A study on the relationship of genetic variation and drug metabolism of an enzyme involved in the metabolism of rifampicin

Abstract

Rifampicin is a key bactericidal drug used for treatment of tuberculosis. Rifampicin has been known to be deacetylated in vivo resulting in its metabolite 25-desacetylrifampicin, but its metabolizing enzyme and its association with genetic variation is not elucidated yet. In this study, the presence of rifampicin metabolizing enzyme was verified and the genetic variation of surrogate enzyme and its association with the drug metabolism was investigated. The activity of rifampicin metabolizing enzyme was measured in human liver microsome (HLM) by measuring the amount of 25-desacetylrifampicin after incubation of various amount of the HLM for various incubation time. After verifying the existence of the enzyme in HLM, carboxylesterase 2 was hypothesized to be the enzyme responsible for the deacetylation of rifampicin. Plasma concentrations of rifampicin and 25-desacetylrifampicin were measured for 35 tuberculosis patients who had been treated with first-line antituberculosis regimen. Their DNA was extracted and sequencing of CES2 gene covering whole 12 exons, 3 UTR, 5 UTR, and intronic regions previously reported to have variations was performed. Ten variations were detected and 2 were in candidate promoter region, 5 in intron, and 3 in 3 UTR. One of the variaion in 3 UTR was novel variation. There was a clear trend of increasing or decreasing concentration of rifampicin as the number of variation increased in 6 variations. The frequency of variation was investigated by SNaPshot analysis of the 10 variation sites using DNAs from 100 healthy persons. The frequency was not different from that in NCBI or the previous reports except for the high frequency of the variation with deletion of 3 base pairs in 3 UTR. Patients with plasma rifampicin concentration ≧ 8 μg/mL had significant higher frequency of c.738A>G, c.4629A>G, c.10748G>A, and c.12027C>T variations. Reconstruced haplotype analysis showed that patients with one H9 haplotype had higher plasma rifampicin concentration. Promoter assay revealed decreased luciferase activity for c.738A>G. The well known variation in CYP3A5 gene, c.6986A>G was not associated with the change of plasma rifampicin concentration. In conclusion, variations in CES2 gene, espcecially c.738A>G may alter the metabolism of rifampicin.