Convallatoxin

Abstract: The relative toxicity of numerous cardiotonic steroids (viz. ouabain, digitoxin, digoxin, convallatoxin, SC4453, bufalin, gitaloxin, digoxigenin, actodigin, oleandrin, digitoxigenin, gitoxin, strophanthidin, gitoxigenin, lanatosides A, B and C, alpha- and beta-acetyl digoxin, alpha- and beta-methyl digoxin) and related compounds towards a number of independent cell lines established from human, monkey, mouse, Syrian hamster, and Chinese hamster have been determined. All cardiac glycosides and their genins, as well as the cardiotonic alkaloid cassaine, exhibited greater than 100-fold higher toxicity towards cultured human and monkey cells in comparison to the cell lines of mouse, Syrian hamster, and Chinese hamster origins. These differences are species-related as all cell lines (both normal as well as transformed) from any one species, as well as cells from the closely related species (e.g., man and monkey or mouse, Chinese hamster, and Syrian hamster), showed similar sensitivity towards these drugs. The failure to see any significant differences in cellular toxicity for a larger number of other compounds which either bear limited structural resemblance to cardiac glycosides (viz. estradiol 17-beta-acetate, testosterone propionate, 21-acetoxy pregnenolone, beta-estradiol, digitonin, tigogenin, and tomatine) or interact with the Na+/K+ ATPase in a different manner (viz. veratridine, sanguinarine nitrate, penicillic acid, vanadium pentoxide, harmaline-HCI,5,5'-diphenyl hydantoin, quindonium bromide, and methyl quinolizinum bromide) provides strong evidence that the observed species-related differences are highly specific for cardiotonic steroids. Studies on the binding of [3H]ouabain show that, in comparison to human and monkey cell lines, no significant binding of the drug is observed in cells derived from the resistant species (i.e., mouse and Chinese hamster). The Na+/K+ ATPase from cells of the resistant species is inhibited at much higher concentrations of ouabain and digitoxin in comparison to the enzyme from human cells, and a good correlation is observed between these concentrations and those reported for inhibition of the enzyme from isolated heart muscles of the same species. These results provide strong evidence that the species-related differences in sensitivity to digitalis have a cellular basis and that the cultured cells from various mammalian species provide a useful model system for investigating the mechanism of action of cardiac glycosides.

Abstract: Cardiac glycosides have been reported to exhibit cytotoxic activity against several different cancer types, but studies against colorectal cancer are lacking. In a screening procedure aimed at identifying natural products with activity against colon cancer, several cardiac glycosides were shown to be of interest, and five of these were further evaluated in different colorectal cancer cell lines and primary cells from patients. Convallatoxin (1), oleandrin (4), and proscillaridin A (5) were identified as the most potent compounds (submicromolar IC50 values), and digitoxin (2) and digoxin (3), which are used in cardiac disease, exhibited somewhat lower activity (IC50 values 0.27-4.1 microM). Selected cardiac glycosides were tested in combination with four clinically relevant cytotoxic drugs (5-fluorouracil, oxaliplatin, cisplatin, irinotecan). The combination of 2 and oxaliplatin exhibited synergism including the otherwise highly drug-resistant HT29 cell line. A ChemGPS-NP application comparing modes of action of anticancer drugs identified cardiac glycosides as a separate cluster. These findings demonstrate that such substances may exhibit significant activity against colorectal cancer cell lines, by mechanisms disparate from currently used anticancer drugs, but at concentrations generally considered not achievable in patient plasma.

Abstract: Larvae ofDanaus plexippus feed almost exclusively on milkweed species of the genusAsclepias, whose characteristic secondary metabolites are cardiac glycosides (CGs). Aposematic last-instar larvae were fed with ouabain and other cardiac glycosides of differing polarities. Time course experiments show that ouabain is sequestered in the integument within 48 hr after feeding, whereas midgut tissue and hemolymph function as transient CG storage compartments. About 63% of ouabain was transferred from larvae to the butterflies, whereas 37% of ouabain was lost with larval and pupal exuviae and with the meconium. The main sites of storage in imagines are wings and integument. If mixtures of CGs are fed toD. plexippus larvae, differential sequestration can be observed: The polar ouabain contributes 58.8% of total CGs, followed by digitoxin (19.6%), oleandrin (10.6%), digoxin (4.9%), digoxigenin (4.6%) and proscillaridin A (1.5%). Thus, uptake and sequestration must be selective processes. Uptake of [(3)H]ouabain in vitro by isolated larval midguts was time-, pH-, and temperature-dependent and displayed an activation energy of 49 kJ/mol. Furthermore, the in vitro uptake of ouabain was inhibited (probably competitively) by the structurally similar convallatoxin. These data provide first evidence that ouabain uptake does not proceed by simple diffusion but with the aid of a carrier mechanism, which would explain the differential cardenolide uptake observed in living larvae.