Pharmaceutical Compositions and Methods for Stimulating H.sub.3 Histamine Receptors

The present invention relates to the therapeutic application of histamine derivatives, and in particular of .alpha.-methylhistamine and its R stereoisomer, the optical rotation of which is laevorotatory and the configuration of which corresponds to that of L-histidine.

The two stereoisomers of .alpha.-methylhistamine were previously prepared as histamine analogs, but pharmacological study of these compounds had only enabled a very weak histaminic activity, which was similar for both stereoisomers, to be demonstrated in respect of the H.sub.1 and H.sub.2 receptors, the only receptors known at the time (Gerhard and Schunack, Arc. Pharm. (Weinheim) 1980, 313, 709). This observation led to the elimination of this product from a detailed pharmacological study and from any therapeutic application.

In fact, we have now discovered the powerful agonistic and highly stereoselective properties of .alpha.-methylhistamine in respect of a new class of receptors (H.sub.3 receptors). Stimulation of the H.sub.3 receptors mainly induces inhibition of the synthesis and release of histamine (Arrang. and al., Nature, 1983, 302, 832; Neuroscience, 1985, 15, 553; Frontiers in Histamine Research, C. R. Ganellin and J. C. Schwartz, eds., Pergamon Press, 1985, p. 143). We also found these properties in a few other compounds which are structurally close to .alpha.-methylhistamine and which are per se known in the literature.

Therefore an objet of the invention is a pharmaceutical composition which comprises a histamine derivative according to the formula ##STR1## in which each of R.sub.1, R.sub.2 and R.sub.4 represents a hydrogen or a methyl, or R.sub.1 and R.sub.2 taken together represent a methylene, and R.sub.3 is a hydrogen, a methyl or a carboxy, with the proviso that R.sub.1, R.sub.2, R.sub.3 and R.sub.4 are not simultaneously methyl groups, and a pharmaceutically acceptable carrier or diluent.

The invention is more particularly concerned with compositions containing the following examples of compounds:

1. .alpha.-methylhistamine or 4-(2-aminopropyl)-imidazole and its stereoisomers R(-) and S(+); (I:R.sub.1 =CH.sub.3 ; R.sub.2 =R.sub.3 =R.sub.4 =H), see Gerhard and Schunack abaove.

2. .alpha.,.alpha.-dimethylhistamine or 4-(2-methyl 2-anino propyl)-imidazole (I:R.sub.1 =R.sub.3 =CH.sub.3 ; R.sub.2 =R.sub.4 =H); disclosed by Schunack, Joint Meeting of the American chemical Society, Div. of Med. Chem, and the American Society for pharmacology and experimental therapeutic, Boston, USA, Aug. 18-22, 1985.

3. .beta.-methylhistamine or 4-(1-methyl 2-amino ethyl)-imidazole and its stereoisomers (I:R.sub.2 =CH.sub.3 ; R.sub.1 =R.sub.3 =R.sub.4 =H) disclosed by Ganellin and al. in J. Med. Chem. 1973, 16, 616 (racemic form) and by Schunack and al. in Frontiers in Histamine Research, C. R. Ganellin and J. C. Schwartz eds. Pergamon Press, 1985, p. 39 (stereoisomers).

4. .beta.,.beta.-dimethylhistamine or 4-(1,1-dimethyl 2-amino ethyl)-imidazole (I:R.sub.1 =R.sub.3 =H; R.sub.2 =R.sub.4 =CH.sub.3), disclosed by Dwart and al., J. Med. chem. 1976, 19, 923.

5. 2-(4-imidazolyl)-cyclopropylamine (I:R.sub.1 -R.sub.2 =CH.sub.2 ; R.sub.3 =R.sub.4 =H), disclosed by Burger and al. J. Med. Chem. 1970, 13, 33.

6. .alpha.-methylhistidine and its stereoisomers (I:R.sub.1 =CH.sub.3 ; R.sub.3 =COOH; R.sub.2 =R.sub.4 =H), disclosed by B. Robinson and T. M. Shepherd J. Chem. Soc. 1961, 5037-8.

The effect of the histamine derivatives according to the invention on the H.sub.3 autoreceptors which control the release of (.sup.3 H) histamine induced by depolarization of slices of rat brain was studied according to the method described by Arrang. and al. (Nature, 1983, 302, 832-837). The derivatives behave as complete agonists of the H.sub.3 receptors and produce a maximal inhibition of release identical to that induced by histamine (approximately 60%). The 50% effective concentration of the R (-) isomer of .alpha.-methylhistamine is 2.+-.1 nM, which corresponds to an activity relative to histamine (EC.sub.50 =60.+-.10 nM) of approximately 3,000%; in contrast, the S (+) isomer has an EC.sub.50 of 700.+-.300 nM and an activity relative to histamine of 9% (stereoselectivity factor 350).

The effect of .alpha.-methylhistamine, like that of histamine, is reversed by various H.sub.3 antagonists, such as impromidine (see Table 2 below).

In contrast, the histamine derivatives as already shown for the two isomers of .alpha.-methylhistamine are of very low activity in respect of the H.sub.1 and H.sub.2 receptors, on which their activity relative to histamine is from 0.5 to 2% (Schunack and al., Frontiers in Histamine Research, C. R. Ganellin and J. C. Schwartz eds., Pergamon Press, 1985, 39). Given the greater sensitivity of the H.sub.3 receptors to the agonists, the outcome is that the R isomer stimulates these latter at concentrations approximately 10,000 to 100,000 times lower than those at which it stimulates the H.sub.1 and H.sub.2 receptors. It hence constitutes the first highly selective agonist of the H.sub.3 receptors. The results are set in Table 1 below.

The effect of .alpha.-methylhistamine was also studied on the H.sub.3 autoreceptors which control the synthesis of histamine in slices of rat brain, according to Arrang. and al., 1985, in Frontiers in Histamine Research, C. R. Ganellin and J. C. Schwartz eds., Pergamon Press, p. 143. .alpha.-Methylhistamine induces the same maximal inhibition as histamine (approximately 70%) of the stimulation by potassium of the synthesis of (.sup.3 H) histamine, with a relative activity of the same order as in the release model. Its effect is reversed competitively by H.sub.3 antagonists such as burimamide (see Table 3 below).

.alpha.-Methylhistamine also inhibits the synthesis of histamine in slices of human cerebral cortex and that of posterior hypothalamus (mamillary), by stimulation of the H.sub.3 receptors.

To study in vivo the effect of (R)-.alpha.-methylhistamine on the synthesis of histamine, the method of Verdiere and al., (Brain Research, 1977, 129, 107) was used with slight modifications.

The rats received 200 .mu.Ci of (.sup.3 H)-L-histidine (i.v.) and the (.sup.3 H) histamine synthesized is measured 10 minutes later (see Table 4 below).

The simultaneous administration of (R)-.alpha.-methylhistamine (10 mg/kg) significantly reduces the synthesis of (.sup.3 H) histamine in the brain and also in the peripheral tissues studied. Since this effect is completely reversed by an antagonist of the H.sub.3 receptors, it results from stimulation of these receptors.

The animals (6-13) received an intravenous injection of 200 .mu.Ci of (.sup.3 H)-L-histidine alone (controls) or with (R)-.alpha.-methylhistamine, 10 mg/kg (treated), and were sacrificed 10 minutes later.

The (.sup.3 H) histamine extracted from the tissues with HClO.sub.4 was isolated.

Thus, the histamine derivatives of formula I powerfully inhibit the release and synthesis of histamine by very selectively stimulating the H.sub.3 receptors. In consequence, they are likely to decrease histaminergic transmission in the digestive tract and in the nervous, cardiovascular and immune systems. They can be used in therapy as a drug having sedative effects, as a sleep regulator, anticonvulsant, regulator of hypothalamo-hypophyseal secretion, antidepressant, modulator of cerebral circulation, and the like.

Furthermore, inhibition of the release of inflammation messengers in various allergic conditions (e.g. asthma) is expected to result from the stimulation of the H.sub.3 receptors of the lung, for example.

In gastroenterology, the inhibition of release of gastric histamine is likely to exert antisecretory and anti-ulcerative effects. Modification of release of the messengers of immune responses is likely to modulate the latter responses.

By virtue of these novel and unexpected properties, the histamine derivatives of formula I and particularly .alpha.-methylhistamine, mainly in the form of its R stereoisomer, may be used for treating diseases which involve histmine synthesis and release in the human or animal body. Thus, the invention provides a method for treating a patient suffering from such a disease by administering a therapeutically effective amount of a histamine derivative of formula I. From their mode of action, their diverse pharmacological effects and their low toxicity in animals, applications of these derivatives may be predicted both in human and veterinary medicine, at doses of the order of 0.1 to 10 mg/kg administered, in particular, orally or parenterally.

They can be presented, in particular, in the form of tablets, dragees, gelatine capsules, aerosols, injectable solutions or suppositories, for example in unit doses containing 10 to 50 mg of a compound (I).