can detect carbovir (abacavir), 3TC and AZT triphosphates, but we are experiencing a little more difficulty with stavudine and ddI. We're now doing studies in Manchester, with Julio Montaner in Vancouver, and also in South Africa."
The assay uses competition between drug triphosphate and radiolabelled dNTP to determine how much of the active drug is present in target cells, so it is quicker to conduct and easier to standardise.
The intracellular assay for nucleoside analogues showed tenfold variations between patients in intracellular levels of abacavir (carbovir) and 3TC triphosphates, suggesting that the development of such assays and their validation in clinical trials may be another helpful application of therapeutic drug monitoring.
Reference
Hoggard P et al. Quantification of nucleoside analogue triphosphates by enzymatic assay. First International Workshop on Clinical Pharmacology of HIV therapy, Noordwijk, abstract 1.1, 2000.