A novel set of inhibitors of the binding interaction between human urokinase plasminogen activator (uPA) and its cell surface receptor (uPAR) has been developed. The inhibitors comprise of peptide fragments, monomeric or in multiple copies attached to a common scaffold, in which the amino acid sequence may include uncommon substituted amino acids to partially comprise of peptoid sequences. The present invention also relates to the use of such peptides in therapy, in particular for the treatment of cancer, having developed a modified non-human mammalian receptor to which the novel inhibitors are antagonistic.