Tazofelone is a new
inflammatory bowel disease agent. The biotransformation of
tazofelone in human livers and the
cytochrome P450 responsible for the biotransformation has been studied. Two metabolites of
tazofelone were formed in vitro. A
sulfoxide metabolite was identified by cochromatography with authentic standards, and a
quinol metabolite of
tazofelone was identified by mass spectrometry and
proton NMR. Sulfoxidation was catalyzed by a single
enzyme system while formation of the
quinol metabolite was catalyzed by a two
enzyme system. The Km and Vmax values for sulfoxidation were 12.4 microM and 0.27 nmol/min/mg
protein, respectively. The high affinity Km and Vmax values for the formation of the
quinol metabolite were 7.5 microM and 0.17 nmol/min/mg
protein, respectively.
Tazofelone was incubated at 20 microM concentration with human microsomes to determine which of the
cytochrome P450 isozyme(s) is involved in the oxidation of
tazofelone. A strong correlation was found between the immunoquantified concentrations of
CYP3A and the rates of formation of the
sulfoxide and
quinol metabolites of
tazofelone. Similarly, significant correlations were observed between the formation of
midazolam 1'-hydroxylation and the rates of formation of both metabolites of
tazofelone. Inhibition studies have indicated that
triacetyloleandomycin, a
CYP3A specific inhibitor, almost completely inhibited the formation of both of these
tazofelone metabolites. Incubations with specific
cDNA expressed microsomes indicated that the formation of both the
sulfoxide and
quinol metabolites was highest with
CYP3A4 containing microsomes. The correlation data was confirmed by inhibition studies and
cDNA expressed
cytochrome P450 systems demonstrating that the biotransformation of
tazofelone to its metabolites is primarily mediated by
CYP3A.