New cardenolide glycosides from the seeds of Digitalis purpurea and their cytotoxic activity.

A chemical investigation of Digitalis purpurea seeds led to the isolation of three new cardenolide glycosides (1, 8 and 11), together with 12 known cardenolide glycosides (2-7, 9, 10 and 12-15). The structures of 1, 8 and 11 were determined by 1D and 2D NMR spectroscopic analyses and the results of an acid or enzymatic hydrolysis. The cytotoxic activity of the isolated compounds (1-15) against HL-60 leukemia cells was examined. Compounds 2, 9, 11 and 12 showed potent cytotoxicity against HL-60 cells with respective 50% inhibition concentration (IC50) values of 0.060, 0.069, 0.038, and 0.034 µM. Compounds 2, 9 and 11 also exhibited potent cytotoxic activity against HepG2 human liver cancer cells with respective IC50 values of 0.38, 0.79, and 0.71 µM. An investigation of the structure-activity relationship showed that the cytotoxic activity was reduced by the introduction of a hydroxy group at C-16 of the digitoxigenin aglycone, methylation of the C-3' hydroxy group at the fucopyranosyl moiety, and acetylation of the C-3' hydroxy group at the digitoxopyranoyl moiety.

Na,K-ATPase activity modulates Src activation: a role for ATP/ADP ratio.

Digitalis-like compounds (DLCs), specific inhibitors of Na,K-ATPase, are implicated in cellular signaling. Exposure of cell cultures to ouabain, a well-known DLC, leads to up- or down regulation of various processes and involves activation of Src kinase. Since Na,K-ATPase is the only known target for DLC binding an in vitro experimental setup using highly purified Na,K-ATPase from pig kidney and commercially available recombinant Src was used to investigate the mechanism of coupling between the Na,K-ATPase and Src. Digoxin was used as a representative DLC for inhibition of Na,K-ATPase. The activation of Src kinase was measured as the degree of its autophosphorylation. It was observed that in addition to digoxin, Src activation was dependent on concentrations of other specific ligands of Na,K-ATPase: Na(+), K(+), vanadate, ATP and ADP. The magnitude of the steady-state ATPase activity therefore seemed to affect Src activation. Further experiments with an ATP regenerating system showed that the ATP/ADP ratio determined the extent of Src activation. Thus, our model system which represents the proposed very proximal part of the Na,K-ATPase-Src signaling cascade, shows that Src kinase activity is regulated by both ATP and ADP concentrations and provides no evidence for a direct interaction between Na,K-ATPase and Src.

A QSAR study on a series of simplified digitalis-like compounds acting on Na+,K(+)-ATPase.

Among the cardiotonics (agents against congestive heart failure), the most important group is of the digitalis cardiac glycosides, but since these compounds suffer from a low therapeutic index, attention has been paid to investigating safer cardiotonic agents through the inhibition of Na+,K(+)-ATPase, the mechanism by which the digitalis cardiac glycosides elicit their action. Recently, a series of perhydroindenes were studied for their Na+,K(+)-ATPase inhibition activity. We report here a QSAR study on them to investigate the physicochemical and structural properties of the molecules that govern their activity in order to rationalize the structural modification to have more potent drugs. A multiple regression analysis reveals a significant correlation between the Na+,K(+)-ATPase inhibition activity of the compounds and Kier's first order valence molecular connectivity index of their R5-substituents and some indicator parameters, suggesting that the R5-substituents of the compounds containing atoms with low valence and high saturation and the R1-substituents having =N-O- moiety will be conducive to the activity.

Elucidation of the Na+, K+-ATPase digitalis binding site.

Despite controversy over their use and the potential for toxic side effects, cardiac glycosides have remained an important clinical component for the treatment for congestive heart failure (CHF) and supraventricular arrhythmias since the effects of Digitalis purpurea were first described in 1785. While there is a wealth of information available with regard to the effects of these drugs on their pharmacological receptor, the Na(+), K(+)-ATPase, the exact molecular mechanism of digitalis binding and inhibition of the enzyme has remained elusive. In particular, the absence of structural knowledge about Na(+), K(+)-ATPase has thwarted the development of improved therapeutic agents with larger therapeutic indices via rational drug design approaches. Here, we propose a binding mode for digoxin and several analogues to the Na(+), K(+)-ATPase. A 3D-structural model of the extracellular loop regions of the catalytic alpha1-subunit of the digitalis-sensitive sheep Na(+), K(+)-ATPase was constructed from the crystal structure of an E(1)Ca(2+) conformation of the SERCA1a and a consensus orientation for digitalis binding was inferred from the in silico docking of a series of steroid-based cardiotonic compounds. Analyses of species-specific enzyme affinities for ouabain were also used to validate the model and, for the first time, propose a detailed model of the digitalis binding site.

Simplified digitalis-like compounds acting on Na(+), K(+)-ATPase.

Digitalis compounds are used in the treatment of congestive heart failure as positive inotropic agents; their action is mainly due to the inhibition of Na(+),K(+)-ATPase. A well-known drawback is their arrhythmogenic potential. Attempts to find safer digitalis-like compounds by means of molecular simplifications of the typical 5beta,14beta-steroidal skeleton, which appeared in the medicinal chemistry literature from 1990 until 2002, are briefly reviewed. Several novel achievements were obtained in order to better understand the requisites of the digitalis binding site on Na(+), K(+)-ATPase. Only minor simplification, such as cleavage of the D ring of the digitalis skeleton, could preserve the desired inotropic activity, while highly simplified digitalis-like compounds failed to give sufficiently high inotropic potency, even in the presence of a powerful pharmacophore, such as the O-aminoalkyloxime group.

Synthesis and inotropic activity of 1-(O-aminoalkyloximes) of perhydroindene derivatives as simplified digitalis-like compounds acting on the Na(+),K(+)-ATPase.

A series of 5-substituted (3aS,7aR)-7a-methylperhydroinden-3a-ol derivatives bearing a 1(S)-(omega-aminoalkoxy)iminoalkyl or -alkenyl substituent was synthesized, starting from the Hajos-Parrish ketol 47, as simplified analogues of very potent 17beta-aminoalkyloximes with digitalis skeleton, previously reported. The target compounds were evaluated in vitro for displacement of the specific [3H]ouabain binding from the dog kidney Na(+),K(+)-ATPase receptor. Some of them revealed IC(50) values in the micromolar range. The most active compounds possess a cyclohexyl group in the 5(S) position and in position 1(S) the same aminoalkyloxime groups already reported for the digitoxigenin-like series in position 17beta. Although the ring conformation of these derivatives was comparable to that of uzarigenin, the binding affinities of the most active ones were 4/8-fold lower in comparison to that standard. Three compounds among those with the highest affinities were assayed in vitro for their inotropic activity on an electrically driven guinea pig left atrium and were found to be less potent than both digoxin, the most widely used inotropic agent, and the corresponding digitalis 17beta-aminoalkyloximes.

Digitalis-like compounds: synthesis and biological evaluation of seco-D and D-homo derivatives.

The synthesis of seco-D and D-homo digitalis derivatives, from the carda-14,20(22)-dienolide 1, is described. Selective ozonolysis gave the seco-D 14-ketoaldehyde 2a. Modification of the two carbonyl groups and of the alpha, beta-unsaturated lactone ring of the seco-D 14-ketoaldehyde 2a allowed preparation of derivatives with a broad range of binding affinity to the Na+, K(+)-ATPase receptor. Some of the seco-D derivatives (10, 11b, and 13b) showed a binding affinity similar to that of digitoxigenin, demonstrating that the D-ring is not essential for recognition by the digitalis receptor. In the class of D-homo derivatives the highest binding value, about 15 times lower than that of digitoxigenin, was that of the C/D cis compound 29b; the C/D trans analog 28b showed a 7-fold decrease in binding affinity, indicating that the C/D configuration plays an important role in D-homo derivatives as in the classical digitalis compounds.

[In vitro measurement of digitalis-like compounds by inhibition of Na+ K+-ATPase: determination of the inhibitory effect]. / In vitro-Messung digitalisähnlicher Verbindungen durch Hemmung des Enzyms Na+/K(+)-ATPase: Bestimmung von Störeinflüssen.

The linked optical test for the analysis of digital-like substances with the enzyme Na+/K(+)-ATPase was investigated with regard to inhibition by components of the reaction medium. Most of the tested inorganic salts influenced the activity of the enzyme. However, the concentrations of the salts in human tissues and fluids are too small to cause measurable effects. Higher concentration of salts, which may be obtained by the preparative treatment of clinical material can influence the test. The determination of a reference value is recommended in these cases. The organic solvents DMSO and Methanol influenced the activity of the Na+/K(+)-ATPase, too. While the influence of DMSO in concentrations below 50% (v/v) was negligible, the measurement of a reference sample at higher DMSO concentrations and for methanolic samples is necessary.