p27(Kip1) V109G as a biomarker for CDK4/6 inhibitors indication in hormone receptor–positive breast cancer

CDK4/6 inhibitors benefit a minority of patients who receive them in the breast cancer adjuvant setting. p27(Kip1) is a protein that inhibits CDK/Cyclin complexes. We hypothesized that single-nucleotide polymorphisms that impaired p27(Kip1) function could render patients refractory to endocrine therapy but responsive to CDK4/6 inhibitors, narrowing the patient subpopulation that requires CDK4/6 inhibitors. We found that the p27(Kip1) V109G single-nucleotide polymorphism is homozygous in approximately 15% of hormone-positive breast cancer patients. Polymorphic patients experience rapid failure in response to endocrine monotherapy compared with wild-type or heterozygous patients in the first-line metastatic setting (progression-free survival: 92 vs 485 days, P < .001); when CDK4/6 inhibitors are added, the differences disappear (progression-free survival: 658 vs 761 days, P = .92). As opposed to wild-type p27(Kip1), p27(Kip1) V109G is unable to suppress the kinase activity of CDK4 in the presence of endocrine inhibitors; however, palbociclib blocks CDK4 kinase activity regardless of the p27(Kip1) status. p27(Kip1) genotyping could constitute a tool for treatment selection.