TFV‐DP and FTC‐TP using volumetric absorptive microsampling compared with DBS among persons receiving TDF/FTC or TAF/FTC

29 Background: The phosphorylated moieties tenofovir-diphosphate (TFV-DP) and emtricitabine-triphosphate (FTC-TP) can be quantified in erythrocytes using dried blood spots (DBS) following tenofovir disoproxil fumarate (TDF) or tenofovir alafenamide (TAF), with emtricitabine (FTC). There are established adherence: concentration benchmarks for DBS TFV-DP/FTC-TP; however, the comparability of volumetric absorptive microsampling (VAMS) methods (e.g., Mitra) has not been extensively evaluated. Material and methods: Paired DBS and VAMS (Mitra 20 μL) samples from two studies were analysed: (A) ATEAM (NCT02968576): subset of 20 participants (35 person-visits) who received single-dose TDF/FTC (300 mg/200 mg) on two occasions (separated by a 14-day washout), measured at 24 h post-dose and (B) TAF-DBS (NCT02962739): subset of six participants (26 person-visits) who received TAF/FTC (25 mg/200 mg) at 33%, 67% or 100% of daily dosing (two participants/adherence level), measured at weeks 1–6. DBS TFV-DP/FTC-TP concentrations were quantified from a 3-mm punch (TDF/FTC) vs. 2*7 mm punches (TAF/FTC); VAMS concentrations were quantified from a 20 μL of tip (units/sample). Mean values of replicate VAMS samples (up to 3 tips) were used for comparison to singlet DBS. Linear mixed-effects models compared log-transformed DBS and VAMS TFV-DP/FTC-TP and results were reported as percentage increases (95% CIs) in DBS for every 10% increase in VAMS. Repeated-measures Bland–Altman analyses calculated DBS:VAMS mean bias ratios/conversion factors (95% CIs), and DBS:VAMS proportional biases (95% CIs) to evaluate if DBS and VAMS differences changed across the range of concentrations. Results: (A) ATEAM (TDF/FTC) participants included 13 (65%) females, 17 (85%) whites, and had a median (range) age of 27 (24, 37) years. Following single-dose TDF/FTC, every 10% increase in VAMS TFV-DP was associated with a DBS TFV-DP increase of 8.5% (7.5%, 9.6%; p < 0.0001); every 10% increase in VAMS FTC-TP was associated with a DBS FTC-TP increase of 6.0% (4.4%, 7.5%; p < 0.0001). DBS concentrations were lower than VAMS (mean bias ratios: TFV-DP, 0.34 [0.19, 0.63; p = 0.001]; FTC-TP, 0.14 [0.10, 0.21; p < 0.0001]). Every unit increase in log-transformed concentrations was associated with a DBS:VAMS proportional bias change of −9.6% (−20.2%, 2.4%; p = 0.11) for TFV-DP and −29.1% (−43.4%, −11.1%; p = 0.004) for FTC-TP. (B) TAF-DBS (TAF/FTC) participants included three (50%) females, six (100%) whites and had a median (range) age of 30 (28, 33) years. Following repeated TAF/FTC dosing, every 10% increase in VAMS TFV-DP was associated with a DBS TFV-DP increase of 9.9% (9.2%, 10.6%; p < 0.0001); every 10% increase in VAMS FTC-TP was associated with a DBS FTC-TP increase of 9.3% (8.2%, 10.5%; p < 0.0001). DBS concentrations were higher than VAMS (mean bias ratios: TFV-DP, 1.43 [0.94, 2.18; p = 0.09]; FTC-TP, 1.69 [1.54, 1.85; p < 0.0001]). Every unit increase in log-transformed concentrations was associated with a DBS:VAMS proportional bias change of 0.1% (−6.4%, 7.1%; p = 0.97) for TFV-DP and −3.4% (−14.5%, 9.2%; p = 0.57) for FTC-TP. Conclusions: Following TAF/FTC, VAMS concentrations can be used to estimate DBS concentrations using the equations, VAMS (TFV-DP)*1.43 = DBS (TFV-DP) and VAMS (FTC-TP)*1.69 = DBS (FTC-TP). TFV-DP/FTC-TP concentrations were low in ATEAM (single-dose TDF/FTC) which led to lower precision; conversion factors following TDF/FTC will require further refinement from repeated dosing studies with paired DBS and VAMS.