Certain compounds of formula (I) below or a pharmaceutically acceptable salt or hydrate thereof: wherein: R.sub.1 is H or C.sub.1-6 alkyl; R.sub.2 is aryl or C.sub.3-7 cycloalkyl or heteroaryl; R.sub.3 is H or C.sub.1-3 alkyl, optionally substituted by one or more fluorines; R.sub.4 is R.sub.8R.sub.9; R.sub.8 is a single bond, C.sub.1-6 alkyl, or aryl; R.sub.9 is H, COO R.sub.10, or NR.sub.11R.sub.12; R.sub.10 is H or C.sub.1-6 alkyl; R.sub.11 and R.sub.12 are independently selected from H and C.sub.1-6 alkyl; R.sub.5 is branched or linear C.sub.1-6 alkyl, C.sub.3-7 cycloalkyl, C.sub.4-7 cycloalkylalkyl, aryl, or a single or fused ring aromatic heterocyclic group; R.sub.6 represents H or up to three substituents independently selected from the list consisting of: C.sub.1-6 alkyl, C.sub.1-6 alkenyl, aryl, C.sub.1-6 alkoxy, hydroxy, halogen, nitro, cyano, carboxy, carboxamido, sulphonamido, C.sub.1-6 alkoxycarbonyl, trifluoromethyl, acyloxy, amino or mono- or di-C.sub.1-6 alkylamino; R.sub.7 is H or halo; a is 1-6; and any of R.sub.2, R.sub.5, R.sub.8, R.sub.10, R.sub.11 and R.sub.12 may optionally be substituted one or more times by halo, hydroxy, amino, cyano, nitro, carboxy or oxo; a process for preparing such compounds, a pharmaceutical composition comprising such compounds and the use of such compounds and composition in medicine.