Abstract 950: Targeting PKMYT1 kinase is an effective treatment strategy in triple negative breast cancers with low molecular weight cyclin E (LMW-E) expression

Abstract Background: Cyclin E is post-translationally modified by neutrophil elastase mediated proteolytic cleavage to generate the low molecular weight isoforms of cyclin E (LMW-E) that are detected in various human cancers. We previously reported that 70% of triple negative breast cancers (TNBC) examined overexpress LMW-E, and these patients have a poor prognosis. Expression of LMW-E promotes genomic instability by causing DNA replication stress. PKMYT1 prevents premature mitotic entry by catalyzing CDK1 phosphorylation at T14, essential for preventing DNA damage and cell death when cyclin E, including LMW-E, is overexpressed. In this study, we tested the hypothesis that LMW-E positive status can be used as a biomarker of response in selecting TNBC patients who are likely to respond to RP-6306, a first in-class and selective inhibitor of PKMYT1 kinase. Results: Assessment of pre-treatment breast biopsies from TNBC patients (n=40) enrolled in a neoadjuvant chemotherapy prospective study for LMW-E and CDK1-pT14 revealed significant positive correlation between these two proteins. Furthermore, positivity of both biomarkers was associated with lack of pathologic complete response (pCR) to neoadjuvant chemotherapy. We next examined the mechanism of response to RP-6306 in vitro and in vivo using TNBC cell lines, patient-derived xenograft (PDX) models and transgenic mouse mammary tumor virus (MMTV) models expressing human LMW-E (hLMW-E). In vitro results using 7 different TNBC cell lines, revealed that high LMW-E levels are significantly predictive of response to RP-6306 (R2=0.78, p= 0.008), while LMW-E knockdown resulted in a 7X increase in IC50 values of RP-6306 (p<0.001). In high LMW-E cells, treatment with RP-6306 resulted in significant (i) downregulation of CDK1-pT14, PKMYT1, WEE1, cyclin B and pRb, (ii) accumulation of sub-G1 and polyploid cell population, (iii) apoptosis, (iv) accumulation of chromosomal breakage, (v) increased DNA damage (increase in γ-H2AX and 53BP1 foci/cell) and lack of DNA repair (downregulation of Rad51), and (vi) premature mitotic entry. Treatment of both breast cancer PDX models and hLMW-E transgenic tumors with RP-6306 revealed that only in animals with high LMW-E tumors, treatment results in significant decrease in tumor volume. However, RP-6306 was ineffective in reducing tumor volume in low cyclin E in vivo models. Immunohistochemical analysis revealed that in vivo efficacy of RP-6306 (in both PDX and transgenic models) was concomitant with increase in γ-H2AX and decrease in CDK1-pT14 and Ki67. Conclusion: Collectively, our results show that overexpression of LMW-E and CDK1-pT14 in TNBC can be used to stratify patients whose tumors are likely to respond to RP-6306. Mechanistically, LMW-E overexpressing TNBC cells activate CDK1 (in vitro and in vivo) to accelerate premature mitotic entry, leading to DNA damage and apoptosis. Citation Format: Amriti Lulla, Tuyen D. Nguyen, Mi Li, Sofia Mastoraki, Yan Wang, Tuyen Bui, Marc Pina, Spiridon Tsavachidis, Gary Marshall, Kelly K. Hunt, Khandan Keyomarsi. Targeting PKMYT1 kinase is an effective treatment strategy in triple negative breast cancers with low molecular weight cyclin E (LMW-E) expression [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 950.