A new approach to the design of novel inhibitors of Na+,K+-ATPase: 17alpha-substituted seco-D 5beta-androstane as cassaine analogues

Progress on Cassaine-Type Diterpenoid Ester Amines and Amides (Erythrophleum Alkaloids)

The structures, spectral characteristics, and bioactivities of 39 natural cassaine-type diterpenoid ester amines and amides (Erythrophleum alkaloids) and 31 synthetic analogues are reviewed. Cassaine-type diterpenoid ester amines and amides, the so called Erythrophleum alkaloids, have the skeleton of cassane-type diterpenoids with a N-containing side chain, and are classified into two groups, ester amines and amides. Cassaine-type diterpenoid ester amines and amides show remarkable inotropic action on the heart, inhibition of Na+/K+-ATPase, cytotoxities, and other major bioactivities. Structural modification of cassaine-type diterpenoid ester amines and amides has been carried out to furnish many derivatives to study the structure-activity relationships.

Erythrofordins D and E, two new cassaine-type diterpenes from Erythrophleum suaveolens

Two new cassaine-type diterpenoids, namely erythrofordins D (1) and E (2), sourced from a Cameroon collection of Erythrophleum suaveolens were isolated and assessed for anti-tumor activity. In the NCI-60 cancer cell assay, erythrofordins D (1) and E (2) were found to be cytotoxic in the low micro molar ranges with a mean GI50 value of 2.45 and 0.71 µM, mean TGI value of 9.77 and 2.29 µM, and a mean LC50 of 26.92 and 11.48 µM for 1 and 2 respectively. Using the COMPARE algorithm, the new compounds were found to have similar NCI-60 response profiles to the known cardiac glycosides hyrcanoside and strophanthin. In addition, in an assay examining the viability and contractile function in human cardiomyocytes derived from induced pluripotent stem-cells, erythrofordins showed cardiotoxicity effects at concentrations as low as 0.03 µg/mL.

Enhancing the potency of lithospermate B for inhibiting Na+/K+-ATPase activity by forming transition metal ion complexes

AIM

To determine whether replacing Mg(2+) in magnesium lithospermate B (Mg-LSB) isolated from danshen (Salvia miltiorrhiza) with other metal ions could affect its potency in inhibition of Na(+)/K(+)-ATPase activity.

METHODS

Eight metal ions (Na(+), K(+), Mg(2+), Cr(3+), Mn(2+), Co(2+), Ni(2+), and Zn(2+)) were used to form complexes with LSB. The activity of Na(+)/K(+)-ATPase was determined by measuring the amount of inorganic phosphate (Pi) liberated from ATP. Human adrenergic neuroblastoma cell line SH-SY5Y was used to assess the intracellular Ca(2+) level fluctuation and cell viability. The metal binding site on LSB and the binding mode of the metal-LSB complexes were detected by NMR and visible spectroscopy, respectively.

RESULTS

The potencies of LSB complexed with Cr(3+), Mn(2+), Co(2+), or Ni(2+) increased by approximately 5 times compared to the naturally occurring LSB and Mg-LSB. The IC50 values of Cr-LSB, Mn-LSB, Co-LSB, Ni-LSB, LSB, and Mg-LSB in inhibition of Na(+)/K(+)-ATPase activity were 23, 17, 26, 25, 101, and 128 μmol/L, respectively. After treatment of SH-SY5Y cells with the transition metal-LSB complexes (25 μmol/L), the intracellular Ca(2+) level was substantially elevated, and the cells were viable for one day. The transition metals, as exemplified by Co(2+), appeared to be coordinated by two carboxylate groups and one carbonyl group of LSB. Titration of LSB against Co(2+) demonstrated that the Co-LSB complex was formed with a Co(2+):LSB molar ratio of 1:2 or 1:1, when [Co(2+)] was less than half of the [LSB] or higher than the [LSB], respectively.

CONCLUSION

LSB complexed with Cr(3+), Mn(2+), Co(2+), or Ni(2+) are stable, non-toxic and more potent in inhibition of Na(+)/K(+)-ATPase. The transition metal-LSB complexes have the potential to be superior substitutes for cardiac glycosides in the treatment of congestive heart failure.

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.

Total synthesis of (+)-cassaine via transannular Diels-Alder reaction

A full account of the total synthesis of (+)-cassaine ( 1) using the transannular Diels-Alder (TADA) reaction as the pivotal construction is described. The strategy began from Evans' oxazolidine 8, the only chiral source used for the total stereochemical outcome of the target molecule. The key intermediate 3 was obtained from 8 in 10 steps in 40% overall yield. Following extensive optimization, the coupling of 3 on both ends with another densely functional partner 2 followed by TADA reaction on macrocycle 4 cleanly furnished the tricycle 5. The stereochemical outcome in 5 was expected via a least-energetic transition state T4. A stereoselective reduction, hydroboration, and methyl cuprate 1,4-addition along with a few other functional interconversions transformed 5 into the key intermediate 37. Final tethering of dimethylaminoethyloxycarbonyl along with epimerization at C8 and alcohol deprotection at C3 yielded the natural product 1.

Magnesium lithospermate B possesses inhibitory activity on Na+,K+-ATPase and neuroprotective effects against ischemic stroke

AIM

To examine if magnesium lithospermate B (MLB) extracted from Danshen, the dried roots of Salvia miltiorrhiza, may act as an active component responsible for the cardiac therapeutic effect of this traditional Chinese herb via the same molecular mechanism triggered by cardiac glycosides, such as ouabain and digoxin. Moreover, we wanted to test if MLB may provide neuroprotection against ischemic stroke as observed for cardiac glycosides.

METHODS

Similarity in the chemical structure and molecular configuration between MLB and ouabain was analyzed. The inhibition potency of MLB and ouabain on Na( +),K( +) -ATPase activity of a commercial product, as well as in purified membrane fractions from rat brain and heart tissues, was examined and compared. Neuroprotective effect of MLB against ischemic stroke was also evaluated using a cortical brain slice-based assay model.

RESULTS

Dose-dependent inhibition on the commercial Na( +),K( +)-ATPase equivalent to that for ouabain was observed for MLB of approximately half dosage by weight. This relative potency of ouabain and MLB was also observed for their inhibition on Na( +),K( +)-ATPase activity of plasma membrane purified from rat tissues, although these 2 inhibitors exhibited somewhat lower competence in these crude extracts. In ischemic gerbil brains, post-treatment with MLB significantly reduced the infarct size, visualized by 2,3,5-triphenyltetrazolium chloride staining, by approximately 55% when compared with the control group.

CONCLUSION

These results evidently suggest that the cardiac therapeutic effect of Danshen should be at least partly attributed to the effective inhibition of Na( +),K( +)-ATPase by MLB, and that MLB provides anti-ischemic neuroprotection in gerbils subjected to focal ischemia and reperfusion.

Total synthesis of 14β-fluorosteroids via a transannular Diels–Alder reaction

14β-Fluorosteroids 3 and 4 were synthesized to give a new class of unnatural cardenolides. The total synthesis of racemic 14β-fluorosteroids was accomplished using a highly diastereoselective transannular Diels–Alder reaction on a trans-cis-cis macrocyclic triene. The α-fluoro analog 4 provided a comparable inhibitory activity to natural digitoxigenin 1.Key words: fluorosteroid, bioisostere, cardiovascular diseases, transannular Diels–Alder reaction (TADA), macrocyclization.

A quantitative structure-activity relationship study on a series of Na+, K+-ATPase inhibitors

A quantitative structure-activity relationship (QSAR) study has been made on a new series of digitalis-like Na+,K+-ATPase inhibitors in which the guanylhydrazone group has been replaced by an aminoalkyloxime group. The correlations obtained have shown that the oxime moiety, primary amine group, overall size, and polarizability of the new type of substituents are higly beneficial to the Na+,K+-ATPase inhibition potency of the compounds and that their effect can be quantitatively assessed. The study also showed that the inotropic activity of the compounds is very well correlated with their Na+,K+-ATPase inhibition potency.

Structure-based design and synthesis of novel potent Na+,K+ -ATPase inhibitors derived from a 5alpha,14alpha-androstane scaffold as positive inotropic compounds

The design, synthesis, and biological properties of novel inhibitors of the Na(+),K(+)-ATPase as potential positive inotropic compounds are reported. Following our model of superposition between cassaine and digitoxigenin, digitalis-like activity has been elicited from a non-digitalis steroidal structure by suitable modifications of the 5alpha,14alpha-androstane skeleton. The strong hydrophobic interaction of the digitalis or cassaine polycyclic cores can be effectively obtained with the androstane skeleton taken in a reversed orientation. Thus, oxidation of C-6 and introduction in the C-3 position of the potent pharmacophoric group recently introduced by us, in the 17 position of the digitalis skeleton, namely, O-(omega-aminoalkyl)oxime, led to a series of substituted androstanes able to inhibit the Na(+),K(+)-ATPase, most of them with an IC(50) in the low micromolar level, and to induce a positive inotropic effect in guinea pig. Within this series, androstane-3,6,17-trione (E,Z)-3-(2-aminoethyl)oxime (22b, PST 2744) induced a strong positive inotropic effect while being less arrhythmogenic than digoxin, when the two compounds were compared at equiinotropic doses.

Pharmacological profile of the novel inotropic agent (E,Z)-3-((2-aminoethoxy)imino)androstane-6,17-dione hydrochloride (PST2744)

The novel Na(+)/K(+)-ATPase inhibitor (E,Z)-3-((2-aminoethoxy)imino)androstane-6,17-dione hydrochloride (PST2744) was characterized for its inotropic and toxic properties. Inhibition potency on dog kidney Na(+)/K(+)-ATPase was comparable (0.43 microM) to that of digoxin (0.45 microM). PST2744 concentration-dependently increased force of contraction in guinea pig atria and twitch amplitude in isolated guinea pig myocytes; in the latter, aftercontractions developed significantly less than with digoxin. Intravenous infusion of 0.2 mg/kg/min PST2744 in anesthetized guinea pigs exerted an immediate and long-lasting inotropic effect (ED(80) of 1.89 +/- 0.37 mg/kg) without causing lethal arrhythmias up to a cumulative dose of 18 mg/kg. Conversely, an equieffective infusion of digoxin (0.016 mg/kg/min; ED(80) of 0.32 mg/kg) caused lethal arrhythmias at a cumulative dose of 0.81 mg/kg. At a higher rate (0.4 mg/kg/min), PST2744 induced lethal arrhythmias, with a lethal dose/ED(80) ratio significantly greater than digoxin (20.2 +/- 6.3 versus 3.23 +/- 0.55, p < 0.05). Decay of the inotropic effect (t(1/2), min) was significantly faster for PST2744 (6.0 +/- 0.39) than for digoxin (18.3 +/- 4.5, p < 0.05). In anesthetized dogs, PST2744 dose-dependently increased maximum velocity of pressure rise (+dP/dt(max)) in the range 32 to 500 microg/kg i.v. and was safer than digoxin. In conscious dogs with a healed myocardial infarction, PST2744 significantly increased resting values of +dP/dt(max), left ventricular pressure, and SPB, and increased +dP/dt(max) throughout treadmill exercise while reverting the increase in left ventricular end diastolic pressure seen in control animals. Digoxin significantly decreased basal heart rate, while not affecting the hemodynamic response to exercise. Thus, PST2744 represents a new class of Na(+)/K(+)-ATPase inhibitors endowed with inotropic activity comparable with that of digitalis but having greater safety.

Inotropic activity of hydroindene amidinohydrazones

Several hydroindenic derivatives (7a-methyl-2,3,5,6,7,7a-hexahydro-1H-indenes), bearing an amidinohydrazone at C-5 and different moieties at C-1, have been synthesized and evaluated for their inotropic and chronotropic effects on right- and left-guinea-pig-atria activity. Three of them showed the same profile as digoxin, although with lower potency. The effect on Na(+),K(+)-ATPase (NKA) was also evaluated for these three compounds, observing that two of them, with the same absolute configuration as natural cardenolides, are also NKA inhibitors, while the compound with the opposite configuration lacks such an effect. More interestingly, both active compounds act without affecting the cardiac rhythm. This could be related to the selective inhibition of the human alpha2beta1 isozyme (associated with the inotropic effect) with respect to the alpha1beta1 isozyme (associated with the maintenance of basal ionic levels in the cell and the toxic effect of cardenolides).

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.

17 alpha-O-(aminoalkyl)oxime derivatives of 3 beta,14 beta-dihydroxy-5 beta-androstane and 3 beta-hydroxy-14-oxoseco-D-5 beta-androstane as inhibitors of Na(+),K(+)-ATPase at the digitalis receptor

The synthesis and binding affinities to the digitalis Na(+),K(+)-ATPase receptor of a series of 3 beta,14 beta-dihydroxy-5 beta-androstane and 3 beta-hydroxy-14-oxoseco-D-5 beta-androstane derivatives bearing a 17 alpha-(aminoalkoxy)imino chain are reported; some derivatives were also studied for their inotropic activity. Our recently proposed model of interaction of molecules with the digitalis receptor was used to design these compounds. On that basis, the possibility to design novel potent inhibitors of Na(+),K(+)-ATPase without being constrained by the stereochemistry of the classical digitalis skeleton in the D-ring region was predicted. The binding affinities of the most potent compounds in the two series, (EZ)-17 alpha-[2-[(2-aminoethoxy)imino]ethyl]-5 beta-androstane-3 beta,14 beta-diol (6f) and (EZ)-3 beta-hydroxy-17 alpha-[2-[(2-aminoethoxy)imino]ethyl]-14,15-seco-5 beta-androstan-14-one (24c) are higher than that of the potent natural compound digitoxigenin, despite the unusual alpha-exit of the substituent in position 17 of 6f or the disruption of the D-ring in 24c. These results further support the validity of our recently proposed model of binding at the digitalis receptor. Results of the inotropic tests on guinea pig atrium deserve further investigation on the pharmacological profile of these derivatives.

Progress toward the total synthesis of cassaine via the transannular diels-alder strategy

The transannular Diels-Alder reaction of trans-trans-trans macrocyclic triene A, bearing two cis substiuents in C(12) and C(13) as well as a gem-dimethyl in C(4), was studied. Under thermal conditions, only the desired trans-anti-cis tricycle B was obtained. This tricycle represents an advanced intermediate toward the total synthesis of cassaine C.

17beta-O-Aminoalkyloximes of 5beta-androstane-3beta,14beta-diol with digitalis-like activity: synthesis, cardiotonic activity, structure-activity relationships, and molecular modeling of the Na(+),K(+)-ATPase receptor

A series of digitalis-like compounds with a 17-aminoalkoxyiminoalkyl or -alkenyl substituent was synthesized and evaluated for inhibition of Na(+),K(+)-ATPase and for inotropic activity. The highest inhibition was found with compounds having the substituent in configuration 17beta and the amino group at a distance of 6 or 7 bonds from C(17) of the digitoxigenin skeleton. The presence of the oxime function strengthens the interaction with the receptor, more if alpha,beta-unsaturated, thus mimicking the electronic situation of the unsaturated lactone in natural digitalis compounds. The most active compounds showed Na(+),K(+)-ATPase inhibitory potencies (IC(50)) 17-25 times higher than the standards digitoxigenin and digoxin and 3-11 times higher inotropic potencies (EC(50)) in isolated guinea pig left atria. These features are supported by a molecular model suggesting the possible interactions of the groups described above with particular amino acid residues in the H1-H2 domains of Na(+),K(+)-ATPase. Some interactions are the classical ones already described in the literature; a new, very strong interaction of the basic group with the Cys138 was found and adds new possibilities to design compounds interacting with this region of the receptor. The most interesting compounds were also studied in vivo in the anesthetized guinea pig for evaluating their inotropic effect versus the lethal dose. Compounds 9 and 12 showed a slightly higher safety ratio than digoxin and deserve further evaluation.