ORM-3819 promotes cardiac contractility through Ca²⁺ 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-2 H-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 Ca²⁺-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/dt_max) (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/dt_max (EC₅₀=0.13±0.05 μM/kg) and improved the rate of LV pressure decrease (-dP/dt_max); (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/dt_max 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 Ca²⁺-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.