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##