Abstract P4-08-02: LOXO-783: A potent, highly mutant selective and brain-penetrant allosteric PI3Kα H1047R inhibitor in combination with standard of care (SOC) treatments in preclinical PI3Kα H1047R-mutant breast cancer models
Authors: Puca Loredana , Dowless Michele S. , Perez-Ferreiro Carmen M. , Ortiz-Ruiz Maria Jesus , Donoho Gregory P. , Capen Andrew , Huber Lysiane , Bogner Sarah M. , Fei Dongling , Manro Jason R. , Yu Chun Ping , Xu Wei Guo , Wang Rui , Chen Shuang , Hicks Mark A. , Zolfaghari Parisa , Faber Andrew , Gilmour Raymond , Ramstetter Monica D. , Chang Matthew T. , Lallena Maria Jose , Gong Xuequian , Hyman David M. , Smyth Lillian M. , Brandhuber Barbara J. , Taylor Barry S. , Klippel Anke 
Published: March 2023

Background Phosphoinositide 3-kinase alpha (PI3Kα) H1047R mutations are activating oncogenic events that occur in ~15% of breast cancers (BC). Early generation PI3Kα inhibitors target both wild-type (WT) and mutant PI3Kα and, as a result, their efficacy may be limited by on-target WT PI3Kα-mediated toxicities, including hyperglycemia, skin rash, and diarrhea. LOXO-783 is an oral, potent and highly mutant-selective, brain-penetrant allosteric PI3Kα H1047R inhibitor that is currently in phase 1 testing. Preclinically, LOXO-783 as a single agent is highly selective for PI3Kα H1047R over WT PI3Kα and other PI3K isoforms, and induces single-agent tumor regressions in ER+, HER2- PI3Kα H1047R-mutant breast cancer models without causing hyperglycemia or increases in plasma insulin/C-peptide. LOXO-783 also demonstrates brain penetration in vivo with dose-dependent tumor growth inhibition in brain metastasis models. Here we report the efficacy of LOXO-783 with SOC treatments in preclinical breast cancer models. Methods Cell proliferation assays and in vivo studies to evaluate combination effects were performed in various PI3Ka H1047R mutant HR+, HER2- and triple negative breast cancer models. For each combination in vitro, a combination index (CI) based on the Loewe Additivity Method was calculated (CI&gt;2 antagonism, 0.5&gt;CI&lt; 2 additivity, CI&lt; 0.5 synergy). For the in vivo studies, the Bliss Independence Method was used to evaluate the statistical significance of the combination effects. Results Combining LOXO-783 with either fulvestrant (FUL; CI at 50% inhibition = 0.28) or imlunestrant (CI at 50% inhibition = 0.43) showed increased efficacy in cell proliferation assays using the HR+, HER2-, PI3Kα H1047R-mutant T47D model. LOXO-783 also demonstrated an additive effect in combination with these endocrine therapies in vivo. Similar results were observed in a T47D model engineered to express ESR1 D538G, as well as in an HR+, HER2- PI3Kα double in-cis mutant model (H1047R/D350G) also harboring ESR1 D538G and derived from a patient who had progressed on prior letrozole plus taselisib. Moreover, LOXO-783 plus abemaciclib demonstrated an additive effect in vitro (CI at 50% inhibition = 0.61), and in T47D xenograft and PDX models in vivo. Combinations of LOXO-783 with abemaciclib plus imlunestrant resulted in a mean tumor regression of –48.1%; LOXO-783 with abemaciclib plus FUL showed mean tumor regression of –43.9% in T47D xenografts. Similar efficacy was not observed in the absence of LOXO-783 (mean tumor regression was –7.3% with abemaciclib plus imlunestrant, and 3.2% with abemaciclib plus FUL). We observed comparable results in PDX models. These data collectively demonstrate the additive effect of LOXO-783 with SOC treatments. Extending these studies to additional treatment settings, LOXO-783 was similarly efficacious as a single agent in abemaciclib-resistant and abemaciclib/FUL double-resistant models, and was additive in combination with paclitaxel in a triple negative breast cancer model in vitro and in vivo. Conclusions LOXO-783 shows additive effects when combined with SOC in breast cancers harboring the PI3Kα H1047R-mutation (as single or double in-cis mutations) in both HR+ and triple negative settings. LOXO-783 is also efficacious in ESR1 mutant as well as in abemaciclib and abemaciclib/FUL double-resistant models. A phase 1 trial of LOXO-783 alone or in combination with anticancer therapies is ongoing (PIKASSO-01; NCT05307705).

Citation Format: Loredana Puca, Michele S. Dowless, Carmen M. Perez-Ferreiro, Maria Jesus Ortiz-Ruiz, Gregory P. Donoho, Andrew Capen, Lysiane Huber, Sarah M. Bogner, Dongling Fei, Jason R. Manro, Chun Ping Yu, Wei Guo Xu, Rui Wang, Shuang Chen, Mark A. Hicks, Parisa Zolfaghari, Andrew Faber, Raymond Gilmour, Monica D. Ramstetter, Matthew T. Chang, Maria Jose Lallena, Xuequian Gong, David M. Hyman, Lillian M. Smyth, Barbara J. Brandhuber, Barry S. Taylor, Anke Klippel. LOXO-783: A potent, highly mutant selective and brain-penetrant allosteric PI3Kα H1047R inhibitor in combination with standard of care (SOC) treatments in preclinical PI3Kα H1047R-mutant breast cancer models [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 P4-08-02.