Certain compounds of formula (I) below or a pharmaceutically acceptable salt or hydrate thereof: wherein: R.sub.1 is H or alkyl; R.sub.2 is aryl, 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 H, R.sub.8NR.sub.9R.sub.10, R.sub.11R.sub.13 or R.sub.11R.sub.12R.sub.13; or R.sub.5 is branched or linear alkyl, cycloalkyl, aryl, arylalkyl, or a single or fused ring aromatic heterocyclic group; a process for preparing such compounds, a pharmaceutical composition comprising such compounds and the use of such compounds and the use of such compounds and composition in medicine.