Abstract PO4-23-10: Restoring the efficacy of standard of care in treatment-refractory ER+ breast cancer by re-activation of cAMP-ROS-DNA damage axis

Abstract Estrogen receptor (ER)-positive (ER+) breast cancer has the highest incidence rate and accounts for around 75% of all cases. Endocrine therapy has been the mainstay therapy for the treatment of both early and late-stage ER+ breast cancer for decades. Although most ER+ breast cancer patients initially respond well to endocrine therapy, resistance is common. The addition of CDK4/6 inhibitors to endocrine therapy led to significant improvements in clinical outcome, and they are now considered one of the standard of care (SOC) therapies. However, a significant proportion of patients still suffer from disease relapse upon prolonged use of endocrine therapy in combination with CDK4/6 inhibitors, representing a major clinical challenge that reduces the long-term benefit and patient survival. Therefore, elucidating the mechanisms of sensitivity and resistance to SOC therapy and identifying novel actionable targets are urgently needed. Here, we show that endocrine therapies and CDK4/6 inhibitors cause toxic PARP1 trapping and generation of a functional BRCAness phenotype by downregulating key DNA repair proteins that ultimately result in increased histone parylation and reduced H3K9 acetylation, leading to transcriptional blockage and cell death. Mechanistically, we found that SOC therapy downregulates phosphodiesterase 4D (PDE4D), resulting in increased cAMP levels, PKA-dependent phosphorylation of mitochondrial COXIV-I, generation of mitochondrial reactive oxygen species (ROS), and DNA damage. Importantly, we identified PDE4D as a novel ER target gene that in turn stimulates ER activity in a feedforward loop in endocrine-responsive models and regulates BRCA1 expression. However, during SOC resistance, an ER-to-EGFR switch induces PDE4D overexpression via c-Jun. Inhibition of PDE4D using BPN14770, the first-in-class PDE4D allosteric inhibitor that has successfully completed Phase II trials in Fragile X syndrome, or its upstream EGFR in combination with SOC therapies in drug-resistant settings, including multiple acquired SOC resistant cell line models, primary cultures and organoids of endocrine resistant ER+ PDXs reinstates PARP1 trapping and BRCAness, leading to drug sensitization in vitro and in vivo. Notably, we demonstrated that high PDE4D mRNA and protein expression is associated with dramatically worse disease-free survival and overall survival in endocrine therapy-treated ER+ breast cancer. Considering the availability of potent and non-toxic PDE4D inhibitors, clinically approved EGFR and PARP1 inhibitors, our findings have great translational potential. Citation Format: Ozge Saatci, Metin Cetin, Meral Uner, Unal Metin Tokat, Ioulia Chatzistamou, Pelin Gulizar Ersan, Elodie Montaudon, Aytekin Akyol, Sercan Aksoy, Aysegul Uner, Elisabetta Marangoni, Sajish Mathew, Ozgur Sahin. Restoring the efficacy of standard of care in treatment-refractory ER+ breast cancer by re-activation of cAMP-ROS-DNA damage axis abstract. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO4-23-10.