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 TDM for the nucleoside analogues (NRTIs) and the non-nucleosides (NNRTIs).

Monitoring nucleoside analogue levels

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 (within cell) levels on virological outcomes.

Nucleoside analogues, e.g. AZT or ddI, require an additional step in their metabolism inside target cells (called phosphorylation) in order to reach their active form. Measuring their concentration in plasma may not provide an accurate reflection of intracellular levels of this active form. See Resistance to nucleoside and nucleotide reverse transcriptase inhibitors in Anti-HIV therapy: Resistance for more information on this subject.

At the First International Workshop on Clinical Pharmacology of HIV Therapy, the University of Liverpool HIV Pharmacology team reported on the development of a simple assay 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.

The development of this new assay is a significant step. Previously, assays were taking two and a half days to do because of the need to separate lymphocytes from a blood sample. In addition, these tests were not very sensitive. The University of Liverpool team has developed a new assay which detects the triphosphates within lymphocytes. 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.

Currently, assays can detect abacavir, 3TC and AZT triphosphates. Research is ongoing to develop assays for d4T and ddI, and to establish the accuracy of the assays.

To date, the intracellular assay has found that abacavir and 3TC levels vary ten-fold between individuals, suggesting that the development of such assays and their validation in clinical trials may be another helpful application of therapeutic drug monitoring.

Monitoring non-nucleoside reverse transcriptase inhibitor levels

An assay has also been developed to measure concentrations of nevirapine in saliva, which are about half the levels found in blood plasma. Researchers are investigating salivary drug testing for other antiretrovirals, which would offer a cheap and easy testing method.

High levels of efavirenz have been associated with increased severity and frequency of central nervous system side-effects, while low levels are associated with higher rates of treatment failure (Marzolini). Consequently, measuring blood levels of efavirenz could be used to maximise efficacy and minimise toxicity. However, no dosage adjustments to counter toxicities have been recommended yet.

On the other hand, a large analysis of 524 patients who received efavirenz in clinical trials found no relationship between efavirenz peak levels or total exposure and the risk of developing central nervous system toxicity (Fiske). See Key research on testing for drug levels in Anti-HIV therapy: Testing drug levels for further details of this study.

References

See Key research on testing for drug levels for references.