Exploiting Our Contemporary Understanding of the Molecular Pharmacology of the Estrogen Receptor to Develop Novel Therapeutics

The estrogen receptor (ER/ESR1) is expressed in the majority of breast and gynecological cancers. As such, drugs that inhibit ER signaling are the cornerstone of pharmacotherapy for these malignancies. Treatment strategies include the Selective Estrogen Receptor Modulator (SERM) tamoxifen, which acts as a competitive antagonist, and aromatase inhibitors (AIs) drugs that inhibit the enzyme responsible for the production of 17- estradiol (E2), the most biologically important estrogen. However, the clinical utility of these treatment strategies are limited by the development of de novo and acquired resistance. The mechanisms underlying resistance to these endocrine therapies are varied and complex include activating genomic alterations in ER (amplification, translocations, and mutations), cell cycle dysregulation and activation of alternative growth factor signaling pathways. Interestingly, it has been observed that ER signaling remains engaged and targetable in the majority of these tumors at all stages of disease. As such, the selective estrogen receptor downregulator (SERD) fulvestrant, which is both a competitive antagonist and downregulator of ER, is often used to treat tumors progressing on AIs or tamoxifen. However, the unfavorable pharmacokinetic properties of this drug have largely limited its use as a monotherapy creating a need for additional ER-modulators.

The field has put much effort into developing orally bioavailable, next-generation SERDs to replace fulvestrant in advanced breast cancer. However, many early efforts to optimize compounds for their degradation activity has not yielded clinically useful drugs. Notwithstanding issues related to drug exposure which may have impacted efficacy there is significant data to suggest that “antagonist activity” is the primary driver of SERD efficacy. To address the need to replace or optimize fulvestrant therapy for advanced breast cancer we undertook both unbiased and biased approaches to define new therapeutic strategies that target ER.

In the first set of studies, we investigated the impact of mutations in ESR1, which occur in metastatic lesions, may have on receptor pharmacology. Specifically, activating point mutations within the ligand binding domain (LBD) of ESR1 have presented as a mechanism of acquired resistance to AIs in metastatic breast cancer; as well as in both de novo and acquired resistance in primary gynecological cancers. Interestingly, these mutations are also resistant/partially resistant to many clinically relevant SERMs and SERDs, including tamoxifen and fulvestrant. Therefore, we undertook a study to elucidate the molecular mechanism(s) underlying ESR1 mutant pharmacology in relevant models of breast cancer. These studies revealed, unexpectedly, that the response of ESR1 mutations to various ligands was dictated primarily by the relative coexpression of ERWT in cells. Specifically, altered pharmacology was only evident in cells in which the mutants were overexpressed relative to ligand-activated ERWT. Importantly, while undertaking an unbiased approach to evaluate all clinically relevant antagonists for activity on the ESR1 mutants, we made the serendipitous discovery that the antagonist activity of the SERM lasofoxifene was not impacted by mutant status. This finding has led to its clinical evaluation as a treatment for patients with advanced ER-positive breast cancer whose tumors harbor ESR1 mutations, with additional studies in patients with gynecological cancer patients likely to be undertaken in the near future.

In addition to the unbiased approach outlined above we also approached the problem of resistance taking a candidate approach to evaluate structurally distinct SERDs, as monotherapy and in combination with CDK 4/6 inhibition, in relevant models of advanced breast cancer. G1T48 is a novel orally bioavailable, non-steroidal small molecule antagonist that we demonstrated both in vitro and in vivo has the potential to be an efficacious oral antineoplastic agent in ER positive breast cancer. While G1T48 can effectively suppress ER activity in multiple models of endocrine therapy resistance, this compound still displayed partial resistance to the ERmuts.

Together, our data supports the hypothesis that novel compounds targeting ER should be optimized based on antagonist potential and not on degradative activity per se. As such, the results of these studies will inform the development of next-generation therapeutics for endocrine therapy resistant cancers, especially those harboring ESR1 mutations.