2(1H)-quinolinones with cardiac stimulant activity. 3. Synthesis and biological properties of 6-imidazol-1-yl derivatives.

A series of 6-imidazol-1-yl-8-methyl-2(1H)-quinolinones was synthesized and evaluated for cardiac stimulant activity in dogs. The majority of compounds were prepared from an appropriate 6-imidazol-1-yl-2(1H)-quinolinone precursor or by sulfuric acid catalyzed cyclization of an N-(4-heteroarylphenyl)-3-ethoxypropenamide. Introduction of a range of 5-substituents into 6-(2,4-dimethylimidazol-1-yl)-8-methyl-2(1H)-quinolinone (1) reduced inotropic activity in anesthetized dogs (percentage increase in dP/dtmax) although replacement of the 2-methyl group by iodo (10) or cyano (11) substituents was well tolerated. The 2-methyl-4-chloro (15) and 2-methyl-4-(methylthio) (22) derivatives displayed similar potency to 1 (40-50% increase in dP/dtmax, 10-12.5 micrograms/kg) and these compounds were 3-5 times more potent than milrinone. Introduction of iodo (14), cyano (16), or acetyl (17) substituents into the 4-position approximately halved inotropic activity. In conscious dogs, 11 (0.25 mg/kg) and 16 and 17 (0.125 mg/kg) produced similar increases in cardiac contractility (decrease in the QA interval) to 1 (0.125 mg/kg) and maximum responses were maintained for at least 3 h. Dose-related (25, 125, 250 micrograms/kg) cardiac stimulant activity was demonstrated by 17 and after the higher doses a marked response (approximately 30% increase in dP/dtmax) was still observed after 7 h, in contrast to milrinone. The substantial increases in cardiac contractility observed with 16 and 17 in the conscious dog were not accompanied by any tachycardia. These compounds also displayed an overwhelming selectivity for increasing the force of cardiac contraction (greater than 120% increase in dP/dtmax) rather than heart rate (5-10 beats/min decrease) in the Starling heart-lung preparation. As a result of this beneficial pharmacological profile, 6-(4-acetyl-2-methylimidazol-1-yl)-8-methyl-2(1H)-quinolinone (17, UK-66,838) was selected for preclinical development studies.