Abstract LB194: Discovery and characterization of a mutant selective PI3KαH1047Xinhibitor with a best-in-class profile
Authors: Buckbinder Leonard , St. Jean David J. , Tieu Trang , Wang Weixue , Kryukov Gregory , Jonsson Philip , Alltucker Jacob , Manimala Samantha , Ladd Brendon , Guzman-Perez Angel , Stuart Darrin 
Published: June 2022

Abstract
Oncogenic PIK3CA mutations are found in 14% of all cancers and most frequently occur at amino acids E542/E545 and H1047, involving the helical and kinase domains, respectively (Zhang 2017). Alpelisib is a PI3Kα isoform-selective, orthosteric kinase inhibitor, having equivalent activity on wild-type (wt) and mutant (mt) forms and was approved to treat PI3Kα mt, HR+/HER2- breast cancer in combination with fulvestrant, nearly doubling progression-free survival (Andre 2019). Hyperglycemia with insulin resistance is an on-target result of wt PI3Kα inhibition, leading to frequent alpelisib dose reductions, interruptions, and discontinuations. H1047X is the most common PIK3CA alteration, found in ~14% of all breast cancer patients (Martínez-Sáez 2020). We considered that selectively targeting this mutant could improve patient outcomes by sparing metabolic dysfunction related to wt inhibition. We have validated this therapeutic concept preclinically with the discovery of ST-814, an allosteric, CNS-penetrant, mutant selective PI3KαH1047X inhibitor, displaying excellent drug-like properties. ST-814 was 14-fold selective for PI3KαH1047R vs PI3Kαwt in biochemical assays (IC50 11 vs 146 nM) with exquisite isoform and kinome selectivity. Cell-based activity and selectivity were characterized in T47D (PI3KαH1047R) and SKBR3 (PI3KαWT) tumor cells grown in 10% FBS, with 8-fold mut-selectivity observed in target engagement (pAKT EC50 38 vs 304 nM) and 11-fold selectivity in viability (GI50 153 vs 1683 nM). ST-814 has robust anti-tumor activity in PI3KαH1047X in xenografts, with efficacy similar or superior to alpelisib dosed at 50 mg/kg (Table 1). To achieve this level of mouse efficacy, alpelisib exposure exceeded that in patients &gt;200% (mouse vs human AUC0-24, 74,000 vs 33,224 ng*hr/mL). As such, repeat dosing of alpelisib at 50 mg/kg, caused profound elevations in serum insulin 1 hr post-dose (*p&lt;0.01), whereas ST-814 showed no significant difference (Table 1). The metabolic consequences of repeated alpelisib (50 mg/kg) and ST-814 (100 mg/kg) dosing was further characterized in an insulin tolerance test. Insulin-mediated glucose clearance (AUC0-4) was significantly impaired by alpelisib (p&lt;0.01, one-way ANOVA), but not by ST-814. These data support the potential of molecules like ST-814 to improve outcomes in patients harboring H1047X mut tumors. A clinical candidate from this program is currently in IND enabling studies.
Table 1. CAL33 H1047R Oral Sq Cell Carcinoma Xenograft Alpelisib TGI% 50 mg/kg ST-814 TGI% 100 mg/kg Alpelisib ΔInsulin ng/mL ST-814 ΔInsulin ng/mL 79% 82% 4.42* 0.40
Citation Format: Leonard Buckbinder, David J. St. Jean, Trang Tieu, Weixue Wang, Gregory Kryukov, Philip Jonsson, Jacob Alltucker, Samantha Manimala, Brendon Ladd, Angel Guzman-Perez, Darrin Stuart. Discovery and characterization of a mutant selective PI3KαH1047Xinhibitor with a best-in-class profile [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 LB194.