WR-2721 protects against the hematologic toxicity of cyclophosphamide: a controlled phase II trial

WR-2721 S-2-(3-aminopropylamino) ethyl phosphorothioic acid, is an organic thiophosphate compound that in the animal model selectively protects against the hematologic toxicity of cyclophosphamide by factors of 1.5 to 2.0. Preliminary data from our controlled phase I trial of WR-2721 and cyclophosphamide suggested that WR-2721 protected against cyclophosphamide-induced granulocytopenia. Since variable drug doses and infusion rates were used in these early studies, we initiated a controlled phase II trial using constant drug doses to establish more precisely WR-2721's level of protection. Initially, 21 patients received 1,500 mg/m2 of cyclophosphamide alone and were retreated 4 weeks later after hematologic recovery was complete with 740 mg/m2 of WR-2721 before the same dose of cyclophosphamide. With WR-2721 pretreatment, 19 of 21 (90%) patients had improved WBC and granulocyte counts. The mean WBC increased from 1,760/mL with cyclophosphamide alone to 2,500/mL with WR-2721 pretreatment (P less than .0005). The mean granulocyte count increased from 541/mL on cyclophosphamide to 1,247/mL with WR-2721 and cyclophosphamide (P less than .0005). Following cyclophosphamide administration alone, neutropenic fevers developed in three patients. No patient experienced a febrile episode following WR-2721 and cyclophosphamide administration. Platelet nadirs below 100,000/mL were only noted in two patients treated with cyclophosphamide alone. Objective partial responses were observed in four of 19 (21%) patients with measurable or evaluable disease. These data suggest that WR-2721 provides significant protection against cyclophosphamide-induced hematologic toxicity.