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Some 2-aryl-5-guanidino-(or N-substituted guanidino)-1,3,4-thiadiazoles and closely related analogues were found to lower blood pressure in metacorticoid (DOCA) hypertensive rats. In the unsubstituted guanidines that exhibited low toxicity, optimum activity resulted when the aryl group was a 2-methylphenyl ring (11). Modifications to the guanidine group did not increase antihypertensive activity, but, in the 2-methylphenyl series, the N-n-butyl- and N-(2-methoxyethyl)guanidines (63 and 78) and the related iminoimidazolidine 93 were of comparable activity to that of the unsubstituted guanidine 11. The iminoimidazolidine 93 showed a somewhat longer duration of action than the guanidine derivatives. Preliminary studies in a pithed rat preparation indicated that these thiadiazole derivatives (11, 63, and 93) lowered blood pressure by a direct relaxant effect on vascular smooth muscle.

Some 2-aryl-5-hydrazino-1,3,4-thiadiazoles have been synthesized and screened for antihypertensive activity. In general, compounds with a 2-substituted phenyl ring had higher activity than their 3- or 4-substituted counterparts or those containing heteroaryl groups. The 2-methylphenyl and 2-ethylphenyl derivatives 7 and 18 were the most potent members of the series. Preliminary studies indicated that the hypotensive action of these compounds was due to a direct relaxant effect on vascular smooth muscle.

Modification of the 1,4-benzodioxan ring present in RX 781094 has not previously been considered. This paper describes a number of analogues of this ring system, including compounds in which one of the oxygen atoms has been replaced by a methylene group and also those in which the ring size has been changed to give, for example, furan and thiophene derivatives. The dihydrobenzofuranylimidazoline compound 7 is the only analogue possessing presynaptic antagonist potency potency and selectivity comparable to that of 1. In view of this result, a number of derivatives was prepared to determine the structure-activity relationships within this series. Many derivatives, as well as the parent compound 7, were found to possess presynaptic alpha 2-adrenoreceptor antagonist and postsynaptic alpha 1-adrenoreceptor partial agonist properties. Two of the selective presynaptic antagonists, 13 and 14 possess greater potency and selectivity than that possessed by 1. The 5-chloro derivative 25 is twice as potent as after oral administration but only about half as potent when given intravenously.

The rational design of RX 781094, 2-(1,4-benzodioxan-2-yl)-2-imidazoline hydrochloride (5), a new potent and selective antagonist of alpha 2-adrenoreceptors, is discussed. A compound that acts as an antagonist at presynaptic alpha 2-adrenoreceptors could be an effective and novel treatment of depression because of its ability to increase the concentration of norepinephrine at central receptor sites. The effects of substituents in the aromatic and imidazoline rings have been examined, as well as the replacement of the imidazoline ring by an amidine function or by other heterocyclic ring systems. None of these derivatives are as potent or selective as 5, although some do display a degree of selectivity as antagonists. Some derivatives were found to possess agonist properties that, with the exception of 23, favored the postsynaptic site. Compounds 9, 12, 16, 21, 30, and 51 possessing presynaptic alpha 2-adrenoreceptor antagonist and postsynaptic alpha 1-adrenoreceptor partial agonist properties were also obtained, and these derivatives could be considered as potential antimigraine agents.

A series of phthalazinyl- and quinoxalinylguanidines has been synthesized and evaluated for potential antihypertensive activity. Unsubstituted guanidines were prepared by treating the appropriate intermediate chloro compounds with guanidine free base. Substituted guanidines were prepared by treating the cyanamides 9 and 16 with the appropriate amine; with hydrazines, cyanamide 9 gave the triazoles 14 and 15. Moderate falls in blood pressure were observed with compounds 10, 11, 14, and 15. The triazole 15 caused a 25% fall in heart rate. Some of the compounds (10, 11, 13, and 18) displayed weak alpha-adrenoceptor antagonist properties in vitro, and this activity was confirmed in the pithed rat (in vivo).

The synthesis and pharmacological activity of a series of 2-substituted derivatives of the selective alpha 2-adrenoreceptor antagonist idazoxan (RX 781094) is described. Substitution in this position by alkyl, alkenyl, cycloalkenyl, and alkoxy groups in many cases gives compounds whose potencies and selectivities are significantly greater than those of the parent compound.