Abstract A023: Preclinical development of PKMYT1 and WEE1 inhibitor combinations
Authors: Gallo David , Shah Fenil , Stocco Rino , Baruah Prasamit , Fourtounis Jimmy , Kryczka Rosie , Hyer Marc L , Roulston Anne , Marshall C Gary , Zimmermann Michal , Morris Stephen , Zinda Michael 
Published: December 2023

Abstract
Background: CCNE1-amplification or overexpression are prevalent alterations in multiple tumor types, including high-grade serous ovarian, endometrial, and gastro-esophageal cancers. CCNE1-amplification is associated with high levels of replication stress, genomic instability, and resistance to chemotherapy. Previously, CRISPR-enabled synthetic lethal screening identified that loss of the CDK1 regulator Membrane-associated tyrosine- and threonine-specific Cdc2-inhibitory kinase (PKMYT1) is synthetic lethal with CCNE1-overexpression, leading to the development of lunresertib - a first-in-class, potent, selective, orally bioavailable PKMYT1 inhibitor currently being investigated in phase I clinical trials (NCT04855656, NCT05147272, NCT05147350). In CCNE1 overexpressing cells, lunresertib treatment leads to reduction of inhibitory CDK1-T14 phosphorylation and activated CDK1 in S-phase, triggering premature mitosis, chromosome pulverization, and ultimately cell death. In CCNE1-amplified tumor models, lunresertib exhibits antitumor activity as monotherapy. We hypothesized that further activation of CDK1 by reducing the inhibitory phosphorylation site at CDK1-pY15 by WEE1 inhibition would increase the antitumor activity of lunresertib.
Methods: In vitro cell viability studies were used to determine the level of synergy between lunresertib and the WEE1 inhibitor Debio-0123 in engineered as well as tumor-derived CCNE1-high models. The lunresertib/Debio-0123 combination mechanism of action was investigated using high content imaging and immunoblotting for DNA damage and cell cycle markers in cultured cells. Finally, the pre-clinical anti-tumor activity and tolerability of combination lunresertib/Debio-0123 was evaluated in vivo using CCNE1-amplified tumor xenografts in mice.
Results: Combination of low doses of lunresertib with Debio-0123 selectively induced synergistic cytotoxicity and catastrophic DNA damage in CCNE1-overexpressing cells, without affecting wild type counterparts. Increased DNA damage resulted from robust activation of CDK1 in S-phase, causing rapid induction of premature mitosis. Combination of lunresertib and Debio-0123 abrogated phosphorylation of both Thr14 and Tyr15 inhibitory CDK1 sites at doses that led to only partial dephosphorylation in both monotherapy settings. Mechanistically, synergistic CDK1 dephosphorylation was partially mediated by activation of CDC25B phosphatase. In a mouse model, the combination of RP-6306 and Debio-0123 was well-tolerated and demonstrated tumor regressions in a CCNE1-overexpressing tumor xenograft model at sub-therapeutic monotherapy doses.
Conclusions: Lunresertib and the WEE1 inhibitor Debio-0123 elicit strong synergistic activity in preclinical models of CCNE1-overexpressoin due to rapid and complete CDK1 activation. In a xenograft tumor model, intermittent dosing of lunresertib/WEE1i lead to tumor regressions at sub-therapeutic monotherapy doses. Together these findings provide mechanistic rationale for clinical development of PKMYT1 and WEE1 inhibitor combinations.
Citation Format: David Gallo, Fenil Shah, Rino Stocco, Prasamit Baruah, Jimmy Fourtounis, Rosie Kryczka, Marc L Hyer, Anne Roulston, C Gary Marshall, Michal Zimmermann, Stephen Morris, Michael Zinda. Preclinical development of PKMYT1 and WEE1 inhibitor combinations [abstract]. In: Proceedings of the AACR-NCI-EORTC Virtual International Conference on Molecular Targets and Cancer Therapeutics; 2023 Oct 11-15; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2023;22(12 Suppl):Abstract nr A023.