Data from Discovery and Clinical Proof-of-Concept of RLY-2608, a First-in-Class Mutant-Selective Allosteric PI3Kα Inhibitor That Decouples Antitumor Activity from Hyperinsulinemia
Authors: Varkaris Andreas , Pazolli Ermira , Gunaydin Hakan , Wang Qi , Pierce Levi , Boezio Alessandro A. , Bulku Artemisa , DiPietro Lucian , Fridrich Cary , Frost Adam , Giordanetto Fabrizio , Hamilton Erika P. , Harris Katherine , Holliday Michael , Hunter Tamieka L. , Iskandar Amanda , Ji Yongli , Larivée Alexandre , LaRochelle Jonathan R. , Lescarbeau André , Llambi Fabien , Lormil Brenda , Mader Mary M. , Mar Brenton G. , Martin Iain , McLean Thomas H. , Michelsen Klaus , Pechersky Yakov , Puente-Poushnejad Erika , Raynor Kevin , Rogala Dipali , Samadani Ramin , Schram Alison M. , Shortsleeves Kelley , Swaminathan Sweta , Tajmir Shahein , Tan Gege , Tang Yong , Valverde Roberto , Wehrenberg Bryan , Wilbur Jeremy , Williams Bret R. , Zeng Hongtao , Zhang Hanmo , Walters W. Patrick , Wolf Beni B. , Shaw David E. , Bergstrom Donald A. , Watters James , Fraser James S. , Fortin Pascal D. , Kipp D. Randal 
Published: February 2024

<div>Abstract<p><i>PIK3CA</i> (PI3Kα) is a lipid kinase commonly mutated in cancer, including ∼40% of hormone receptor–positive breast cancer. The most frequently observed mutants occur in the kinase and helical domains. Orthosteric PI3Kα inhibitors suffer from poor selectivity leading to undesirable side effects, most prominently hyperglycemia due to inhibition of wild-type (WT) PI3Kα. Here, we used molecular dynamics simulations and cryo-electron microscopy to identify an allosteric network that provides an explanation for how mutations favor PI3Kα activation. A DNA-encoded library screen leveraging electron microscopy-optimized constructs, differential enrichment, and an orthosteric-blocking compound led to the identification of RLY-2608, a first-in-class allosteric mutant-selective inhibitor of PI3Kα. RLY-2608 inhibited tumor growth in <i>PIK3CA</i>-mutant xenograft models with minimal impact on insulin, a marker of dysregulated glucose homeostasis. RLY-2608 elicited objective tumor responses in two patients diagnosed with advanced hormone receptor–positive breast cancer with kinase or helical domain <i>PIK3CA</i> mutations, with no observed WT PI3Kα-related toxicities.</p>Significance:<p>Treatments for <i>PIK3CA</i>-mutant cancers are limited by toxicities associated with the inhibition of WT PI3Kα. Molecular dynamics, cryo-electron microscopy, and DNA-encoded libraries were used to develop RLY-2608, a first-in-class inhibitor that demonstrates mutant selectivity in patients. This marks the advance of clinical mutant-selective inhibition that overcomes limitations of orthosteric PI3Kα inhibitors.</p><p><i><a href="https://aacrjournals.org/cancerdiscovery/article/doi/10.1158/2159-8290.CD-23-1392" target="_blank">See related commentary by Gong and Vanhaesebroeck, p. 204</a></i>.</p><p><i><a href="https://aacrjournals.org/cancerdiscovery/article/doi/10.1158/2159-8290.CD-23-0704" target="_blank">See related article by Varkaris et al., p. 227</a></i>.</p><p><i><a href="https://aacrjournals.org/cancerdiscovery/article/doi/10.1158/2159-8290.CD-14-2-ITI" target="_blank">This article is featured in Selected Articles from This Issue, p. 201</a></i></p></div>