In vitro bioactivation of a selective estrogen receptor modulator (2S,3R)-(+)-3-(3-hydroxyphenyl)-2-[4-(2-pyrrolidin-1-ylethoxy)phenyl]-2,3-dihydro-1,4-benzoxathiin-6-ol (I) in liver microsomes: formation of adenine adducts.

As part of our efforts to develop safer selective estrogen receptor modulators (SERMs), compound I {(2S,3R)-(+)-3-(3-hydroxyphenyl)-2-[4-(2-pyrrolidin-1-ylethoxy)-phenyl]-2,3-dihydro-1,4-benzoxathiin-6-ol} was previously identified as a lead for further development. Subsequent studies showed that compound I is genotoxic in both in vitro Chinese hamster ovary (CHO) cells and in vivo mouse studies. To better understand the possible mechanisms for the observed genetoxicity effects, in vitro incubations of I with liver microsomes of human, monkey, and mouse in the presence of adenine were performed, which led to the detection of five adenine adducts. The formation of these adducts was NADPH-dependent, suggesting the involvement of oxidative bioactivation catalyzed by cytochrome P450 enzymes. The mechanism for the formation of the major adenine adduct (A1) involves the formation of a reactive ring-opened para-quinone intermediate. The formation of four other adenine adducts may involve the formation of a reactive epoxide or ortho-quinone intermediate. Furthermore, incubations of compound I with human hepatocytes showed dose-dependent DNA damages in Comet assays. All of the above suggest that some reactive metabolites of compound I, formed through bioactivation mechanisms, have a potential to interact with DNA molecules in vitro and in vivo. This may be one of the causes of the genotoxicity observed preclinically both in vitro and in vivo. This case study demonstrated an approach using in vitro DNA trapping assays for assessing the genotoxicity potential of drug candidates.