A small peptide antagonist of integrin alpha5beta1 with potential antineoplastic activity. ATN-161 selectively binds to and blocks the receptor for integrin alpha5beta1, thereby preventing integrin alpha5beta1 binding. This receptor blockade may result in inhibition of endothelial cell-cell interactions, endothelial cell-matrix interactions, angiogenesis, and tumor progression. Integrin alpha5beta1 is expressed on endothelial cells and plays a crucial role in endothelial cell adhesion and migration.