Efficacy of CDK4/6 Inhibition in colorectal cancer and the role of p16 expression in predicting drug resistance.

Introduction: Colorectal cancer (CRC) is a leading cause of cancer-related mortality worldwide. The use of sequential polychemotherapies has improved the survival of patients with advanced metastatic disease. However, the survival rates achieved are low, and chemotherapy-related side effects are significant. Therefore, new, efficient, and tolerable therapies are urgently needed. In this study, we investigate the efficacy of pharmacological cyclin D-dependent kinase (CDK) 4/6 inhibition and explore the relevance of p16 as predictors of susceptibility to CDK 4/6 therapy.

Materials and methods: CDK 4/6 inhibitors were evaluated in native and FOLFOX- or ribociclib-resistant CRC, hepatocellular carcinoma (HCC), and breast cancer (BC) cell lines using viability, colony formation, and flow cytometry (FC)-based assays. Western blotting was employed to assess the expression of Rb and members of the INK4 family. SiRNA-based knockdown of CDK4/6 was utilized to gain insights into mechanisms of action or resistance. Tissue from 185 CRC patients was examined for the expression of p16 and its relevance for progression-free and overall survival. The prognostic relevance of cyclin-dependent kinase inhibitor 2 A (CDKN2A) mRNA expression data was derived from The Cancer Genome Atlas (TCGA) data.

Results: Ribociclib demonstrates significant antitumoral effects in various CRC, HCC, and BC cell lines, similar to two other approved CDK4/6 inhibitors (palbociclib and abemaciclib). Ribociclib-resistant cell lines (Hep-3B, HCC-1937, and BT-549) exhibited higher p16 expression compared to ribociclib-sensitive cell lines. In ribociclib-sensitive cell lines, CDK4/6 inhibition led to G1 phase arrest, whereas resistant cells did not exhibit such effects. A similar phenotype could be observed upon dual siRNA based CDK4/6 knockdown in ribociclib-sensitive HuH-7 and ribociclib-resistant Hep-3B cell lines. All CRC cell lines tested showed sensitivity to ribociclib, including the FOLFOX-resistant SW620 cell line. Low mRNA expression of CDKN2A (p16) was associated with favorable prognosis in CRC patients. No prognostic significance was found for p16 protein expression in an early-stage CRC cohort (n = 185).

Conclusion: Ribociclib demonstrates significant antitumoral effects across a large panel of cancer cell lines and chemoresistant models, especially in CRC. Resistance towards ribociclib is associated with high p16 expression, which is a negative prognostic marker for patients with CRC. Our findings underscore p16 as a promising biomarker for predicting ribociclib responsiveness and emphasize the need for further mechanistic studies and combination therapy approaches to overcome resistance in p16^high patients.