Effect of a Dihydropyridine Analogue, 2-[Benzyl(phenyl)amino]ethyl l,4-Dihydro-2,6- dimethyl-5-(5,5-dimethyl-2-oxo-l,3,2-dioxaphosphorinan-2-yl)-l-(2-morpholino- ethyl)-4-(3-nitrophenyl)-3-pyridinecarboxylate on Reversing in Vivo Resistance of Tumor Cells to Adriamycin1

A newly synthesized dihydropyridine analogue, 2-|benzyl(phenyl)amino]ethyl l,4-dihydro-2,6-dimethyl-5-(5,5-dimethyl-2-oxo-l,3,2-dioxaphosphorinan-2-yl)-l-(2-morpholinoethyl)-4-(3-nitrophenyl)-3-pyridinecarboxylate (PAK-200), at 5 pM inhibited the efflux of |'H|vincristine from KB-C2 cells and increased the accumulation of |}H)vincristine in KB-C2 cells to a level similar to that in KB-3-1 cells. PAK-200 inhibited the photoaffinity labeling of P-glycoprotein in KB-C2 mem branes by |3H]azidopine. At 5 JIM,PAK-200 enhanced the cytotoxic effect of Adriamycin on drug-sensitive KB-3-1 cells, multidrug-resistant KB8-5 cells, and two human colorectal carcinoma tumor lines, COK-28LN and COK-36LN, by factors of 2, 5, 2, and 3 times, respectively. The calcium antagonistic activity of PAK-200 was about 1000 and 5 times lower than that of another dihydropyridine analogue, nicardipine, and of verapamil, respectively. PAK-200 in combination with Adriamycin completely suppressed the growth of KB-3-1 and COK-36LN and partially suppressed the growth of KB-8-5 but had no significant effect on COK-28LN cells xenografted in nude mice. The level of MDRl expression of COK-36LN was about 3 times higher than that of COK-28LN, but lower than that of KB-8-5 cells. These results suggest that the interaction of PAK-200 with P-glyco protein may be partly correlated with the enhancement of the antitumor effect of Adriamycin on xenografted KB-8-5 and COK-36LN cells in nude mice.