Dimeric isoxazolyl-1,4-dihydropyridines have enhanced binding at the multi-drug resistance transporter

A series of dimeric isoxazolyl-1,4-dihydropyridines (IDHPs) were prepared by click chemistry and examined for their ability to bind the multi-drug resistance transporter (MDR-1), a member of the ATP-binding cassette superfamily (ABC). Eight compounds in the present study exhibited single digit micromolar binding to this efflux transporter. One monomeric IDHP m-Br-1c, possessed submicromolar binding of 510nM at MDR-1. Three of the dimeric IDHPs possessed <1.5µM activity, and 4b and 4c were observed to have superior binding selectivity compared to their corresponding monomers verses the voltage gated calcium channel (VGCC). The dimer with the best combination of activity and selectivity for MDR-1 was analog 4c containing an m-Br phenyl moiety in the 3-position of the isoxazole, and a tether with five ethyleneoxy units, referred to herein as Isoxaquidar. Two important controls, mono-triazole 5 and pyridine 6, also were examined, indicating that the triazole - incorporated as part of the click assembly as a spacer - contributes to MDR-1 binding. Compounds were also assayed at the allosteric site of the mGluR5 receptor, as a GPCR 7TM control, indicating that the p-Br IDHPs 4d, 4e and 4f with tethers of from n=2 to 5 ethylenedioxy units, had sub-micromolar affinities with 4d being the most efficacious at 193nM at mGluR5. The results are interpreted using a docking study using a human ABC as our current working hypothesis, and suggest that the distinct SARs emerging for these three divergent classes of biomolecular targets may be tunable, and amenable to the development of further selectivity.