Cardiotonic steroids have been widely used clinically to treat heart failure and cardiac arrhythmias. It has long been accepted that the cardiotonic effect of CTS results from their ability to inhibit Na pumps (Na,K-ATPase) [151
] and thereby promote Ca2+
entry via NCX [152
]. The CTS include two structurally distinct groups – the cardenolides, in which the steroid is attached to a five member singly unsaturated lactone ring (, ), and the bufadienolides, in which the lactone has six members and is doubly unsaturated. When one or more sugars are attached to the CTS at carbon 3, they are termed ‘cardiac glycosides’; common examples include ouabain and digoxin. With the exception of the bufanolide, proscillaridin, the steroid nucleus (aglycone) in the common bufadienolides is usually not glycosylated, but it may be conjugated with suberyl arginine or various other congeners.
Fig. 3 Prototypical cardenolide steroid skeleton. The primary feature is a steroid skeleton with the rings fused in a cis-trans-cis arrangement. The cardenolides discussed here have a 14βOH, an unsaturated lactone ring attached via C17 in the β (more ...)
Some relevant cardenolides and their substituents.
Cardenolides and bufadienolides are synthesized in certain plants, some amphibians and insects, and possibly all higher animals. Crude as well as highly enriched extracts from plants and the parotid secretions of the toad have been used in homeopathic remedies to treat heart failure and some cancers, and as general tonics for metabolism and immune function, especially in China (e.g., Chan Su) and Japan (Senso). The advent of modern pharmacology, coupled with the desire to use purer preparations in therapy, led to extensive studies on the Digitalis and Strophanthus glycosides and their aglycones, and the more prominent entities in toad secretions, including bufalin and resibufagenin.
Most research on CTS, and on various natural and synthetic analogs, has focused on the positive inotropic response (enhanced contraction) of heart preparations, and on the inhibition of isolated kidney enzyme (Na,K-ATPase). Thus, the bulk of knowledge about the structure-activity relationships is relevant to the heart, or to (renal) Na+ pumps with an α1 catalytic subunit. Overall, the inotropic response appears to be correlated with the ability to bind and inhibit the Na+ pump. Introduction of various substitutions in the steroid nucleus and lactone ring indicate that the configuration of the steroid is crucial for these effects.
The classic adrenocortical, ovarian and testicular steroids lack the cis-trans-cis fusion of the AB, BC, and CD rings found in the CTS ( and ), and do not bind to, or inhibit, the Na+
pump. Certain steroids with trans-trans-cis ring fusions are cardiotonic [154
], while those with cis-trans-trans ring fusions (e.g., common bile salts, 14α-digitoxigenin and 14α-artebufogenin) are either inactive or very weak [156
]. Addition of one or more sugars to the cardenolide steroid nucleus increases the potency, while inversion of the lactone configuration at C17 from β to α [158
], or saturation (e.g., dihydroouabain and dihydrodigoxin) of the lactone ring, reduces the cardiotonic activity 10-30 fold. These fundamental relationships, obtained with cardiac preparations, have been widely assumed to be valid in other physiological systems.
Both ouabain and digoxin, when administered acutely in vivo
, and often in high doses, induce vasoconstriction [95
]. Nanomolar ouabain, however, augments myogenic constriction in rodent isolated arteries [36
]. The first experimental evidence of a previously-unrecognized cardenolide structure-activity relationship was the observation that the prolonged administration of digoxin, also an Na,K-ATPase inhibitor [167
], does not
raise BP in normal rats, whereas ouabain does () [168
]. This result has been confirmed by several investigators [99
]. Moreover, while digoxin itself does not raise BP [171
], digoxin and a related CTS, digitoxin, are very effective in lowering the elevated BP in rats with ouabain-induced hypertension () [99
]. Importantly, digoxin also is known to lower BP in hypertensive humans [172
]. These remarkable observations can only be explained by structural differences between the Strophanthus
(e.g., ouabain) and Digitalis
steroids, even though they are ostensibly similar Na+
pump inhibitors. The sugar(s) is(are) not crucial for these effects: the aglycone of ouabain, ouabagenin (), is also pro-hypertensive [173
], while Rostafuroxin, a derivative of digitoxigen [174
], is anti-hypertensive in humans and rats [105
]. Thus, differences in the steroid moieties of digoxin/digitoxin and ouabain account for their disparate effects on long term BP. Excluding the common oxygen at C3, ouabain is hydroxylated in positions 1,5,11,14 and 19, while digoxin is hydroxylated in positions 12 and 14 (). The major structural difference between the two steroids therefore lies in the extensive hydroxylation of ouabain in the A and B rings (and well away from the lactone ring) and the 12 hydroxyl group in digoxin. Like digoxin and ouabain, Rostafuroxin has a steroid nucleus that is cis-trans-cis fused and has a 14β hydroxyl group. However, it lacks the ouabain hydroxyls at positions 1,5,11 and 19 and the lactone has been replaced with a doubly unsaturated furane [174
In sum, the key structural components that underlie the long term pressor activity of the cardenolides appear to include a steroid nucleus whose rings are fused in a cis-trans-cis configuration with oxygenation of the AB ring at C5. The depressor activity of the cardenolides appears to be linked with the cis-trans-cis steroid configuration, deoxygenation of the AB ring at C5 and substituents at C17 that augment potency as Na+ pump inhibitors including unsaturated 5- and 6-member lactone rings. Many of the naturally occurring cardenolides are mixtures of structural features at opposite ends of the steroid nucleus that confer prolonged pressor and depressor activity in vivo. Synthetic analogs with either improved pressor or depressor activity, the latter exemplified by Rostafuroxin, may be of clinical relevance. Clearly, the physiology and pharmacology of these agents is still full of surprises.