LAUSR

Background: Three CDK 4/6 inhibitors (inh) are FDA approved in combination with endocrine therapy (ET) for HR+/HER2- metastatic breast cancer (MBC), however, there are no validated predictive markers of response to this class of drugs. Approximately 30-40% of patients (pts) have little to no response to these agents, with disease progression occurring in weeks to a few months after therapy initiation. We conducted an open-label, multicenter clinical trial (NCT03195192) to utilize cutting edge proteomic technologies to map the functional activation of the signaling architecture of pre-treatment tumor tissue from HR+/HER2- MBC pts receiving first line CDK4/6 inh plus ET, and to correlate these functional phosphoprotein-based signaling patterns with 1-year progression-free survival (1-yr PFS). Methods: We enrolled 29 of 100 planned pts, then the study closed early due to slow accrual. All pts were followed up to 12 months from starting a CDK4/6 inhibitor or until disease progression if it occurred earlier. The primary objective of this trial is to assess the correlation between baseline phosphorylated Rb levels that indicate activated Rb in tumor tissue and 1-yr PFS. We hypothesized that high levels of activated Rb will identify pts who are more likely to respond to CDK4/6 inh. Secondary objective is to evaluate the correlation between 1-yr PFS and 8 pre-specified qualifying (protein/phosphoprotein CDK 4/6 kinase pathway biomarkers: total Rb, Rb (S780), total Cyclin D1, Cyclin D1 (S286), total p16INK, total p27, p27 (T187), FoxM1 (T600). Results: Pre-treatment diagnostic FFPE biopsy material available from 24 evaluable pts were analyzed by a Laser Capture Microdissection (LCM) Reverse Phase Protein Microarray (RPPA) workflow. Seventeen of 24 pts (71%) were White, 7 (29%) African American; median age 65 (range 36-79); 22 pts (92%) received an aromatase inh and 2 (8%) had fulvestrant. The primary analysis expressed the relationship between phospho-RB and 1-yr PFS as a 2x2 table of frequencies summarized either above or below the median values observed in all pt values using a Pearson chi-squared test. Similarly, we analyzed the qualified protein/phosphoprotein markers quantified by RPPA by median dichotomization. Univariate analysis showed that 1-yr PFS correlated with below median levels of phospho and total RB (Chi-sq = 8.71, p-value = 0.003); phospho-FoxM1 (Chi-sq = 4.44, p-value = 0.035); Cyclin D1 S286 (Chi-sq = 4.44, p-value = 0.035); total and phospho-p27 and Ki67 (Chi-sq = 4.44, p-value = 0.035). None of these biomarkers were individually significant as continuous variables on multivariate analysis by logistic regression. Conclusions: Functional CDK 4/6 drug target pathway mapping analysis is possible from the pre-treatment diagnostic FFPE material. Our results indicate that pts whose tumors had low inherent proliferative and CDK 4/6 kinase activity were more likely to respond to first line CDK4/6 inh plus ET indicating possible prognostic determinants of the biomarkers. Citation Format: Maysa M Abu-Khalaf, Christos Hatzis, K. Alex Hodge, Frances Valdes, William Sikov, Monica Mita, Neelima Denduluri, Kris Awerkamp, Rita Murphy, Daniel Zelterman, Bryant Dunetz, Emanuel Petricoin, Mariaelena Pierobon. Multiomic advanced diagnostics for CDK 4/6 drug target activation mapping of HR+/HER2- metastatic breast cancer [abstract]. In: Proceedings of the 2020 San Antonio Breast Cancer Virtual Symposium; 2020 Dec 8-11; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2021;81(4 Suppl):Abstract nr PS5-28.