Abstract P2-26-09: Synergistic activity of PI3K inhibitor in combination with AZD6738, ATR inhibitor in breast cancer preclinical model via DNA damage response pathway

Abstract Background: The PI3K inhibition has been an appealing approach for anticancer therapy due to its crucial role in cell growth, proliferation, and survival in various cancers. Emerging data suggest the PI3K pathway is also involved in DNA replication and genome stability, making DNA damage response (DDR) inhibitors as an attractive combination treatment for PI3K pathway blockades. Here, to enhance the efficacy of PI3K inhibitors, we investigated the novel approach combining PI3K inhibitors (alpelisib, AZD8835, AZD8186, NVP-BKM120) with DDR blockade using ATR inhibitor (AZD6738) in breast cancer preclinical models. Methods: Hormone receptor-positive breast cancer cells, MCF7 (PIK3CA E545K mutation) and T47D (PIK3CA H1047R mutation) were used. First, cell viability and the synergistic effect of combination drug was carried out by MTT assy. Next, western blot, cell cycle analysis, apoptosis assay, and immunocytochemistry were conducted to validate the hypothesis. Finally, tumor xenograft experiments were conducted using BALB/c nude mice to validate the in vitro data. Results: A synergistic antiproliferative effect was observed over a wide range of combination (PI3K inhibitor + ATR inhibitor) concentrations (even in a lower concentration as 1/10 x IC50) in vitro, suggesting that when alpelisib is combined with an ATR inhibitor, there may be possibility of lowering the effective dosage of alpelisib. After confirmation of the strong synergistic effect in sub-G1/G0-G1 arrest-mediated apoptosis by the combination treatment, we demonstrated that expressions of phospho ATR, ATM, and CHK1 were suppressed but -H2AX markedly increased by combination treatment as compared to PI3K inhibitors and ATR inhibitor alone. This suggests that combination treatment activates apoptotic pathway through enhanced DNA double strand breaks. Finally, the in vivo study showed that the combination treatment group tended to show a greater tumor growth inhibition compared with PI3K inhibitor or ATR inhibitor alone. Conclusion: The combined inhibition of PI3K and ATR showed a synergistic anticancer effect in vitro and in vivo. ATR inhibitor in combination with PI3K inhibitor merits further clinical investigation to enhance the activity of PI3K inhibitor for the treatment of PIK3CA mutated breast cancer patients. Citation Format: Yong Wha Moon, Mithun Gosh, Nahee Park, Kamal Pandey, Nar Bahadur Katwal, Sa Deok Hong. Synergistic activity of PI3K inhibitor in combination with AZD6738, ATR inhibitor in breast cancer preclinical model via DNA damage response pathway [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr P2-26-09.