The present invention relates to compounds of formula (I) wherein: Ar is an optionally substituted aromatic or heteroaromatic group, R1 is H, C1-6alkyl, CF3, phenyl, benzyl, carboC1-6alkoxy, carbamoyl, or methyl(N-C1-6alkylcarbamoyl), R2 is H, C1-6alkyl, aminoC1-6alkyl, hydroxyC1-6alkyl,(mono- or di- C1-6alkyl)aminoC1-6alkyl,(aminoC1-3alkylphenyl)C1-3alkyl, amidinothio C1-6alkyl, R3 is H or C1-6alkoxy, R4 is H or together with R2, forms an annulated ring which may be substituted by hydroxyC1-3alkyl or amidinothioC1-3alkyl, or aminoC1-3alkyl; and pharmaceutically acceptable salts thereof; and the use of such compounds in medical therapies.