Abstract
OBJECTIVES: To evaluate the effects of concomitant efavirenz-based ART and genetic polymorphism on the variability in rifampicin and 25-desacetylrifampicin pharmacokinetics. PATIENTS AND METHODS: Plasma concentrations of rifampicin and 25-desacetylrifampicin from 63 patients coinfected with TB and HIV were analysed by LC-MS/MS followed by non-linear mixed-effects modelling. Patients were genotyped for SLCO1B1 (463 C>A, 388 A>G, 11187 G>A, rs4149015, 521 T>C and 1436 G>C) and SLCO1B3 (334 T>G). RESULTS: One-compartment disposition models described the observations adequately. The oral clearances of rifampicin and 25-desacetylrifampicin were 140% and 110% higher, respectively, in patients on concomitant efavirenz-based ART. Rifampicin bioavailability was also lower in patients on concomitant ART. Further, although not included in the final model, a lower relative bioavailability in carriers of WT SLCO1B3 334 T>G compared with carriers of mutations in the genotype was estimated. CONCLUSIONS: The results presented indicate both pre-systemic and systemic induction by efavirenz-based ART affecting rifampicin pharmacokinetics. The described drug-drug interaction has a clinical impact on rifampicin exposure prior to steady state and may impact the early bactericidal activity in patients on efavirenz-based ART.