Deactivation of anti-cancer drug letrozole to a carbinol metabolite by polymorphic cytochrome P450 2A6 in human liver microsomes

1. Cytochromes P450 (P450) involved in letrozole metabolism were investigated. Among 13 recombinant P450 forms examined, only P450 2A6 and 3A4 showed activities in transforming letrozole to its carbinol metabolite with small K_m and high V_max values yielding apparent V_max/K_m values of 0.48 and 0.24 nl min^?1 nmol^?1 P450, respectively.

2. The metabolic activities of individual human liver microsomes showed a significant correlation with coumarin 7-hydroxylase activities (P450 2A6 marker) at a letrozole concentration of 0.5 μM, while a good correlation was also seen with testosterone 6β-hydroxylase activities (P450 3A4 marker) at 5 μM substrate concentration with different inhibition by 8-methoxypsolaren.

3. Significantly low carbinol-forming activities were seen in human liver microsomes from individuals possessing CYP2A6*4/*4 (whole CYP2A6 gene deletion) at a letrozole concentration of 0.5 μM. A V_max/K_m value measured for CYP2A6.7 (amino acid substitution type) in human liver microsomes, in the presence of anti-P450 3A4 antibodies, was approximately seven-fold smaller than that for CYP2A6.1 (wild-type).

4. These results demonstrate that P450 2A6 and 3A4 catalyse the conversion of letrozole to its carbinol metabolite in vitro at low and high concentrations of letrozole. Polymorphic variation of CYP2A6 is considered to be relevant to inter-subject variation in therapeutic exposure of letrozole.