Tamoxifen metabolites as active inhibitors of aromatase in the treatment of breast cancer

Abstract

The mechanism of tamoxifen action in the treatment of breast cancer is believed to be via active metabolites that act as potent estrogen receptor antagonists. Attempts to identify relationships between active metabolite concentrations and clinical outcomes have produced mixed results. Since anti-estrogenic effects may be brought about not only by estrogen antagonism, but also by reduced estrogen synthesis, we tested the ability of tamoxifen and its principal metabolites to inhibit aromatase in vitro. The activity of human aromatase in both recombinant and placental microsomal preparations was measured using the rate of generation of a fluorescent metabolite in the presence and absence of multiple concentrations of tamoxifen, endoxifen, N-desmethyl-tamoxifen, and Z-4-hydroxy-tamoxifen. Aromatase inhibition was further characterized by measuring the inhibition of testosterone metabolism to estradiol. The biochemical mechanisms of inhibition were documented and their inhibitory potency was compared. Using recombinant human aromatase, endoxifen, and N-desmethyl-tamoxifen were able to inhibit aromatase activity with K (i) values of 4.0 and 15.9 μM, respectively. Detailed characterization of inhibition by endoxifen and N-desmethyl-tamoxifen indicated non-competitive kinetics for both inhibitors. Similarly, endoxifen-inhibited testosterone metabolism via a non-competitive mechanism. No appreciable inhibition by tamoxifen or Z-4-hydroxy-tamoxifen was observed at similar concentrations. The relative inhibitory potency was: endoxifen > N-desmethyl-tamoxifen >>> Z-4-hydroxy-tamoxifen > tamoxifen. Similar data were obtained in human placental microsomes. Endoxifen and N-desmethyl-tamoxifen were found to be potent inhibitors of aromatase. Inhibition by these tamoxifen metabolites may contribute to the variability in clinical effects of tamoxifen in patients with breast cancer. Relationships between tamoxifen metabolite concentrations and clinical outcomes may be complex, and the biologic mechanisms that underlie these relationships may include aromatase inhibition.

The tamoxifen metabolite norendoxifen is a potent and selective inhibitor of aromatase (CYP19) and a potential lead compound for novel therapeutic agents

“…The antiestrogenic activity of the most widely used SERM tamoxifen has been thought to be mediated exclusively by antagonism of estrogen binding to the estrogen receptors [3,4]. However, recent studies have shown that important metabolites of tamoxifen, N-desmethyl-tamoxifen and endoxifen, can also act as AIs in vitro [5]. These data are consistent with the observed similarity in side effects of tamoxifen and AIs, including musculoskeletal aches [6] and hot flashes [7,8] seen with both [9,10].…”

“…In previous studies, we have shown that tamoxifen metabolites, including endoxifen and N-desmethyl-tamoxifen, can act as AIs in vitro with K i values of 4 and 15.9 lM, respectively [5]. The present studies demonstrate that norendoxifen is a potent and selective inhibitor of human aromatase with a K i value in the nanomolar range, close to the potency of the positive control used: letrozole (IC 50 of 5.3 nM) [5], which is the most potent AI that is available for clinical use.…”

Inhibition of Cytochrome P450 Enzymes by the E- and Z-Isomers of Norendoxifen

“…Among tamoxifen metabolites, 4HT and endoxifen have been shown to be SERMs, serving as antagonists of estrogen binding to the ERs (Lim et al, 2005). N-DMT and endoxifen act as AIs with IC 50 values of 6.1 and 20.7 mM, respectively, via noncompetitive mechanisms (Lu et al, 2012a). In previous studies, we have characterized the inhibitory ability of norendoxifen against recombinant CYP19, CYP2B6, CYP2D6, human liver CYP2C9, and CYP2C19.…”

“…It has a complex metabolic profile involving both active and inactive metabolites (Jin et al, 2005). Previous studies have shown that two tamoxifen metabolites, N-desmethyltamoxifen (N-DMT) and 4-hydroxy-N-desmethyltamoxifen (endoxifen), can act as AIs in vitro (Lu et al, 2012a). Recently, N, N-didesmethyl-4-hydroxy-tamoxifen (norendoxifen), another active metabolite of tamoxifen, has also been shown to be a potent AI.…”

Genetic polymorphisms of CYP2D6*10 and the effectiveness of combined tamoxifen citrate and testosterone undecanoate treatment in infertile men with idiopathic oligozoospermia

“…Tamoxifen is a trans isomer of clomiphene citrate, which is a combination of two isomers that exert both anti-estrogenic and estrogenic effects simultaneously and human studies have confirmed its estrogenic activity to be minimal or negligible (Lin et al, 2010;Lu et al, 2012). Tamoxifen citrate enhances spermatogenesis by increasing FSH, leydig cells sensibility to LH, and testosterone levels, which lacks an intrinsic oestrogenic effect, so it may be more appropriate to use in male infertility (Buvat et al, 1983;Kadioglu et al, 1999).…”