Cardiac profile of EGIS-9377, a novel cardiotonic agent as a Ca2+ sensitizer with bradycardiac activity

The cardiac profile of EGIS-9377 {2-(1-methylthio)-5-(2-morpholinoethylamino)-8,9-dihydro-7H-thiopyrano[3,2-d][1,2,4]triazolo[1,5-a]pyrimidine dihydrochloride}, a newly synthesized cardiotonic agent, was compared with those of pimobendan and isoprenaline in cardiac preparations isolated from guinea pigs. The positive inotropic potency and efficacy of EGIS-9377 were equal to those of pimobendan in electrically paced papillary muscles, with each agent maximally increasing force of contraction by 30–35% of the maximum effect of isoprenaline. The positive inotropic effects of EGIS-9377 and pimobendan were accompanied by an increase in the relaxation time of the isometric contraction curve, whereas that of isoprenaline was associated with an abbreviation of this parameter. Pimobendan significantly increased the spontaneously beating frequency of right atria, and its positive chronotropic effect amounted to 40% of the maximum effect of isoprenaline. In contrast, EGIS-9377 exerted a significant negative chronotropic action, which resulted in a 30% decrease in the basal frequency. In β-escin-skinned trabecular muscles, both EGIS-9377 and pimobendan substantially enhanced contractions induced by Ca2+. EGIS-9377 at concentrations to cause a significant negative chronotropic action produced a marked prolongation of action potential duration (APD) in guinea pig papillary muscle and greatly inhibited the delayed rectifier potassium current (I K) in guinea pig ventricular single cells. This suggests that the negative chronotropic effect of EGIS-9377 may, at least in part, be due to the prolongation of APD as a result of the I K inhibition. The present results indicate that EGIS-9377 efficiently increases myocardial contractile force possibly due to its Ca2+-sensitizing activity, and yet produces a substantial negative chronotropic action. This cardiac profile of EGIS-9377 is suggested to be a clinically favorable feature compared with the inotropic agents having cyclic AMP generation or phosphodiesterase inhibition as their action mechanisms.