Inhibitors of dihydrofolate reductase as antitumor agents: design, synthesis and biological evaluation of a series of novel nonclassical 6-substituted pyrido[3,2-d]pyrimidines with a three- to five-carbon bridge.

Bridge homologation of the previously reported nonclassical two-carbon-bridged antifolate, 2,4-diamino-6-phenethylpyrido[3,2-d]pyrimidine (wm-5a), afforded the three-, four- and five-carbon-bridged antifolate analogues 3.1-3.5, 4.1-4.2 and 5.1-5.5. The target compounds, with substituents at various positions on the carbon bridges, were efficiently synthesized by aldol condensation or Wittig reaction and followed by reduction. Elongation of the two-carbon bridge to three-, four- or five-carbon bridges, and also saturation of the carbon bridges, provided compounds with good inhibitory activity against recombinant human DHFR (rhDHFR). Analogue 3.5, which has a three-carbon bridge, inhibited the proliferation of HL-60 and HCT116 cells to a greater extent than the other analogues. Compound 3.5 was also the most potent inhibitor of rhDHFR (IC50  = 0.06 μM), and was approximately 38-fold more potent than the two-carbon-bridged lead compound. Docking studies revealed that both the length and flexibility of the saturated carbon bridge in 3.5 were important for high potency. Flow cytometry studies indicated that compound 3.5 arrested HL-60 cells in the S-phase and induced apoptosis. Western blot analysis of HL-60 cells treated with 3.5 showed a dose-dependent upregulation of DHFR protein levels.