X-ray crystal structure analysis of elacestrant (RAD1901), a novel selective estrogen receptor degrader (SERD), bound to estrogen receptor alpha ligand binding domain.

e15647 Background: Antiestrogens are a mainstay of treatment for estrogen receptor positive (ER+) breast cancer in both the adjuvant and the advanced/metastatic settings. Elacestrant is a mixed activity selective estrogen receptor (SER) alpha (ERα) antagonist, acting as a SER modulator (SERM) at low doses and a SER degrader (SERD) at high doses. It has shown activity in hormone sensitive wild type (WT) ERα and insensitive estrogen receptor gene 1 (ESR1) mutation-harboring (Y537S and D538G) ERα breast cancer, both in preclinical models and in clinical studies. It also possesses a unique pharmacology compared to other competitive ER antagonists in its ability to cross the blood brain barrier. Competitive ERα antagonists are typically comprised of a core that sits in the ligand binding pocket and an arm that manipulates the structure to achieve SERM or SERD activities. In these molecules, the arm is attached in the same position as the triphenylethylene core of tamoxifen. However, elacestrant possesses a novel site of attachment. As such, we hypothesized that elacestrant adopts an alternative binding orientation in the ERα ligand binding pocket to achieve its unique pharmaceutical profiles. Methods: X-ray crystallography was used to solve a co-crystal structure of elacestrant in complex with WT ERα ligand binding domain to 2Å. Results: Overall, elacestrant promotes the formation of a canonical ERα ligand binding domain antagonist conformation, whereby helix 12 (H12) is docked into the activating function-2 cleft. However, elacestrant adopts a novel vector in the ERα ligand binding pocket that places it in close proximity to helix 12. As a result, it forms a bifurcated hydrogen bond that is not observed in other competitive antiestrogens and samples a chemical space known to increase H12 mobility and induce SERD activity. This novel vector also places it near positions 537 and 538, the two most common sites of somatic mutation. Conclusions: The high-resolution x-ray crystal structure of elacestrant suggests that the unique binding mode it adopts enables novel pharmacology and positions it to achieve potency in the WT and activating somatic ERα mutated breast cancer setting.