Abstract 5298: Activity and tolerability of combinations of trastuzumab deruxtecan (T-DXd) with inhibitors of the DNA damage response in preclinical models

Abstract Background: Trastuzumab deruxtecan (T-DXd) is an antibody-drug conjugate composed of an anti-HER2 antibody, a cleavable tetrapeptide-based linker, and a cytotoxic topoisomerase I inhibitor. T-DXd is approved for patients with HER2+ metastatic breast cancer who have received two or more anti-HER2 based regimens as well as HER2+ gastric cancer, and has demonstrated antitumor activity in both HER2+ and HER2-low cancers clinically. Trapping of Topo1cc by DXd in targeted cells, induces replication stress (RS) and double strand breaks (DSB), activating a DNA Damage Repair (DDR) response. Therefore we aimed to investigate if combination of T-DXd with DDR inhibitors enhances efficacy and durability of response. Methods: We evaluated the antiproliferative effect of the combination of T-DXd with AZD6738 (ATR inhibitor), AZD1775 (WEE1 inhibitor), AZD1390 (ATM inhibitor), in a panel of 27 breast cancer cell lines in an in vitro 7-day viability assay. Combinations were also evaluated in vivo in the HER2-high gastric NCI-N87 cell line xenograft. To evaluate the specificity of the combination activity in tumor cells (vs normal tissue), we further evaluated the combination in a 2D in vitro human bone marrow progenitor assay. Results: We found that the combination with all DDRi significantly enhanced in vitro cell killing activity over single agents in many of the models tested (12/27 for AZD6738, 9/27 for AZD1775, 6/27 for AZD1390). This was not limited to models with DNA damage repair mutations (e.g. BRCA1, BRCA2, or ATM). Mechanistically, T-DXd activated PARP and the ATR, Wee1, ATM pathways, inducing cell cycle arrest. Combination of T-DXd with AZD6738 and AZD1775 inhibited the RS response (pATR-T1989, pChk1-S345) and cell cycle arrest (pCdc2-Y15), while AZD1390 inhibited the DSB response pathway (pATM-S1981, pRad50-S685, pKap1-S824, pChk2-T68). All DDR inhibitors exacerbated DNA damage (pRPA-S4/8, gH2AX) and cell death (cCasp3). In the in vitro human bone marrow (BM) assay, T-DXd had no synergistic interaction with AZD6738 and AZD1775 inhibitors, but synergy with AZD1390. In vivo, combination therapy with all DDRi tested achieved superior Tumor Growth Inhibition (TGI) compared to monotherapy alone. At day 41 in NCI-N87, 3mg/kg of T-DXd provided TGI of 74% while monotherapy arms of 25mg/kg BID AZD6738, 60mg/kg (5-days on/2-days off) AZD1775 or 10mg/kg AZD1390 provided 68%, 47%, 31% TGI respectively. The combinations of T-DXd with AZD6738, AZD1775 and AZD1390 provided TGI of >100% (19% regression), 90%, and 99% respectively. Conclusions: These results suggest that AZD6738, AZD1775, and AZD1390 all potentiate T-DXd activity in vitro and in vivo, and the combination interactions with AZD6738 and AZD1775 were much stronger in some tumor models over bone marrow cells in vitro. These data suggest further investigation of these combinations in clinical studies may be warranted. Citation Format: Yann Wallez, Theresa Proia, Azadeh Cheraghchi-Bashi-Astaneh, Ankur Karmokar, Zena Wilson, Suzanne Randle, Mark Anderton, Stephen Durant, Elisabetta Leo, Alan Lau, Mark O'Connor, Jerome Mettetal. Activity and tolerability of combinations of trastuzumab deruxtecan (T-DXd) with inhibitors of the DNA damage response in preclinical models [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 5298.