Plasma Pharmacokinetics and Urinary Excretion of Tenofovir Following Cessation in Adults with Controlled Levels of Adherence to Tenofovir Disoproxil Fumarate

Abstract

Objectives:

To fully characterize the plasma and urine washout pharmacokinetics of TFV in adults following 6 weeks of controlled levels of TDF adherence to inform the utility of clinic-based adherence testing.

Design:

A 3-arm randomized, open-label study in adult volunteers. Participants were randomized to receive TDF 300mg/emtricitabine (FTC) 200mg either: (i) 7 doses/week (Perfect Adherence), (ii) 4 doses/week (Moderate Adherence), or (iii) 2 doses/week (Low Adherence). Plasma and urine samples were regularly collected during the six-week dosing phase and four weeks following drug cessation.

Results:

Twenty-eight adults were included in this analysis. Median (range) age was 33 (20-49) years. No differences in TFV PK parameters during the washout were observed across arms. Small differences in TFV plasma concentrations occurred across arms between 4 to 10 hours post-dose. The cumulative amount of TFV excreted in urine was not different 24 hours post-dose, but at 148 hours was 24.8, 21.0 and 17.2 mg for the Perfect, Moderate and Low Adherence arms, respectively (p=0.043).

Conclusions:

Among adults with different TDF adherence patterns, relative differences in plasma concentrations and cumulative urine extraction of TFV were minor following cessation. TFV measurement in plasma or urine is more indicative of last drug ingestion rather than differentiating recent adherence patterns.

Introduction

Tenofovir disoproxil fumarate (TDF) is a key drug for both HIV prevention and antiretroviral treatment (ART) regimens. TDF was approved by the US Food and Drug Administration (FDA) in 2001 for the treatment of HIV/AIDS in adults, and is the backbone of preferred first-line antiretroviral therapy (ART) regimens recommended by the World Health Organization (WHO)¹. In 2012, TDF was approved in combination with emtricitabine (FTC) for use in HIV pre-exposure prophylaxis (PrEP)² in the U.S. and recommended by the WHO in 2015 for worldwide use³. TDF was developed as a prodrug of tenofovir (TFV) to improve its bioavailability. The oral bioavailability of a 300 mg dose of TDF in humans was estimated to be 25% in the fasted state and 39% in the fed state⁴. After absorption TDF is rapidly converted to TFV via esterase hydrolysis and subsequently converted intracellularly to its active form of TFV-diphosphate (TFV-DP), which inhibits reverse transcriptase activity⁵. Plasma concentrations of TFV from TDF follow a biexponential decay pattern in plasma with a terminal half-life of 17 hours⁶.

Consistent adherence to daily oral ART and PrEP regimens is critical for efficacy but can often be a major challenge. Accurate assessment of adherence to HIV prevention and treatment is difficult for health care workers. Classical methods such as self-report, pill counts, diaries, home visits, along with the more recent innovations of electronic monitoring tools (e.g. MEMS caps, Wisepill boxes, Mobile Apps) provide some level of adherence assessment but do not measure actual drug ingestion. Direct measurement of drugs in clinical samples can serve as objective markers of drug adherence, and detection of plasma concentrations of TFV or intracellular levels of TFV-DP correlate with the success or failure of TDF-based PrEP and treatment^7–10. Studies aimed at identifying adherence 'benchmarks' of TFV-moieties in various biological samples have been performed, as the HPTN066 trial¹⁴, but data in urine are limited. Directly measuring drug levels to ascertain adherence, especially in whole blood, plasma and peripheral mononuclear cell samples, is generally limited to research settings because of the time, training and costs associated with sample collection, processing, and bioanalytical testing.

A point-of-care (POC) tenofovir test that can provide real-time monitoring for health care workers and subsequent feedback to patients would be a major step forward to help with the clinical management and care of individuals receiving PrEP or ART. Urine collection is non-invasive and is widely employed for screening of other drugs. Tenofovir is primarily excreted in the urine both via glomerular filtration and active tubular secretion following TDF administration. Thus, a urine-based POC test to detect or semi-quantify TFV could provide useful information on the recent dosing patterns of TDF and help identify individuals who may benefit from additional adherence counselling or other interventions.

Data describing the pharmacokinetics of TFV in urine among adults with different patterns of adherence to TDF are needed to guide the development and interpretation of a TFV-based POC assay. To this end, we conducted a randomized pharmacokinetic trial to fully characterize the plasma an ¹ urinary pharmacokinetics of TFV in adults who received directly-observed therapy with TDF simulating patterns of adherence associated with different levels of PrEP efficacy. Herein, we report data on concentrations of TFV in plasma and urine excretion of TFV after cessation of TDF/FTC among adults with controlled levels of adherence.

Methods

Study Design

The TARGET study (Tenofovir Adherence to Rapidly Guide and Evaluate PrEP and HIV Therapy) was a single center, randomized, open-label, pharmacokinetic study conducted among healthy adult volunteers (ClinicalTrials.gov #NCT0301260). All participants were enrolled at Sanpatong Hospital in Chiang Mai, Thailand and signed written informed consent prior to the study. A paper solely describing the TARGET clinical trial protocol has been previously published¹¹. Briefly, adults aged 18-49-years-old were eligible if they were HIV-uninfected, hepatitis B surface Ag (HBsAg) negative, and had normal renal function [defined as an estimated glomerular filtration rate (eGFR) >60 mL/minute by the Cockcroft-Gault equation] within 14 days of enrollment. Participants were excluded if they were pregnant, had been using PrEP or were eligible to receive PrEP, or had a Grade ≥3 abnormality in neutrophil count, hemoglobin, platelets, aspartate aminotransferase, or alanine aminotransferase (Defined by Division of AIDS Table for Grading the Severity of Adult and Pediatric Adverse Events, Version 2.0, Nov. 2014). The study protocol and consent documents were approved by Ethics Committees at the Institute for the Development of Human Research Protections at the Medical Sciences Department, Thai Ministry of Public Health (IHRP), the Ethics Committee of Sanpatong Hospital, the Faculty of Associated Medical Sciences, Chiang Mai University and the University of Washington Institutional Review Board in Seattle.

At entry, participants were randomized (1:1:1) to receive one of three directly-observed dosing schedules of TDF 300mg/emtricitabine (FTC) 200mg (Truvada®, Gilead Sciences) for six weeks (with the last dose at the start of week 7): (i) ‘Perfect’ Adherence: daily dosing, (ii) ‘Moderate’ Adherence: 4 doses/week (Monday, Wednesday, Friday, and Saturday), or (iii) ‘Low’ Adherence: 2 doses/week (Monday and Thursday). A total of 30 participants (10 per study arm) was planned to account for potential loss to follow-up or withdrawals. All participants started TDF/FTC in the morning of their entry visit, received their last dose at the start of Week 7 and were followed for a further four weeks during the drug washout period. Multiple adherence monitoring methods were implemented to ensure strict compliance to the randomized TDF dosing schedule. Directly observed therapy (DOT) was performed at the study clinic for each study drug intake on weekday mornings. On weekends, DOT was performed via video call or mobile phone application and an electronic Wisepill box (Wisepill Technologies Inc., Somerset West, South Africa) provided to each participant to electronically monitor the opening of their pill box.

Multiple clinical samples, including whole blood, plasma, urine, oral fluid, red blood cells, and peripheral blood mononuclear cells (PBMCs), were collected over the 10-week study period, with the sample collection schedule described in detail in the protocol publication¹¹. Sample collection included both intensive plasma blood sampling and collection of multiple urine samples. Here we report data on concentrations of TFV in plasma and urine samples after the last dose of TDF/FTC administered at the start of week 7. Specifically, one plasma sample was drawn at week 7 pre-dose. A single TDF/FTC tablet was then administered on an empty stomach (at least six-hour fast beforehand). Venous blood was then collected at 1, 2, 4, 6, 8, 10, 12, and 24, 48, 72, 96, 120, 144, 168, 240, and 336 hours post-dose. Urine samples were collected over the intervals of 0-4 hours, 4-8 hours, 8-12 hours, and 12-24 hours following the administered TDF/FTC dose. After the 24 hours, the participant was discharged from the hospital and was asked to collect a 24-hour urine sample in a single container over the next 6 days and bring each of these samples to the clinic each morning. Renal and liver function tests for safety evaluations were performed at baseline, week 3, week 5, week 7, and week 10.

Measurement of Tenofovir Plasma and Urine Drug Concentrations

All plasma and urine samples were stored at −70°C until analysis. TFV concentrations were measured using validated liquid chromatography-mass spectrometer (LC-MS/MS) assays over the concentration ranges of 3 to 2,500 ng/mL in plasma and 50 to 50,000 ng/mL in urine. Each LC-MS/MS assay was validated in accordance with guidelines on method validation from the U.S. National Institutes of Health (NIH)’s Clinical Pharmacology Quality Assurance (CPQA) Program which are based on the US Food and Drug Administration Guidance for Industry Bioanalytical Method Validation recommendations^{12, 13}. The plasma concentrations were analyzed in the AMS-PHPT pharmacology laboratory at Chiang Mai University and the urine levels analyzed in the University of California San Francisco (UCSF) Hair Analytical Laboratory. Both laboratories participate in external quality control programs through their participation in the NIH-supported HIV clinical trial networks.

Statistical Analyses

The primary outcomes were descriptive pharmacokinetics endpoints. A non-compartmental pharmacokinetic analysis was performed using Phoenix WinNonLin (Certara, Princeton, NJ, USA) on the plasma TFV concentration data. Calculated pharmacokinetic parameters included area-under-the-curve (AUC_τ), maximum plasma concentration (Cmax), time to Cmax (Tmax), apparent oral clearance (CL/F), and apparent volume of distribution (Vd/F). In a 300 mg dose (tablet) of TDF there is a 136 mg dose of TFV and this was used in all calculations of TFV pharmacokinetic parameters. Plasma and urine TFV concentrations below the LLOQ were excluded for the calculation of pharmacokinetic parameters. AUC_τ was determined using the log-linear trapezoidal method Cmax, Cmin and Tmax were taken directly from the observed concentration-time data. CL/F was calculated as dose/AUC_τ. The terminal slope λz was determined from the log- linear portion of the curve and, when appropriate, the half-life calculated as 0.693/λz. The total amount of TFV excreted (Ae) was calculated by the summation of drug excretion (urine concentration multiplied by urine volume) during the urine collection period. The fraction of TFV excreted in the urine (Fe) was determined as the Ae divided by the administered dose. Renal TFV clearance (CL_R) was calculated by the Ae_0-t/AUC_0-t ratio. Median (range), means (standard deviations), and geometric means with 95% confidence intervals for each pharmacokinetic parameter were calculated. Non-pharmacokinetic statistical analyses were performed using Stata software (Version 11.0, StataCorp LP, Texas, USA). For the comparison of raw TFV plasma concentrations samples at specific time-point, samples with concentrations below the LLOQ were imputed as LLOQ/2. Tenofovir concentrations and pharmacokinetic parameters were compared between the three study arms using an ANOVA test on log-transformed parameters.

Results

The CONSORT diagram of recruitment, randomization and follow-up for the TARGET study was presented as part of our report describing TFV plasma and urine concentrations in our study at steady-state¹⁴. Briefly, 31 of 32 adults screened were randomised to either Perfect (n=11), Moderate (n=10) or Low (n=10) simulated TDF Adherence (one adult was not eligible due to HBsAg positivity). Three subjects enrolled were excluded: one subject in the Perfect Adherence arm withdrew due to their inability to comply with the visit schedule, and two subjects due to confirmed lack of compliance with drug administration [one in the Perfect and one in Low Adherence arm]. Among the remaining subjects no doses were missed and all intakes were documented via DOT. All 28 adults (57% male) included were Asian and the baseline characteristics by adherence arm are shown in Table 1. There was one serious adverse event reported, a dengue infection, which was considered unrelated to study participation and resolved with appropriate medical care. No other significant adverse events were reported.

Table 1:

Baseline Characteristics

	TDF Adherence Arms			Overall
	Low (n=9)	Moderate (n=10)	Perfect (n=9)	(n=28)
Male:Female	5:4	8:2	3:6	16:12
Age, Years	38 (21-45)	32 (20-49)	34 (25-44)	33 (20-49)
Body Weight, kg	67 (47-84)	67 (48-96)	52 (43-70)	61 (43-96)
Body Surface Area, m2	1.8 (1.4-2.0)	1.7 (1.5-2.2)	1.5 (1.4-1.8)	1.7 (1.4-2.2)
Hemoglobin, g/dL	14.7 (9.9-15.4)	14.5 (12.6-16.2)	12.4 (10.1-15.8)	14.1 (9.9-16.2)
Hematocrit, %	43 (33-47)	44 (39-48)	38 (32-48)	42 (32-48)
Lymphocytes, %	33 (29-42)	29 (19-38)	30 (21-35)	32 (19-42)
BUN, mg/dL	10.5 (5.7-14.6)	10.4 (6.3-14.1)	9.3 (4.5-15.8)	10.3 (4.5-15.8)
Creatinine (plasma), mg/dL	0.86 (0.59-1.2)	0.84 (0.54-1.1)	0.73 (0.49-0.96)	0.82 (0.49-1.2)
eGFR a, mL/min	108 (60-126)	124 (97-149)	91 (83-116)	108 (60-149)

Values: median (range); BUN: Blood urea nitrogen.

^aestimated glomerular filtration rate per Cockcroft-Gault equation; TDF adherence arms: Perfect Adherence: 7 dose/week; Moderate Adherence: 4 doses/week; Low Adherence: 2 doses/week.

Plasma TFV Pharmacokinetics following Cessation by TDF Adherence Arm

The mean plasma concentrations time curves following the last dose of TDF by Adherence arm are shown in Figure 1 and a summary of the pharmacokinetic parameters reported in Table 1. Pre-dose plasma concentrations prior to the last dose were influenced by the study arm as the time of the penultimate dose was dependent on the randomization arm. The absorption of TFV following TDF administration was similar between adherence arms (Tmax 1.0 hour), with no significant difference in Cmax (Table 1). No significant differences in plasma PK parameters, AUC_0-24, AUC_0-τ, AUC_0-inf, CL/F or terminal half-life, were observed when comparing across all three arms. Following peak TFV concentrations, although small in terms of real concentrations, significant differences (p<0.05) in TFV plasma concentrations were observed at 4, 6, 8 and 10 hours post-dose across the arms during the 24h-intensive PK sampling (Figure 1).

Figure 1.

Median (range) tenofovir concentrations in plasma in adults following the cessation of TDF/FTC after 6 weeks of controlled TDF/FTC adherence in the Perfect (daily), Moderate (four times weekly), and Low (two times weekly) Adherence arms. LLOQ: lower limit of quantification, 3.0 ng/mL. *p<0.05 across three study arms

Urinary Excretion of TFV following Cessation by TDF Adherence Arm

The mean cumulative urinary excretion of TFV following the last dose of TDF by adherence arm is shown in Figure 2. The cumulative amount of TFV excreted over the first 24 hours was not significantly different by arm (p=0.051), while differences were observed at 48 (p=0.016) and 144 (p=0.043) hours post-dose. After 148 hours, the cumulative amount of TFV excreted was 24.8, 21.0 and 17.2 mg for the Perfect, Moderate and Low Adherence arms, respectively (Table 2). For each of the Adherence arms, approximately 65% of the total amount of TFV excreted occurred within the first 24 hours, increasing to 85% after 48 hours (Figure 2). The CL_R of TFV was similar between arms (Table 2) with an overall geometric mean (95% CI) of 110 (100-120) mL/min.

Figure 2.

Mean cumulative excretion of tenofovir in urine following the last dose of TDF/FTC in adults who had followed six weeks of a Perfect (daily), Moderate (four times weekly), or Low (two times weekly) Adherence dosing regimen. *p<0.05 across three study arms

Table 2.

Plasma and urine pharmacokinetic parameters for tenofovir by adherence arm.

	TDF Adherence Arm
Plasma pharmacokinetics	Low (N=9)	Moderate (N=10)	Perfect (N=9)	P-value
AUC0-24 (ng*h/mL)	1,924 (1,554-2,384)	2,053 (1,746-2,414)	2,526 (1,881-3,392)	0.153
AUC0-τ (ng*h/mL)a	2,649 (2,101-3,341)	2,587 (2,195-3,050)	2,526 (1,881-3,392)	0.948
AUC0-inf (ng*h/mL)	2,778 (2,168-3,559)	3,037 (2,554-3,612)	3,766 (2,875-4,932)	0.116
CL/F (mL/min)	856 (678-1,079)	876 (743-1,033)	897 (668-1,205)	0.948
Cmax, ng/mL	429.5 (178-670)	362.8 (226-541)	395.7 (176-686)	0.703
Tmax (hr)	1.0 (1.0-2.0)	1.0 (1.0-2.0)	1.0 (0.9-4.2)	-
Half-life (hr)	20 (11-34)	19 (16-34)	23 (18-28)	0.702
Urine pharmacokinetics
Ae (0-24 hr) (mg)	11.0 (8.7-13.8)	13.5 (11.2-16.2)	15.8 (12.1-20.7)	0.051
Ae (0-48 hr) (mg)	14.1 (11.0-18.1)	18.1 (15.6-21.1)	21.4 (16.8-27.2)	0.016
Ae (0-148 hr) (mg)	17.2 (13.4-22.2)	21.0 (18.0-24.5)	24.8 (19.5-31.7)	0.043
CLR (mL/min)	103 (82-130)	115 (104-128)	110 (91-132)	0.601
CLR (mL/min/m2)	59.9 (45.7-78.5)	65.0 (55.7-75.9)	70.5 (57.9-86.0)	0.473

Values are median (range), except for AUC and CL/F which are geometric mean (95% CI);

^aAUC_0-τ was calculated as 24 hours for Perfect adherence arm, 48 hours for moderate adherence arm, and 96 hours for low adherence arm; C_max: Maximum concentration; Tmax: time to maximum concentration; Ae: Cumulative amount of drug excreted in urine (over 6 days following last dose); CL_R: renal clearance; CL/F: Oral clearance.

Discussion

We present the first study to fully characterize the washout urinary excretion of tenofovir in adults receiving TDF under different simulated patterns of adherence with directly-observed therapy dosing. Among adults provided different patterns of dosing of TDF for 6 weeks simulating high, moderate and low adherence, no differences in TFV PK parameters during the washout were observed across arms. Although small differences in plasma concentrations and cumulative urine extraction of TFV following cessation were observed at certain time-points following cessation, the measurement of TFV in plasma or urine is more indicative of last drug ingestion rather than being able to differentiate recent adherence patterns.

Direct measurement or detection of antiretroviral drug concentrations in biological samples can provide an objective measurement of medication consumption and in turn, act as a surrogate for long-term adherence patterns^{15, 16}. To date, different drug moieties have been utilized as surrogates with the prodrug of TDF, with short-half-life moieties -such as tenofovir in plasma - inferring recent adherence, and long-half moieties - such as tenofovir-diphosphate (TFV-DP) in dried blood spots (DBS) and TFV in hair - inferring cumulative adherence^{17, 18}. Quantification of these drug moieties have been performed using LC-MS/MS-based methodologies, which, while accurate, are relatively complex and costly, thereby largely restricting their utility to research studies¹⁹. In order to enable drug level measurement in more routine clinic-based settings, alternative assays are needed. Our research goal has been to develop cheap, user-friendly, rapid, POC assays to detect drugs in biological specimens. LC-MS/MS-based assays are not rapid which is why an antibody-based (immunologic) assay has been sought and developed. While a rapid assay to quantify TFV-DP in DBS or TFV in hair would be preferable in order to measure long-term adherence, sample collection/processing for intracellular or hair-based quantification make this a major challenge for rapid clinic-based testing. Thus, as a first step, a POC assay to measure TFV in either plasma or urine is being developed, with urine being the preferred biologic sample²⁰.

Among adults in the Perfect Adherence arm of TARGET (e.g. steady state), the plasma AUC_0-24, maximum and minimum concentrations of TFV were similar to those observed in other studies performed in adults at steady state, including those in Asian²¹ and non-Asian adults^{22, 23}. Moreover, a study describing plasma concentrations of TFV following cessation of TDF among adult volunteers in the U.K. at steady-state following standard 300 mg once daily dosing reported an AUC_0-24 of 2,573 ng.hr/mL, comparable to 2,526 ng.hr/mL in our study, although we observed a higher Cmax (227 vs. 396 ng/mL)²⁴. We have previously shown that the pre-dose plasma and spot urine concentrations at steady state are different among the different adherence arms¹⁴. In this study we showed that, following cessation of TDF, differences in plasma pharmacokinetic profiles in the immediate post-dose period, including Cmax, were minimal across the three arms. This finding is consistent with demonstrating no observed differences in total plasma drug exposure as measured by AUC_0-24, AUC_0-τ and AUC_0-inf during the washout period in this study. In sum, these data indicate that, following a TDF 300 mg dose, plasma concentration profiles are comparable among individuals receiving their last TDF dose 24 or 96 hours beforehand. While differences in TFV plasma concentrations do exist at certain time points, the ability to set thresholds that could accurately and reproducibly distinguish between different adherences patterns will be extremely challenging for this short-term adherence assay, such as a POC TFV-based plasma test.

While the plasma pharmacokinetics of TFV have been relatively well defined, the urinary excretion of TFV in adults with different levels of adherence are not. TFV is primarily excreted unchanged in the urine via glomerular filtration and active tubular secretion following TDF administration²⁵. Initial studies of TFV following IV administration (1 mg/kg, intravenous) reported a renal clearance of 161 mL/hr/kg (or approx. 188 mL/min/70 kg) and 70–80% of the dose was recovered in the urine within 72 hours after administration. The oral bioavailability of tenofovir after a 300 mg TDF dose is approximately 25% in the fasted state²⁶. Among adults with a plasma creatinine clearance (CL_CR) between 12 to 101 mL/min who received a single oral 300 mg dose of TDF, the renal clearance of TFV ranged from 10.6 to 275 mL/min²⁵. The cumulative amount of TFV excreted in urine ranged from 20 to 22 mg across subjects with normal renal function (CL_CR >80 mL/min), to severe renal impairment (CL_CR R 10–29 mL/min). In our study the cumulative amount of TFV excreted in urine (0-148 hr) was similar to that in previously-reported data but, as perhaps expected, slightly different between adherence groups (25 mg Perfect vs. 17 mg Low adherence).

The mean renal clearance of TFV was also similar across the three adherence arms (range across all groups: 82 to 132 ml/mL). However, renal clearance values in our study were lower than those in adults with ‘normal’ renal function of 246 ml/min defined by a previously-reported study N=3²⁵. Other studies have reported a range of renal clearance values of TFV. In 30 HIV-infected adults on TDF paired with either a non-nucleoside reverse transcriptase inhibitors (NNRTIs) or lopinavir/ritonavir (LPV/r), the overall TFV renal clearance was 188 mL/min; and was 17.5% slower with LPV/r compared to NNRTIs after adjusting for renal function²⁷. A study in HIV-infected Thai adults with renal impairment found that the total oral clearance of TFV was reduced with concomitant LPV/r use when compared to NNRTIs²⁸. In HIV-infected adolescents the renal clearance of TFV was 120 (21–230) mL/min/m² when TDF was administered with efavirenz but 75.3 (25–148) mL/min/m² when coadministered with atazanavir/ritonavir. The renal clearance of TFV at steady-state among 18 HIV-infected children between 10 to 16 years old ranged from 59–197 mL/min/m²⁹. The renal clearance in the present study is towards the lower end of the range observed in these previous adult and pediatirc studies and the reasons for this are unclear as differences in study design (single or multi dose), or duration of urine sample collections (i.e. over 24 hours or until last detectable level) are unlikely to fully explain the observed differences.

In conclusion, our study reveals that urine-based TFV measurements, like plasma-based metrics, are short-term metrics of adherence and largely provide information on the timing of the last dose taken. A major step in the development of a POC drug test for short-term metrics, defining the thresholds of detection, which in this context would differentiate time since last dose taken. This study has provided a detailed description of the plasma pharmacokinetics and urinary excretion of TFV following cessation among adults receiving different controlled levels of TDF. Using the urine data generated in TARGET we are developing and validating a urine based POC test to detect TFV and develop an appropriate threshold based on ‘time since last TDF intake’. We have recently published data reporting a novel immunological-based assay to quantify TFV in urine³⁰, which in turn has been transferred into an immunochromatographic (IC) or lateral flow assay (LFA) strip test²⁰. The initial data of this IC strip in diverse populations are encouraging³¹, but will need to be field tested among individuals on TDF-based PrEP or ART to understand the acceptability of the test by health care professionals and patients and to estimate its impact on both monitoring and supporting long-term adherence.

Highlights.

• Tenofovir disoproxil fumarate (TDF) is a key drug for HIV prevention and treatment

• Maintaining adherence to TDF is critical for efficacy but can be challenging

• A urine Point-of-care (POC) to detect tenofovir could help clinical management

• Development of a urine POC test requires fully characterizing urinary tenofovir excretion

• Among adults with three different adherence patterns differences in plasma and cumulative urinary TFV excretion were minor following TDF cessation, suggesting measurement is more indicative of last drug ingestion, rather than prior dose patterns