OBJECTIVE Suramin has been shown to be of interest as a potential new anticancer agent because of its capacity to inhibit the binding of several growth factors to their receptors and to inhibit the growth of cancer cells in vitro. Since multi-autocrine loops involving growth factors which are antagonized by suramin have been demonstrated in colorectal cancer, we previously evaluated the activity of suramin in patients with advanced colorectal cancer. Interestingly, in this study three patients who had received 5-FU+LV after suramin, although heavily pretreated with fluoropyrimidines, obtained an objective response. This observation was intriguing as it might have been that suramin had changed the biology of the tumor, making it sensitive to 5-FU+LV. We therefore conducted the present study to investigate the possibility that suramin might overcome the resistance to 5-FU+LV. METHODS Only colorectal cancer patients with metastatic and progressive disease during 5-FU+LV-based chemotherapy were eligible for this study. Suramin was administered for eight weeks at doses determined by means of a computer-assisted dosing algorithm that used Bayesian pharmacokinetics to maintain suramin plasma concentrations of 200-250 microg/ml. 5-FU was administered weekly at a dosis of 450 mg/m2 halfway through a two-hour infusion of I-LV 250 mg/m2 starting one week after the initiation of suramin for a maximum of 26 weeks. RESULTS Treatment was relatively well tolerated, but no objective responses were observed after the accrual of 13 patients in the first stage of the trial. Consequently, the trial was interrupted according to the initial two-stage sampling design. CONCLUSIONS The present study does not support the hypothesis that suramin might overcome resistance to 5-FU+LV and its use in colorectal cancer is not recommended.