Abstract LB436: VRN16: A novel PKMYT1 kinase selective inhibitor with wide therapeutic window

Abstract PKMYT1, a serine/threonine kinase, selectively regulates CDK1 during the G2/M phase transition to prevent mitotic entry in the presence of DNA damage, whereas WEE1 kinase regulates both CDK1 and CDK2. Inhibition of PKMYT1 forces cancer cells to prematurely enter mitosis without repairing DNA damage, thereby leading to cell death. Therefore, PKMYT1 is an attractive target for synthetic lethality in DNA damage-sensitive cancer cells, such as those with hyperactive CCNE1/CDK2 signaling and p53 mutations. CCNE1 amplification (Copy number ≥ 6) is observed in approximately 8% of all solid tumor patients and is particularly prevalent in ovarian cancer (31%), uterine carcinosarcoma (37%), gastric cancer (14%), and breast cancer (8%). Despite the significant patient population harboring this biomarker, no targeted therapy has been approved, emphasizing a critical unmet medical need. CCNE1 amplification drives premature S-phase entry, resulting in genome instability due to DNA replication stress. VRN16 is a novel PKMYT1 kinase inhibitor with high target selectivity. KINOMEScanTM showed that VRN16 has fewer off-target kinases (1% with S(10)) than lunresertib (7% with S(10)), a clinical stage PKMYT1 inhibitor. Notably, VRN16 spares WEE1, PLKs, and other off-target kinases, including BRAF. In contrast, lunresertib inhibits BRAF kinase, confirmed to cause paradoxical activation of phospho-MEK and phospho-ERK signaling in the cells, similar to the effects observed with vemurafenib, a BRAF inhibitor. Indeed, in CCNE1+ cancer cells, VRN16 demonstrates higher proliferation inhibition potency than lunresertib. Conversely, in CCNE1-normal cells, VRN16 exhibits lower cytotoxicity compared to lunresertib. In three CCNE1+ CDX mouse models (OVCAR3, MKN1, and HCC1569), VRN16 demonstrated superior efficacy without observed toxicity compared to lunresertib which showed adverse effects such as stomatitis, keratosis, and edema at its MTD, 10 or 20 mg/kg, BID. Additionally, in a CCNE1+ ovarian cancer PDX model, VRN16 demonstrated dose-dependent efficacy as both monotherapy and in combination therapy with gemcitabine. Although CDK2 inhibitors are also undergoing clinical trials with CCNE1+ as a major biomarker, VRN16 demonstrated superior efficacy in combination with gemcitabine in the OVCAR3 CDX, compared to the combination of BLU-222, a CDK2 inhibitor, with paclitaxel or carboplatin. In a 2-week repeated-dose oral toxicity study, VRN16 achieved a wide preclinical safety margin, more than 8-fold, compared to lunresertib with a limited safety margin. This wide therapeutic window highlights VRN16 as a novel therapeutic option for patients with CCNE1+ solid tumors or DNA damage sensitive tumors. Citation Format: Kwangwoo Hwang, Seohyun Jo, Ho-Soo Lee, Youngyi Lee, Donghyun Park, Songyee Han, Kyungah Seo, Jiseon Choi, Soochan Kim, Jiyeon Kim, Donghyuk Seo, Younho Lee, Yikyung Ko, Hyunjong Kwak, Ye Ji Son, Jeonghun Lee, Soyeon Je, Hong-ryul Jung, Daekwon Kim, Sunghwan Kim, Jaeyoung Ahn. VRN16: A novel PKMYT1 kinase selective inhibitor with wide therapeutic window [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 2 (Late-Breaking, Clinical Trial, and Invited Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_2):Abstract nr LB436.