Modified peptide antagonists of interleukin 5 exhibit extended in vivo persistence but restricted species specificity

AF18748 is disulphide-linked homodimeric peptide with 19 amino acids in eac h chain that antagonises the action of the eosinophil-specific cytokine, in terleukin 5 (IL-5). We have generated a set of N-terminally truncated pepti des derived from AF18748 and demonstrated that the first five amino acids o f the peptide do not contribute to receptor binding activity. The shortened peptide blocked IL-5-dependent adhesion of eosinophils with an 1C(50) of 3 50 pM, and had no effect on stimulation by IL-3, granulocyte-macrophage col ony-stimulating factor (GM-CSF), tumour necrosis factor (TNF)-alpha or fMet -Leu-Phe. The peptides were rapidly broken down in mouse plasma through cle avage of a single chain of the dimer. However, this breakdown did not corre late with loss of biological activity, indicating that the asymmetric pepti de fragment retains full receptor binding capacity. The activity of AF18748 disappeared rapidly from the blood following intravenous injection into mi ce. Coupling of polyethylene glycol to the N-terminus of AF18748 resulted i n a moderate loss in biological potency (IC50 30 nM), but the resulting con jugate persisted in the circulation for more than 8 h after injection. Desp ite its high potency at the human IL-5 receptor, AF18748 was unable to anta gonise the activity of IL-5 on murine B13 cells, or on canine eosinophils, indicating that the peptide is highly specific for the human IL-5 receptor. (C) 2001 Academic Press.