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

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