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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>2 antagonism, 0.5>CI< 2 additivity, CI< 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.

Supplementary Materials and Methods

Although Aurora A, B, and C kinases share high sequence similarity, especially within the kinase domain, they function distinctly in cell-cycle progression. Aurora A depletion primarily leads to mitotic spindle formation defects and consequently prometaphase arrest, whereas Aurora B/C inactivation primarily induces polyploidy from cytokinesis failure. Aurora B/C inactivation phenotypes are also epistatic to those of Aurora A, such that the concomitant inactivation of Aurora A and B, or all Aurora isoforms by nonisoform–selective Aurora inhibitors, demonstrates the Aurora B/C-dominant cytokinesis failure and polyploidy phenotypes. Several Aurora inhibitors are in clinical trials for T/B-cell lymphoma, multiple myeloma, leukemia, lung, and breast cancers. Here, we describe an Aurora A–selective inhibitor, LY3295668, which potently inhibits Aurora autophosphorylation and its kinase activity in vitro and in vivo, persistently arrests cancer cells in mitosis, and induces more profound apoptosis than Aurora B or Aurora A/B dual inhibitors without Aurora B inhibition–associated cytokinesis failure and aneuploidy. LY3295668 inhibits the growth of a broad panel of cancer cell lines, including small-cell lung and breast cancer cells. It demonstrates significant efficacy in small-cell lung cancer xenograft and patient-derived tumor preclinical models as a single agent and in combination with standard-of-care agents. LY3295668, as a highly Aurora A–selective inhibitor, may represent a preferred approach to the current pan-Aurora inhibitors as a cancer therapeutic agent.

Supplementary Tables

Supplementary Figures

Although Aurora A, B, and C kinases share high sequence similarity, especially within the kinase domain, they function distinctly in cell-cycle progression. Aurora A depletion primarily leads to mitotic spindle formation defects and consequently prometaphase arrest, whereas Aurora B/C inactivation primarily induces polyploidy from cytokinesis failure. Aurora B/C inactivation phenotypes are also epistatic to those of Aurora A, such that the concomitant inactivation of Aurora A and B, or all Aurora isoforms by nonisoform–selective Aurora inhibitors, demonstrates the Aurora B/C-dominant cytokinesis failure and polyploidy phenotypes. Several Aurora inhibitors are in clinical trials for T/B-cell lymphoma, multiple myeloma, leukemia, lung, and breast cancers. Here, we describe an Aurora A–selective inhibitor, LY3295668, which potently inhibits Aurora autophosphorylation and its kinase activity in vitro and in vivo, persistently arrests cancer cells in mitosis, and induces more profound apoptosis than Aurora B or Aurora A/B dual inhibitors without Aurora B inhibition–associated cytokinesis failure and aneuploidy. LY3295668 inhibits the growth of a broad panel of cancer cell lines, including small-cell lung and breast cancer cells. It demonstrates significant efficacy in small-cell lung cancer xenograft and patient-derived tumor preclinical models as a single agent and in combination with standard-of-care agents. LY3295668, as a highly Aurora A–selective inhibitor, may represent a preferred approach to the current pan-Aurora inhibitors as a cancer therapeutic agent.

Phosphoinositide 3-kinase alpha (PI3Kα) H1047R mutations are activating oncogenic events that occur in ~15% of advanced breast cancers. While there is one PI3Kα inhibitor FDA-approved for patients with PI3Kα-mutated breast cancer, and many others in clinical development, all of these agents inhibit wild-type PI3Kα and its mutated form with approximate equal potency. As a result, their efficacy is limited by toxicities associated with on target wild-type PI3Kα inhibition, notably hyperglycemia as well as cutaneous and GI toxicity. LOX-22783 is a highly potent, mutant-selective and brain-penetrant allosteric PI3Kα H1047R inhibitor. Here, we describe the preclinical profile of LOX-22783. H1047R selectivity was measured using biochemical kinase activity and cell-titer Glo and signal transduction assays. Tumor growth inhibition, pharmacokinetic and pharmacodynamic effects were assessed in in vivo studies using xenograft and patient-derived xenograft (PDX)-models. LOX-22783 inhibited growth and signaling responses in multiple H1047R-driven breast cancer cell lines and demonstrated high selectivity for H1047R mutated PI3Kα (EC50 values 250 nM) as well as the other wild-type PI3K isoforms (beta, gamma, and delta, all EC50 >250nM). In enzyme and cell-based assays, LOX-22783 dissociated from PI3Kα H1047R at a slower rate (3-6 hrs) compared to alpelisib (≤10 mins), potentially allowing for extended inhibition of PI3Kα H1047R by LOX-22783. LOX-22783 also normalized the EGF-stimulated membrane-localization of PI3Kα H1047R while alpelisib did not. LOX-22783 was highly kinome-selective when assayed at 3 µM, with no inhibitory activity on 17 lipid kinases or 374 protein kinases. In preclinical species, LOX-22783 demonstrated high oral bioavailability, including exposure in the CNS, a common site of metastases for patients with breast cancer. In vivo, LOX-22783 demonstrated dose-dependent tumor regression in H1047R breast cancer models without inducing hyperglycemia or other toxicities. Tumor pharmacodynamic analyses confirmed successful pathway inhibition. At doses resulting in 90% pathway inhibition, tumor regressions of ≥60% were observed. This wide therapeutic index is predicted to allow for maximizing dose intensity and efficacy in patients, without wild-type PI3Kα inhibition limiting target coverage for the H1047R mutant form. These data demonstrate that LOX-22783 potently and selectively inhibits mutant H1047R, but not wild-type PI3Kα, or other PI3K isoforms. LOX-22783 binds to an allosteric pocket distinct from the ATP binding site used by the approved and investigational PI3Kα inhibitors. We hypothesize that this profile will lead to differentiated efficacy and tolerability for patients with PI3Kα H1047R-mutated cancers, with the additional potential to address brain metastases. An IND submission is planned for 2022. Citation Format: Anke Klippel, Rui Wang, Loredana Puca, Andrew Lee Faber, Weihua Shen, Shripad V. Bhagwat, Kannan Karukurichi, Feiyu Fred Zhang, Carmen Perez, Ramon Rama, Ana Ramos, Yi Zheng, Zahid Bonday, James Thomas, Harold B. Brooks, Lisa J. Kindler, Sarah M. Bogner, Parisa Zolfaghari, Mark Hicks II, Sophie Callies, Brian Mattioni, Laurie LeBrun, Jim Durbin, Erin Anderson, Chris Mayne, Edward Kesicki, Gabrielle Kolakowski, Steven W. Andrews, Barbara J. Brandhuber. Preclinical characterization of LOX-22783, a highly potent, mutant-selective and brain-penetrant allosteric PI3Kα H1047R inhibitor [abstract]. In: Proceedings of the AACR-NCI-EORTC Virtual International Conference on Molecular Targets and Cancer Therapeutics; 2021 Oct 7-10. Philadelphia (PA): AACR; Mol Cancer Ther 2021;20(12 Suppl):Abstract nr P142.