Inhibition of cyclic AMP phosphodiesterase by 2,6-dimethyl-4-(3-nitrophenyl)-1,4-dihydropyridine-3,5-dicarboxylic acid 3-[2-(N-benzyl-N-methylamino)] ethyl ester 5-methyl ester hydrochloride (YC-93), a potent vasodilator.

A new, potent vasodilator (YC-93), 2,6-dimethyl-4-(3-nitrophenyl)-1,4-dihydropyridine-3,5-dicarboxylic acid 3-2-(N-benzyl-N-methyl amino)] ethyl ester 5-methyl ester hydrochloride, competitively inhibited cyclic adenosine 3′,5′-monophosphate (cyclic AMP) phosphodiesterase in the 105,000 g supernatant solutions from canine basilar, carotid, coronary and femoral arteries. The inhibition constant (K_i) of YC-93 for these enzyme preparations was in the range of 2.0–4.3 μM at substrate concentrations near the low K_m (about 1 μM for each enzyme preparation), and was 4.0–12 μM at substrate concentrations near the high K_m (50–70 μM). The potency of YC-93 for inhibition of coronary phosphodiesterase at 1 μM cyclic AMP and 50 μM cyclic AMP was much greater than that of papaverine and 3-isobutyl-l-methyl xanthine (IBMX). Commercially available cyclic AMP phosphodiesterase purified from beef heart was also strongly inhibited by YC-93 in a competitive manner and its K_i value was 2.0 μM in the wide range of substrate concentrations tested. Studies on the structure-activity relationship using low K_m phosphodiesterase from canine coronary artery and high K_m phosphodiesterase from beef heart, demonstrated that 3,5-diethoxycarbonyl-1,4-dihydro-2, 4,6-collidine, the simplest 1,4-dihydropyridine derivative (tested in the present studies) resulted in slightly less inhibition than papaverine, and the inhibitory potency of the former compound was greatly increased mainly by two structural modifications. Firstly, addition of a nitrophenyl group at position 4 of the 1,4-dihydropyridine ring, secondly, the replacement of ethylester at position 3 of the 1,4-dihydro-pyridine ring by N-benzyl-N-methylaminoethyl ester. A few dihydropyridine derivatives together with YC-93 were the most potent inhibitors of cyclic AMP phosphodiesterase among the compounds tested. The finding that the level of cyclic AMP in canine arterial strips was increased by 64 per cent (P < 0.01) even after 1 min exposure to 1 μM YC-93 supports the possibility of at least a partial involvement of phosphodiesterase inhibition in vasodilation by the drug.