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 (CYP 3A4), 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 r educed activity after incubation with 1 mu M tiamulin and was complete ly reduced to background level after incubation with 2, 5 and 10 mu M tiamulin. The CYP3A4-expressing cell line was used in combination with a shuttle vector containing the bacterial lacZ' gene to study the eff ect of tiamulin on CYP3A4-mediated mutagenicity of aflatoxin B-1. The mutation frequency of aflatoxin B-1 could be completely inhibited by t iamulin in CYP3A4-expressing cells, but no effect was observed on the mutation frequency of the direct mutagen ethylmethanesulphonate. Weste rn blotting of homogenates of the CYP3A4-expressing cell line showed s tabilization of CYP3A4 protein after incubation with tiamulin, support ing the hypothesis that the mechanism of inhibition is by binding of t iamulin to the cytochrome.