Enantioselectivity of muscarinic antagonists. Isomeric 2-cyclohexyl-2-phenyl-5-[(dimethylamino)methyl]-1,3-oxathiolane 3-oxide methiodides.

The eight isomers of 2-cyclohexyl-2-phenyl-5-[(dimethylamino)methyl]-1, 3-oxathiolane 3-oxide methiodides were prepared and their absolute configurations were attributed by synthesis and by X-ray crystallography. The compounds were tested on guinea pig bladder, ileum, and heart and their antimuscarinic potency was evaluated and expressed as pA2. The absolute configuration of the most potent isomer [(+)-(2R,3R,5R)-7] is identical with that of the corresponding agonist [(2R,3R,5R)-c-2-methyl-r-5-[(dimethylamino)methyl]-1,3-oxathiolane t-e-oxide methiodide],3 which further supports our previous hypothesis that muscarinic agonists and antagonists of this series recognize a common binding site. While some of the racemates (3,4) show different enantioselectivity on the different tissues, the most potent and the most enantioselective one (7) does not discriminate between muscarinic receptors as it shows eudismic ratios of the same order for all tissues examined.

Structure-activity relationships among benextramine-related tetraamine disulfides. Chain length effect on alpha-adrenoreceptor blocking activity.

Several N'-substituted N,N''-(dithiodi-2,1-ethanediyl)bis(1, omega-alkanediamines) were prepared and evaluated for their blocking activity on alpha-adrenoreceptors in the isolated rat vas deferens and human blood platelets. The results were compared with those obtained for benextramine (N,N''-(dithiodi-2,1-ethanediyl)bis[N'-[(2-methoxyphenyl)-methyl]- 1 ,6- hexanediamine], 10). Bendotramine (N,N''-(dithiodi-2,1-ethanediyl)bis[N'-[(2-methoxyphenyl)- methyl]-1,12-dodecanediamine], 16) proved to be as active as 10 on alpha 1-adrenoreceptors, showing that optimum activity is associated with two carbon chain lengths separating inner from outer nitrogens of tetraamine disulfides. On the other hand, 16 had no activity up to 20 microM at alpha 2-adrenoreceptors. The optimum activity at this receptor subtype was associated with a six to eight carbon chain (10-12). Furthermore, 10 proved to be more selective toward alpha 2-adrenoreceptors whereas 16 was a selective alpha 1-antagonist. The tetraamine disulfides were shown also to be potent inhibitors of human platelet aggregation induced by ADP or epinephrine. The potency increased with the carbon chain length. However, the results on platelets did not parallel those found in the rat vas deferens, indicating that differences exist between the alpha-adrenoreceptor subtypes investigated. In conclusion, 10 may be a useful tool in characterizing alpha 2-adrenoreceptors whereas 16 might help in investigating alpha 1-adrenoreceptors.

Structure-activity relationships in prazosin-related compounds. Effect of replacing a piperazine ring with an alkanediamine moiety on alpha 1-adrenoreceptor blocking activity.

Several prazosin-related compounds were synthesized in which the piperazine ring of prazosin (1) was replaced by an alkanediamine chain and were evaluated for their blocking activity on alpha 1- and alpha 2-adrenoreceptors in isolated rat vas deferens. All the compounds investigated proved highly selective toward the alpha 1-adrenoreceptor owing to a very low affinity for alpha 2-adrenoreceptors. Furthermore, compounds 2, 9, and 13 were also investigated in vivo to determine their hypotensive effect on anesthetized rats, which were compared with that of prazosin (1). It was confirmed that the piperazine moiety of 1 is not essential for potency. However, optimum activity depends on two parameters: carbon-chain length of the alkanediamine moiety and N-methylation of both the amide and the 2-amino functions. In the desmethyl series, optimum activity was associated with the lower homologues (2-4) bearing a chain of two to four methylenes whereas in the N,N'-dimethyl series peak potency was observed with a six-carbon chain as in 13. Compound 13 proved the most active of the series and was more potent than prazosin (1) in both in vivo and in vitro assays. It is hypothesized that the alpha 1-adrenoreceptor incorporates a lipophilic area that is located between the binding sites for the quinazoline and the furoyl moieties and is able to accommodate a polymethylene chain.

Structure-activity relationships among methoctramine-related polymethylene tetraamines. Chain-length and substituent effects on M-2 muscarinic receptor blocking activity.

Several polymethylene tetraamines related to methoctramine (1) were prepared and evaluated for their blocking activity on M-2 muscarinic receptors in guinea pig atria and ileum. It turned out that antimuscarinic potency depends on the following parameters: (a) nature of the substituent on both inner and outer nitrogens and (b) carbon chain length separating the inner nitrogens as well as the inner and outer nitrogens. Optimum activity at cardiac M-2 muscarinic receptors was associated with the chain lengths present in 1, that is, eight methylenes between the inner nitrogens and six methylenes between the inner and outer nitrogens. With regard to the substituents, replacement of the benzylic moiety of 1 by a 2-furyl or a 5-methyl-2-furyl nucleus resulted in enhanced potency toward cardiac M-2 muscarinic receptors. In fact, furtramine (18) and mefurtramine (19) proved to be more potent and more selective than 1. Moreover, N-methylation of the four nitrogens of 1 gave different effects: methylation of the outer nitrogens, giving 22, caused a significant decrease in activity whereas methylation of the inner nitrogens, yielding 23, resulted in an increase in activity in both atria and ileum.

Fluoronorepinephrines: further pharmacological evaluation in vitro and in pithed rats.

The effect of 6F-, 5F- and 2F-norepinephrine (6F-, 5F- and 2F-NE) in rat vas deferens, guinea-pig ileum and pithed rats was compared to that of norepinephrine (NE). The rank order of potency on postsynaptic alpha 1-adrenoreceptors, determined from the isometric contraction of vas deferens, was 6F-NE = 5F-NE = NE greater than 2F-NE. A similar pattern was found for presynaptic alpha 2-adrenoreceptor activity in both noradrenergic nerve terminals of vas deferens and cholinergic nerve terminals of the ileum, determined from the inhibition of contraction elicited by electrical field stimulation. The only exception was the 5F isomer which was 7 times less active than NE to activate the alpha 2-adrenoreceptors of vas deferens. Thus, ring fluorination markedly alters both alpha 1- and alpha 2-agonist properties of NE. Moreover, alpha 1/alpha 2 selectivity, at least as far as rat vas deferens is concerned, is not significantly influenced by the introduction of a fluorine atom in the NE molecule. 6F-NE was about 3-4 times more active than NE in pithed rats. In turn, NE was equiactive with 5F-NE. 2F-NE was the least active isomer, being 30- and 100-fold less active than NE and 6F-NE, respectively.

Antimuscarinic action of methoctramine, a new cardioselective M-2 muscarinic receptor antagonist, alone and in combination with atropine and gallamine.

The antimuscarinic effects of methoctramine (N,N'-bis[6- [(2-methoxybenzyl)amino]hexyl]-1,8-octanediamine tetrahydrochloride) were investigated in vitro in isolated paced left (force) and spontaneously beating right (force and rate) atria of guinea pigs as well as ileum of guinea pig and rat. Methoctramine was a potent competitive antagonist of M-2 muscarinic receptors in myocardium and pacemaker cells over a wide range of concentrations. The pA2 values ranged from 7.74 to 7.93. They were not significantly different in the two cardiac preparations and were independent of the agonist used (muscarine and carbachol). A combination of methoctramine with atropine resulted in addition of the dose ratios for left atria, which is expected for two antagonists interacting competitively with the same receptor site. In contrast, a combination of methoctramine with gallamine produced a less than additive shift of the dose-response curve for carbachol, confirming that gallamine acts as an allosteric antagonist at cardiac muscarinic receptors. Methoctramine was 54 to 132-fold less potent in ileal than in atrial preparations (pA2 values ranging from 5.81 to 6.20) which makes it the most cardioselective antimuscarinic agent now available. A combination of methoctramine with atropine gave a slight supra-additive antagonism on guinea pig ileum, which suggests that methoctramine interacts to some extent with a second independent site. These results strongly reinforce the view that M-2 muscarinic receptors are not a homogeneous population.

Chiral muscarinic agonists possessing a 1,3-oxathiolane nucleus: enantio- and tissue-selectivity on isolated preparations of guinea-pig ileum and atria and of rat urinary bladder.

Racemate and corresponding enantiomers of muscarinic agonists carrying a 1,3-oxathiolane nucleus were studied on isolated preparations of guinea-pig ileum and atria and of rat urinary bladder. The efficacy of these agonists were determined according to the method of Furchgott and Bursztyn (1967) and enantio-selectivity and tissue-selectivity were investigated. The enantio-selectivities of the most potent compounds studied (expressed as the ratio of potencies or affinities of the enantiomers) vary significantly from tissue to tissue, supporting the view that M2 receptors are not homogeneous. In particular, the data all indicate that the ileal receptors are different to the atrial and bladder ones.

Synthesis and Ca-antagonist activity of some benzhydryl derivatives.

The preparation of a series of benzhydryl derivatives is described. Their activities as Calcium-antagonists were evaluated on the taenia coli of the guinea-pig. These new compounds show lower activities as Ca-antagonists than Cinnarizine.