Phase I Clinical Trial of a Combination of Dipyridamole and Acivicin Based Upon Inhibition of Nucleoside Salvage

A Phase I clinical trial of simultaneous 72-h infusions of dipyridamole and acivicin was carried out in patients with advanced malignancies. The objective of this trial was to determine the maximum tolerated dose of dipyridamole when administered as a 72-h infusion in combination with acivicin. The development of this combination is of interest because of in vitro observations which demonstrate that dipyridamole potentiates the cytotoxic action of acivicin by blocking nucleoside salvage. Patients were treated with concomitant i.v. infusions of dipyridamole and acivicin for 72 h. The acivicin dose infused remained constant during the trial at 60 mg/m²/72 h. The maximum tolerated dose (MTD) of dipyridamole was 23.1 mg/kg/72 h. Limiting toxicities at the MTD of dipyridamole with acivicin were severe gastrointestinal and constitutional symptoms which appeared to be caused by the high doses of dipyridamole administered. Escalation of dipyridamole did not potentiate the mild myelosuppression or the neurotoxicity which occurs with acivicin alone. At a dose of dipyridamole which was well below the MTD, one patient experienced symptomatic orthostatic hypotension, and another patient with coronary artery disease developed dizziness and transient electrocardiogram abnormalities. However, no other hypotensive or cardiovascular events occurred as dipyridamole was escalated to the MTD. Phlebitis occurred at the site of infusion when the dose of dipyridamole exceeded 13.5 mg/ kg/72 h. Because of this local toxicity, it was necessary to administer dipyridamole through a central venous catheter to achieve maximum plasma levels. At the MTD of dipyridamole, steady-state total and free plasma levels of 11.9 μM and 27.8 nM, respectively, were attained by 24 h. These are free dipyridamole levels which in vitro were sufficient to block cytidine salvage and to potentiate the biochemical and cytotoxic effects of acivicin against human colon cancer cells (P. H. Fischer et al., Cancer Res., 44:3355-3359,1984).