Selective Estrogen Receptor Degraders Induce Bradycardia by Modulating Nuclear Estrogen Signaling

Selective estrogen receptor degraders (SERDs) are emerging therapies for estrogen receptor-positive (ER+) breast cancer. However, certain oral SERDs, including giredestrant and camizestrant, have been associated with bradycardia in clinical trials. To elucidate the underlying molecular mechanism, we used chemical biology and genetic approaches in a zebrafish model of SERD-induced bradycardia. Giredestrant and camizestrant induced significant bradycardia in wild-type zebrafish embryos, whereas fulvestrant and amcenestrant (SERDs that do not induce bradycardia in patients) did not alter heart rate. Cotreatment with ER agonists rescued bradycardia, suggesting an ER-mediated mechanism. Mutations in gper, esr2a, and esr2b did not confer resistance to SERD-induced bradycardia, whereas esr1 mutant embryos were protected. These findings demonstrate that SERD-associated bradycardia is mediated through Esr1 signaling, supporting an on-target adverse effect.