Antifolates have been used to treat
cancer for the last 50 years and remain the mainstay of many therapeutic regimes.
Nucleoside salvage, which depends on plasma membrane transport, can compromise the activity of
antifolates. The
cardiovascular drug dipyridamole inhibits
nucleoside transport and enhances
antifolate cytotoxicity in vitro, but its clinical activity is compromised by binding to the
plasma protein alpha(1)-acid glycoprotein (AGP). We report the development of a novel pyrimidopyrimidine analogue of
dipyridamole,
NU3153, which has equivalent potency to
dipyridamole, remains active in the presence of physiologic levels of AGP, inhibits
thymidine incorporation into
DNA, and prevents
thymidine and
hypoxanthine rescue from the multitargeted
antifolate,
pemetrexed. Pharmacokinetic evaluation of
NU3153 suggested that a soluble
prodrug would improve the in vivo activity. The
valine prodrug of
NU3153, NU3166, rapidly broke down to
NU3153 in vitro and in vivo. Plasma
NU3153 concentrations commensurate with rescue inhibition in vitro were maintained for at least 16 hours following administration of NU3166 to mice at 120 mg/kg. However, maximum inhibition of
thymidine incorporation into
tumors was only 50%, which was insufficient to enhance
pemetrexed antitumor activity in vivo. Comparison with the cell-based studies revealed that
pemetrexed enhancement requires substantial (> or =90%) and durable inhibition of
nucleoside transport. In conclusion, we have developed non-AGP binding
nucleoside transport inhibitors. Pharmacologically active concentrations of the inhibitors can be achieved in vivo using
prodrug approaches, but greater potency is required to evaluate inhibition of
nucleoside rescue as a therapeutic maneuver.