Phase I/pharmacokinetic trial of the novel thioxanthone SR233377 (WIN33377) on a 5-day schedule

SR233377 (WIN33377) is a novel 4-aminomethyl thioxanthone derivative with promising preclinical activity against solid tumors at doses substantially below the MTD. We performed a phase I trial to determine a suitable phase II dose of SR233377 when administered as a 2-h intravenous infusion for five consecutive days.

A group of 25 patients with a range of solid tumor diagnoses and good performance status received SR233377 at eight dose levels ranging from 4.8 mg/m2 per day to 74.7 mg/m2 per day. Cycles were repeated every 35 days and patients were evaluated for response following two cycles of treatment. Doses were escalated in cohorts of three using a modified Fibonacci scheme. Pharmacokinetic sampling was performed during the first cycle in all patients.

Toxicities of SR233377 on this schedule included neutropenia, fever, nausea, and dyspnea but all were mild and not dose-limiting. Asymptomatic prolongation of the corrected QT (QTc) interval during infusion in all patients monitored at the 74.7 mg/m2 dose level prompted closure of the study. QT lengthening correlated with increasing plasma concentrations of SR233377. SR233377 Cmax values increased linearly with dose, but substantial interpatient variability in SR233377 AUC, clearance, and half-life was noted. There was no evidence of drug accumulation when day 1 and day 5 AUC and Cmax values were compared. Seven patients displayed tumor growth inhibition lasting for 4 months or more.

We conclude that SR233377 administered on a 5-day schedule is associated with tolerable clinical symptoms and some activity against a range of solid tumors but dosing is limited by QTc prolongation, a condition that predisposes to ventricular arrhythmias. Phase II development on this schedule is not recommended based on the occurrence of this concentration-dependent effect. Further investigation of alternative schedules of administration and of SR233377 analogues is warranted.