Compounds of formula (I) or (II) are dihydrofolate reductase inhibitors, useful for the treatment of, for example, cell proliferative diseases: wherein A and D are independently —CHR7— or —NR7—; E and G are independently ═CR7— or ═N—; each R6 independently represents hydrogen or —OR7; R7 is hydrogen or C1-C6 alkyl; R1 is a carboxylic acid group (—COOH), or an ester group which is hydrolysable by one or more intracellular carboxylesterase enzymes to a carboxylic acid group; R2 is the side chain of a natural or non-natural alpha amino acid which does not contain a carboxyl, or carboxyl ester group; Y is a bond, —C(═O)—, —S(═O)2—, —C(═O)NR3—, —C(═S)—NR3, —C(═NH)NR3 or —S(═O)2NR3— wherein R3 is hydrogen or optionally substituted C1-C6 alkyl; L1 is a divalent radical of formula -(Alk1)m(Q)n(Alk2)p- wherein m, n and p are independently 0 or 1, and Q, Alk1 and Alk2 are as defined in the claims; X1 represents a bond; —C(═O); or —S(═O)2—; —NR4C(═O)—, —C(═O)NR4—, —NR4C(═O)NR5—, —NR4S(═O)2—, or —S(═O)2NR4— wherein R4 and R5 are independently hydrogen or optionally substituted C1-C6 alkyl; and z is 0 or 1.