Title : Emtricitabine-Triphosphate in Dried Blood Spots as a Marker of Recent 1 Dosing 2 3 Running title : FTC-TP in DBS to quantify recent dosing 4 5

32 Background: New objective measures of antiretroviral adherence are needed. We 33 determined if emtricitabine triphosphate (FTC-TP) in dried blood spots (DBS) can be 34 used as a marker of recent dosing to tenofovir disoproxil fumarate-emtricitabine (TDF35 FTC). 36 Methods: The half-life of FTC-TP was estimated in DBS samples obtained from an 37 intensive pharmacokinetic (PK) study of co-formulated TDF-FTC in HIV-negative and 38 HIV-infected participants. The concordance of quantifiable FTC-TP in DBS with 39 tenofovir (TFV)/FTC in plasma was evaluated utilizing paired plasma/DBS samples from 40 participants enrolled in 2 large pre-exposure prophylaxis (PrEP) open-label trials. The 41 time to FTC-TP undetectability after TDF-FTC dosing was evaluated utilizing DBS from 42 HIV-negative participants enrolled in a directly-observed therapy study of variable 43 adherence to TDF-FTC. 44 Results: The mean (95% CI) terminal half-life of FTC-TP in the PK study was 35 (23 to 45 47) hrs. A total of 143/163 (88%) samples obtained 0-48 hours post TDF-FTC dose had 46 quantifiable FTC-TP in DBS, compared with 2/93 (2%) and 0/87 (0%) obtained >48 and 47 >96 hours post dose. In 746 paired plasma/DBS samples from 445 participants enrolled 48 in PrEP trials, when both TFV/FTC in plasma were below the limit of quantification FTC49 TP was as well in 98.9% of samples, and when either TFV or FTC in plasma were 50 quantifiable, FTC-TP was as well in 90.5% of samples. 51 Conclusions: The half-life of FTC-TP in DBS is short relative to TFV-diphosphate 52 (TFV-DP), making it a surrogate for TFV-FTC detection in plasma. FTC-TP can be 53 on M ay 3, 2021 by gest httpaac.asm .rg/ D ow nladed fom