[Breast Cancer‒Associated Allergy Caused by Fosnetupitant-A Report of Four Cases].

We describe the cases of 4 patients with breast cancer who developed an allergy to fosnetupitant (Pro‒NETU). Case 1: A 67‒year‒old woman with breast cancer and bone metastasis received premedication that included Pro‒NETU. Minutes after administration, she complained of flushing, tachycardia, and dyspnea. Administration was discontinued. Minutes after discontinuation, the patient's symptoms were alleviated. She experienced no further complaints of such symptoms, and her premedication subsequently excluded Pro‒NETU. Case 2: A 50‒year‒old woman with early stage breast cancer received premedication that included Pro‒NETU. Minutes after administration, she complained of flushing, dyspnea, and drowsiness. Administration was discontinued. Minutes after discontinuation, her symptoms ameliorated. Case 3: A 41‒year‒old woman with early stage breast cancer received premedication that included Pro‒NETU. Minutes after administration, she complained of flushing, tachycardia, and dyspnea. Administration was thus discontinued, and she received an H2‒blocker and corticosteroid. Minutes after discontinuation, her symptoms were alleviated. Case 4: A 50‒year‒old woman with early stage breast cancer received premedication that included Pro‒NETU. Minutes after administration, she complained of flushing and tachycardia. Administration was discontinued. Minutes after discontinuation, her symptoms ameliorated. Fosaprepitant did not cause these allergies. The patients in cases 2, 3, and 4 had known allergies to docetaxel, and the cause of allergy in case 4 was unknown. Polysorbate 80, contained in docetaxel, fosaprepitant, and Pro‒NETU, was suspected to be the cause of allergy in cases 1, 2, and 3.