Stereoselectivity of actions of the calcium sensitizer [+]-EMD 60263 and its enantiomer [-]-EMD 60264

Abstract

The thiadiazinone derivative [+]-EMD 60263 ((+)-5-(l-(alpha-ethylimino-3,4-dimethoxybenzyl)-1,2,3,4-tetrah ydroquinoline -6-yl)-6-methyl-3,6-dihydro-2H-1,3,4 -thiadiazine-2-on) is a Ca(2+)-sensitizing agent with only minor phosphodiesterase inhibitory activity. Our aim was to characterize the inotropic and electrophysiological effects of [+]-EMD 60263 and its enantiomer [-]-EMD 60264 in several cardiac muscle preparations. The Ca(2+)-sensitizing activity resided in the [+]-enantiomer only. [+]-EMD 60263 (3 microM) shifted the EC50 of Ca2+ for contractile activation of skinned fibers of pig heart from 2.41 microM to 0.73 microM, whereas [-]-EMD 60264 (30 microM) was ineffective. In Langendorff-perfused guinea pig hearts, [+]-EMD 60263 and [-]-EMD 60264 induced concentration-dependent positive and negative inotropic effects, respectively; both enantiomers reduced spontaneous heart rate but did not influence perfusion pressure. The maximum increase in force of human atrial trabeculae was 35% of pre-drug control with [+]-EMD 60263 in comparison to 113% with forskolin. In guinea-pig papillary muscles, [+]-EMD 60263 and [-]-EMD 60264 had opposite inotropic responses, however, both agents similarly prolonged action potential duration. Both enantiomers concentration-dependently blocked the rapidly activating component IKr of the delayed rectifier in guinea-pig myocytes. The block saturated at potentials positive to +30 mV, closely resembling the effects of the antiarrhythmic agent E-4031 which had been originally used to define IKr.

[+]‐EMD 60263—A Novel, Selective, Calcium‐Sensitizing Agent

“…Similar inotropic responses were obtained in isolated papillary muscles of the guinea pig heart: only [+I-EMD 60263 increased the force of contraction, while [-1-EMD 60264 had a negative inotropic effect despite its PDE-inhibitory action (21).…”

“…The compound also exhibits strong stereoselectivity, with the Ca"-sensitizing effect residing in the [+I-enantiomer (21). The effect of [+I-EMD 60263 on the force-Ca2+ relationship in porcine skinned ventricular fibers is depicted in Fig.…”

“…Under control conditions, pCa for maximum force activation is 4.5 and for half-maximum activation, pCa,, is 5.61. There are two concentrationdependent effects of the drug: it shifts the force-Ca2+ relationship to lower concentrations of Ca2+ (3 p M [+I-EMD 60263 increases pCa,, to 6.14) and increases the maximum of force development to 128% of pre-drug control ( = 100%) (21 pimobendan, see above). Thus, [+I-EMD 60263 is one of the most selective Ca2' sensitizers known at present.…”

“…The ICs0 of [+I-CGP 48506 must be >lo00 pM, as this concentration inhibits by only about 30% the type I11 isoenzyme of PDE which is responsible for the positive inotropic effect (18). However, [+I-CGP 48506 is much less potent than [+I-EMD 60263 with respect to force increase in isolated ventricular myocardium: the concentration ranges for minimum to maximum force increase were: [+I-CGP 48506, 10 to 100 pM (29); and [+I-EMD 60263,0.3 to 3 pM (21). In contrast to [+I-EMD 60263, however, CGP 48506 appears to be a Ca2+-channel activator at high concentrations (100 pM); therefore, this drug may not be considered to be a pure Ca2+ sensitizer (7).…”

“…The effects of [+I-EMD 60263 and in part also of its stereoisomer [-1-EMD 60264 on contractile performance have been studied extensively in several in vitro and in vivo models, such as skinned cardiac fibers (see above, 2), single cardiomyocytes, isolated papillary muscles, Langendorff-perfused hearts (21,22), blood-perfused rabbit hearts (1 2), and open-chest pig heart preparations (26).…”