Targeting the Transition State

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Science's STKE  20 Jul 2004:
Vol. 2004, Issue 242, pp. tw259-TW259
DOI: 10.1126/stke.2422004TW259

Because epidermal growth factor receptor (EGFR) hyperactivity is associated with many epithelial cancers, it is a popular target for the development of cancer therapies. Some tumors contain a variant form of EGFR (de2-7 EGFR) that has a deletion mutation in the extracellular domain, resulting in low constitutive activity. Johns et al. investigated the mechanism by which a monoclonal antibody mAb 806 binds the de2-7 EGFR effectively, but binds wild-type EGFR, when at normal concentrations on the cell surface, minimally. This antibody has been therapeutically effective against de2-7 EGFR-expressing tumors and tumors with amplified wild-type EGFR that have increased abundance of cell surface EGFR. The epitope for mAb 806 was identified to be residues 287 to 302 of the extracellular domain by immunoblotting of EGFR fragments and yeast surface display. The binding of mAb 806 to a peptide consisting of residues 287 to 302 was analyzed by surface plasmon resonance and the measured affinity (30 nM) correlated with the affinity for mAb 806 to de2-7 EGFR on cells. Analysis of the published crystal structures of the EGFR suggests that the receptor exists in three conformations: (i) a tethered form that is inactive and represents 95% of the unliganded receptor; (ii) an untethered form that is in equilibrium with the tethered state; and (iii) the dimerized, active form, which is abundant in the presence of ligand. Because mAb 806 binds to only a small fraction of wild-type EGFR on cells, the authors suggest that it is unlikely that this antibody binds the tethered form of the receptor. The dimerized receptor also does not have additional portions of the mAb 806 epitope exposed compared with the tethered form. However, in the untethered conformation, more of the mAb 806 epitope is exposed. Analysis of the binding of mAb 806 to cells expressing EGFR mutants that cannot form the tethered or dimerized conformations, which would be similar to de2-7 EGFR, indicated that mAb 806 does recognize the untethered conformation of the receptor. The authors propose that selectivity for the de2-7 EGFR tumors comes from the presence of this mutant receptor in the untethered form. The ability of mAb 806 to inhibit EGFR in cells with amplified EGFR compared to cells with normal concentrations of EGFR may arise because an increased proportion of the amplified EGFR is in the untethered conformation, thus providing selectivity for the effects of mAb 806.

T. G. Johns, T. E. Adams, J. R. Cochran, N. E. Hall, P. A. Hoyne, M. J. Olsen, Y.-S. Kim, J. Rothacker, E. C. Nice, F. Walker, G. Ritter, A. A. Jungbluth, L. J. Old, C. W. Ward, A. W. Burgess, K. Dane Wittrup, A. M. Scott, Identification of the epitope for the epidermal growth factor receptor-specific monoclonal antibody 806 reveals that it preferentially recognizes an untethered form of the receptor. J. Biol. Chem. 279, 30375-30384 (2004). [Abstract][Full Text]

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