Concurrent inhibition of CDK2 adds to the anti-tumour activity of CDK4/6 inhibition in GIST.

Abstract

Advanced gastrointestinal stromal tumour (GIST) is characterised by genomic perturbations of key cellcycle regulators. Oncogenic activation of CDK4/6 results in RB1 inactivation and cell cycle progression. Given that single-agent CDK4/6 inhibitor therapy failed to show clinical activity in advanced GIST, we evaluated strategies for maximising response to therapeutic CDK4/6 inhibition. Targeted next-generation sequencing and multiplexed protein imaging were used to detect cellcycle regulator aberrations in GIST clinical samples. The impact of inhibitors of CDK2, CDK4 and CDK2/4/6 was determined through cellproliferation and proteindetection assays. CDK-inhibitor resistance mechanisms were characterised in GIST cell lines after long-term exposure. We identify recurrent genomic aberrations in cellcycle regulators causing co-activation of the CDK2 and CDK4/6 pathways in clinical GIST samples. Therapeutic co-targeting of CDK2 and CDK4/6 is synergistic in GIST cell lines with intact RB1, through inhibition of RB1 hyperphosphorylation and cell proliferation. Moreover, RB1 inactivation and a novel oncogenic cyclin D1 resulting from an intragenic rearrangement (CCND1::chr11.g:70025223) are mechanisms of acquired CDK-inhibitor resistance in GIST. These studies establish the biological rationale for CDK2 and CDK4/6 co-inhibition as a therapeutic strategy in patients with advanced GIST, including metastatic GIST progressing on tyrosine kinase inhibitors.