The hidden impact of renal impairment on ribociclib exposure and co-medications: a case report

Ribociclib, a cyclin-dependent kinase (CDK) 4/6 inhibitor, is widely used in combination with endocrine therapy for advanced hormone receptor-positive breast cancer. However, drug exposure may be highly variable in older patients due to organ dysfunction and polypharmacy, raising the need for therapeutic drug monitoring (TDM).

We report the case of an 88-year-old woman with metastatic lobular breast carcinoma treated with ribociclib and tamoxifen, who developed progressive renal impairment and was co-prescribed apixaban. Serial TDM demonstrated ribociclib overexposure (Cmin 1022-1318 ng/mL vs. ~ 500 ng/mL expected at steady state), associated with markedly increased plasma concentrations of tamoxifen, endoxifen, and apixaban (923 ng/mL vs. reference peak ~ 206 ng/mL). Monte Carlo simulations incorporating patient covariates confirmed that measured ribociclib concentrations exceeded predicted distributions, likely due to the accumulation of uremic toxins.

TDM of CDK4/6 inhibitors provides valuable insight into drug exposure in complex geriatric oncology settings. Beyond monitoring ribociclib itself, this approach facilitates a broader discussion on individualized dosing, comedication safety, and management of organ dysfunction in elderly patients with cancer.