Cyclopenta[g]quinazolines of the formula (I): wherein: A is a group OR or NR0R1 wherein R0 and R1 are each independently hydrogen C1-4 alkyl, C3-4 alkenyl, C3-4 alkynyl, C2-4 hydroxyalkyl, C2-4 halogenoalkyl or C1-4 cyanoalkyl, or R0 and R1 together with the intermediate N form a five- or six-membered heterocyclic ring; p is an integer in the range 1 to 4; R2 is hydrogen, C1-4 alkyl, C3-4 alkenyl, C3-4 alkynyl, C2-4 hydroxyalkyl, C2-4 halogenoalkyl or C1-4 cyanoalkyl; Ar1 is phenylene, thiophenediyl, thiazolediyl, pyridinediyl or pyrimidinediyl which may optionally bear one or two substituents selected from halogeno, hydroxy, amino, nitro, cyano, trifluoromethyl, C1-4 alkyl and C1-4 alkoxy; and R3 is a group of one of the following formulae: -A1-Ar2-A2-Y1-A5-CON(R)CH(Y4) Y5-A8-X—Ar4 and pharmaceutically acceptable salts or esters thereof are of therapeutic value particularly in the treatment of cancer.