Would you consider a CDK 4/6 inhibitor in combination with endocrine therapy for a patient with ER low (1-9%) metastatic breast cancer?

Whereas ER 1-9% represents a heterogeneous group of tumors with relatively poor response to endocrine monotherapy, there are few if any data on whether this group of patients should be excluded from the use of CDK 4/6 inhibitors. For example, Finn et al. demonstrated that in the PALOMA-2 study, ER H score <25 vs > 25 was not a predictor of palbociclib response (Finn et al., PMID 31527167). Furthermore, Finn et al. demonstrated that in both the ESR1 high- and low-expression groups, the addition o...