Cardiotonic steroids. I. Importance of 14 beta-hydroxy group in digitoxigenin

Quantitative structure-activity relationships of cardiotonic agents

Quantitative structure-activity relationships (QSARs) of different cardiotonic agents are presented. A critical analysis of all QSARs provides a very vivid picture of the mechanisms of varying cardiotonic agents. The cardiotonics can be broadly put into 2 categories: cardiac glycosides and nonglycoside cardiotonics, which include phosphodiesterase of type III (PDE III) inhibitors, sympathomimetic (adrenergic) stimulants, A1-selective adenosine antagonists, Ca2+ channel activators and vasopressin antagonists. For cardiac glycosides, QSARs reveal that the position of carbonyl oxygen in their lactone moiety and shifting of the lactone ring from its original position or its replacement by another group would be crucial for their activity. The carbonyl group or its isostere like CN is indicated to be the sole binding entity and the hydrogen bonding through this group is considered to be the most likely binding force. For nonglycoside cardiotonics that include PDE III inhibitors and A1-selective antagonists, a five-point model has been established for their activity, the salient features of which are: (1) the presence of a strong dipole, (2) an adjacent acidic proton, (3) a methyl-sized lipophilic space, (4) a relatively flat overall topography and (5) a basic or hydrogen-bond acceptor site opposite to the dipole. For Ca2+ channel activators, the importance of steric, electrostatic, lipophilic and hydrogen-bonding properties of molecules is indicated, while for vasopressin antagonists the lipophilic and electronic properties are suggested to be the most important.

Comparative hemodynamic study of a new aminosteroid: LND-623 with its 20 alpha-isomer: LND-369, and with digoxin and digoxigenin-rhamnoside in the anesthetized dog

Hemodynamic effects of LND-623, a new aminosteroid lacking the C17 lactone ring and the C14 hydroxyl group common to the natural glycosides, were studied in the pentobarbital-anesthetized dog and compared to those of its 20 alpha-isomer LND-369 and of digoxin and digoxigenin-rhamnoside (DRh). Twenty-four mongrel dogs were divided into 4 groups. Group I received either LND-623 or saline on study day 1 and the other drug or saline 1 wk later. Saline was replaced by digoxin in group II, digoxigenin-rhamnoside in group III, and LND-369 in group IV. All drugs except LND-369 were infused as 3.10(-9) mol.kg-1.min-1 over 20 minutes. LND-369 was infused at twice the dose. LND-623 increased left ventricular dP/dt for at least 3 h with a peak at end-infusion or 15 min later, accompanied by a transient vasopressor effect. LND-369 induced, at twice the dose, an inotropic effect of comparable magnitude but of shorter duration. Inversely, it provoked a more marked and prolonged vasopressor effect than its 20 beta-isomer, LND-623. Maximal digoxin inotropic effect occurred later but was of comparable magnitude to that induced by LND-623. Its vasopressor effects reached a plateau rapidly and remained sustained until min 200. Digoxigenin-rhamnoside inotropic but not vasopressor effects are weaker than those of LND-623. It is concluded that LND-623, although lacking the most common structural features of the natural cardiac glycosides, provoked rapid and sustained inotropic activities with transient vasopressor effects. These time-course effects differ from digoxin, and these differences are unrelated to their sugar-moiety characteristics. LND-623 inotropic effect is twice as potent as its 20 alpha-isomer.

Influence of 16 beta formylation on Na, K-ATPase inhibition by cardiac glycosides

The inhibitory effect of formylated cardiac steroids (gitaloxin and its derivatives) on guinea-pig heart Na, K-ATPase was compared to that of other cardiac steroids with various hydroxy substituents. The decreasing order of potency of aglycones at equilibrium was as follows: gitaloxigenin greater than digitoxigenin greater than ouabagenin greater than digoxigenin greater than gitoxigenin greater than diginatigenin. This sequence was different to the sequence of drugs hydrophobic character. The compounds with hydroxy groups in the vicinity of the lactone ring (gitoxigenin, diginatigenin) were less potent than the hydrophilic compound ouabagenin. We propose that intramolecular bounding between 16 beta-OH and the lactone ring contributes to the relatively low potency of gitoxigenin and diginatigenin. The formylation of 16 beta-OH increased the potency of gitoxigenin by a factor of 41. The formylated compound (gitaloxigenin) was 5-fold more potent than digitoxigenin. The 3 beta-glycosylation of digoxigenin lead to pseudo-irreversible inhibitors of Na, K-ATPase. The half-time to achieve the equilibrium (for 5 mumol/l) was equal to 54 s, 90 s and 108 s respectively for digoxigenin monodigitoxoside, digoxin and desacetyllanatoside C. However, at equilibrium the three glycosides were equipotent, suggesting the existence of steric effects at the sugar site of the receptor. The sequence of potency observed for monodigitoxosides, monodigitalosides and tridigitoxosides after 60 min incubation was similar to that observed for the corresponding aglycones. These results suggest that the strongly negative inductive group 16 beta-OCHO is tightly bound to Na, K-ATPase, possibly to the same receptor site than that which is thought forming hydrogen and ionic bonds with the lactone ring. They show that the high toxicity of gitaloxin in guinea-pig heart is likely due to its high potency as Na, K-ATPase inhibitor.

Digitalis genin activity: side-group carbonyl oxygen position is a major determinant

The Na+,k+-adenosine triphosphatase-inhibiting activity of digitalis genins and their analogs is a function of side-group carbonyl (C = O) oxygen position. For each 2.2 angstroms that this oxygen is displaced from its position in digitoxigenin, activity drops by one order of magnitude. This quantitative relation resolves previously proposed models which have attempted to describe the molecular basis of genin activity. A multidisciplinary (crystallographic, conformational energy, synthetic, biological) approach to structure-activity relations is described.

Cardiotonic steroids II: 3-deoxycardenolides and 3-deoxycardanolides

A series of compounds related to 3-deoxydigitoxigenin was prepared and assayed for inhibition of myocardial Na+,K+- adenosine triphosphatase. Although the relatively high activity of 3-deoxydigitoxin was confirmed, the corresponding 3beta,4beta-epoxide and a mixture of 2,3-olefins and 3,4-olefins were less active. 3-Deoxy compounds with variations at the 14-position and the butenolide ring were much less active than the corresponding 3beta-hydroxy analogs. Thus the activity of 3-deoxydigitoxigenin appears to be particularly susceptible to structural changes elsewhere in the molecule.