Substituted isocytosines having histamine H2 -antagonist activity

Claim 1

A compound of the formula: ##STR12## wherein Het is 2- or 4-imidazolyl optionally monosubstituted by lower alkyl, halogen, trifluoromethyl or hydroxymethyl; 2-pyridyl optionally substituted by one or two groups (which may be the same or different) selected from lower alkyl, lower alkoxy, halogen, amino and hydroxy; 2-pyridyl with a phenyl, carbocyclic or cyclic ether ring having 2 oxygen atoms fused to it; 2-thiazolyl; 3-isothiazolyl optionally monosubstituted by chlorine or bromine; 3-(1,2,5)-thiadiazolyl optionally monosubstituted by chlorine or bromine, or 2-(5-amino-1,3,4-thiadiazolyl); Y is sulphur or a methylene group; m is 0, 1 or 2 and n is 2 or 3 such that their sum is 3 or 4 or when Y is methylene and Het is other than an imidazole ring, 2; Z is hydrogen or lower alkyl; X is oxygen or sulphur; A is a straight or branched alkylene chain having from 1-5 carbon atoms or --(CH 2 ) p W(CH 2 ) q -- where W is oxygen or sulphur and p and q are such that their sum is from 1 to 4; Het' is a 5 or 6 membered heterocyclic ring selected from pyridine, pyridine-N-oxide, furan, thiophen, thiazole, oxazole, isothiazole, imidazole, pyrimidine, pyrazine, pyridazine or thiadiazole, which ring is optionally substituted by one or two (which may be the same or different) of the groups selected from lower alkyl, lower alkoxy, halo, hydroxy and amino, or Het' is a pyridine ring with a carbocyclic or cyclic ether ring having two oxygen atoms fused to it, or Het' is a pyridine, imidazole or thiazole ring which has a benzene ring fused to it, said Het' being attached at a ring carbon atom; or a pharmaceutically acceptable salt thereof.