Retinoblastoma Expression and Targeting by CDK4/6 Inhibitors in Small Cell Lung Cancer

Area of Science:

• Oncology
• Molecular Biology
• Genetics

Background:

• The canonical model of small cell lung cancer (SCLC) involves dual inactivation of TP53 and RB1.
• Genomic studies reveal SCLC tumors lacking RB1 mutations, challenging the canonical model.
• RB1 protein expression and function in SCLC require further investigation.

Purpose of the Study:

• To investigate RB1 protein expression and its functional implications in SCLC.
• To determine if RB1-positive SCLC is sensitive to CDK4/6 inhibitors.
• To identify biomarkers for predicting response to CDK4/6 inhibitors in SCLC.

Main Methods:

• Immunohistochemistry (IHC) analysis of RB1 expression in 62 human SCLC tumors.
• Assessment of SCLC cell and xenograft tumor sensitivity to palbociclib, a CDK4/6 inhibitor.
• CRISPR knockout to confirm RB1 dependency for drug sensitivity.
• Analysis of ctDNA and tumor mass changes in a patient treated with abemaciclib.
• Development and validation of a transcriptomics-based RB1 loss-of-function signature.

Main Results:

• Approximately 14% of SCLC tumors exhibited abundant RB1 protein expression, associated with neuroendocrine gene and YAP1 expression.
• SCLC cells and xenografts with RB1 expression were sensitive to palbociclib growth inhibition, dependent on RB1.
• A patient with RB1-positive SCLC showed a significant decrease in mutant TP53 ctDNA and tumor mass upon treatment with abemaciclib.
• A transcriptomics-based RB1 loss-of-function signature was identified and validated in the responsive patient.

Conclusions:

• RB1 protein is an actionable target in a subset of SCLC patients.
• CDK4/6 inhibitors represent a potential therapeutic strategy for RB1-positive SCLC.
• The identified transcriptomic signature may predict response to CDK4/6 inhibitors in SCLC.