Abstract Fri077: Selective Estrogen Receptor Degraders (SERDs) Induce Bradycardia by Modulating Nuclear Estrogen Signaling

Introduction: Selective estrogen receptor degraders (SERDs) are emerging as a promising therapeutic strategy for estrogen receptor-positive (ER+) breast cancer. However, clinical trials of next-generation oral SERDs, such as giredestrant and camizestrant, have reported a dose-dependent reduction in heart rate (sinus bradycardia) in treated patients. Whether this bradycardia occurs due to on-target effects on nuclear estrogen receptor (ERα) signaling, the non-canonical G-coupled estrogen receptor (GPER), or other off-target pathways remains unknown. Hypothesis: We hypothesized that SERDs induced bradycardia is due to an on-target effect on estrogen signaling. Aims: We aimed to investigate the molecular mechanisms of SERD-induced bradycardia in an embryonic zebrafish model. Methods: We used a chemical biology approach, coupled with genetic mutation of gper and the nuclear estrogen receptors esr1 (zebrafish homolog of human ESR1) and esr2a/esr2b (zebrafish homologs of human ESR2), to investigate the mechanisms contributing to bradycardia induced by SERDs in zebrafish embryos. For RT-qPCR, an unpaired Student’s t-test compared two groups, while one-way ANOVA with Tukey’s post hoc test was used for heart rate analysis. A p-value of <0.05 was considered statistically significant. Results: Consistent with clinical data in patients, wild-type (WT) zebrafish embryos exposed to giredestrant and camizestrant exhibited a decrease in heart rate compared to controls, whereas those exposed to SERDs that have not been associated with bradycardia in patients (i.e., fulvestrant and amcenestrant) maintained a normal heart rate. We evaluated known regulators of heart rate and found no alteration in thyroid hormone levels or expression of cardiac conduction genes. Treatment with competitive ER agonists protected against SERD-induced bradycardia in zebrafish, suggesting an on-target/off-tissue effect. Treatment with SERDs did not affect Gper signaling in zebrafish, as assessed by ERK phosphorylation. Concordant with this observation, giredestrant-induced bradycardia was intact in zebrafish with gper mutation, as well as in zebrafish with mutant esr2a and esr2b. However, zebrafish carrying esr1 mutation were resistant to giredestrant-induced bradycardia, indicating that Esr1 signaling is necessary for the reduction in heart rate. Conclusions: Chemical and genetic studies in an embryonic zebrafish model indicate that SERD-associated bradycardia is mediated by the Esr1 nuclear estrogen receptor.