Tiamulin inhibits human CYP3A4 activity in an NIH/3T3 cell line stably expressing CYP3A4 cDNA.

Tiamulin is an antibiotic frequently used in veterinary medicine. The drug has been shown to produce clinically important interactions with other compounds that are administered simultaneously. An NIH/3T3 cell line, stably expressing human cytochrome P450 (EC 1.14.14.1) cDNA (CYP3A4), was used to study the effect of tiamulin on CYP3A4 activity. The 6 beta-hydroxylation activity of testosterone, which is increased in CYP3A4-expressing cells compared to vector-transfected cells, showed reduced activity after incubation with 1 microM tiamulin and was completely reduced to background level after incubation with 2, 5 and 10 microM tiamulin. The CYP3A4-expressing cell line was used in combination with a shuttle vector containing the bacterial lacZ' gene to study the effect of tiamulin on CYP3A4-mediated mutagenicity of aflatoxin B1. The mutation frequency of aflatoxin B1 could be completely inhibited by tiamulin in CYP3A4-expressing cells, but no effect was observed on the mutation frequency of the direct mutagen ethylmethanesulphonate. Western blotting of homogenates of the CYP3A4-expressing cell line showed stabilization of CYP3A4 protein after incubation with tiamulin, supporting the hypothesis that the mechanism of inhibition is by binding of tiamulin to the cytochrome.