CDK4 phosphorylation status and a linked gene expression profile predict sensitivity to Palbociclib [lines]

Cyclin D-CDK4/6 are the first CDK complexes to be activated in G1 phase in response to oncogenic pathways. The specific CDK4/6 inhibitor PD0332991 (Palbociclib) was recently approved by the FDA to treat advanced ER+ breast tumors. Unfortunately, reliable predictive tools are lacking to identify potentially responsive or insensitive tumors. We have shown that the activating T172 phosphorylation of CDK4 is the central rate-limiting event that initiates the cell cycle decision and signals the presence of active CDK4. Here, we found that, in breast cancer cell lines, the CDK4 T172 phosphorylation best correlates with the sensitivity to PD0332991. Moreover, the modification profile of CDK4 differs among breast tumors and it associates with their subtypes and risk. A gene expression signature faithfully predicted CDK4 modification profiles in tumors and cell lines. This surrogate biomarker identifies tumors that are unlikely to respond to CDK4/6 inhibitors and could allow extending the indication of the drug to HER-2 positive and some basal-like tumors.

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