Selection between Diastereomeric Kinetic vs Thermodynamic Carbonyl Binding Modes Enables Enantioselective Iridium-Catalyzed anti-(α-Aryl)allylation of Aqueous Fluoral Hydrate and Difluoroacetaldehyde Ethyl Hemiacetal

Enantioselectivity increases with increasing carbonyl electrophilicity in 2-propanol-mediated reductive couplings of aldehydes with branched aryl-substituted allylic acetates to form products of carbonyl anti-(α-aryl)allylation. This unusual phenomenon is caused by aldehyde coordination to diastereomeric kinetic vs thermodynamic carbonyl binding sites that deliver enantiomeric products. Exploiting this effect, anti-diastereo- and enantioselective (α-aryl)allylations of fluoral hydrate and difluoroacetaldehyde ethyl hemiacetal were developed.

A conjugate of pyridine-4-aldoxime and atropine as a potential antidote against organophosphorus compounds poisoning

A conjugate of pyridine-4-aldoxime and atropine (ATR-4-OX) was synthesized and its antidotal efficiency was tested in vitro on tabun- or paraoxon-inhibited acetylcholinesterase (AChE) of human erythrocytes as well as in vivo using soman-, tabun- or paraoxon-poisoned mice. Its genotoxic profile was assessed on human lymphocytes in vitro and was found acceptable for further research. ATR-4-OX showed very weak antidotal activity, inadequate for soman or tabun poisoning. Conversely, it was effective against paraoxon poisoning both in vitro and in vivo. All animals treated with 5 % or 25 % LD(50) doses of the new oxime survived after administration of 10.0 or 16.0 LD(50) doses of paraoxon, respectively. Based on the persistence of toxicity symptoms in mice, the atropine moiety had questionable effects in attenuating such symptoms. It appears that ATR-4-OX has a therapeutic effect related to the reactivation of phosphylated AChE, but not to receptor antagonization.

Photoaffinity labeling of the solubilized, partially purified muscarinic acetylcholine receptor from porcine atria by p-azidoatropine methyl iodide

The synthesis of a tritiated photoaffinity analogue of the muscarinic antagonist atropine, [3H]-p-azidoatropine methyl iodide is described. The compound appeared to bind to a single class of sites in membrane-bound, solubilized, and partially purified preparations of muscarinic receptor from porcine atria with a dissociation constant (determined by competition vs. [3H]-L-quinuclidinyl benzilate) of about 1.0 X 10(-7) M. This value was in agreement with the apparent dissociation constant (8.5 X 10(-8)M) determined by measuring the concentration dependence of covalent incorporation into a partially purified receptor preparation. Competition experiments indicated that the specific covalent labeling could be blocked by the muscarinic agonist carbamylcholine and the antagonists L-quinuclidinyl benzilate and atropine. An apparent molecular weight of 75 000 +/- 5000 was found for specifically labeled peptide(s) in a solubilized, partially purified receptor preparation by sodium dodecyl sulfate-polyacrylamide gel electrophoresis.

Cardiovascular studies on different classes of soft drugs

Based on the inactive metabolite approach, three different classes of soft drugs were designed and synthesized. Their cardiovascular effects and duration of actions were studied in anesthetized male Sprague-Dawley rats compared to the traditional drugs. During the experiments ECG (leads II, aVF) and beat-to-beat blood pressure (BP) from the left carotid artery were recorded (except during the anticholinergic studies). The soft anticholinergic methoxycarbonylphenylcyclopentyl-N,N-dimethyltropinium methyl sulfate was as potent as atropine in the prevention of carbachol induced bradycardia; however, its action only lasted up to 15-30 min, compared to 2 h of that of atropine. In the isoproterenol-induced tachycardia model, while bufuralol at an i.v. dose of 3.8 mumol/kg (1 mg/kg) diminished heart rate (HR) for at least 2 h, the effects of the soft drugs lasted for only 30-40 min at equimolar doses. The methyl-, ethyl-, isopropyl-, and tert-butyl ester-analogs of the carboxylic acid metabolite of bufuralol showed the highest beta-blocking potencies (i.e., 30-50% of that of bufuralol). When these compounds were infused for 10 min at doses ranging from 2-4 mumol/kg/min, they caused a 20-40% decrease in HR and a 30-40% reduction in mean arterial pressure (MAP). These effects were similar to those elicited by esmolol at a dose of 20 mumol/kg/min in respect of the kinetics and in the extent of the reductions in heart rate and MAP. The isopropyl-, the sec-butyl-, and the neopentyl-esters of the acidic metabolite of amiodarone, with plasma hydrolytic half-lives of 60, 240 and 300 min, were tested in the benzene/adrenaline induced ventricular tachycardia (VT) model of the rat. All drugs were administered at a dose of 5 mumol/kg i.v. bolus immediately followed by an infusion at 15 mumol/kg/h for 2 h. It was found, that amiodarone resulted a complete suppression of VTs at 30 min after the start of drug administration, but its effect lasted up to the total course of the experiment (up to 180 min). On the contrary, both the sec-butyl and the isopropyl-analog resulted in complete suppression of VTs already during the first benzene/adrenaline challenge after drug administration (i.e., at 5 min). However, their effects disappeared between 15 and 30 min after discontinuation of the drug infusions in accordance with the enzymatic inactivation (ester hydrolysis) of these soft drugs. All these three classes of soft cardioactive drugs are good examples for highly potent but short acting drugs whose side effects might also be reduced via the retrometabolism based drug design.

Synthesis and enantioselectivity of the enantiomers of PG9 and SM21, new potent analgesic and cognition-enhancing drugs

The enantiomers of two alpha-tropanyl esters, SM21 (1) and PG9 (2), derived from (+)-R-hyoscyamine, that act by increasing the central cholinergic tone, were obtained by esterification after resolution of the corresponding racemic acids [(-)-S-1, (-)-R-2 and (+)-S-2] and by stereospecific synthesis [(+)-R-1]. Their analgesic and cognition-enhancing activities were tested in mice and their ACh-releasing properties determined on rat parietal cortex. These compounds show enantioselectivity in analgesic and cognition-enhancing tests on mice, the eutomers being the isomers which possess the same spatial arrangement of the groups on the chiral atom as (+)-R hyoscyamine [(+)-R-SM21, (+)-S-PG9]. The ACh-releasing effect of the enantiomers of SM21 in rats is in agreement with the results in mice, while PG9 enantiomers do not show any appreciable enantioselectivity in this test. On the basis of the different effects of the 5-HT4 antagonist SDZ 205557 on analgesia induced by the enantiomers of 1 and 2 and by (+)-R-hyoscyamine and the alpha-tropanyl ester of 2-phenylpropionic acid 3, a mechanism of action is proposed for this class of compounds.

Soft drugs--XVI. Design, evaluation and transdermal penetration of novel soft anticholinergics based on methatropine

Atropine has been reported to produce unwanted systemic side effects on topical administration into the eye. The same problem could arise when atropine is used topically as a suppressant of eccrine sweating. In this study, the principles of soft drug design were applied to methatropine. A hypothetical carboxylate metabolite of methatropine was reactivated by esterification with cyclic and alicyclic alcohols to yield a series of compounds (3a-g). In vitro evaluation by guinea pig ileum assay indicated that the compounds are potent anticholinergics and the lead carboxylate metabolite is about 60 times less potent than the most active compound of the series. The activity was found to decrease with the increasing side chain length. The n-octanol/water partition coefficients were found to be directly dependent on the chain length for the compounds made with straight chain alcohols. The transdermal permeability coefficients across the hairless mice skin were found to be directly dependent on the partition coefficients. The soft drugs are found to metabolize extensively during the penetration process compared to the unmetabolizable nature of methatropine. The soft drugs reported in this study will probably be able to elicit a local action at the site of application but will probably be metabolized rapidly in the systemic circulation, thereby avoiding the systemic side effects with a consequent increase in the therapeutic index.

Soft drugs--XIV. Synthesis and anticholinergic activity of soft phenylsuccinic analogs of methatropine

Three soft drug analogs and a metabolite of methatropine based on phenylsuccinic structural moiety were synthesized and tested for activity. In an in vivo assay, the soft drugs were found to be two orders of magnitude less potent than methatropine while the carboxylate metabolite was found to be one order of magnitude less potent than the soft drugs. A structural isomer of compound 4a was found to be less potent. All the soft drugs tested elicited shorter durations of mydriatic action in rabbit eyes compared to atropine. The untreated eye was dilated in the atropine treated animals while no dilation occurred in the soft drug treated animals indicating facile systemic metabolism of the soft drugs to inactive moieties, possibly the carboxylate metabolite. In in vitro stability studies, the soft drugs have been found to be more hydrolytically labile than atropine. The shorter duration of mydriatic action of compound 4a coupled with increased hydrolytic lability make this a candidate for further study.

Synthesis and biological evaluation of new antimuscarinic compounds with amidine basic centers. A useful bioisosteric replacement of classical cationic heads

Amidines (guanidine, formamidine, and acetamidine) were introduced as substitutes for the cationic heads present in atropine, scopolamine, and corresponding quaternary derivatives. Amidine systems are intermediate in structure between tertiary amines and quaternary compounds, at least as regards ionization and electronic properties, but differ from the latter in shape (planar not tetrahedral). They have additional binding opportunities on account of their hydrogen-bond-forming capacity. The effect of the introduction of these cationic heads on the affinity for different muscarinic acetyl choline receptor (m-AcChR) subtypes was investigated in vitro, in binding displacement studies, and in functional tests on isolated organs. All new compounds (3a,b-5a,b) showed high affinity for the m-AcChR considered, comparable or slightly inferior to that of the parent drugs (1a-e). The new amidine derivatives proved effective as spasmolytic agents, with little tendency to cause central effects. However, no separation was achieved of spasmolytic and other untoward effects, like inhibition of salivation. Thus, amidine moieties are effective bioisosteric substitutes for conventional cationic heads present in antimuscarinic agents. Their unusual physical-chemical properties make them useful tools when modulation of pharmacokinetic or pharmacodynamic effects is required.

Novel 'soft' anticholinergic agents

The concepts involved in the design of 'soft' drugs (drugs which, after achieving their therapeutic role, are metabolised in a predictable manner and at a controlled rate to non-toxic moieties) have been further applied in the case of atropine. Selected aliphatic and cycloaliphatic esters of a hypothetical, inactive acidic metabolite of atropine were designed and found to have atropine-like activity, and to revert to the inactive metabolite in rat liver homogenates. Peak anticholinergic activity, measured by the degree of antagonism of carbachol-induced spasms of guinea pig ileum strips, was observed when the esters contained a quaternary group. The in vitro stability of the esters was determined in human plasma, in pH 12 buffer solution, and in rat liver homogenate; the fastest rate of hydrolysis occurred in rat liver homogenate, and least sterically hindered esters degraded more rapidly than hindered esters. Synthesis of the esters was achieved in two stages. Phenylmalonic acid and the appropriate alcohol gave phenylmalonic acid monoesters, which, after reaction with tropine, afforded the required diesters. The tertiary amine group on the tropine moiety of the diesters then allowed the preparation of quaternary derivatives.

[Synthesis and pharmacology of N-stereoisomers of quaternary tropane compounds]

The synthesis of (8r)-8-(4-biphenylmethyl)-atropinium bromide (3) as N-isomeric form of xenytropium bromide, (8s)-8-(4-biphenylmethyl)-atropinium bromide (2), is reported. It was revealed, that the equatorial alkylation predominates in quaternization of tertiary tropane compounds. Furthermore, the preparation of other N-isomeric pairs of quaternary tropane derivatives in pure form is described. Xenytropium bromide is more active--16 times on the parasympathic endplate and 8 times on the ganglion--than its N-isomer.

Synthesis of the bronchospasmolytic agent flutropium bromide and of some homologous and configuration isomeric compounds

A technically practicable synthesis of unknown N-beta-fluoroalkyl-substituted benzilic acid nortropine esters, among them (8r)-8-(2-fluoroethyl)-3 alpha-hydroxy-1 alpha H, 5 alpha H-tropanium bromide benzilic acid ester (flutropium bromide, Ba 598 BR), and their configuration isomeric quaternary salts via benzilic acid imidazolide is described. The quaternization which takes place with sufficiently high stereo-selectivity leads to configuration isomers which differ not only in their physico-chemical properties but also in their profile of pharmacological action.

Preparation of diastereomeric urethane derivatives of atropine and l-hyoscyamine using (-)-1-phenylethylisocyanate

Diastereomeric urethane derivatives of atropine (d,l-hyoscyamine) and l-hyoscyamine were prepared by reacting the alkaloids with (-)-1-phenylethylisocyanate. The derivatives, as the picrate ion-pairs, were characterized by their melting points, optical rotations, elemental analyses, and IR, NMR, and UV-visible spectra.

Structure-activity relationship in a new series of atropine analogues. 1. N,N'-Disubstituted 6,7-diazabicyclo[3.2.2]nonane derivatives

The synthesis of a new series of N,N'-disubstituted 6,7-diazabicyclo[3.2.2]nonane derivatives is described. The antimuscarinic potency of these drugs was evaluated in the guinea pig ileum and compared to that of atropine sulfate. All the drugs tested competitively inhibited the acetylcholine-induced contractions. Kd values were calculated and, in several cases, compared to those obtained by direct binding to the muscarinic receptor from mouse brain. The order of potencies followed that which is known for various tropine and pseudotropine esters; that is, the 3alpha configuration is more potent than the 3beta configuration, and the quaternary analogues are more potent than the tertiary ones. The antimuscarinic activity of the drugs is dicussed in terms of their acetylcholine-like molecular arrangement that gives rise to a characteristic interaction pharmacophore.