In vivo evidence of positive inotropism of EMD 57033 through calcium sensitization

Interaction of cardiac troponin with cardiotonic drugs: a structural perspective

Over the 40 years since its discovery, many studies have focused on understanding the role of troponin as a myofilament based molecular switch in regulating the Ca(2+)-dependent activation of striated muscle contraction. Recently, studies have explored the role of cardiac troponin as a target for cardiotonic agents. These drugs are clinically useful for treating heart failure, a condition in which the heart is no longer able to pump enough blood to other organs. These agents act via a mechanism that modulates the Ca(2+)-sensitivity of troponin; such a mode of action is therapeutically desirable because intracellular Ca(2+) concentration is not perturbed, preserving the regulation of other Ca(2+)-based signaling pathways. This review describes molecular details of the interaction of cardiac troponin with a variety of cardiotonic drugs. We present recent structural work that has identified the docking sites of several cardiotonic drugs in the troponin C-troponin I interface and discuss their relevance in the design of troponin based drugs for the treatment of heart disease.

Structural based insights into the role of troponin in cardiac muscle pathophysiology

Troponin is a molecular switch, directly regulating the Ca2+-dependent activation of myofilament in striated muscle contraction. Cardiac troponin is subject to covalent and noncovalent modifications; phosphorylation modulates myofilament physiology, mutations are linked to familial hypertrophic cardiomyopathy, intracellular acidification causes myocardial infarction, and cardiotonic drugs modify myofilament response to Ca2+. The structure of troponin provides insights into the mechanism of this molecular switch and an understanding of the effects of protein modification under pathophysiological conditions. Although the structure of troponin C has been solved in various Ca2+-bound states for some time, structural information on troponin I and troponin T has only emerged recently. This review summarizes recent advances on the structure of complexes of troponin subunits with the aim of assessing how these proteins interact with each other to execute its role as a molecular switch and how covalent and noncovalent modifications affect the structure of troponin and the switch mechanism. We focus on pinpointing the specific amino acid residues involved in phosphorylation and mutation and the pH sensitive regions in the structure of troponin. We also present recent structural work that have identified the docking sites of several cardiotonic drugs on cardiac troponin C and discuss their relevance in the direction of troponin based drug design in the therapy of heart disease.

The therapeutic potential of novel cardiotonic agents

During the course of treatment of heart failure patients, cardiotonic agents are inevitable for improvement of myocardial dysfunction. Clinically available agents, such as beta-adrenoceptor agonists and selective phosphodiesterase 3 inhibitors, act mainly via cyclic AMP/protein kinase A-mediated facilitation of Ca(2+) mobilisation (upstream mechanism). These agents are associated with the risk of Ca(2+) overload leading to arrhythmias, myocardial cell injury and premature cell death. In addition, they are energetically disadvantageous because of an increase in activation energy and metabolic effects. Cardiac glycosides act also via an upstream mechanism and readily elicit Ca(2+) overload with a narrow safety margin. No currently available agents act primarily via an increase in the myofilament sensitivity to Ca(2+) ions (central and/or downstream mechanisms). Novel Ca(2+) sensitisers under basic research may deserve clinical trials to examine the therapeutic potential to replace currently employed agents in acute and chronic heart failure patients. Molecular mechanisms of action of Ca(2+) sensitisers are divergent. In addition, they show a wide range of discrete pharmacological profiles due to additional actions associated with individual compounds. Therefore, the outcome of clinical trials has to be explained carefully based on these mechanisms of actions.

Mechanisms of action of novel cardiotonic agents

Regulation of myocardial contractility by cardiotonic agents is achieved by an increase in intracellular Ca2+ mobilization (upstream mechanism), an increase in Ca2+ binding affinity to troponin C (central mechanism), or facilitation of the process subsequent to Ca2+ binding to troponin C (downstream mechanism). cAMP mediates the regulation induced by Ca2+ mobilizers such as beta-adrenoceptor agonists and selective phosphodiesterase III inhibitors acting through the upstream mechanism. These agents act likewise on the central mechanism to decrease Ca2+ sensitivity of troponin C in association with the cAMP-mediated phosphorylation of troponin I. In addition to such a well-known action of cAMP, recent experimental findings have revealed that Ca2+ sensitizers, such as levosimendan, OR-1896, and UD-CG 212 Cl, require the cAMP-mediated signaling for induction of Ca2+ sensitizing effect. These agents shift the [Ca2+] -force relationship to the left, but their positive inotropic effect (PIE) is inhibited by carbachol, which suppresses selectively the cAMP-mediated PIE. These findings imply that cAMP may play a crucial role in increasing the myofilament Ca2+ sensitivity by cross-talk with the action of individual cardiotonic agents. No clinically available cardiotonic agents act primarily via Ca2+ sensitization, but the PIE of pimobendan and levosimendan is partly mediated by an increase in myofilament Ca2+ sensitivity. Evidence is accumulating that cardiotonic agents with Ca2+ sensitizing action are more effective than agents that act purely via the upstream mechanism in clinical settings. Further clinical trials are required to establish the effectiveness of Ca2+ sensitizers in long-term therapy for congestive heart failure patients.

Beneficial effects of the Ca2+ sensitizer EMD 57033 in exercising pigs with infarction-induced chronic left ventricular dysfunction

1. It is unknown how cardiac stimulation by Ca(2+) sensitization modulates the cardiovascular response to exercise when left ventricular (LV) function is chronically depressed following a myocardial infarction. We therefore investigated the effects of EMD 57033 at rest and during exercise and compared these to those of the mixed Ca(2+)-sensitizer/phosphodiesterase-III inhibitor pimobendan. 2. Pigs were chronically instrumented for measurement of cardiovascular performance. At the time of instrumentation, infarction was produced by coronary artery ligation (MI, n=12). Studies in MI were performed in the awake state, 2 - 3 weeks after infarction. 3. MI were characterized by a lower resting cardiac output (18%), stroke volume (30%) and LVdP/dt(max) (18%), and a doubling of LV end-diastolic pressure, compared to normal pigs (N, n=13). 4. In 11 resting MI, intravenous EMD 57033 (0.2 - 0.8 mg kg(-1) min(-1)) increased LVdP/dt(max) (57+/-5%) and stroke volume (26+/-6%) with no effect on heart rate, LV filling pressure, and myocardial O(2)-consumption, similar to N. 5. In MI, the effects of EMD 57033 (0.4 mg kg(-1) min(-1), IV) on stroke volume and LVdP/dt(max) were maintained during treadmill exercise up to 85% of maximal heart rate, while heart rate was lower compared to control exercise (all P<0.05). In contrast, the effects of EMD57033 gradually waned in N at increasing intensity of exercise. 6. Compared to N, the cardiostimulatory effects of pimobendan (20 microg kg(-1) min(-1), IV) were blunted in MI both at rest and during exercise compared to N. 7. In conclusion, the positive inotropic actions of the Ca(2+) sensitizer EMD 57033 are unmitigated in resting and exercising MI compared to N, while those of the mixed Ca(2+)-sensitizer/phosphodiesterase-III inhibitor pimobendan are blunted.

Structure of the C-domain of human cardiac troponin C in complex with the Ca2+ sensitizing drug EMD 57033

Ca(2+) binding to cardiac troponin C (cTnC) triggers contraction in heart muscle. In heart failure, myofilaments response to Ca(2+) are often altered and compounds that sensitize the myofilaments to Ca(2+) possess therapeutic value in this syndrome. One of the most potent and selective Ca(2+) sensitizers is the thiadiazinone derivative EMD 57033, which increases myocardial contractile function both in vivo and in vitro and interacts with cTnC in vitro. We have determined the NMR structure of the 1:1 complex between Ca(2+)-saturated C-domain of human cTnC (cCTnC) and EMD 57033. Favorable hydrophobic interactions between the drug and the protein position EMD 57033 in the hydrophobic cleft of the protein. The drug molecule is orientated such that the chiral group of EMD 57033 fits deep in the hydrophobic pocket and makes several key contacts with the protein. This stereospecific interaction explains why the (-)-enantiomer of EMD 57033 is inactive. Titrations of the cCTnC.EMD 57033 complex with two regions of cardiac troponin I (cTnI(34-71) and cTnI(128-147)) reveal that the drug does not share a common binding epitope with cTnI(128-147) but is completely displaced by cTnI(34-71). These results have important implications for elucidating the mechanism of the Ca(2+) sensitizing effect of EMD 57033 in cardiac muscle contraction.

Positive inotropic effects of calcium sensitizers on normal and failing cardiac myocytes

Calcium sensitizers increase myocardial contractile function without affecting Ca2+ homeostasis, which might be beneficial in the treatment of patients with heart failure. However, it remains uncertain whether Ca sensitizers induce quantitatively similar inotropic responses in control and failing hearts. To compare their effects in normal versus failing hearts at the cellular level, shortening mechanics and intracellular calcium ([Ca2+]i) transient were simultaneously measured in the left ventricular myocytes isolated from normal dogs (n = 8) and dogs with rapid pacing-induced heart failure (n = 7). CGP 48506 and EMD 57033 exerted a positive inotropic effect in a dose (0.1-3 microM)-dependent manner in both normal and heart failure myocytes. The percent increase of cell shortening magnitude was comparable between the two groups. CGP 48506 and EMD 57033 did not affect the diastolic cell length and resting [Ca2+]i level. They did not affect the duration of [Ca2+]i transient dynamics. Thus Ca2+ sensitizers exerted comparable positive inotropic effects without affecting the rest cell length and rest [Ca2+]i in normal and heart failure myocytes.

Overall cardiac functional effect of positive inotropic drugs with differing effects on relaxation

Recent interest in so-called calcium-sensitizing positive inotropic drugs has highlighted the potential problem of a positive effect on force development being offset, at least partially, by the negative effect that many of these drugs have on relaxation. The purpose of this study was to examine the interplay of contraction and relaxation in determining the overall cardiac effect of different positive inotropic drugs. Using a buffer-perfused isolated rabbit heart preparation, we studied four drugs (calcium, dobutamine, EMD 57033, and CGP 48506) that were given at doses sufficient to increase similarly left ventricular pressure-generating capability by approximately 20%. We show that, even though they produce equivalent changes in pressure-generating capability, these four agents produce dissimilar changes in relaxation capability, with dobutamine speeding relaxation, EMD 57033 slowing relaxation, and calcium and CGP 48506 having little effect of relaxation. Similar relative effects were observed for drug-induced changes in the timing of pressure-generation events. These effects combine to produce different drug-induced changes in overall cardiac pump function judged by the relation between cardiac output and heart rate. Dobutamine shifted the maximal cardiac output to a higher heart rate. In contrast, both calcium sensitizers shifted the maximum in cardiac output to a lower heart rate, whereas calcium had no effect. Thus even though positive inotropic drugs may have similar effects on left ventricular pressure generation, the overall benefit of such drugs on ventricular pump function will depend on how the drug also affects ventricular relaxation and ejection capabilities.

Improved mechanoenergetics and cardiac rest and reserve function of in vivo failing heart by calcium sensitizer EMD-57033

BACKGROUND

Myofilament Ca(2+) sensitizers enhance contractility but can adversely alter diastolic function through sensitization to low intracellular Ca(2+) concentration. Concomitant phosphodiesterase III inhibition (PDE3I) may offset diastolic changes but limit the mechanoenergetic benefits. We tested whether selective Ca(2+) sensitization in vivo with the use of EMD-57033 enhances both systolic and diastolic function in failing hearts at minimal energetic cost.

METHODS AND RESULTS

Pressure-dimension data were measured with sonomicrometry/micromanometry in conscious dogs before (CON, n=9) and after tachycardia-induced heart failure (HF, n=11). In contrast to blunted dobutamine (DOB) responses in HF, low-dose EMD-57033 (0.4 mg. kg(-1). min(-1) for 20 minutes) markedly enhanced contractility, doubling end-systolic elastance and raising fractional shortening similarly in CON-treated and HF hearts. EMD-57033 effects were achieved at a reduced heart rate, without vasodilation. EMD-57033 augmented blunted heart rate-dependent contractility responses in HF at a rate of twice that of DOB, despite matched basal inotropic responses. EMD-57033 also improved diastolic function, lowering left ventricular end-diastolic pressure and increasing the filling rate. At equipotent inotropic doses and matched preload, EMD-57033 lowered the oxygen cost of contractility by -11.4+/-5.8%, whereas it rose 64+/-18% with DOB (P=0.001) and 28+/-11% with milrinone. Doubling EMD-57033 dose further augmented positive inotropy in CON and HF, accompanied by vasodilation, increased heart rate, and other changes consistent with PDE3I coactivity, but the oxygen cost of contractility remained improved compared with the use of DOB.

CONCLUSIONS

Selective Ca(2+) sensitization with minimal PDE3I in vivo is achieved with the use of EMD-57033, improving basal and rate-stimulated contractility and mechanoenergetics of HF without compromising diastolic function. Despite PDE3I activity at higher doses, energetic benefits persist.

Different chronotropic and inotropic effects of EMD 57033 and EMD 53998, Ca2+ sensitizers, on isolated, blood-perfused dog heart preparations

To investigate whether a Ca2+ sensitizer increases sinus rate, we studied the effects of racemic thiadiazinone, EMD 53998 (a Ca2+ sensitizer with phosphodiesterase inhibitory action) and its (+)-enantiomer EMD 57033 (a relatively pure Ca2+ sensitizer) on isolated, blood-perfused spontaneously beating right atria and paced left ventricles of the dogs. EMD 53998 increased sinus rate dose-dependently, but EMD 57033 did not. Both substances increased atrial and ventricular contractile force. Propranolol did not affect the responses to each substance. These results suggest that the Ca2+ sensitizing action induced by EMD 57033 does not affect pacemaker currents directly.