Hypersensitivity reaction to a metabolite of cyclophosphamide.

Generalized urticaria and fever were noted in a patient with IgA (kappa) myeloma after intravenous cyclophosphamide. Intradermal skin testing revealed no reaction to cyclophosphamide and its analog, isophosphamide. However, phosphoramide mustard, a principle metabolite of cyclophosphamide, evoked an immediate wheal-and-flare response. Subsequent therapy with isophosphamide was well tolerated. These findings suggest that acute hypersensitivity reactions to cyclophosphamide are due to its metabolites and can be delineated with skin testing.