Metesind glucuronate

Cellular uptake of hydrophilic antifolates proceeds via the reduced folate carrier whereas lipophilic antifolates enter cells by diffusion. Recently we have shown that transfectant cells overexpressing the mutant G482 ABCG2 displayed 120-6,250-fold resistance to hydrophilic antifolates than untransfected cells upon 4 h drug exposure, but lost almost all their antifolate resistance upon 72 h drug exposure (Shafran et al. in Cancer Res 65:8414-8422, 2005). Here we explored the ability of the wild type (WT) R482-as well as the mutant G482-and T482 ABCG2 to confer resistance to lipophilic antifolate inhibitors of dihydrofolate reductase (trimetrexate, piritrexim, metoprine and pyrimethamine) and thymidylate synthase (AG337, AG377 and AG331). Lipophilic antifolate resistance was determined using growth inhibition assays upon 72 h drug exposure. Cells overexpressing these mutant efflux transporters displayed up to 106-fold resistance to lipophilic antifolates relative to untransfected cells; this resistance was reversed by the specific and potent ABCG2 efflux inhibitor Ko143. In contrast, cells overexpressing the WT R482 ABCG2 exhibited either no or only a low-level of lipophilic antifolate resistance. These results provide the first evidence that overexpression of the mutant G482- and T482 but not the WT R482 ABCG2 confers a high-level of resistance to lipophilic antifolates. The high membrane partitioning of lipophilic antifolates along with the large confinement of ABCG2 to the plasma membrane suggest that these mutant ABCG2 transporters may possibly recognize and extrude lipophilic antifolates from the lipid bilayer. The potential implications to cancer chemotherapy as well as the mechanism of anticancer drug extrusion by these mutant exporters are discussed.

This study showed that small, lipophilic folate analogs AG337 and AG331, optimized to bind human thymidylate synthase (TS) at low nanomolar concentrations, bound Plasmodium TS 130-fold more weakly.Based on thermodn. principles, it should be possible to identify reciprocal small mols. that bind Plasmodium TS better than human TS.Data suggest that species-specific inhibitor design may be realized by exploiting subtle host-parasite differences in enzyme active-site conformations that are dictated by distant amino acids in the proteins.It is proposed that, in the absence of folate-derivatives, human and Plasmodium TS active sites exist in slightly different ground-state conformations.Upon meeting the full-length 5,10-methylenetetrahydrofolate substrate, or the full size inhibitor 1843U89, favorable interactions could provide sufficient intrinsic binding energy to both change the conformation of the enzymes to the catalytically active form and still maintain effective enzyme inhibition.