Compounds of formula (1) are described: in which X is an --O-- or --S-- atom or --N(R.sup.2)-- group and R.sup.1 is a group Ar.sup.1L.sup.2Ar.sup.2Alk- in which Ar.sup.1 is an optionally substituted aromatic or heteroaromatic group, L.sup.2 is a covalent bond or a linker atom or group, Ar.sup.2 is an optionally substituted bicyclic heteroarylene group and Alk is a chain --CH.sub.2--CH(R)--, --CH.dbd.C(R)--, OR--CH(CH.sub.2R)-- in which R is a carboxylic acid (--CO.sub.2H) or a derivative or biostere thereof. The compounds are selective inhibitors of alpha 4 integrins such as .alpha..sub.4.beta..sub.1 and are of use in modulating cell adhesion for the prophylaxis or treatment of inflammatory diseases or disorders, such as rheumatoid arthritis, in which the extravasculation of leukocytes plays a role. ##STR00001##