Does Cyclin-dependent Kinase 4/6 Inhibitor Toxicity Predict Treatment Response in Metastatic Hormone Positive Breast Cancer?

In estrogen and/or progesterone receptor expression (hormone receptor [HR]-positive) without human epidermal growth factor receptor 2 (HER2) overexpression (HR + /HER2 − ) metastatic breast cancer (MBC), a significant progression-free survival (PFS) benefit has been obtained with cyclin-dependent kinase 4/6 (CDK 4/6) inhibitor and endocrine therapy (ET) combinations in the first-line treatment. We mainly aimed to investigate whether the toxicities of CDK 4/6 inhibitors predict treatment response. This study was designed retrospectively. A total of 191 patients diagnosed with MBC were treated with CDK 4/6 inhibitors plus ETs between 2019 and 2021, in four centers included. One hundred six patients received ribociclib, and 85 patients received palbociclib. The most common adverse event in both groups was neutropenia. We found that toxicities didn’t predict response rates (RRs). Additionally, the RRs in patients with albumin levels above 4.1g/dl was better than that in patients with albumin levels 4.1g/dl and below in multivariate analysis when all patients were considered (OR,4.76;95%CI,1.30–17.46; p  = 0.018). When the multivariate analysis was performed separately for those who received ribociclib and those who received palbociclib, it was seen that this difference was due to ribociclib (OR,49.89;95%CI,2.49–999.16; p  = 0.011). Toxicities of CDK4/6-inhibitors didn’t predict RRs. However, pretreatment albumin level may predict ribociclib response.