Compounds of formula (I), and their pharmaceutically acceptable derivatives, wherein R1 and R2 are optional substituents and independently represent C1-6 alkyl, C2-6 alkenyl, optionally substituted by CO2H or CO2(C1-6 alkyl), C2-6 alkynyl, halogen, C1-3 perfluoroalkyl, (CH2)mAr1, (CH2)mHet1, (CH2)mCONR7R8, (CH2)mCO2R8, O(CH2)qCO2R8, (CH2)mCOR8, (CH2)mOR8, O(CH2)pOR8, (CH2)mNR7R8, CO2(CH2)qNR7R8, (CH2)mCN, S(O)nR8, SO2NR7R8, CONH(CH2)mAr1 or CONH(CH2)mHet1; R3 represents H, C1-6 alkyl, (CH2)pNR9R10, SO2R10, SO2NR9R10, (CH2)mCO2R10, C2-6 alkenyl, C2-6 alkynyl, (CH2)mCONR9R10, (CH2)mCO2R10, (CH2)pCN, (CH2)pR10 or (CH2)pOR10; R4 represents H or C1-6 alkyl; R5 represents H or OH; R6 represents phenyl optionally fused to a heterocyclic ring, the group as a whole being optionally substituted; R7-10 are fully defined herein and may independently represent Ar2 or Het2; Z represents CO2H, CONH(tetrazol-5-yl), CONHSO2O(C1-4 alkyl), CO2Ar3, CO2(C1-6 alkyl), tetrazol-5-yl, CONHSO2Ar3, CONHSO2(CH2)qAr3 or CONHSO2(C1-4 alkyl); Ar1-3 independently represent phenyl, naphthyl, or an aromatic heterocycle, which groups are optionally fused and optionally substituted; and Het1 and Het2 independently represent a non-aromatic heterocycle which is optionally substituted; are useful in the treatment of restenosis, renal failure and pulmonary hypertension.