- Integrins are cell adhesion receptors that mediate cell-to-cell, or cell-to-extracellular matrix adhesion. They represent an attractive target for treatment of multiple diseases. Two classes of small molecule integrin inhibitors have been developed. Competitive antagonists bind directly to the integrin ligand binding pocket and thus disrupt the ligand-receptor interaction. Allosteric antagonists have been developed primarily for ?(L)?(2)- integrin (LFA-1, lymphocyte function-associated antigen-1). Here we present the results of screening the Prestwick Chemical Library using a recently developed assay for the detection of ?(4)?(1)-integrin allosteric antagonists. Secondary assays confirmed that the compounds identified: 1) do not behave like competitive (direct) antagonists; 2) decrease ligand binding affinity for VLA-4 ?2 orders of magnitude; 3) exhibit antagonistic properties at low temperature. In a cell based adhesion assay in vitro, the compounds rapidly disrupted cellular aggregates. In accord with reports that VLA-4 antagonists in vivo induce mobilization of hematopoietic progenitors into the peripheral blood, we found that administration of one of the compounds significantly increased the number of colony-forming units in mice. This effect was comparable to AMD3100, a well known progenitor mobilizing agent. Because all the identified compounds are structurally related, previously used, or currently marketed drugs, this result opens a range of therapeutic possibilities for VLA-4-related pathologies.