Richard C. Dage

Biochemical studies on the mechanism of cardiotonic activity of MDL 17,043.

MDL 17.043 (1,3-dihydro-4-methyl-5-[4-(methylthio)-benzoyl]-2H-imidazol-2-one) is a new drug with cardiotonic properties. Its effects on several biochemical systems considered to be important in myocardial contraction were investigated and compared with those produced by amrinone and theophylline. Dog cardiac phosphodiesterases (PDEs) were separated into three major forms and labeled PDE I, II, and III according to the order of elution during isolation by column chromatography. PDE I and II, considered to be “high-λm.. enzymes for cyclic AMP, were not inhibited by MDL 17.043, amrinone, or theophylline in concentrations of 50 μM. PDE III, a ”low-Km.. enzyme, was strongly inhibited by MDL 17.043. Kinetic studies showed the inhibition to be characteristic of partial competitive inhibition. At 0.25 μM cyclic AMP, 1.3 μM MDL 17.043 caused 50% inhibition of PDE III (150), while the 150 for amrinone and theophylline were estimated to be 19.5 μM and 119 μM, respectively. Dog kidney Na., K.-ATPase was inhibited 54% by 100 μM MDL 17.043 while amrinone caused an 18% inhibition at the same concentration. Ca2.-ATPase and Ca2. uptake by dog sarcoplasmic reticulum vesicles were unchanged by MDL 17.043 concentrations up to 300 μM and 100 μM, respectively. It is suggested that the inhibition of PDE III is related to the cardiotonic effects produced by MDL 17.043 and amrinone, although inhibition of Na., K.-ATPase may also play a role at high concentrations of these drugs.

Cardiovascular properties of a new cardiotonic agent: MDL 17,043 (1.3-dihydro-4-methyl-5-[4-(methylthio)-benzoyl]-2H-imidazol-2-one).

The cardiovascular properties of a new noncatechol, nonglycoside cardiotonic agent, MDL 17.043, were investigated in anesthetized and conscious dogs and the dog heart-lung preparation. MDL 17.043 (0.1–1 mg/kg), administered to anesthetized dogs by intravenous injection, produced dose-related increases in cardiac contractile force lasting more than 1 h. It also produced relatively minor and shorter-lasting increases in heart rate, and brief, decreases in blood pressure. These effects were not blocked by propranolol. Of these effects, the increase in cardiac contractile force was, by far, the most prominent. The cardiac effects were also observed in the dog heart-lung preparation. When administered to anesthetized dogs by constant intravenous infusion. MDL 17.043 (0.03 and 0.1 mg/kg/min) produced a marked and sustained increase in cardiac contractile force and a sustained decrease in blood pressure without altering heart rate, suggesting a wide separation between the inotropic and chronotropic effects of this drug. Given orally to conscious, chronically instrumented dogs. MDL 17.043 (3–30 mg/kg) produced a sustained increase in dP/dt without altering heart rate or blood pressure. It reversed the depressant effect of pentobarbital on the ventricular function curve in the dog heart-lung. When the hemodynamic characteristics of compensated heart failure were produced by propranolol in anesthetized dogs. MDL 17.043 reversed these effects. These studies suggest that MDL 17.043 may have a beneficial effect in the treatment of heart failure.

Characterization of the Cardiovascular Activities of a New Cardiotonic Agent, Mdl 17043 (1,3-dihydro-4-methyl-5-[4-(methylthio)-benzoyl]-2h-imidazol-2-one)

Summary The cardiovascular actions of 1,3-dihydro-4-methyl-5[4-(methylthio)-benzoyl]-2H-imidazol-2-one (MDL 17043), a new non-catecholamine, nonglycoside cardiotonic agent, were examined in vivo in anesthetized dogs and in vitro in isolated cat atrial and papillary muscle preparations and guinea pig atria. In the anesthetized dog, intravenous administration of MDL 17043 (0.1 - 1 mg/kg) produced marked increases in cardiac contractile force which were accompanied by minor increases in heart rate and small decreases in blood pressure. These effects were not altered by α-or β-adrenergic receptor blockade, catecholamine depletion produced by reserpine, bilateral vagotomy, or by bilateral carotid sinus denervation. MDL 17043 (105-103 M) produced positive inotropic effects in isolated papillary muscle and left atrial strips of the cat that were much greater than the positive chronotropic effects seen in the spontaneously beating right atrium of the cat. The in vitro inotropic effects of MDL 17043 in guinea pig electrically driven left atrial strips were not modified by adrenergic β-receptor or histamine H1-receptor blockade. The vasodilatory effect of MDL 17043 in the canine isolated pump-perfused hindlimb preparation was not attenuated by surgical sympathectomy, α- or β-adrenergic receptor blockade, cholinergic or histaminergic receptor blockade, or by prostaglandin synthesis inhibition, indicating that MDL 17043 produces direct relaxation of vascular smooth muscle. MDL 17043 was found to be safe and effective when administered acutely in combination with ouabain, hydralazine, nitroglycerin, or furosemide, agents commonly used in the treatment of congestive heart failure.

Dual Inhibition of Angiotensin-Converting Enzyme and Neutral Endopeptidase in Rats with Hypertension

Summary: Angiotensin converting enzyme (ACE) and neutral endopeptidase (NEP), are two mechanistically similar enzymes involved in the metabolism of several vasoactive peptides. Selective inhibitors of ACE are effective antihypertensive agents in high-renin, renovascular rats and normal-renin, spontaneously hypertensive rats (SHR), but are not effective in the low-renin, deoxy-corticosterone acetate (DOCA)-salt hypertensive rats. In contrast, NEP inhibitors are only effective in the low-renin model of hypertension. Treatment with a combination of selective inhibitors or with a dual inhibitor of both enzymes produces an antihypertensive response regardless of basal plasma renin activity. In this study, we compared the activities of MDL 100,173, a novel subnanomolar inhibitor of both ACE and NEP, with those of equimolar doses of captopril, a selective ACE inhibitor, following intravenous administration in these three rat models of hypertension. Treatment with MDL 100,173 significantly lowered blood pressure compared to vehicle treatment in all three models, whereas captopril treatment lowered blood pressure in the renovascular and SHR models only. Administration of MDL 100,173 also significantly elevated diuresis and natriuresis compared to either vehicle or captopril treatment in the SHR and DOCA-salt rats. Urinary excretion of atrial natriuretic peptide (ANP) was increased by MDL 100,173 treatment in all three models of hypertension. Treatment with captopril did not alter urine, sodium, or ANP excretion in any of the models. However, plasma-renin activity was elevated by both MDL 100,173 and captopril administration in all three groups. These results indicate that MDL 100,173 is an effective antihypertensive agent across a spectrum of rat models of hypertension, regardless of basal plasma-renin activity, whereas captopril is effective only in high- or normal-renin hypertensive rats. In addition, MDL 100,173 also possesses diuretic and natriuretic activity as a result of its ability to inhibit NEP.

PROTECTION AGAINST POSTISCHEMIC MYOCARDIAL DYSFUNCTION IN ANESTHETIZED RABBITS WITH SCAVENGERS OF OXYGEN-DERIVED FREE RADICALS : SUPEROXIDE DISMUTASE PLUS CATALASE, N-2-MERCAPTOPROPIONYL GLYCINE AND CAPTOPRIL

Postischemic myocardial dysfunction in canine myocardium has been reported to be reduced by scavengers of oxygen-derived free radicals. One potential source of oxygen-derived free radicals in canine myocardium is xanthine oxidase, but human and rabbit myocardium either lack or possess very low levels of this enzyme. Therefore, the effects of scavengers of oxygen-derived free radicals on postischemic myocardial dysfunction produced by 15 min of ischemia and 3 h of reperfusion were evaluated in vivo in the rabbit. Superoxide dismutase (SOD) (45,000 U/kg) and catalase (55,000 U/kg) were given into the left atrium 10 min before ischemia, and followed by an additional 45,000 U/kg of SOD and 55,000 U/kg of catalase given over 85 min. This treatment reduced postischemic myocardial dysfunction, as did sulfhydryl-containing free radical scavengers N-2-mercaptopropionyl glycine (4 mg/kg, i.v.) and captopril (3 mg/kg, i.v.) given 5 min before and 60 min after reperfusion. SOD given alone at the same dose was ineffective, as was enalaprilat (0.3 mg/kg, i.v.), an angiotensinconverting enzyme inhibitor that does not scavenge oxygen-derived free radicals. Thus, postischemic myocardial dysfunction was reduced by scavengers of oxygenderived free radicals in vivo in a species that is deficient in myocardial xanthine oxidase. This suggests that oxygen-derived free radicals derived from a source other than xanthine oxidase play a role in postischemic myocardial dysfunction.

Pharmacology of enoximone

Enoximone is a new cardiotonic agent, active by both intravenous and oral routes of administration, that is being studied clinically for the treatment of patients with congestive heart failure. The animal pharmacology pertinent to the clinical development of enoximone is reviewed. Direct positive inotropic, positive chronotropic and vasodilator properties have been demonstrated for enoximone in several in vivo and in vitro preparations. However, positive inotropism and vasodilation are the principal effects of this agent with the inotropic effect being the most prominent. In anesthetized dogs, the cardiovascular effects produced by enoximone (0.1 to 1 mg/kg) were not accompanied by significant alterations in myocardial oxygen consumption. Cardiac function was improved by enoximone in anesthetized dogs given myocardial depressant amounts of propranolol. Studies in vivo and in vitro have indicated that the actions of enoximone are direct and not mediated by stimulation of adrenergic receptors, histaminic receptors, cholinergic receptors, Ca++-adenosine triphosphatase, Mg++-adenosine triphosphatase, adenyl cyclase or inhibition of Na+, K+-adenosine triphosphatase. However, enoximone reversed the depressant effects of verapamil in the dog heart-lung preparation; this suggests that its action resulted in the activation of slow calcium channels. Enoximone was found to be potent and highly selective inhibitor of a high affinity cyclic adenosine monophosphate-phosphodiesterase type IV-phosphodiesterase from dog heart, whereas standard inhibitors (e.g., 3-isobutyl-1-methylxanthine and papaverine) inhibit all 3 cardiac phosphodiesterases. Further, enoximone produced an increase in cyclic adenosine monophosphate, but not cyclic guanosine monophosphate, in the isolated, blood perfused dog papillary muscle during the peak inotropic effect.(ABSTRACT TRUNCATED AT 250 WORDS)

Cardiovascular properties of a new cardiotonic agent, MDL 19205.

The cardiovascular properties of a new cardiotonic agent, MDL 19205, were investigated in anesthetized and conscious dogs and in a dog heart-lung preparation. MDL 19205 (0.1–1 mg/kg), administered to anesthetized dogs by intravenous injection, produced a dose-related increase in cardiac contractile force lasting up to 1 h. It also produced a relatively minor increase in heart rate and a brief decrease in blood pressure. These effects did not involve β-adrenergic receptor stimulation, as they were observed in dogs after a myocardial-depressant dose of propranolol. Given orally to conscious dogs, MDL 19205 (1 and 3 mg/kg) produced a dose-related increase in dP/dt without producing a significant alteration in heart rate or blood pressure. When administered to anesthetized dogs by constant intravenous infusion, MDL 19205 (0.1 mg/kg/min) produced a marked and sustained increase in cardiac contractile force and decreases in blood pressure and left atrial pressure, but did not produce a significant change in cardiac output or stroke volume, indicating an enhancement of cardiac pump function. Intravenous MDL 19205 reversed the hemodynamic characteristics of heart failure produced by propranolol in anesthetized dogs. In addition, it reversed the depressant effect of pentobarbital on cardiac function in a dog heart-lung preparation. These studies show that MDL 19205 is a potent, direct-acting cardiotonic agent in animals.

Tissue distribution and selective inhibition of subtypes of high affinity cAMP phosphodiesterase

High affinity cAMP phosphodiesterase (PDE), also referred to as PDE III, or low Km PDE occurs as two subtypes. One subtype is sensitive to inhibition by cGMP while the other is relatively insensitive. To be consistent with previously recommended nomenclature, these subtypes were designated Types IV and V PDEs respectively. Tissue distribution of these subtypes of high affinity cAMP PDE was investigated using comparative potencies of specific inhibitors. Of the tissues examined, dog heart contained the highest proportion of the cGMP inhibitable form (Type IV PDE), whereas dog kidney cortex and brain were composed almost entirely of the cGMP non-inhibitable form (Type V PDE). Enoximone and other new cardiotonic drugs that inhibit high affinity cAMP PDE were shown to be specific for the cGMP inhibitable form, whereas rolipram was specific for the cGMP non-inhibitable form. The apparently partially competitive kinetics shown by one of these drugs, enoximone, was due to the presence of both subtypes of the enzyme. When the activity of the cGMP non-inhibitable form was suppressed by rolipram, competitive inhibition of the cGMP inhibitable subtype by enoximone was observed. Rat heart high affinity cAMP PDE activity contained a higher proportion of the cGMP non-inhibitable subtype than did the enzyme from dog heart. It is suggested that this may account for the relative insensitivity of rats to the cardiotonic PDE inhibitors.

Studies on the mechanism of the cardiotonic activity of MDL 19205: effects on several biochemical systems.

MDL 19205, 4-ethyl-l, 3-dihydro-5-(4-pyri-dinyl-carbonyl)-2 H-imidazol-2-one, is a new drug with cardiotonic properties. Its effects on several biochemical systems considered to be important in myocardial contraction were investigated. Cyclic nucleotide phospho-diesterases (PDEs) from dog hearts were separated into three isoenzymes, F I, F II, and F III, and the effect of the drug on these enzymes was tested. MDL 19205 inhibited F III PDE specifically and produced little or no inhibition of F I and F II PDEs. The IC50 for inhibition of F III PDE was 8.6 μM when 0.5 μM cyclic AMP (cAMP) was used, whereas no more than 10% inhibition of F I and 18% of F II PDEs occurred at drug concentrations up to 200 μM when 1 μM cAMP was used. Concentrations of MDL 19205 up to 100 μM had no effect on Ca2+-adenosine triphosphatase (ATPase) or Ca2+ uptake by dog cardiac sarcoplasmic reticulum. At 100 μM, the drug produced a weak (18%) inhibition of Na+,K+ -ATPase. It is suggested that inhibition of F III PDE may be the primary mechanism by which MDL 19205 produces its cardiotonic effect. Inhibition of Na+, K+ -ATPase may also be involved at very high concentrations of this drug.

Cardiovascular properties of medroxalol, a new antihypertensive drug.

Summary Medroxalol is a new antihypertensive agent that is presently undergoing clinical trial. Its cardiovascular properties were studied using spontaneously hypertensive rats (SHR), anesthetized dogs, and isolated tissues. Medroxalol produced a long-lasting fall in blood pressure when given by the oral route to SHR. It was more potent than phentolamine in antihypertensive effectiveness. Given intravenously to dogs, medroxalol reduced blood pressure and heart rate at doses that did not greatly reduce cardiac output. The hypotensive effect of medroxalol was reduced but not abolished following α- and β-adrenergic-receptor blockade. Medroxalol inhibited heart rate and blood pressure responses to isoproterenol and phenylephrine in dogs. In vitro medroxalol resembled a competitive antagonist at α-adrenergic receptors in rabbit aortic strips (pA2 6.09) and β-adrenergic receptors in guinea pig atria (pA2 7.73). It was 0.02 as potent as phentolamine at α-receptors and 0.09 as potent as propranolol at β-receptors. It was concluded that the principal action of medroxalol was to produce a fall in blood pressure by decreasing peripheral vascular resistance more than cardiac output. Adrenergic α- and β-receptor blockade alone does not satisfactorily explain the hypotension. A contribution by an active vasodilatory component appears likely.