326P Patient selection biomarkers for CDK7 inhibitor samuraciclib (SAM; CT7001) combined with selective estrogen receptor degrader (SERD) in hormone receptor-positive advanced breast cancer (HR+ ABC) post-CDK4/6 inhibitor (CDK4/6i)

CDK7 enhances oncogene transcription, upregulates anti-apoptotic genes, accelerates the cell cycle and activates estrogen receptors driving resistance to endocrine therapy. SAM is a small molecule, ATP competitive, selective oral (PO) inhibitor of CDK7. SAM was administered to patients (pts) with HR+ ABC with fulvestrant (FUL), an intramuscular (IM) SERD, or giredestrant (GIR), a highly potent, non-steroidal, PO SERD, in 2 independent studies: CT7001_001 (NCT03363893) Module 2A and MORPHEUS (NCT04802759) [Coombes et al.