Development and validation of a pharmacokinetically based fixed dosing scheme for suramin

Purpose: We used population pharmacokinetic-parameter estimates and designed a fixed dosing schedule to maintain plasma suramin concentrations between 100 and 300 μg/mL and then evaluated its performance. Materials and Methods: On day 1, patients received a 200-mg test dose and 1,000-mg/m² loading dose. On days 2, 3, 4, and 5, patients received 1-hour infusions of 400, 300, 250, and 200 mg/m², respectively. Subsequent 1-hour infusions of 275 mg/m² were given on days 8, 11, 15, 19, 22, 29, 36, 43, 50, 57, 67, and 78. Therapy was discontinued for dose-limiting toxicity (DLT) or progressive disease (PD). Patients were to be removed from the fixed dosing schedule if, after day 5, three consecutive peak plasma suramin concentrations were greater than 300 μg/mL. Results: Forty-two patients, including 40 with hormone-refractory prostate cancer (HRPC), received 700 infusions. Forty patients were assessable for toxicity; 38 were assessable for response. Two patients with preexisting pulmonary disease died early of respiratory insufficiency. Treatment was discontinued in five patients due to DLT and in seven due to PD. No patient had treatment discontinued due to repeated peak plasma suramin concentrations ≥ 300 μg/mL. The fixed dosing schedule was precise, unbiased, and well tolerated. DLT consisted of grade 4 nephrotoxicity (n = 2), neurotoxicity (n = 2), and corticosteroid-induced psychosis (n = 1). Three patients, who received all 18 doses of suramin per protocol, developed severe, but not dose-limiting, malaise, fatigue, and lethargy. Twenty-four of 36 assessable patients with elevated serum prostate- specific antigen (PSA) levels had a ≥ 50% reduction, lasting more than 4 weeks, and 18 had a ≥ 75% reduction, lasting more than 4 weeks. Twelve of 23 (52%) symptomatic HRPC patients noted a subjective improvement in pain. There were no measurable responses in four patients with measurable disease. The estimated median survival time in 38 assessable patients with HRPC was 18.8 months. The estimated median time to progression in 35 patients, for whom data were available, was 10.1 months. Conclusion: This easily implemented schedule allowed suramin to be administered safely as an intermittent bolus injection. Toxicity was manageable and reversible.