ORM-3819 promotes cardiac contractility through Ca(2+) sensitization in combination with selective PDE III inhibition, a novel approach to inotropy.

This study is the first pharmacological characterization of the novel chemical entity, ORM-3819 (L-6-{4-[N'-(4-Hydroxi-3-methoxy-2-nitro-benzylidene)-hydrazino]-phenyl}-5-methyl-4,5-dihydro-2H-pyridazin-3-one), focusing primarily on its cardiotonic effects. ORM-3819 binding to cardiac troponin C (cTnC) was confirmed by nuclear magnetic resonance spectroscopy, and a selective inhibition of the phosphodiesterase III (PDE III) isozyme (IC50=3.88±0.3 nM) was revealed during in vitro enzyme assays. The Ca(2+)-sensitizing effect of ORM-3819 was demonstrated in vitro in permeabilized myocyte-sized preparations from left ventricles (LV) of guinea pig hearts (ΔpCa50=0.12±0.01; EC50=2.88±0.14 µM). ORM-3819 increased the maximal rate of LV pressure development (+dP/dtmax) (EC50=8.9±1.7 nM) and LV systolic pressure (EC50=7.63±1.74 nM) in Langendorff-perfused guinea pig hearts. Intravenous administration of ORM-3819 increased LV+dP/dtmax (EC50=0.13±0.05 µM/kg) and improved the rate of LV pressure decrease (-dP/dtmax); (EC50=0.03±0.02 µM/kg) in healthy guinea pigs. In an in vivo dog model of myocardial stunning, ORM-3819 restored the depressed LV+dP/dtmax and improved % segmental shortening (%SS) in the ischemic area (to 18.8±3), which was reduced after the ischaemia-reperfusion insult (from 24.1±2.1 to 11.0±2.4). Our data demonstrate ORM-3819 as a potent positive inotropic agent exerting its cardiotonic effect by a cTnC-dependent Ca(2+)-sensitizing mechanism in combination with the selective inhibition of the PDE III isozyme. This dual mechanism of action results in the concentration-dependent augmentation of the contractile performance under control conditions and in the postischemic failing myocardium.

Stereoselective binding of levosimendan to cardiac troponin C causes Ca2+-sensitization.

The effects of the Ca(2+) sensitizer levosimendan and that of its stereoisomer dextrosimendan on the cardiac contractile apparatus were studied using skinned fibers obtained from guinea pig hearts. Levosimendan was found to be more effective than dextrosimendan in this model. The respective concentrations of levosimendan and dextrosimendan at EC(50) were 0.3 and 3 microM. In order to explain the difference in efficacy as Ca(2+) sensitizers, the binding of the two stereoisomers on cardiac troponin C was studied by nuclear magnetic resonance in the absence and presence of two peptides of cardiac troponin I. The two stereoisomers interacted with both domains of cardiac troponin C in the absence of cardiac troponin I. In the presence of cardiac troponin I-(32-79) and cardiac troponin I-(128-180), the binding of both levosimendan and dextrosimendan to the C-terminal domain of cardiac troponin C was blocked and only the binding to the N-terminal domain was observable. Differences in the overall binding behavior of the two isomers to cardiac troponin C were highlighted in order to discuss their structure to activity relation. Our data are consistent with the notion that the action of levosimendan as a Ca(2+) sensitizer and positive inotrope relates to its stereoselective binding to Ca(2+)-saturated cardiac troponin C.

The contractile apparatus as a target for drugs against heart failure: interaction of levosimendan, a calcium sensitiser, with cardiac troponin c.

Cardiac failure is one of the leading causes of mortality in developed countries. As life expectancies of the populations of these countries grow, the number of patients suffering from cardiac insufficiency also increases. Effective treatments are being sought and recently a new class of drugs, the calcium sensitisers, was developed. These drugs cause a positive inotropic effect on cardio-myocytes by interacting directly with the contractile apparatus. Their mechanism of action is not accompanied by an increase in intracellular calcium concentration at therapeutic doses, as seen for the older generation of positive inotropic drugs, and thus does not induce calcium-related deleterious effects such as arrhythmias or apoptosis. Levosimendan is a novel calcium sensitiser which has been discovered by using cardiac troponin C (cTnC) as target protein. This drug has been proved to be a well-tolerated and effective treatment for patients with severe decompensated heart failure. This review describes the basic principles of muscle contraction, the main components of the contractile apparatus and their roles in the heart contraction. The regulatory proteins troponin C (cTnC), troponin I (cTnI), troponin T (cTnT), and tropomyosin (Tm) and their interactions are discussed in details. The concept of calcium sensitisation is thereafter explained and a few examples of calcium sensitisers and their putative mechanisms are discussed. Finally, the binding of levosimendan to cTnC and its mechanism of action are described and the results discussed under the light of the action of this drug in vitro and in vivo.