Validation of the REverSe TRanscrIptase Chain Termination assay for measuring tenofovir diphosphate in dried blood spots from a clinical pharmacokinetic trial.

BACKGROUND: Tenofovir diphosphate concentration in red blood cells is an objective measure of long-term oral pre-exposure prophylaxis (PrEP) or antiretroviral therapy (ART) adherence. However, current methods for measuring tenofovir diphosphate are equipment and capital intensive, limiting widespread adoption. OBJECTIVES: Low cost, rapid diagnostics for measuring tenofovir diphosphate may drive clinical adoption of routine drug level measurement as a tool for adherence monitoring of tenofovir disoproxil fumarate-based PrEP or ART. We validate a simple and accessible enzymatic assay [REverSe TRanscrIptase Chain Termination (RESTRICT)] for measuring tenofovir diphosphate in dried blood spots (DBS) obtained from a directly observed therapy study of individuals on PrEP. METHODS: We performed RESTRICT measurements on 74 DBS samples from individuals on tenofovir disoproxil fumarate/emtricitabine regimens. We compared RESTRICT measurements with those from a gold-standard method of liquid chromatography tandem mass spectrometry (LC-MS/MS). The ability of RESTRICT to correctly classify DBS tenofovir diphosphate concentrations to established steady-state adherence benchmark concentrations was determined using area under receiver operating characteristic curves (AUCs). RESULTS: The RESTRICT measurements of DBS samples were highly correlated with LC-MS/MS measurements of tenofovir diphosphate from DBS (r = -0.90; P < 0.0001). The RESTRICT assay correctly classified DBS samples as above or below established steady-state adherence benchmark concentrations corresponding to low (AUC = 0.974), moderate (AUC = 0.936) and high (AUC = 0.955) levels of adherence. CONCLUSIONS: The enzymatic RESTRICT assay can accurately measure tenofovir diphosphate concentrations in DBS specimens using simple procedures and readily available laboratory equipment, offering accessible objective adherence monitoring for persons receiving tenofovir disoproxil fumarate-based PrEP or ART.