Compounds of the formula (I) ##STR1## where X is a --CO.sub.2 H or --CONHOH group; Y and Z are independently sulphur or oxygen, at least one being sulphur; R.sub.1 is hydrogen, hydroxy, (C.sub.1 -C.sub.6)alkyl, (C.sub.2 -C.sub.6)alkenyl, or (C.sub.3 -C.sub.8 cycloalkyl; R.sub.2 is a (C.sub.1 -C.sub.24)alkyl, phenyl(C.sub.1 -C.sub.6)alkyl, or phenyl(C.sub.0 -C.sub.6 alkyl)O)C.sub.1 -C.sub.6)alkyl, any of which may be optionally substituted by (C.sub.1 -C.sub.6)alkyl, (C.sub.1 -C.sub.6)alkoxy, halo or cyano (CN); R.sub.3 is the characterising side chain of a natural .alpha.-amino acid in which any functional groups may be protected, (C.sub.1 -C.sub.6)alkyl which may be optionally substituted, or cycloalkyl(C.sub.1 -C.sub.6)alkyl; R.sub.4 is hydrogen, (C.sub.1 -C.sub.6)alkyl, phenyl(C.sub.1 -C.sub.6)alkyl, optionally substituted phenyl or heteroaryl, or a group of formula --(Q--O).sub.n --Q where Q is a straight or branched (C.sub.1 -C.sub.6)alkyl, where n is an integer >1 and no continuous linear sequence of atoms in the group R.sub.4 is >12; or a salt, hydrate or solvate thereof. The compounds are useful for treating various medical conditions such as tissue breakdown and inflammation.