No relationship between drug transporter genetic variants and tenofovir plasma concentrations or changes in glomerular filtration rate in HIV-infected adults.

Host genetics of drug transporters located in the renal tubule may contribute towards tenofovir disoproxil fumarate (TDF)-associated kidney disease susceptibility1, 2. Genetic variants in the ATP-binding cassette (ABC) transporters ABCC2 (1249 G>A) and ABCC4 (3463A>G) have been associated with a greater decline in creatinine clearance (CrCL) over 96 weeks of TDF-based treatment3. High plasma tenofovir (TFV) trough concentrations have been correlated with a decrease in glomerular filtration rate4. A recent study in Thailand reported that tenofovir ‘mid-dose’ plasma concentrations were significantly lower in ABCC2 -24 CT/TT carriers compared to CC carriers after 24 weeks of TDF/3TC/EFV treatment and independently associated with decreased glomerular rate after 48 weeks5. We investigated the association between drug transporter genetic polymorphisms implicated in tenofovir excretion and/or associated nephrotoxicity with TFV plasma concentrations and changes in kidney glomerular filtration rate in antiretroviral naive HIV-infected adults initiating TDF as part of a NNRTI-based regimen. This was a retrospective analysis of adults enrolled in an observational cohort study in Thailand [NCT00433030]. Criteria for treatment initiation were: CDC clinical stage B/C or CD4 <250 cells/mm3. Tenofovir-DF was prescribed 300 mg once-daily. Patients were follow-up monthly during the first 3 months of treatment and then 3-monthly thereafter. A single random blood sample was collected at each visit and plasma frozen at -70°C. The exact time of last drug intake and blood draw were recorded. Creatinine clearance was calculated using the Cockcroft–Gault equation to estimate glomerular filtration rate (eGFR). Local institutional review boards approved the protocol and signed informed consent was obtained from all subjects. Human genomic DNA isolated from EDTA cell pellets stored at −20°C. Ten Single Nucleotide Polymorphisms (SNPs) linked with TFV excretion and/or associated nephrotoxicity were genotyped: ABCC2: -24C>T (rs717620), 1249G>A (rs2273697), 3563 T>A (rs8187694), 3972C>T (rs3740066) and 4544 G>A (rs8187710); ABCC4: 3463 A>G (rs1751034); ABCC10: G>A (rs9349256), 2843 T>C (rs2125739) and SLC22A2: -1604 T>C (rs3127573), G>A, (rs316009). Primers/probes were designed by the TaqMan® “Assays-by-Design®” SNP Genotyping Assay Service (Applied Biosystems, CA, USA) and SNPs identified using Real-time PCR. Plasma TFV concentrations were measured using a validated reversed-phase high-performance liquid chromatography assay6. This assay was internally validated and the average accuracy was 99–102% and precision (inter/intra-assay) was <5% of the coefficient of variation. Population means and variances of TFV pharmacokinetic parameters were estimated using non-linear mixed effects regression (Monolix v4.2, http://www.lixoft.eu)7. Individual patient characteristics including weight, sex, age, body mass index, serum creatinine and CrCL were evaluated for their inclusion in the model using a stepwise forward inclusion and backward elimination procedure. Validity of the model was evaluated using a visual predictive check. Post-hoc subject specific pharmacokinetic parameters were used to estimate individual TFV trough concentrations (C24) and area-under the concentration time curve (AUC0-24h). With ~240 subjects, we had 90% power to detect a difference in TFV C24 of 0.02 μg/mL between combined variants (homo- and heterozygous) and homozygosity of the common allele provided at least 6% (~14 subjects) were in the combined variant group (using a two-sided test, with a standard deviation of 0.02 μg/mL). Differences in TFV C24 between variants (homo- and heterozygous) and homozygosity of the common allele were assessed by Wilcoxon rank-sum tests (due to non-normal C24 distribution). To control the false discovery rate (FDR) q-values were determined8. CrCL was assumed normally distributed so t-tests were used to compare differences in change in CrCL between variants and homozygosity of the common allele (adjusted for baseline CrCL level). All statistical tests performed were two-sided using STATA (v11.1).