Drug Interaction Report

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 CLOSELY: Coadministration with folate therapy may potentiate the pharmacologic effects of 5-fluorouracil (5-FU). The exact mechanism of interaction is unknown. Although enhancement of 5-FU cytotoxicity may be used to advantage in some cancer patients, increased toxicity should also be considered. Deaths from severe enterocolitis, diarrhea, and dehydration have been reported in elderly patients receiving weekly leucovorin and fluorouracil. In a clinical study consisting of 148 patients with advanced untreated colorectal cancer, weekly administration of 5-FU (600 mg/m2) in combination with leucovorin (500 mg/m2) was associated with a higher response rate than 5-FU alone (23% versus 8%). However, the combination was also more toxic than 5-FU alone, as evidenced by a higher incidence of grade 3 to 4 diarrhea (19.5% versus 8.5%) and conjunctivitis (26.5% versus 5.6%), as well as one recorded toxic death versus none. No differences in median survival and time to progression were observed between the two groups. Similar results were observed in another study with capecitabine, a prodrug of 5-FU. The interaction has also been reported with folic acid. A published case report describes two patients who were hospitalized for presumed 5-FU toxicity (anorexia, severe mouth ulceration, bloody diarrhea, vaginal bleeding) during concomitant treatment with a multivitamin containing folic acid (0.5 mg in one and 5 mg in the other). Both patients tolerated subsequent courses of 5-FU at the previous dosage following discontinuation of the multivitamin. Another published report describes a breast cancer patient who died during treatment with capecitabine (2500 mg/m2 daily for 14 days every 3 weeks) while taking folic acid 15 mg/day. The patient developed diarrhea, vomiting, and hand-foot syndrome eight days after starting capecitabine therapy. Her condition improved briefly following discontinuation of capecitabine and then folic acid, but she subsequently developed necrotic colitis and died from septic shock and vascular collapse.

MANAGEMENT: Caution is advised if 5-FU or any of its prodrugs (e.g., capecitabine, tegafur) are prescribed in combination with leucovorin. A lower dosage of 5-FU or the prodrug may be required. Therapy with leucovorin and fluorouracil should not be initiated or continued in patients with symptoms of gastrointestinal toxicity until such symptoms have resolved. Closely monitor patients with diarrhea until it resolves. Monitor for other potential toxicities of 5-FU such as neutropenia, thrombocytopenia, stomatitis, cutaneous reactions, and neuropathy. Patients should be instructed to avoid taking folic acid supplementation or multivitamin preparations containing folic acid without first speaking with their physician.