ABSTRACT
The novel Na(+)/K(+)-ATPase inhibitor (E,Z)-3-((2-aminoethoxy)imino)androstane-6,17-dione hydrochloride (PST2744) was characterized for its inotropic and toxic properties. Inhibition potency on dog kidney Na(+)/K(+)-ATPase was comparable (0.43 microM) to that of digoxin (0.45 microM). PST2744 concentration-dependently increased force of contraction in guinea pig atria and twitch amplitude in isolated guinea pig myocytes; in the latter, aftercontractions developed significantly less than with digoxin. Intravenous infusion of 0.2 mg/kg/min PST2744 in anesthetized guinea pigs exerted an immediate and long-lasting inotropic effect (ED(80) of 1.89 +/- 0.37 mg/kg) without causing lethal arrhythmias up to a cumulative dose of 18 mg/kg. Conversely, an equieffective infusion of digoxin (0.016 mg/kg/min; ED(80) of 0.32 mg/kg) caused lethal arrhythmias at a cumulative dose of 0.81 mg/kg. At a higher rate (0.4 mg/kg/min), PST2744 induced lethal arrhythmias, with a lethal dose/ED(80) ratio significantly greater than digoxin (20.2 +/- 6.3 versus 3.23 +/- 0.55, p < 0.05). Decay of the inotropic effect (t(1/2), min) was significantly faster for PST2744 (6.0 +/- 0.39) than for digoxin (18.3 +/- 4.5, p < 0.05). In anesthetized dogs, PST2744 dose-dependently increased maximum velocity of pressure rise (+dP/dt(max)) in the range 32 to 500 microg/kg i.v. and was safer than digoxin. In conscious dogs with a healed myocardial infarction, PST2744 significantly increased resting values of +dP/dt(max), left ventricular pressure, and SPB, and increased +dP/dt(max) throughout treadmill exercise while reverting the increase in left ventricular end diastolic pressure seen in control animals. Digoxin significantly decreased basal heart rate, while not affecting the hemodynamic response to exercise. Thus, PST2744 represents a new class of Na(+)/K(+)-ATPase inhibitors endowed with inotropic activity comparable with that of digitalis but having greater safety.
Subject(s)
Cardiotonic Agents/pharmacology , Digoxin/pharmacology , Animals , Cardiotonic Agents/toxicity , Dogs , Dose-Response Relationship, Drug , Etiocholanolone/analogs & derivatives , Etiocholanolone/pharmacology , Guinea Pigs , Heart Atria/drug effects , In Vitro Techniques , Myocardial Contraction/drug effects , Myocardial Infarction/physiopathology , Myocardium/cytology , Myocardium/enzymology , Physical Exertion/physiology , Radioligand Assay , Sodium-Potassium-Exchanging ATPase/metabolism , Ventricular Function, Left/drug effectsABSTRACT
The synthesis and binding affinities to the digitalis Na(+),K(+)-ATPase receptor of a series of 3 beta,14 beta-dihydroxy-5 beta-androstane and 3 beta-hydroxy-14-oxoseco-D-5 beta-androstane derivatives bearing a 17 alpha-(aminoalkoxy)imino chain are reported; some derivatives were also studied for their inotropic activity. Our recently proposed model of interaction of molecules with the digitalis receptor was used to design these compounds. On that basis, the possibility to design novel potent inhibitors of Na(+),K(+)-ATPase without being constrained by the stereochemistry of the classical digitalis skeleton in the D-ring region was predicted. The binding affinities of the most potent compounds in the two series, (EZ)-17 alpha-[2-[(2-aminoethoxy)imino]ethyl]-5 beta-androstane-3 beta,14 beta-diol (6f) and (EZ)-3 beta-hydroxy-17 alpha-[2-[(2-aminoethoxy)imino]ethyl]-14,15-seco-5 beta-androstan-14-one (24c) are higher than that of the potent natural compound digitoxigenin, despite the unusual alpha-exit of the substituent in position 17 of 6f or the disruption of the D-ring in 24c. These results further support the validity of our recently proposed model of binding at the digitalis receptor. Results of the inotropic tests on guinea pig atrium deserve further investigation on the pharmacological profile of these derivatives.
Subject(s)
Androstanes/chemical synthesis , Androstanols/chemical synthesis , Enzyme Inhibitors/chemical synthesis , Oximes/chemical synthesis , Secosteroids/chemical synthesis , Sodium-Potassium-Exchanging ATPase/antagonists & inhibitors , Androstanes/chemistry , Androstanes/pharmacology , Androstanols/chemistry , Androstanols/pharmacology , Animals , Atrial Function , Binding, Competitive , Digitoxigenin/chemistry , Dogs , Enzyme Inhibitors/chemistry , Enzyme Inhibitors/pharmacology , Guinea Pigs , Heart Atria/drug effects , In Vitro Techniques , Kidney/chemistry , Male , Models, Molecular , Myocardial Contraction/drug effects , Oximes/chemistry , Oximes/pharmacology , Radioligand Assay , Secosteroids/chemistry , Secosteroids/pharmacologyABSTRACT
A new three-dimensional model for the relative binding mode of cassaine 1 and digitoxigenin 2 at the digitalis receptor site is proposed on the basis of the structural and conformational similarities among 1, 2 and its 14,15-seco analogues 3 and 4. Accordingly, the speculation that also 17alpha-substituted derivatives of the digitalis 5beta,14beta-androstane skeleton could efficiently bind to the Na+,K+-ATPase receptor is put forward and verified through the synthesis of some related compounds. The binding affinity shown by 2-(N,N-dimethylamino)ethyl 3beta, 14-dihydroxy-5beta,14beta-androstane-17alpha-acrylate 6 (IC50 = 5.89 microM) and, much more significantly, by the corresponding 14, 15-seco-14-oxo derivative 9 (IC50 = 0.12 microM) substantiates the new hypothesis and opens new prospects to the design of novel inhibitors of Na+,K+-ATPase as potential positive inotropic compounds.
