Selective potentiation of lometrexol growth inhibition by dipyridamole through cell-specific inhibition of hypoxanthine salvage

Abstract

Summary The novel antifolate lometrexol (5,10-dideazatetrahydrofolate) inhibits de novo purine biosynthesis, and co-incubation with hypoxanthine abolishes its cytotoxicity. The prevention of hypoxanthine rescue from an antipurine antifolate by the nucleoside transport inhibitor dipyridamole was investigated for the first time in nine human and rodent cell lines from seven different tissues of origin. In A549, HeLa and CHO cells, dipyridamole prevented hypoxanthine rescue and so growth was inhibited by the combination of lometrexol, dipyridamole and hypoxanthine, but in HT29, HCT116, KK47, MDA231, CCRF CEM and L1210 cells dipyridamole had no effect and the combination did not inhibit growth. Dipyridamole inhibited hypoxanthine uptake in A549 but not in CCRF CEM cells. Dipyridamole prevented the hypoxanthine-induced repletion of dGTP pools, depleted by lometrexol, in A549 but not in CCRF CEM cells. Thus, the selective growthinhibitory effect of the combination of lometrexol, dipyridamole and hypoxanthine is apparently due to the dipyridamole sensitivity (ds) or insensitivity (di) of hypoxanthine transport. Both the human and murine leukaemic cells are of the di phenotype. If this reflects the transport phenotype of normal bone marrow it would suggest that the combination of lometrexol, dipyridamole and hypoxanthine might be selectively toxic to certain tumour types and have reduced toxicity to the bone marrow.

Resistance-Modifying Agents. 11. Pyrimido[5,4-d]pyrimidine Modulators of Antitumor Drug Activity. Synthesis and Structure−Activity Relationships for Nucleoside Transport Inhibition and Binding to α1-Acid Glycoprotein

“…The cardiovascular and antithrombotic agent dipyridamole (DP) functions as an indirect purinergic agonist by inhibiting the cellular reuptake of adenosine, and this is thought to account, at least in part, for the activity of the drug . In contrast to NBT, DP inhibits both ENT1 and ENT2 transporters and has been employed successfully to potentiate the in vitro cytotoxicity of a number of antifolate antitumor agents. − However, clinical studies of antimetabolites in combination with DP have been disappointing, and the drug did not markedly increase either antiproliferative toxicity or antitumor activity in a number of studies. − DP binds avidly to the serum protein α 1 -acid glycoprotein (AGP), the levels of which can be elevated in the plasma of cancer patients. , As a consequence, free levels of DP may not reach those necessary to inhibit nucleoside transport adequately, and this effect on potency has been clearly demonstrated in experimental systems, employing physiological concentrations of AGP…”

“…21 In contrast to NBT, DP inhibits both ENT1 and ENT2 transporters and has been employed successfully to potentiate the in vitro cytotoxicity of a number of antifolate antitumor agents. [22][23][24][25][26][27] However, clinical studies of antimetabolites in combination with DP have been disappointing, and the drug did not markedly increase either antiproliferative toxicity or antitumor activity in a number of studies. [27][28][29][30][31][32][33] DP binds avidly to the serum protein R 1 -acid glycoprotein (AGP), the levels of which can be elevated in the plasma of cancer patients.…”

Dipyridamole potentiates the in vitro activity of MTA (LY231514) by inhibition of thymidine transport

“…Similarly, the cytotoxicity of methotrexate, which inhibits both de novo thymidylate and purine biosynthesis, is increased by DP in a variety of human and animal cells through inhibition of thymidine and/or hypoxanthine uptake (Nelson and Drake, 1984;Van Mouwerik et al, 1987;Hughes and Tattersall, 1989;Chan and Howell, 1990). DP has been shown to inhibit hypoxanthine transport in some cell lines but not others (Plagemann and Wohlhueter, 1984a), and has been used to produce a cell line-specific blockade of hypoxanthine rescue from antipurine antifolate-induced growth inhibition (Turner et al, 1997;Marshman et al, 1998).…”

Nucleoside Transport Inhibitors: Structure−Activity Relationships for Pyrimido[5,4-d]pyrimidine Derivatives That Potentiate Pemetrexed Cytotoxicity in the Presence of α1-Acid Glycoprotein

“…Diastereoisomers (10 and 11) of 6 were prepared as described previously by reaction of 2,6-dichloro-N,N 0 -bis(3,4-dimethoxybenzyl)pyrimido [5,4-d]pyrimidine-4,8-diamine with an excess of the appropriate enantiomer of 1-amino-2-propanol. 35 The required aryl carbamate derivatives (12)(13)(14)(15)(16)(17)(18)(19) were readily synthesized in satisfactory yield by treatment of 6 with the appropriate aryl isocyanate in THF, followed by separation of the mono-and symmetrical diaryl carbamates by chromatography on silica (Scheme 1). With the exception of the mono-and diglycine carbamates (20 and 21), a similar approach was also adopted for the preparation of the target amino acid carbamate derivatives (22)(23)(24)(25)(26).…”

“…Efforts to overcome the unfavorable pharmaceutical properties of 1 through the synthesis of lipophilic derivatives (e.g., 2 ) have recently been reported . The established antithrombotic and cardiovascular drug dipyridamole (DP, 3 ) is a potent ENT inhibitor, , which has been demonstrated to potentiate the cytotoxicity and antitumor activity of a number of antimetabolite drugs by blocking nucleoside uptake for salvage. − Moreover, by abrogating cytotoxic nucleoside efflux, 3 can also act synergistically with base- or nucleoside-derived antimetabolite drugs, e.g., 5-fluorouracil, as well as exacerbating the nucleotide pool imbalances elicited by these agents . Interestingly, 3 has very recently been reported to augment the replication of oncolytic Herpes simplex viral vectors utilized for tumor therapy, through a mechanism involving replicative amplification as a consequence of NT inhibition .…”