Optimization of HIV and tuberculosis co-treatment in Tanzanian patients : emphasis on pharmacogenetics and drug interactions

Abstract

Treatment of HIV and its comorbidities particularly tuberculosis is complicated by wide inter-individual variations in drug exposure due to drug-interactions and variable expressions of drug metabolizing enzymes and transporters. Efavirenz in particular, is an antiretroviral characterized by wide inter-individual variation in exposure. Unfortunately, it also has narrow therapeutic range resulting into considerable proportions of patients with sub-therapeutic or supra-therapeutic concentrations. This thesis describes sources of variation in EFV plasma levels and evaluates its enzyme induction and hepatotoxicity; so as to inform appropriate dose optimization.

We recruited HIV infected patients with tuberculosis (arm2) and without (arm1) both in Tanzania and Ethiopia. Patients took EFV 600mg based antiretroviral therapy (HAART) and for arm2, rifampicin (RIF) based anti-TB was initiated 4 weeks before HAART. They were followed for 48 (arm1) and 52 (arm2) weeks to collect information on plasma efavirenz exposure (week 4 and 16), immunological and virological outcomes (weeks 0, 12, 24, 36 and 48), liver and renal functional tests and complete blood count (weeks 0, 2, 4, 6, 8, 12, and monthly thereafter) and genotyping for SNPs in EFV metabolizing enzymes CYP2B6*6 c.516G>T, CYP3A5 (*3, *6, *7), UGT2B7 -372G>A and drug transporters ABCB1 (c.3435C>T and c4036A>G) and SLCO1 (388A>G and 521T>C).

Among arm1, patients with CYP2B6 homozygous wild type (fast metabolizers) had significant decrease in mean EFV plasma level between week 4 and 16. Consequently, a significantly large proportion these patients had sub-therapeutic plasma level at week 16 compared to week 4, indicating prolonged auto-induction in these individuals.

Among arm2, only fast metabolizers had significantly lower efavirenz level, 4 weeks after HAART, compared to their counterpart in arm 1 implying that rifampicin induction occurred only in these individuals. Due to prolonged auto-induction in arm1 fast metabolizers, plasma levels between arm1 and arm2 were comparable at week 16 even when stratified by genotype implying that CYP2B6 genetic polymorphisms, but not rifampicin, influence efavirenz exposure after prolonged treatment.

ABCB1 4036A>G SNP was associated with higher EFV levels at week 4 while CYP3A5 (*3, *6, *7) alleles combined, were partially associated with variability in efavirenz metabolic ratio changes between week 4 and 16 among patients with CYP2B6*6/*6 genotype (poor metabolizers).

Ethiopians, even after controlling for genetic and other differences, had lower efavirenz exposure and lower immunological outcomes compared to Tanzanians.

Efavirenz induction of CYP3A4/5 among arm1 was highest (about 5 times) in poor and lowest (about 2 times) in fast metabolizers. After completion of TB therapy, induction dropped to 60% of its maximum, suggesting continued but lower induction of CYP3A4/5 by efavirenz.

EFV with or without rifampicin was associated with mild and transient elevation of liver enzymes. Only CYP2B6*6/*6 genotype and hepatitis C co-infection were associated with such elevations, suggesting that efavirenz is safe for both treatments but caution and monitoring of plasma levels among poor metabolizers and those co infected with hepatitis C should be exercised.

Time on therapy (with and without rifampicin co treatment), CYP2B6c516G>T, ABCB1c4036A>G, ethnicity and CYP3A5 (*3, *6, *7) alleles combined, can be used as priori for Bayesian estimation of individual pharmacokinetic parameters during dose adjustments.

The SNP, CYP2B6c516G>T, influences induction of CYP3A4/5 in gene dose dependent manner, therefore it should be considered during dose optimization of concomitant drugs taken with efavirenz.

It may be necessary to lower doses for concomitant CYP3A4/5 substrate drugs, whose doses were elevated during HIV/TB co treatment, after completion of TB therapy if such drugs have narrow therapeutic range.