Synthesis and biological activity of semisynthetic digitalis analogs

Concise Enantioselective Synthesis of Oxygenated Steroids via Sequential Copper(II)-Catalyzed Michael Addition/Intramolecular Aldol Cyclization Reactions

A new scalable enantioselective approach to functionalized oxygenated steroids is described. This strategy is based on chiral bis(oxazoline) copper(II) complex-catalyzed enantioselective and diastereoselective Michael reactions of cyclic ketoesters and enones to install vicinal quaternary and tertiary stereocenters. In addition, the utility of copper(II) salts as highly active catalysts for the Michael reactions of traditionally unreactive β,β'-enones and substituted β,β'-ketoesters that results in unprecedented Michael adducts containing vicinal all-carbon quaternary centers is also demonstrated. The Michael adducts subsequently undergo base-promoted diastereoselective aldol cascade reactions resulting in the natural or unnatural steroid skeletons. The experimental and computational studies suggest that the torsional strain effects arising from the presence of the Δ(5)-unsaturation are key controlling elements for the formation of the natural cardenolide scaffold. The described method enables expedient generation of polycyclic molecules including modified steroidal scaffolds as well as challenging-to-synthesize Hajos-Parrish and Wieland-Miescher ketones.

Amino acid substitutions of Na,K-ATPase conferring decreased sensitivity to cardenolides in insects compared to mammals

Mutagenesis analyses and a recent crystal structure of the mammalian Na,K-ATPase have identified amino acids which are responsible for high affinity binding of cardenolides (such as ouabain) which at higher doses block the enzyme in the phosphorylated state. Genetic analysis of the Na,K-ATPase of insects adapted to cardenolides in their food plants revealed that some species possess substitutions which confer strongly increased resistance to ouabain in the mammalian enzyme such as the substitution T797A or combined substitutions at positions 111 and 122. To test for the effect of these mutations against the background of insect Na,K-ATPase, we here expressed the ouabain sensitive Na,K-ATPase α-subunit of Drosophila melanogaster together with the β-subunit Nrv3 in baculovirus-infected Sf9 cells and introduced the substitutions N122H, T797A, Q111T-N122H, Q111V-N122H, all of which have been observed in cardenolide-adapted insects. While all constructs showed similar expression levels, ouabain affinity of mutated Na,K-ATPases was reduced compared to the wild-type fly enzyme. Ouabain sensitivity of the ATPase activity in inhibition assays was significantly decreased by all mutations, yet whereas the IC₅₀ for the single mutations of N122H (61.0 μM) or T797A (63.3 μM) was increased roughly 250-fold relative to the wild-type (0.24 μM), the double mutations of Q111V-N122H (IC₅₀ 550 μM) and Q111T-N122H (IC₅₀ 583 μM) proved to be still more effective yielding a 2.250-fold increased resistance to ouabain. The double mutations identified in cardenolide-adapted insects are more effective in reducing ouabain sensitivity of the enzyme than those found naturally in the rat Na,K-ATPase (Q111R-N122D) or in mutagenesis screens of the mammalian enzyme. Obviously, the intense selection pressure on cardenolide exposed insects has resulted in very efficient substitutions that decrease cardenolide sensitivity extremely.

A single H:CDR3 residue in the anti-digoxin antibody 26-10 modulates specificity for C16-substituted digoxin analogs

We constructed Fab libraries of bacteriophage-displayed H:CDR3 mutants in the high-affinity anti-digoxin antibody 26-10 to determine structural constraints on affinity and specificity for digoxin. Libraries of mutant Fabs randomized at five or 10 contiguous positions were panned against digoxin and three C16-substituted analogs, gitoxin (16-OH), 16-formylgitoxin and 16-acetylgitoxin. The sequence data from 83 different mutant Fabs showed highly restricted consensus patterns at positions H:100, 100a and 100b for binding to digoxin; these residues contact digoxin in the 26-10:digoxin co-crystal structure. Several mutant Fabs obtained following panning on digoxin-BSA showed increased affinity for digoxin compared with 26-10 and retained the wild-type (wt) Trp at position 100. Those Fabs selected following panning on C16-substituted analogs showed enhanced binding to the analogs. Replacement of H:Trp100 by Arg resulted in mutants that bound better to the analogs than to digoxin. This specificity change was unexpected, as C16 lies on the opposite side of digoxin from H:CDR3. Substitution of wt Trp by Arg appears to alter specificity by allowing the hapten to shift toward H:CDR3, thereby providing room for C16 substituents in the region of H:CDR1.

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.

Structural insights into the binding of cardiac glycosides to the digitalis receptor revealed by solid-state NMR

Several biologically active derivatives of the cardiotonic steroid ouabain have been made containing NMR isotopes ((13)C, (2)H, and (19)F) in the rhamnose sugar and steroid moieties, and examined at the digitalis receptor site of renal Na(+)/K(+)-ATPase by a combination of solid-state NMR methods. Deuterium NMR spectra of (2)H-labeled inhibitors revealed that the sugar group was only loosely associated with the binding site, whereas the steroid group was more constrained, probably because of hydrogen bonding to residues around the K(+)-channel region. Crosspolarization magic-angle spinning NMR showed that chemical shifts of inhibitors (13)C-labeled in the sugar group moved downfield by 0.5 ppm after binding to the digitalis site, suggesting that the sugar was close to aromatic side groups. A (19)F, (13)C- rotational-echo double-resonance NMR strategy was used to determine the structure of an inhibitor in the digitalis receptor site, and it showed that the ouabain derivatives adopt a conformation in which the sugar extends out of the plane of the steroid ring system. The combined structural and dynamic information favors a model for inhibition in which the ouabain analogues lie across the surface of the Na(+)/K(+)-ATPase alpha-subunit with the sugar group facing away from the surface of the membrane but free to move into contact with one or more aromatic residues.

Positive inotropic and vasoconstrictor responses to 14 beta-aminopregnane derivatives in isolated tissues from the guinea-pig

1. The objective of this study was to compare the positive inotropic and vasoconstrictor responses in guinea-pig isolated tissues of the aminosteroid derivatives LND 623 and LND 796 and the cardiac glycoside ouabain. 2. Ouabain (-log EC50, 6.66 +/- 0.04, n = 11) was more potent as a positive inotropic agent on guinea-pig right ventricular papillary muscles than either LND 623 (-log EC50, 6.26 +/- 0.08, n = 6) or LND 796 (-log EC50 5.66 +/- 0.05, n = 7). All compounds gave similar maximal increases in force of contraction. However, the ratio of the maximally effective concentration to the concentration giving 25% of the maximal response was approximately doubled for the aminosteroids compared with ouabain. 3. Ouabain (-log EC50 6.27), LND 623 (-log EC50 6.24) and LND 796 (-log EC50 5.43) produced slowly developing contractions of guinea-pig thoracic aortic rings. 4. These results indicate that, while the aminopregnane derivatives and ouabain produce qualitatively similar results, the therapeutic range for the positive inotropic responses is greater for the aminosteroids.

Development of a specific assay for cardiac glycoside-like compounds based on cross-resistance of human cell mutants

The cross-resistance patterns of single- and two-step mutants of HeLa cells resistant to SC4453 (a digoxin analog) and digoxin, which involve specific alteration in Na+, K+-ATPase towards numerous other compounds, have been examined. The mutants exhibited increased resistance to all of the steroidal compounds known to elicit a digitalis-like positive inotropic response (viz. various cardiac glycosides and their genins, erythrophleum alkaloid cassaine), but they showed no cross-resistance to any of a large number of other compounds which do not show cardiac glycoside (CG)-like biological activity. Based on the above characteristics of the mutants, a new cross-resistance assay for identifying compounds that show CG-like activity has been developed. In this assay, a sample is considered to possess CG-like activity if, in comparison to the parental HeLa cells, the CGR mutants exhibit increased resistance to it. From the known D10 value (drug concentration which reduces cloning efficiency of cells to 10%) of the drug for HeLa cells and the sample dilution necessary to produce equivalent cytotoxicity, the concentration of CGs in a given sample can be estimated. In blind studies the assay correctly identified all of the samples containing CGs; none of the other samples which lacked such activity tested positive. In the blind studies the assay also provided a good estimate of the concentration of CGs (+/- 50% of the actual concentration) that was not affected by the presence of either serum components or a 20-fold excess of various steroidal compounds known to interfere in other assays. In view of the high specificity of the present assay for CG-like compounds, it should prove very useful in establishing/characterizing the presence of such activity in various biological (namely endogenous digitalis-like substances) and other samples.

Synthetic approach to analogues of 19-norsteroids with an acyclic side chain

Racemic 14 beta-hydroxy-3-methoxy-8 alpha,9 alpha-1,3,5(10)-estratriene-17-one (I), obtained by total synthesis, was converted into a derivative with alkoxycarbonyl-ethylenic side chain, rac-(20E)-21-methoxycarbonyl-19-nor-8 alpha,9 alpha-pregna- 1,3,5(10),20-tetraene-3,14 beta-diol 3-methyl ether (XII) using two Wittig reactions. Analogous derivatives of 5 alpha-androstane were prepared as synthetic models. In the estrane series the stereochemistry of attachement of the side chain in position 17, biological activity of some compounds, and their chromatographic properties were investigated.

Lipophilicity and pharmacodynamics of cardiotonic steroids in guinea-pig isolated heart muscle preparations

1. The inhibitory potency of cardiotonic steroids on myocardial Na+-K+-ATPase increases with increasing lipophilicity. Employing the inotropic action on guinea-pig isolated left atria, the relationship between lipophilicity, chemical structure and the pharmacodynamic properties of cardiac glycosides was further examined. Nineteen digitoxigenin- and digoxigenin-derivatives, whose lipophilic nature and inhibitory effects on the myocardial Na+-K+-ATPase have been previously investigated, were tested. 2. All steroids exhibited positive inotropic effects which varied with the lipophilicity of these drugs. The dependence of the relationship between these two parameters on structural transformations of the steroids showed qualitatively very close parallelism to that between lipophilicity and their inhibitory effects on myocardial Na+-K+-ATPase. 3. The positive inotropic effects and the inhibitory effects on myocardial Na+-K+-ATPase also correlated very well, exhibiting similar patterns in their respective correlations with the individual lipophilicity parameters. 4. It is inferred that (a) the positive inotropic effects of the cardiac glycosides vary with their lipophilicity, exhibiting trends similar to those shown for their inhibitory effects on myocardial Na+-K+-ATPase, (b) this interdependence is secondary to the influence of structural changes, particularly on the steroid nucleus, and (c) both the lipophilic nature and pharmacodynamic behaviour of the cardiac glycosides almost exclusively depend on the steroid nucleus itself.

Reversed-phase high-performance liquid chromatographic separation of steroids with the beta-crotonate side chain

An isocratic high-performance liquid chromatographic separation of some ethyl 3-stereoidyl crotonates [ethyl-24-nor-20(22)-cholen-23-oate derivatives] was developed. The separations were achieved by reversed-phase chromatography (Separon Si C18) using methanol, methanol-water, methanol-0.1 M formic acid and ethanol-0.01 M aqueous phosphoric acid mixtures as mobile phases. The steroidal crotonates were detected at 230 and 240 nm.

The lead structure in cardiac glycosides is 5 beta, 14 beta-androstane-3 beta 14-diol

The purpose of the present study was to determine the lead structure in cardiac glycosides at the receptor level, i.e. the minimal structural requirement for specific and powerful receptor recognition. Accordingly 73 digitalis-like acting steroids were characterized as to the concentration effecting half-maximum inhibition of Na,K-ATPase from human cardiac muscle under standardized turnover conditions. Since the Ki value equaled the apparent KD value, K'D was expressed in terms of the apparent standard Gibbs energy change delta G degrees' of steroid interaction with Na,K-ATPase. This allowed the use of the extrathermodynamic approach as a rational way of correlating in a quantitative manner, the potency and structure of the various steroidal compounds. The results of the present analysis taken in conjunction with relevant findings reported in the literature, favour the following conclusions. Cassaine, canrenone, prednisolone- and progesterone-3,20-bisguanylhydrazone, and chlormadinol acetate are compounds that are not congeneric with digitalis. The butenolide ring of cardenolides or the analogous side-chains at C17 beta of 5 beta, 14 beta-androstane-3 beta, 14-diol are not pharmacophoric substructures, but merely amplifiers of the interaction energy of the steroid lead. All modifications of the structure, geometry and spatial relationship between the steroid nucleus and butenolide side chain of digitoxigenin all at once weaken the close fit interaction with the steroid and butenolide binding subsites of the enzyme in such way that the cardenolide derivatives interact with the receptor binding site area in whatever orientation that will minimize the Gibbs energy of the steroid-receptor-solvent system. The "butenolide carbonyl oxygen distance model" (Ahmed et al. 1983) for the interpretation of the differences in potency of the cardenolide derivatives describes the change in interaction energy through structural modification as a function of the entire molecule. 5 beta, 14 beta-androstane-3 beta, 14-diol, the steroid nucleus of cardiac glycosides of the digitalis type, is the minimum structure for specific receptor recognition and the key structure for inducing protein conformational change and thus Na,K-ATPase inhibition. It is also the structural requirement for maximum contributions of the butenolide substituent at C17 beta and the sugar substituent at C3 beta-OH to the overall interaction energy, i.e. this steroid nucleus is the lead structure.(ABSTRACT TRUNCATED AT 400 WORDS)

Synthesis and biological activity of some cardiotonic compounds related to gitoxigenin

The four diastereomeric 3,16-diacetates 6, 9, 10 and 11 were prepared from gitoxin 1 by the routes shown in Schemes 1 and 2 and tested for inotropic activity in the isolated guinea-pig atrial preparation. In line with earlier findings in the digitoxigenin series, derivatives with a 3 alpha-acetoxy function, viz 9 and 10, possessed high biological activity.

Quantitative structure-activity relationships of cardiotonic steroids using empirical molecular electrostatic potentials and semiempirical molecular orbital calculations

Empirical molecular electrostatic potentials and a rigid receptor H atom model are used to calculate differences in the interaction energy of cardiotonic steroids with the digitalis receptor. An attempt is made to map the receptor H binding sites for two different steroid conformations with respect to 17 beta side-chain orientation. The calculated interaction energies using single-crystal X-ray structure data indicate linear relationships with the Na+, K+-ATPase inhibitory activity. On the basis of these correlations, the activity of 8 so far pharmacologically not investigated steroids containing C = O in 17 beta substituents are estimated with the help of structural data determined by means of the semiempirical CNDO molecular orbital theory.

Binding of dihydrodigitoxin to beef and human cardiac (Na+ + K+)-ATPase: evidence for two binding sites in cell membranes

The specific binding of three cardiac glycosides, 3H-ouabain, 3H-digitoxin and 3H-dihydrodigitoxin, to beef cardiac (Na+ + K+)-ATPase was compared. Non-specific binding was defined as that in the presence of 0.1 mM unlabelled compound, or in the absence of ligands. The dissociation constants (KD-values) calculated from the inhibition of 3H-ouabain binding were: ouabain, 2.9 X 10(-9)M; digitoxin, 1.1 X 10(-9)M; and dihydrodigitoxin 2.7 X 10(-8)M. The concentrations which inhibited beef cardiac (Na+ + K+)-ATPase by 50% were: ouabain, 5.9 X 10(-9)M; digitoxin, 1.6 X 10(-9)M; and dihydrodigitoxin, 2.5 X 10(-8)M. Ouabain and digitoxin showed straight Scatchard plots for one site of high affinity (ouabain, KD = 2.6 X 10(-9)M; digitoxin, KD = 1.7 X 10(-9)M). However, dihydrodigitoxin gave a curved Scatchard plot. Analysis of this binding by the methods of M. J. Weidemann, H. Erdelt and M. Klingenberger (Eur. J. Biochem. 16, 313 (1970) for two binding sites gave the following results: for Mg2+,Pi-supported binding, the KD of the high affinity site was 1.6 X 10(-8)M with a capacity similar to that for ouabain of about 30 pmole/mg protein. For binding supported by Na+,ATP,Mg2+, the KD-value of the high affinity site was 5.3 X 10(-8)M of similar capacity. The low affinity binding site (KD = 4.0 X 10(-6)M for Mg2+,Pi; KD = 5.5 X 10(-6)M for Na+,ATP,Mg2+) bound about 350 pmole/mg protein. The low affinity site but not the high affinity site was also present in heat-denatured enzyme. Binding supported by Mg2+,Pi showed one low affinity site only for ouabain and dihydrodigitoxin in the presence of 200 mM Na+. The high affinity sites for these three cardiac glycosides were further characterized by measurement of the association and dissociation rate constants. The specific binding of 3H-ouabain and 3H-dihydrodigitoxin to human cardiac (Na+ + K+)-ATPase was measured. 3H-Ouabain showed a straight Scatchard plot for one high affinity site only (KD = 4.5 X 10(-9) M, capacity about 15 pmole/mg protein). 3H-Dihydrodigitoxin gave two binding sites: a high affinity site (KD = 1.8 X 10(-8) M) of similar capacity to ouabain, and a low affinity site (KD = 2.0 X 10(-6) M) of about 10-fold greater capacity.(ABSTRACT TRUNCATED AT 400 WORDS)

Digitalis structure-activity relationship analyses. Conclusions from indirect binding studies with cardiac (Na+ + K+)-ATPase

We have performed direct and indirect binding studies with [3H]ouabain or [3H]digitoxin on beef or guinea pig cardiac (Na+ + K+)-ATPase to measure the potencies of a broad range of cardiotonic steroids for structure-activity relationship (SAR) studies for comparison with previously determined positive inotropic potencies. The positive inotropic potencies of twelve compounds on contracting guinea pig left atria correlated well with the equilibrium dissociation constants (KD values) from the inhibition of [3H]ouabain binding to guinea pig cardiac (Na+ + K+)-ATPase (r = 0.98 for seven 5 beta-compounds, r = 0.95 for five 5 alpha-compounds). Further we calculated KD values from the inhibition of [3H]ouabain binding data for a total of 33 digitalis derivatives on the digitalis-sensitive beef cardiac (Na+ + K+)-ATPase. For the 27 compounds tested on both beef cardiac (Na+ + K+)-ATPase and guinea pig left atria, the potencies showed a significant correlation (r = 0.92 for 22 5 beta-compounds, r = 0.96 for five 5 alpha-compounds. For seven compounds, KD values were measured on beef cardiac (Na+ + K+)-ATPase using inhibition of binding of [3H]digitoxin. These values correlated well (r = 0.99) with the KD values from the [3H]ouabain studies. These results show that: (1) The significant correlation observed between KD values on guinea pig cardiac (Na+ + K+)-ATPase and positive inotropic potency in guinea pig left atria is further evidence that the pharmacological receptor for inotropy is part of the enzyme, (2) Inhibition of the binding of [3H]ouabain or [3H]digitoxin can be used to determine the relative potencies of unlabelled digitalis derivatives. The similar relative potencies on beef and guinea pig cardiac (Na+ + K+)-ATPase of a broad range of digitalis derivatives indicate that the binding site is similar for both species; and (3) SAR studies indicate that functional groups on these steroids have the same influence on potency on either the positive inotropy or cardiac (Na+ + K+)-ATPase studies.

Synthesis and biological activity of some 3-amino compounds related to digoxigenin

3 alpha-Amino 1a and 3 beta-amino 1b analogues of digoxigenin 14 and their 12 beta-acetate derivatives 2a and 2b were prepared and tested for inotropic activity in the isolated guinea-pig atrial preparation. The 3 alpha-amino compounds were inactive whereas the 3 beta-amino compounds showed comparable activity to their 3 beta-hydroxy counterparts. The replacement of the 17 beta-butenolide ring by other ring systems was investigated. Compounds with a 3'-furyl ring, 9b and 16 were found to possess appreciable activity. A compound with a 4'-pyridazinyl ring 13b exhibited weak activity, whereas the isomeric butenolide compound 11b proved inactive. N-monomethylation of the amine 2b reduced activity and N-dimethylation abolished activity. Acetylation of the 12 beta-hydroxyl function gave less active compounds.

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.

The cardioactive properties of SC4453, a digoxin analogue with a C17 beta-pyridazine ring

SC4453 is a digoxin analogue with a pyridazine instead of a lactone ring in C17 beta. SC4453 was compared with digoxin with respect to effect on contractility and on activity of the sodium pump in guinea-pig isolated left atria stimulated at 3.3 Hz. The two glycosides stimulated the sodium pump at low concentrations and inhibited at high concentrations. At the time to peak inotropic effect, for a similar inhibition of the Na pump, the increase in systolic tension was higher with SC4453 than with digoxin, whereas the increase in diastolic tension was similar for both. These observations confirm that the inhibition of the Na pump is not the only mechanism responsible for the positive inotropic effect of cardiac glycosides.

Cardenolide analogues: 10--characterization of cardiac glycosides by chemical ionization mass spectrometry

The potential of chemical ionization mass spectrometry for the characterization of naturally occurring and semi-synthetic cardiac glycosides has been investigated. Methane, isobutane and ammonia were used as reactant gases. With the exception of ouabain, the ammonia chemical ionization mass spectra of the cardiac glycosides examined in this work contained abundant [M + NH4]+ions and abundant fragment ions formed by cleavage of glycoside bonds. Ammonia chemical ionization mass spectrometry was found to provide a rapid and sensitive method for the characterization of the products of glycosidation reactions. In contrast, the methane and isobutane chemical ionization mass spectra of the cardiac glycosides, with the exception of ouabain, did not contain protonated molecular ions and did not contain abundant fragment ions above m/z 400.

Cardenolide analogs. An explanation for the usual properties of AY 22241

AY 22241 (AY) was compared with digitoxigenin with respect to effects on myocardial contractility and inhibition of Na+, K+-ATPase. Ay was less potent than digitoxigenin but otherwise showed the same type of rapid onset and rapid reversal of activity usually associated with genins. Since AY is a glycoside, its behaviour was regarded as anomalous since glycosides usually show both slow onset and slow reversal of activity. Attempts were made to account for the genin-like properties of AY in terms of the model for the digitalis receptor proposed by Thomas et al. (1974a,b). It is suggested that the low potency and genin-like properties of AY may be due to the fact that the steroid is attached to the lactone through the carbon atom that is alpha to the carbonyl group. This feature could result in non-alignment of the sugar residue with the sugar binding site and might also reduce the effectiveness of the interaction of the steroid moiety with its binding site. A notable feature of AY toxicity was its extremely rapid progression to irreversible fibrillation.

Electrophysiological and inotropic effects of natural amino-deoxyglyco-cardenolides

The electrophysiological, inotropic and chronotropic effects of the two first natural amino-dexoyglyco-cardenolides, mitiphylline and holarosine B, were studied. Mitiphylline has qualitatively the same properties as those of common cardiac glycosides but is more potent; holarosine B has a similar action on cardiac electrophysiological parameters of the guinea pig heart but is less potent. The difference in activity between these two compounds could be attributed to the configuration in 5-and 17-positions of the genins. Holarisone B has the particularity of increasing the action potential plateau amplitude and its duration in rat heart; this could be part of the role of the aminosugar moiety.

Cardenolide analogues. VI. The metabolism of the prodrug diacetylcymarol in the rat

1. Diacetylcymarol is an acetylated glycoside which is better absorbed than the parent glycoside, cymarol. 2. Diacetyl[19-3H]cymarol was rapidly metabolized and excreted by the rat following intraperitoneal administration. 3. The drug was metabolized extensively to polar compounds with the principal pathway involving loss of the C-19 acetyl group and probable demethylation of the sugar. 4. The bulk of the radioactive material was excreted in the bile and there was little reabsorption. 5. The results show that acetylation was successful in converting the poorly absorbed glycoside, cymarol, into a derivative that was rapidly absorbed from the peritoneal cavity. 6. Following or during absorption, the biologically inactive diacetylcymarol was converted to polar derivatives with potential therapeutic activity. However, subsequent elimination was so rapid that little therapeutic benefit could be expected.

Thiocardenolides II: synthesis and pharmacological evaluation of 3beta-thioacetyl-14beta-hydroxy-5beta-card-20(22)-enolide

The synthesis of a 3beta-thioacetylcardenolide is described. The thioacetate exhibited effects similar to those seen with digitoxigenin acetate on the isolated frog and guinea pig hearts at 1 X 10(-7) dilution. In the intact rat heart, the lethal dose was 5 mg/kg for the thioacetate and 2.5 mg/kg for digitoxigenin acetate. The thioacetate inhibited sodium- and potassium-activated adenosine triphosphatase to the same extent as digitoxigenin, but it was somewhat less inhibitory than digitoxigenin acetate.