Drug Interactions between letrozole / ribociclib and radium 223 dichloride

MONITOR CLOSELY: Coadministration of radium Ra 223 dichloride (Ra-223 dichloride) with other agents that can cause bone marrow suppression or myelosuppression may result in additive toxicity. Ra-223 dichloride alone is associated with thrombocytopenia, neutropenia, pancytopenia, and leukopenia; death from bone marrow failure has also been reported. In a randomized clinical trial in patients with metastatic castration-resistant prostate cancer with bone metastases, 2% of the patients on Ra-223 dichloride experienced bone marrow failure or ongoing pancytopenia compared to no patients in the placebo group. Grade 3-4 adverse reactions of thrombocytopenia and neutropenia were more commonly reported in patients who had received prior docetaxel. However, data from clinical drug interaction studies are lacking.

MANAGEMENT: Caution and close monitoring for additive hematologic toxicity are recommended if concomitant use of Ra-223 dichloride with other agents that can cause bone marrow suppression or myelosuppression is required. The manufacturer advises that Ra-223 dichloride be discontinued in patients requiring administration of chemotherapy, other systemic radioisotopes, or hemibody external radiotherapy. If concomitant use is required, the manufacturer's product labeling should be consulted for specific hematologic monitoring and dose adjustment recommendations. Some authorities recommend not initiating subsequent systemic cancer treatment for at least 30 days after the last administration of Ra-223 dichloride. Patients should be advised to contact their physician if they develop signs or symptoms of myelosuppression or infection including but not limited to pallor, dizziness, fatigue, lethargy, fainting, easy bruising or bleeding, fever, chills, sore throat, body aches, and/or other influenza-like symptoms.

MONITOR: Coadministration with ribociclib may increase the plasma concentrations and pharmacologic effects of drugs that are substrates of CYP450 3A4. The proposed mechanism is decreased clearance due to ribociclib-mediated inhibition of CYP450 3A4 metabolism. In healthy study subjects, administration of midazolam, a sensitive CYP450 3A4 substrate, with multiple 400 mg daily doses of ribociclib increased the midazolam peak plasma concentration (Cmax) and systemic exposure (AUC) by 2.1-fold and 3.8-fold, respectively, compared to midazolam administered alone. When given at a clinically relevant dose of 600 mg daily, ribociclib is predicted to increase midazolam Cmax and AUC by 2.4-fold and 5.2-fold, respectively.

MANAGEMENT: Caution is advised when ribociclib is used concomitantly with drugs that undergo metabolism by CYP450 3A4, particularly those with a narrow therapeutic range. Dosage adjustments as well as clinical and laboratory monitoring may be appropriate for some drugs whenever ribociclib is added to or withdrawn from therapy.