Specificity and selectivity of action of arylhaloalkylamine adrenergic blocking agents
Stone, Clement Addison
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Aryl and/or alkyl substitution of one or two of the B-chloroethyl groups of bis or tris-B-chloroethylamine (nitrogen mustards) was shown in 1946 to reduce markedly the toxicity of these substances and to confer new and interesting pharmacologic properties upon the 2-haloalkylamine grouping. These compounds, of which N, N, dibenzyl-B-chloroethylamine (Dibenamine) was the forerunner (1), possess the ability to block and reverse many of the pharmacologic effects of epinephrine, as well as reduce or abolish the effects mediated over the sympatho-adrenal nervous system. Evidence accumulated indicating that Dibenamine was the most specific, albeit not the most potent, adrenergic blocking drug, as compared to notoriously non-specific and toxic members existing in the same class (ergot derivatives, yohimbine, dioxanes, and so forth. (See (2) for reference and discussion). In 1947 it was shown by Loew (3) and, subsequently by Kerwin,et al. (4), Hunt (5), Henderson, et al. (6), and Nickerson (7), that the dibenzyl radical was not the only aryl substituent which conferred low toxicity and adrenergic blocking properties to the B-chloroethyl group in nitrogen mustards. Hence, alpa-naphthylmethyl, biphenylyloxy, phenylethyl, phenoxy, benzhydryl, and 9-fluorenyl substituted derivatives were found to be active adrenergic blocking drugs, some homologues of which were far more potent than Dibenamine in this respect. However, certain of these derivatives were found not to be as specific as Dibenamine. Hence, compounds possessing appreciable antihistaminic properties (alpha-naphthyl-methyl and biphenylyloxy derivatives) were described (8); and others proved to be cholinergic (e.g., N, N-dimethyl-2-chloro-2-phenylethylamine, (9). From the available evidence it appeared that aryl and/or alkyl substitution of one or two of the B-chloroethyl groups of the nitrogen mustards altered the toxicity but did not reduce the potential chemical reactiveness inherent in the B-chloroethyl radical (see (10) for discussion of chemical reactivity of nitrogen and sulfur mustards). It was apparent, however, that the nature of the aryl substituent modified this reactiveness of this group so that it became more or less specific. This in turn was reflected in predominant pharmacologic effects of a given derivative, in contrast to the rather wide-spread and manifold pharmacologic effects of the nitrogen mustards (10). Nonetheless, other pharmacologic effects which appear in a given compound are in addition to prominent adrenergic blocking properties; thus most aryl derivatives of B-chloroethylamine which have been described are classified in this category. In view of the possible uses of adrenergic blocking drugs in medicine, and the fundamental usefulness of these drugs in delimiting functions of the sympathetic nervous system, the importance of knowing the specificity and potency of drugs is evident. Aryl derivatives possess desirable properties in that they are long-acting and highly potent and effective. Unfortunately, as pointed out above, not all derivatives appeared as specific as desired and, in addition, specificity among these agents had not yet been placed in quantitative terms. With these points in mind, studies of the specificity of action of some of the more potent and better known aryl-haloalkylamines were made on the isolated seminal vesicle of the guinea pig by the Magnus technique. Specificity of drug action was inferred from the relative concentrations of each adrenergic blocking drug required to inhibit the spasmogenic effects of equi-effective doses of epinephrine, nor-epinephrine, histamine, and acetylcholine, as compared to control contractions made in the absence of the antagonist. The measurement data obtained were converted to all-or-none data by designating that a given trial with an antagonistic drug was a positive trial if the per cent reduction was 50 per cent or greater; any value below this was a negative trial. Thus, small doses of antagonist yielded relatively few positive trials, and increasingly larger doses resulted in greater proportions of positive trials. The data were analyzed statistically by the graphic analysis of Litchfield and Wilcoxon (11) to forecast the relative concentration (in mcg./100 cc.) producing 50 per cent or greater inhibition of the spasm induced by the active drug in 50 per cent of the trials (ED50) and the 95 per cent confidence limits of these estimates. Comparisons of the ED5O values obtained with the adrenergic blocking drugs antagonizing epinephrine revealed that N-ethyl-N-(2-chloroethyl)-9-fluorenamine.HCl (Sy-21, ED50Epi = 0.52) was the most effective antagonist of epinephrine. This was followed in order by: N-ethyl-N-(2-chloroethyl)benzhydrylamine.HCl (SY-2, ED50Epi = 0.75); N-ethyl-N-(2-bromoethyl)-naphthalenemethylamine. HBr (SY-28, ED50Epi = 1.37); N, N-(2-chloroethyl)-dibenzylamine.HCl (Dibenamine, ED50Epi = 2.60) and, 2-(2-biphenylyloxy)-2'-chlorotriethylamine.HCl (SY-8, ED50Epi = 3.03). As to be expected, atropine sulfate (ED50Epi = 1330) and pyranisamine (ED50Epi = ca. 12-24,000) were highly ineffective in antagonizing epinephrine under the conditions employed. Comparison of ED50 Norepi values yielded the same order of potency. The individual values were uniformly less than the corresponding ED50Epi. The ED50Norepi to ED5OEpi ratio was 0.8 in each case examined and indicates that less drug was required to inhibit nor-epinephrine than an equieffective amount of epinephrine, although the difference was statistically significant in only 2 of the 4 comparisons made. This result suggests that the greater susceptibility to blockade by these drugs of epinephrine pressor effects, as compared to nor-epinephrine. is due to significantly greater vasodilating properties of the former, and not to differences in the amines, per se, or site of action of the pressor agents. Antihistaminic properties, as judged by the ability to reduce histamine spasm, were revealed in all of the aryl-haloalkylamines exrunined. The most potent aryl-haloalkylamine in this respect (SY-28, ED50Hist = 0.71), however, was far less effective than pyranisamine (Neo-antergan maleate, ED50Hist = .053) tested under the same conditions. The antihistaminic activity of the aryl-haloalkylamines (as was the anti-epinephrine activity) was irreversible (and hence, probably non-competitive) under these conditions since the drugs, once added to the muscle bath, could not be washed from the tissue. The ratio, ED50Hist/ED50Epi, each from the same antagonist gives some measure of the relative specificity. This ratio, calculated for each antagonist, varied from 44 (SY-2) to 0.5 (SY-28). Hence, the same relatioship that existed between epinephrine and nor-epinephrine (ED50Norepi to ED50Epi ratio for each antagonist = 0.8) does not exist between histamine and epinephrine and does not support the concept that epinephrine and histamine act in similar manners or on the same receptors. Comparison of the ratio, ED50Hist to ED50Epi, revealed that SY-2 (Ratio = 44) was the most discriminating under these conditions, followed by SY-21 (20), Dibenamine (13), SY-8 (2) and SY-28 (0.5). The ratio of 0.5 obtained with SY-28 indicates that this drug is more effective in antagonizing histamine than epinephrine, and as such, should be classified as an antihistaminic. While the properties of these aryl-haloalkylamines were qualitatively similar with respect to histamine, epinephrine, and nor-epinephrine antagonism, marked differences were apparent with respect to antagonism of acetylcholine. Thus, SY-2 and Dibenamine, were found to be incapable of blocking acetylcholine-induced spasm in quantities that did not produce spasm themselves (700 to 1400 times the respective ED50Epi). SY-21 potentiated acetylcholine spasm markedly in doses of 500-1000 mcg./100 cc., quantities in themselves slightly spasmogenic. SY-28 and SY-8 were found to block acetylcholine. The concentrations required to do so were much above those of atropine sulfate but only 8 (SY-8) and 68 (SY-28) times their respective ED50Epi. The blocking properties of SY-28 and SY-8 were partially reversible by washing and differ in this respect from anti-epinephrine and anti-histamine activity. The modifying importance of the aryl radical is brought out by these results since the drugs studied differed only in the nature of the aryl radical (all were aryl-ethyl-B-chloroethylamines except Dibenamine--an aryl-benzyl-B-chloroethylamine). The results of these experiments revealed that aryl-haloalkylamine adrenergic blocking drugs possess pharmacologic properties other than anti-epinephrine activity, which may approach or overshadow the adrenergic blocking activity. The great pharmacologic potentialities of the B-chloroethylamine group appears to have been rendered more specific by juxtaposition of a suitable aryl radical, although the specificity exhibited does not approach certain other antagonists of different classes (e.g., atropine vs. acetylcholine, and pyranisamine vs. histamine). Finally, two of the most specific drugs, as determined above, were selected and studied in intact animals in order to assess, in some measure, their range of activity and site of action. The method employed was one in which the blocking activity of the drugs against epinephrine and adrenergic reflex pressor responses were obtained in the anesthetized dog by 1) carotid occlusion and 2) anoxia (nitrogen inhalation). In addition, adrenergic effects, manifested as a rise in blood pressure, was induced by injections of nicotine (25 mcg./kgm., i.v.) acting primarily at the ganglionic level, and epinephrine (1 mcg./kgm., i.v.) acting at the effector cell level. Both SY-2 (20 mgm./kgm., i.v.) and SY-21 (1.0 mgm./kgm., i.v.) reduced, blocked or reversed the adrenergic responses due to carotid occlusion (reduced), anoxia (reversed), nicotine (reversed), and epinephrine (reversed). The data obtained suggested that SY-21 was more potent than SY-2, even more so than the differential in doses indicate. The data are interpreted as to suggest that a peripheral site of action is responsible for blockade of all the responses. In addition, no effects were observed, under the conditions employed, that could not be explained as due to peripheral blockade of the sympatho-adrenal nervous system. The experiments support the previous conclusion that SY-2 and SY-21 are among the most specific aryl-haloalkylamine adrenergic blocking drugs available.
Thesis (Ph.D.)--Boston University