Besides undergoing O-demethylation in vivo, the triarylethylene
antiestrogen nitromifene [1-(4-(2-pyrrolidinylethoxy)phenyl)-1-(4-methoxy)-phenyl-2-phenyl- 2- nitroethene, 1] undergoes biotransformation via nitroreduction,
ethene bond cleavage, and
pyrrolidine ring oxidation affording
ketone metabolites 2 and 3 and a
lactam metabolite 4.
Estrogen receptor (ER) affinities of 1, 2, and 4 were, in turn, 1.7, 0.1, and 3.8% that of
estradiol in MCF 7 human
breast cancer cells, and these compounds inhibited by 50% the proliferation of MCF 7 cells at respective concentrations of 1.1, 5.6, and 2.0 microM. The inhibitory effect of 4 was fully reversible by
estradiol, but that of 2 was only partially reversible. Also 3, which did not interact with ER, inhibited proliferation by 44% at a concentration of 10 microM. These results suggested that in contrast to 4, the effects of 2 and 3 were due in part to interaction with sites distinct from ER.
Antiestrogen binding sites and
calmodulin have been suggested to mediate antiproliferative effects of drugs. Interaction of
ligands with the former sites has been proposed to antagonize the growth promoting effect of
histamine. Although 2 and 3 had high affinities for these sites, their inhibitory effects on MCF 7 cell growth were largely unaffected by the presence of
histidine, the source of intracellular
histamine. Thus, the relationship between
antiestrogen binding site affinity and antiproliferative effects of 2 and 3 was not clarified. In contrast, MCF 7 cell growth suppression potencies paralleled
calmodulin antagonist potencies of 1 and 2 suggesting that interaction of 1 and 2 with
calmodulin may contribute to their anticancer effects.