Tumor genomics and response to CDK 4/6 inhibitors for patients with hormone receptor-positive (HR+) metastatic breast cancer (MBC).
1046 Background: The combination of endocrine therapy with a cyclin-dependent kinase (CDK) 4/6 inhibitor, such as palbociclib, has changed the treatment paradigm of HR+ MBC, particularly as 1stline therapy. However, there are no predictive biomarkers at present, and little is known about the impact of tumor genomics on outcomes. For example, mutations in TP53 could impact the ability of p53 to negatively regulate p21, thereby promoting cell cycle progression despite CDK 4/6 inhibition. The aim of this study was to evaluate the association between tumor genomics, particularly PIK3CA and TP53 mutations, and response to CDK 4/6 inhibitors in HR+ MBC. Methods: All HR+/HER2- MBC patients at our institution receiving a CDK 4/6 inhibitor in the second line or beyond were identified. Tumor genomics were analyzed utilizing the institutional tumor genotyping next generation sequencing (NGS) assay known as “Snapshot-NGS assay” on DNA isolated from the tumor, covering key oncogenic driver mutations and tumor suppressor genes. The log-rank test was used for statistical analysis. Results: A total of 83 patients with HR+/HER2- MBC were identified, of which 61 had available tumor genotyping results available (52 metastatic specimens). The median line of therapy was three. Median progression-free survival (PFS) on CDK 4/6 inhibitor-based therapy, as second line or beyond, was 9.2 months (mo) overall. No significant difference in PFS was seen among patients with PIK3CA (n = 31) mutations (7.1 vs. 9.7 mo; p = 0.28) or with any alteration in the PI3K/Akt/mTOR (n = 36) pathway (8.2 vs. 9.3 mo; p = 0.40). The presence of a p53 mutation (n = 9) or complex tumor genomics (n = 14) demonstrated a trend towards shorter PFS (5.5 vs. 9.2 mo; 5.5 vs. 9.5 mo, respectively), although statistical significance was not reached due to small numbers (p = 0.76; p = 0.31). In a multivariable analysis adjusting for age and prior lines of therapy, similar results were observed. Conclusions: This study suggests patients with HR+ MBC harboring p53 mutations or complex tumor genomics may experience shorter PFS on CDK 4/6 inhibitor-based therapy in the second line and beyond, and these novel findings require validation in additional studies.