A compound of formula (I): ** Formula ** 5 in which: G is -CH>； = D is an optionally substituted phenyl ring； R6 is hydrogen； P represents hydrogen and U represents a radical of formula (IA)； or U represents hydrogen and P represents a radical of formula (IA)； -A- (CH2) 2-X1-L1-Y-NH-CHR1R2 (IA) in which A represents an optionally substituted phenyl or cyclohexyl ring； z is or 1； Y is a link, -C (>； = O) -, -S (>； = O) 2-, -C (>； = O) NR3-, -C (>； = S) -NR3, -C (>； = NH ) NR3 or -S (>； = O) 2NR3- where R3 is hydrogen or optionally substituted C1-C6 alkyl； L1 is a divalent radical of the formula - (Alq1) m (Q) n (Alq2) p- in which m, n and p are independently 0 or 1, Q is (i) a divalent, carbocyclic or heterocyclic, mono or bicyclic radical, optionally substituted having 5-13 ring members, or (ii), in the case where myp is both 0, a divalent radical of the formula -X2-Q1- or -Q1-X2- in which X2 is -O-, S- or NRA- in which RA is optionally substituted hydrogen or C1-C3 alkyl, and Q1 is a divalent, carbocyclic or heterocyclic, mono or bicyclic, optionally substituted radical having 5-13 ring members, Alq1 and Alq2 independently represent optionally substituted C3-C7 cycloalkyl radicals, or optionally substituted C1-C6 alkylene, C2-C6 alkenylene or C2-C6 alkynylene, linear or branched, which may end in, or optionally contain, an ether (-O-) bond, thioether (-S-) or amino (-NRA-) where RA is hydrogen or optionally substituted C1-C3 alkyl； and X1 represents a link； -C (>； = O)； or -S (>； = O) 2-； -NR4C (>； = O) -, -C (>； = O) NR4 -, - NR4C (>； = O) NR5-, -NR4S (>； = O) 2- or -S (>； = O) 2NR4- where R4 and R5 are independently hydrogen or optionally substituted C1-C6 alkyl； R1 is an ester group that is hydrolysable by one or more intracellular carboxylesterase enzymes to give a carboxylic acid group； R2 is the side chain of a natural or non-natural alpha amino acid selected from: (i) C1-C6 alkyl, phenyl, 2-, 3- or 4-hydroxyphenyl, 2-, 3- or 4-met
