Plasma cell myeloma: response of melphalan-resistant patients to high-dose intermittent cyclophosphamide.

High-dose intermittent cyclophosphamide was used in the treatment of 19 patients with plasma cell myeloma who had demonstrated resistance to melphalan. Good objective improvement was found in six patients and partial responses in five. Patients have survived from three to more than 36 months (estimated median 21 months) after starting cyclophosphamide, and 11 are still alive. The important implications of the lack of cross-resistance between these two alkylating agents are discussed in terms of the mechanisms of cellular resistance to alkylating agents.