Plasma uridine changes in cancer patients treated with the combination of dipyridamole and N-phosphonacetyl-L-aspartate.

Dipyridamole (DP) and N-phosphonacetyl-L-aspartate (PALA) act synergistically in vitro against many cell lines and in vivo against human ovarian carcinoma xenografts. We have conducted a phase I clinical trial of DP p.o. (50 mg/m2, every 6 h) in combination with PALA (starting at 500 mg/m2 i.v. with 300-mg dose escalations). Sixty-five patients were entered into this study, and we have established the maximum tolerated dose of PALA to be 4.5 g/m2 when combined with DP, which is approximately 80% of the previously reported maximum tolerated dose for PALA alone. The observed toxicities of DP plus PALA were mild and were similar to those reported for PALA alone. Bone marrow toxicities were not evident at any PALA dose. Ten patients with a mean pretreatment plasma uridine concentration of 3.49 +/- 1.28 (SD) microM had their plasma uridine reduced to 2.29 +/- 0.70 microM 9 h after DP p.o. A peak plasma DP concentration of 1.86 +/- 0.99 microM was achieved approximately 2 h after p.o. dosing. Nine patients who had a reduced plasma uridine concentration of 2.46 +/- 0.61 microM after 1 week of DP had their plasma uridine further reduced by PALA to 0.87 +/- 0.23 microM 7 h post-PALA. Daily plasma uridine measurements in two patients during their DP treatment confirmed the previously described pattern for Day 1, but the data suggest a slight recovery (15%) in plasma uridine by Day 2. Daily sampling in two other patients after a single PALA dose of 4.2 g/m2 showed that their plasma uridine declined 5 h after the PALA dose and remained depressed for 6 days in one patient and 11 days in the other. These results suggest that DP worked in synergy with PALA to lower circulating uridine in cancer patients. The mechanism for the ability of DP to reduce plasma uridine is not known, but there is evidence that DP can inhibit cellular efflux of uridine as well as its uptake. DP may reduce plasma nucleoside pools in addition to blocking nucleoside salvage and therefore have general applicability in other chemotherapy regimens.