The cardiovascular and antithrombotic agent dipyridamole (DP) has potential therapeutic utility as a modulator of the activity of antimetabolite antitumor agents by virtue of its inhibition of nucleoside transport. However, the activity of DP can be compromised by binding to the acute phase serum protein, alpha(1)-acid glycoprotein (AGP). Analogues of DP were synthesized and evaluated as inhibitors of (3)H-thymidine uptake into L1210 leukamia cells in the presence and absence of 5 mg/mL AGP. Compounds with potency similar to that of DP were identified where the piperidino substituents at the 4,8-positions were replaced by 4'-methoxybenzylamino, 3',4'-dimethoxybenzylamino, or piperonylamino groups. Replacement of the diethanolamino groups at the 2,6-positions of DP by alkylamino or alkoxy substituents was tolerated, although at least one oxygen-bearing function (hydroxyl or alkoxy) was required in the side chain for activity comparable to that of DP. Whereas AGP completely ablated the activity of DP, the majority of the newer compounds synthesized retained significant activity in the presence of excess AGP, although replacement of the piperidino groups at the 4,8-positions by N-methylbenzylamino substituents did, in some cases, restore susceptibility to AGP. Selected compounds have been demonstrated to prevent rescue from antifolate cytotoxicity, mediated by nucleoside salvage.

Membrane transport of nucleosides or nucleobases is mediated by transporters including the equilibrative nucleoside transporters (ENTs), and resistance to antitumor antimetabolite drugs may arise via salvage of exogenous purine or pyrimidine nucleosides or nucleobases by ENT transporters. The therapeutic utility of dipyridamole (3), a potent ENT inhibitor, is compromised by binding to the serum protein α(1)-acid glycoprotein (AGP). Derivatives and prodrugs of the ENT inhibitor 4,8-bis[(3,4-dimethoxybenzyl)amino]-2,6-bis[(2-hydroxypropyl)amino]pyrimido[5,4-d]pyrimidine (6, NU3108) are described, with improved in vivo pharmacokinetic properties and reduced AGP binding relative to dipyridamole. The mono- and diglycine carbamate derivatives were at least as potent as 6 and showed no reduction in potency by AGP. In a [(3)H]thymidine incorporation assay, employing COR-L23 cells, the diastereoisomers of 6 (IC(50) = 26 nM) exhibited activity comparable with 3 (IC(50) = 15 nM). The monophenyl carbamate and mono-4-methoxyphenyl carbamate exhibited the best ENT-inhibitory activity in the COR-L23 assay (IC(50) = 8 and 4 nM, respectively). All of the new prodrugs were also highly effective at reversing thymidine/hypoxanthine rescue from pemetrexed cytotoxicity in the COR-L23 cell line.