Mechanism‐based discovery as an approach to identify the next generation of estrogen receptor modulators

The estrogen receptor (ER) signal transduction pathway is the primary target of the hormonal therapies used for the treatment and prevention of breast cancer. Interestingly, although animal studies performed over a century ago first suggested a link between the ovaries (estrogen production) and growth of breast tumors, definition of the molecular basis for this association and its pharmaceutical exploitation has occurred relatively recently. In the realm of breast cancer chemotherapy, compounds like tamoxifen have emerged which function as antiestrogens by directly blocking the binding of estrogen(s) to their receptors in breast tumors. The aromatase inhibitors, such as letrazole and anastrazole, inhibit estrogen production while high dose progestins attenuate ER signaling by an as yet unknown mechanism. Together, these hormonal therapies have been extremely successful and have improved both mortality and morbidity in this disease. However, as we begin to fully understand the complexities of estrogen signaling, it is clear that new classes of drugs that regulate important components of this signaling pathway are going to emerge that will increase the armamentarium of the oncologist and improve overall prognosis in this disease. Estrogen-containing medicines have been marketed in the United States for the relief of climacteric symptoms in postmenopausal women since 1942 and for contraception since 1960. However, at the time of approval, little was known about the mechanism(s) underlying the biological responses to these drugs in target tissues. In the late 1950s, Elwood Jensen and colleagues identified a high affinity intracellular binding protein (now known as ER ) that was found only in estrogen-responsive tissues (1). This was followed some years later by work from Bert O’Malley’s group which demonstrated that ER was in fact a liganddependent transcription factor, whose actions dictated the cellular response to estrogens (2). These discoveries, and those of other investigators, in the early years of molecular endocrinology enabled the development of a simple model to describe the actions of estrogens in target cells, the essence of which has stood the test of time. Specifically, these models proposed that in the absence of hormone, ER resided in cells in an inactive form that underwent a biochemical “activating” event upon binding an estrogen. The agonist-bound ER was then capable of interacting with specific DNA sequences within the regulatory regions of target genes and positively or negatively regulating their transcription. By extension, it was proposed that antiestrogens, much like competitive enzyme antagonists, functioned by binding to the hormone-binding pocket of ER, thus blocking agonist access. Interestingly, there were early indications that this elegant model and proposed mechanism of action was over simplified and did not explain the full complexity of ER pharmacology. Notably, Harper and Walpole observed in their early studies of the antiestrogen tamoxifen that whereas it functioned as a potent antagonist of estrogen action in some organs, it manifested robust agonist activity in others (3). These observations were not followed up on to any great extent until the studies of Love et al. in 1992 revealed that tamoxifen, in the context of adjuvant chemotherapy for breast cancer, exhibited a paradoxical agonist activity in bone (4). Indeed, so dramatic were the results in these bone studies that it led to the rebirth of ER as a drug discovery target, this time for drugs useful for the treatment and prevention of osteoporosis. Expectedly, the heightened interest in ER modulators for osteoporosis and the accompanying advancement in our understanding of estrogen action have also had a positive impact on the development of ER modulators for breast cancer. One of the most significant advances in drug discovery efforts aimed at identifying new ER modulators is the dramatic move in recent years from empirical to predictive mechanism-based drug screens. Traditionally, discovery programs for ER modulators used receptor-binding assays as a primary screen with the subsequent use of a “predictive” secondary assay to evaluate the relative activity of selected compounds in relevant endpoints. Compounds with desired properties were then assayed in animal models of the targeted disease for efficacy. Most, if not all, of the currently approved ER modulators emerged from programs that used this general methodology. For the identification of compounds