Proscillaridin A Sensitizes Human Colon Cancer Cells to TRAIL-Induced Cell Death

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) is a cytotoxic cytokine that induces cancer cell death by binding to TRAIL receptors. Because of its selective cytotoxicity toward cancer cells, TRAIL therapeutics, such as recombinant TRAIL and agonistic antibodies targeting TRAIL receptors, have garnered attention as promising cancer treatment agents. However, many cancer cells acquire resistance to TRAIL-induced cell death. To overcome this issue, we searched for agents to sensitize cancer cells to TRAIL-induced cell death by screening a small-molecule chemical library consisting of diverse compounds. We identified a cardiac glycoside, proscillaridin A, as the most effective TRAIL sensitizer in colon cancer cells. Proscillaridin A synergistically enhanced TRAIL-induced cell death in TRAIL-sensitive and -resistant colon cancer cells. Additionally, proscillaridin A enhanced cell death in cells treated with TRAIL and TRAIL sensitizer, the second mitochondria-derived activator of caspase mimetic. Proscillaridin A upregulated TRAIL receptor expression, while downregulating the levels of the anti-cell death molecules, cellular FADD-like IL-1β converting enzyme-like inhibitor protein and Mcl1, in a cell type-dependent manner. Furthermore, proscillaridin A enhanced TRAIL-induced cell death partly via O-glycosylation. Taken together, our findings suggest that proscillaridin A is a promising agent that enhances the anti-cancer efficacy of TRAIL therapeutics.

Proscillaridin A inhibits hepatocellular carcinoma progression through inducing mitochondrial damage and autophagy

Hepatocellular carcinoma (HCC) is one of the leading causes of cancer-related deaths globally. At present, drug options for systemic treatment of HCC are very limited. There is an urgent need to develop additional effective drugs for HCC treatment. In the present study, we found that proscillaridin A (ProA), a cardiac glycoside, exerted a strong anticancer effect on multiple HCC cell lines. ProA significantly inhibited the cell proliferation, migration, and invasion of HCC cells. ProA also had a marked inhibitory effect on the progression of HCC in the MHCC97H xenograft nude mouse model. ProA-mediated suppression of HCC was closely related to cell apoptosis. ProA-treated HCC cells displayed significant mitochondrial damage and elevated reactive oxygen species production, resulting in profound cell apoptosis. Meanwhile, ProA also played a role in autophagy induction in HCC cells. Defects in autophagy partially relieved ProA's anticancer effect in HCC cells. Our findings demonstrate that ProA can effectively inhibit HCC progression and may serve as a potential therapeutic agent for HCC treatment.

Proscillaridin A induces apoptosis and suppresses non-small-cell lung cancer tumor growth via calcium-induced DR4 upregulation

Non-small-cell lung cancer (NSCLC) is the predominant histological type of lung cancer and is characterized by the highest mortality and incidence rates among these types of malignancies. Cardiac glycosides, a class of natural products, have been identified as a potential type of chemotherapeutic agent. This study aims to investigate the anti-cancer effects and the mechanisms of action of Proscillaridin A (P.A) in NSCLC cells. In vitro sodium-potassium pump (Na+/K+ ATPase) enzyme assays indicated that P.A is a direct Na+/K+ ATPase inhibitor. P.A showed potent cytotoxic effects in NSCLC cells at nanomolar levels. Treatment mechanism studies indicated that P.A elevated Ca2+ levels, activated the AMPK pathway and downregulated phosphorylation of ACC and mTOR. Subsequently, P.A increased death receptor 4 (DR4) expression and downregulated NF-κB. Interestingly, P.A selectively suppressed EGFR activation in EGFR mutant cells but not in EGFR wild-type cells. In vivo, P.A significantly suppressed tumor growth in nude mice compared to vehicle-treated mice. Compared with the Afatinib treatment group, P.A displayed less pharmaceutical toxicity, as the body weight of mice treated with P.A did not decrease as much as those treated with Afatinib. Consistent changes in protein levels were obtained from western blotting analysis of tumors and cell lines. Immunohistochemistry analysis of the tumors from P.A-treated mice showed a significant suppression of EGFR phosphorylation (Tyr 1173) and reduction of the cell proliferation marker Ki-67. Taken together, our results suggest that P.A is a promising anti-cancer therapeutic candidate for NSCLC.

Proscillaridin A induces apoptosis, inhibits STAT3 activation and augments doxorubicin toxicity in prostate cancer cells

Cardiac glycosides are natural compounds used for the treatment of congestive heart failure and cardiac arrhythmias. Recently, they have been reported to exhibit anticancer activity. Proscillaridin A (PSN-A), a cardiac glycoside constituent of Urginea maritima has been shown to exhibit anticancer activity. However, the cellular targets and anticancer mechanism of PSN-A in various cancers including prostate cancer remain largely unexplored. In the present study, we have shown that PSN-A inhibits proliferation and induces apoptosis in prostate cancer cells in a dose-dependent manner. Further mechanistic study have shown that anticancer activity of PSN-A in prostate cancer cells is associated with ROS generation, Bcl-2 family proteins modulation, mitochondrial membrane potential disruption and ultimately activation of caspase-3 and cleavage of PARP. Moreover, we found that PSN-A inhibits JAK2/STAT3 signaling and augments doxorubicin toxicity in prostate cancer cells. Of note, LNCaP cells were found to be more sensitive to PSN-A treatment as compared to DU145 cells. Taken together, the data provided first evidence of the anticancer activity and possible molecular mechanism of PSN-A in prostate cancer. Further study is needed to develop PSN-A into a potential lead compound for the treatment of prostate cancer.

The traditional medical uses and cytotoxic activities of sixty-one Egyptian plants: discovery of an active cardiac glycoside from Urginea maritima

ETHNOPHARMACOLOGICAL RELEVANCE

Medicinal plants from the Sinai desert are widely used in traditional Bedouin medicine to treat a range of conditions including, cancers, and may thus be useful sources of novel anti-tumor compounds. Information on plants used in this way was obtained through collaboration with Bedouin herbalists.

AIM OF THE STUDY

To document the traditional uses of 61 species from 29 families of Egyptian medicinal plants and to investigate their biological activity using a cytotoxicity assay.

MATERIAL AND METHODS

MeOH extracts of the 61 plant species investigated were dissolved in 10% DMSO and their cytotoxic activity was evaluated. The extracts were tested in duplicate on three separate occasions at three different concentrations (1, 10 and 100μg/ml) against human lymphoma U-937 GTB. The most active extract was subjected to bioassay-guided fractionation using HPLC and LC/ESI-MS to isolate and identify its active components.

RESULTS AND DISCUSSION

The most potent extracts were those from Asclepias sinaica, Urginea maritima, Nerium oleander and Catharanthus roseus, followed by those from Cichorium endivia, Pulicaria undulate and Melia azedarach. Literature reports indicate that several of these plants produce cardiac glycosides. Bioassay-guided fractionation of alcoholic U. maritima extracts led to the isolation of a bioactive bufadienolide that was subsequently shown to be proscillaridin A, as determined by 1D and 2D NMR spectroscopy. This result demonstrates the value of plants used in traditional medicine as sources of medicinally interesting cytotoxic compounds.

Inhibition of DNA topoisomerases I and II by G3 PAMAM-NH2 dendrimer-modified digoxin and proscillaridin A conjugates in a cell free system

Two modified glycosides--digoxin and proscillaridin A conjugated to a generation 3 of polyamidoamine dendrimer (G3 PAMAM-NH2) were evaluated as DNA topoisomerase II inhibitors. The ability of these compounds (PAMAM-Dig and PAMAM-Prosc) to inhibit topoisomerase I and II activity was quantified by measuring the action on supercoiled DNA substrate as a function of increasing concentration of the test compounds by the use of agarose gel electrophoresis. The obtained results suggest that a conjugation of the modified glycosides with G3 PAMAM-NH2 significantly improved the ability of the parent compounds to an inhibition of DNA topoisomerases.

Cytotoxic effects of cardiac glycosides in colon cancer cells, alone and in combination with standard chemotherapeutic drugs

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.

Inhibition of DNA topoisomerases I and II, and growth inhibition of breast cancer MCF-7 cells by ouabain, digoxin and proscillaridin A

We evaluated the cytotoxicity and underlying mechanisms of cardiac glycosides, including digoxin, ouabain and proscillaridin A, on the proliferation of breast cancer MCF-7 cells. In terms of inhibition of cell proliferation of MCF-7 cells, the compounds rank in the order proscillaridin A>digoxin>ouabain. While both digoxin and ouabain inhibited topoisomerase II catalytic activity at nanomolar concentrations (100 nM), neither agent inhibited topoisomerase I catalytic activity even at concentrations as high as 100 microM. On the other hand, proscillaridin A was a potent poison of topoisomerase I and II activity at nanomolar drug concentrations (30 nM, 100 nM, respectively), suggesting that this agent may produce its cytotoxic activity by targeting both enzymes simultaneously. These studies suggest that the stabilization of DNA-topoisomerase II complexes is closely linked to the mechanism of digoxin, ouabain and proscillaridin A cytotoxicity. The potential DNA-binding properties of the cardiac glycosides have been assessed by measuring the displacement of ethidium bromide from calf thymus DNA. These results indicate that digoxin, ouabain and proscillaridin A neither intercalate nor interact with the minor groove of DNA.

The T-cell suppressive effect of bufadienolides: structural requirements for their immunoregulatory activity

Many studies indicate that substances similar to cardenolides and bufadienolides naturally occur in mammals. The majority of previous studies focused on their cardiovascular, renal, and central nervous action. We analyzed the immunoregulatory property of 52 bufadienolides. Human T-cells were stimulated "in vitro" with mitogens or alloantigens in the presence of bufadienolides. The most active compound totally inhibited T-cell activity at a concentration of 0.75 pmol/10(5) cells. This effect is 16,384 x stronger than that of cortisol and 256 x stronger than that of cyclosporin A or tacrolimus. Preactivated T cells were downregulated and, most importantly, suppressed viable T cells could not be restimulated. Lack of the 17 beta-lactone ring dramatically reduced the activity of bufadienolides. Substitution at C3 also affected their function: components with a 3-OH group were up to 1000 x stronger than those without. The replacement of 14 beta-OH with an epoxy-group slightly decreased the activity. Because there is evidence that the latter change abolishes the cardiac activity, this finding is relevant for therapeutic applications in which immunosuppression without the risk of cardiotoxicity is attempted. One of the substances analyzed in this study was Proscillaridin A. A similar bufadienolide occurs naturally in mammals. We speculate that bufadienolides represent an important bioregulatory link between the cardiovascular, nervous and immune systems.

Cardiac glycoside-induced elevation of intracellular Na+ ion concentration in human erythrocytes studied by 23Na NMR spectroscopy: relationship between inotropy speed and elevation rate of intracellular Na+ ion concentration

Elevation of intracellular sodium ion concentration in human erythrocyte induced by the cardiac glycoside, proscillaridin, and its four derivatives was measured using 23Na NMR spectrometry. In this examination, there was a significant correlation between the time to half maximum inotropic effect and the time to maximum of Na+ concentrations in human erythrocyte, determined by 23Na NMR.

[Studies on cardiac ingredients of plants. X. Preparation of nitrates of tetrahydroproscillaridin and their pharmacological activities]

To reduce the vascular contracting effect of the hydrogenated cardiac glycosides, 20-(R)- and 20-(S)-tetrahydroproscillaridins (THPs, 1a, 1b), and to extend the concentration-dependent range, mono- and dinitrates of THPs were prepared. The pharmacological activities of the nitrates of THP were evaluated by use of isolated guinea-pig papillary muscle preparations and Na+,K(+)-adenosine triphosphatase preparations from dog kidney. Furthermore, the effect for smooth muscle was examined using the helical strips isolated from 13-week-old spontaneously hypertensive rat. The positive inotropic effects of mononitrates (11a, 11b, 2a, 2b, 8a, and 8b) were more potent than those of THPs. Nitration of the sugar moiety in THPs resulted in a vascular relaxing effect unobserved in the case of THPs.

Studies on cardiac ingredients of plants. IX. Chemical transformation of proscillaridin by utilizing its 1,4-cycloadducts as key compounds and biological activities of their derivatives

Three aromatic compounds (2-4) possessing a carbomethoxyl group or a dimethoxyphthaloyl group, prepared by the Diels-Alder reaction of the cardiac glycoside, proscillaridin (1), with dimethyl acetylenedicarboxylate and methyl propiolate, were transformed into alcohols, carboxylic acids and amides. The biological activities of the resulting derivatives were evaluated by the use of Na+, K(+)-adenosine triphosphatase (Na+,K(+)-ATPase) from dog kidney and isolated guinea-pig papillary muscle. Although the biological activities of the resulting derivatives were less potent than that of 1, a para-substituted benzylalcohol (5), methylbenzamides (9a and 10a), and ethylbenzamides (9b and 10b) inhibited the activity of Na+,K(+)-ATPase almost as potently as naturally occurring cardiac glycosides such as digoxin and digitoxin.

[Studies on cardiac ingredients of plants. VIII. Preparation of nitrates of proscillaridin and their pharmacological activities]

To reduce the vascular contracting effect of the cardiac glycoside, proscillaridin (1), all kinds of its nitrates were prepared by utilizing effectively an isopropylidene function as a protective group. The pharmacological activities of proscillaridin nitrates were evaluated by the use of isolated guinea-pig papillary muscle preparations and Na+,K(+)-adenosine triphosphatase preparations from the dog kidney. Furthermore, the effect for smooth muscle using the helical strips isolated from 13-week old spontaneously hypertensive rat was examined. The positive inotropic effects and Na+, K(+)-adenosine triphosphatase inhibition activities of mononitrates (6, 9, 15) and dinitrates (3, 4, 5) were a little less potent than 1, but those of trinitrate (2) were much reduced. Every nitrate did not exhibited a vascular contracting effect but a relaxing effect. Among them, the vascular relaxing effects of 2',3'-dinitrate (3) and 2',4'-dinitrate (4) were more potent than those of the other nitrates.

Studies on cardiac ingredients of plants. VII: Chemical transformation of proscillaridin by means of the Diels-Alder reaction and biological activities of its derivatives

The Diels-Alder reactions of a cardiac glycoside, proscillaridin (1), with some dienophiles were investigated. The reaction of 1 with alkenes such as methyl vinyl ketone and methyl acrylate afforded 3-oxo-2-oxabicyclo[2.2.2]oct-7-enes (2-5) and para-substituted benzene derivatives (6 and 7), while 1 reacted with alkynes (3-butyn-2-one, methyl propiolate) to yield para- or meta-substituted benzene derivatives (6-9). The biological activities of the resulting derivatives were evaluated by the use of isolated guinea-pig papillary muscle preparations and Na+,K(+)-adenosine triphosphatase (ATPase) preparation from dog kidney. Among the proscillaridin derivatives, compounds 4 and 7 moderately inhibited Na+,K(+)-ATPase activity. Furthermore, the concentration range of 7 over which its positive inotropic effect on guinea-pig papillary muscle preparations, increased from 5% to 95% of maximum was broader than that of 1, i.e., concentration dependency was maintained over a greater range of concentration.

Hemodynamic effect of methylepoxyproscillaridin (P35) in patients with congestive myocardiopathy

The hemodynamic effect of methylepoxyproscillaridin (3'-methyl-4'-5'-epoxy-proscillaridin) (P35) was studied according to the Swan Ganz thermodilution method in 6 patients with latent cardiac insufficiency. Both at rest and under ergometer exercise, the stroke volume and the systolic blood pressures increased under P35. Moreover, under physical exercise the heart rate decreased significantly, while cardiac output increased. The hemodynamic effect of P35 can therefore be regarded as typical of cardiac glycosides.

A comparative dose-effect study with cardiac glycosides assessing cardiac and extracardiac responses in normal subjects

We tested the hypothesis that differences exist in the pharmacodynamic pattern of different cardiac glycosides. We conducted a randomized, placebo-controlled study in normal volunteers and evaluated the effects of weekly increased oral dosing of digoxin (n = 10; from 0.25 to 1.0 mg/day), meproscillarin (n = 10; from 0.5 to 2.0 mg/day), and placebo (n = 5). To determine the glycoside effects, corrected electromechanical systole (QS2c) was used to measure inotropy and the PQ interval to test dromotropy. Red-green discrimination and critical flicker fusion (CFF) assessed visual functions. Subjective complaints were collected using rating lists. Both glycosides dose dependently shortened QS2c and prolonged PQ interval. PQ prolongations over +20 ms occurred in seven of 10 digoxin subjects, in two of 10 meproscillarin, and in one of five placebo. Equi-inotropic response, identified at 12 ms mean QS2c shortening, revealed the relative potency of digoxin to be 2.4 times higher than meproscillarin; this ratio increased to sevenfold for equi-effective negative dromotropic effects at 12 ms mean PQ prolongation. Each drug was associated with a dominant subjective complaint: digoxin with anergy and meproscillarin with diarrhea. Red-green discrimination was better under meproscillarin and CFF was depressed by digoxin. The results indicate that pharmacodynamic differences exist between cardiac glycosides. A differential use of various glycosides should be considered and tested clinically.

[Effect of plant glycosides on resistance and capacitance vessels]

In the anaesthetized cat, SCOA ( Miroton ), a product which contains extracts from Scilla , Convallaria , Oleander and Adonis , displays not only its well-known positive inotropic effect but has also constrictor effects on veins when applied in intravenous doses of 21.5-100 GPU /kg ( GPU = guinea-pig units, i.e. cardiotoxic equivalents related to 1 g body weight of guinea-pigs). The latter effect differs in that it is somewhat more prolonged. With intraduodenal administration the doses required to achieve equal peak effects as with intravenous injection are about 4 times larger and this suggests a relatively good enteral availability in the cat. SCOA constricts not only veins but also arteries. However, this latter effect is comparatively small and occurs only after intraarterial infusion of high doses (9.1 and 91 GPU /min, respectively).--The cardiac glycosides contained in the drug product primarily account for its vasoactive qualities. The venous constrictor effect correlates with the guinea-pig units. In qualitative respects, the pure glycosides cymarin , convallatoxin , proscillaridin , and scillaren exert equal effects. There is, however, evidence that the correlation between the effect on veins and on the heart differs for the glycosides tested. Based on equal guinea-pig units, the adonis extract, for instance, acts on capacitance vessels about twice as much as scilla , oleander and convallaria extracts. Cymarin , too, has a stronger effect on veins than would be expected from its cardiotoxic effect. The action on arteries and veins are based on different mechanism.(ABSTRACT TRUNCATED AT 250 WORDS)

Electrophysiological studies of some semisynthetic cardiac glycoside derivatives in isolated papillary muscle of the guinea-pig

The effects of digitoxin, 3 alpha-methyl-digitoxigenin-3 beta-monoglucoside (3 alpha-MDM), 3 alpha-methyl-digitoxigenin (3 alpha-MD), proscillaridin, 4, 5-methylene-procillaridin (4, 5-MP), and 3 beta-hydroxy-4, 5-methylene-A, B-trans-scillarenin (3 beta-HMTS) on force of contraction and on the transmembrane action potentials were examined in isolated papillary muscles of guinea-pigs. All derivatives exhibited the typical cardiac glycoside effects: i.e. they increased the force of contraction and shortened the action potential duration at 20% (plateau phase) and 90% of repolarization. With digitoxin, 3 beta-HMTS and 4, 5-MP a transient prolongation in action potential duration was observed at the lower concentrations. The action potential amplitude and the resting membrane potential were reduced consistently only with the higher concentrations used. The onset of the positive inotropic effects of 3 alpha-MDM, 3 alpha-MD and 3 beta-HMTS was more rapid than that of digitoxin and proscillaridin. The increment in contractile force reached a maximum well before the full shortening effect on the action potential duration had developed. The shortening of the action potential is thought to be responsible for the biphasic nature of the positive inotropic effect. With 3 alpha-MD and 3 alpha-MDM even toxic effects, e.g. increase in baseline tension, were completely reversible after washing in drug-free solution. The dose-response curves for the positive inotropism can only be compared reliably once the equilibrium of drug action has been established. This steady state is probably reflected by the development of the full shortening in action potential duration.

[Threshold of action of meproscillarin following intravenous administration]

A placebo-controlled double-blind trial was conducted, to determine the onset of effect of Meproscillarin (injected intravenously). 28 days apart six healthy volunteers were given double-blind 1,0 mg Meproscillarin or placebo and 60 days later 0,6 mg Digoxin, each drug as short infusion over four minutes. The total electromechanical systole corrected for heart rate (QS2c), was used as criterion of the inotropic effect of the glycoside. After placebo there no changes were observed over a period of two hours. Both glycosides shortened QS2c for about--16 ms. From Meproscillarin a 40% higher dosis was required, to achieve the same effect as with digoxin. After use of Meproscillarin the peak effect was reached after ten minutes; after digoxin a period of 60--120 minutes were required. The results show that intravenously given Meproscillarin is a fast acting glycoside.

[Effect of proscillaridin-4'-methylether on pressure rise velocity in the left ventricle of patients with coronary heart disease (author's transl)]

In a randomized, controlled, double-blind study 2 groups of 7 patients each with coronary heart disease received either 1 mg methylproscillaridin (MP) a cardiac glycoside of the squill intravenously, or placebo to test the inotropic effect of MP. Pressures and dp/dt max were measured in the left ventricle before and after coronary angiography after an average of 62 min (60 to 70 min). There was a significant (p less than 0.05) increase of dp/dt max in the MP-group (Wilcoxon-Mann-Whitney test). Left ventricular enddiastolic pressure was decreased from 16.1 to 13.6 mm Hg in the MP-group, and increased in theplacebo group from 12 to 13.7 mm Hg (mean values). Thus, a positive inotropic effect of the glycoside may be assumed.

[Echocardiographic evaluation of left ventricular function during therapy with cardiovascularly effective drugs]

The task of this study was to examine the feasibility of applying echocardiography in determining alterations of left ventricular performance during administration of vasoactive drugs. Patients suffering from congestive heart failure were treated with Digitalis, other patients with normal hearts were subjected to infusion therapy of Isoproterenol, Propranolol, Verapamil and Calcium chloride. A new concept for interpretation of function curves relating the velocity of circumferential fiber shortening (VCF) and time was introduced to establish the inotropic, bathmotropic and chronotropic effects of vasoactive drugs to the myocardium. It can be concluded that echocardiography is a feasible tool in determining changes of left ventricular dynamics during administration of vasoactive drugs, in indicating the individual dosage of a vasoactive drug in each patient and the follow-up the course of the therapeutic success.

Pharmacokinetics and pharmacodynamics of methyl proscillaridin in healthy man

The aim of this study was to obtain data about the pharmacological properties of a new glycoside derivative in man. Plasma concentrations and ECG parameters were measured after oral and intravenous administration of a single dose of 1.2 mg methyl proscillaridin in 16 healthy volunteers, using a strictly randomized, two-period change-over design. Glycoside concentrations were measured using a modified 86Rb-erythrocyte-assay. QT-duration, corrected for frequency (QTc), was the principal variable measured in the ECG. By either route, there was a maximum plasma level after 1 hour, which had decreased to a minimum at 3 hours, followed by a second peak at 4 to 10 hours (orally greater than iv). From 10 to 72 hours the concentrations decreased with a median t 1/2 of 23.3 hours (iv) and 33.0 hours (orally). Comparison of the ratio of plasma concentrations following oral and iv administration resulted in a bioavailability of 69% using the 48 hour plasma levels, and 59% using the areas under the concentration-time curves. The mean QTc was maximally shortened to 28 msec at 1 hour after iv and to 19 msec at 10 hours after the oral dose. A distinct similarity between time-concentration and time-QTc curves was seen after the initial distribution phase, both after oral and intravenous administration. The new derivative shows a rapid elimination. Its bioavailability is reasonably high.

In vitro stability of proscillaridin A

In an in vitro study, proscillardin A was found to be rapidly inactivated at low pH. More than 50 per cent of its activity, measured by 86Rb assay, was lost after incubation for 15 minutes at pH 1 and 37 degrees C. Compared with proscillaridin, the rate of inactivation of digoxin was lower in these experiments. The rapid inactivation of proscillaridin might be of clinical importance when treating patients with this glycoside.