Ginger Chao

Bispecific designed ankyrin repeat proteins (DARPins) targeting epidermal growth factor receptor inhibit A431 cell proliferation and receptor recycling

Background: The EGF receptor (EGFR) is an important therapeutic target. Results: Bispecific anti-EGFR designed ankyrin repeat proteins (DARPins), alternative targeting molecules efficiently produced in bacteria, were shown to inhibit A431 cell proliferation and receptor recycling. Conclusion: One bispecific construct containing four DARPins showed a biological activity superior to that of the registered antibody cetuximab. Significance: Bispecific DARPins may form building blocks for tomorrows cancer therapeutics. The EGF receptor (EGFR) has been implicated in the development and progression of many tumors. Although monoclonal antibodies directed against EGFR have been approved for the treatment of cancer in combination with chemotherapy, there are limitations in their clinical efficacy, necessitating the search for robust targeting molecules that can be equipped with new effector functions or show a new mechanism of action. Designed ankyrin repeat proteins (DARPins) may provide the targeting component for such novel reagents. Previously, four DARPins were selected against EGFR with (sub)nanomolar affinity. As any targeting module should preferably be able to inhibit EGFR-mediated signaling, their effect on A431 cells overexpressing EGFR was examined: three of them were shown to inhibit proliferation by inducing G1 arrest, as seen for the Food and Drug Administration-approved antibody cetuximab. To understand this inhibitory mechanism, we mapped the epitopes of the DARPins using yeast surface display. The epitopes for the biologically active DARPins overlapped with the EGF-binding site, whereas the fourth DARPin bound to a different domain, explaining the lack of a biological effect. To optimize the biological activity of the DARPins, we combined two DARPins binding to different epitopes with a flexible linker or with a leucine zipper, leading to a homodimer. The latter DARPin was able to reduce surface EGFR by inhibiting receptor recycling, leading to a dramatic decrease in cell viability. These results indicate that multispecific EGFR-specific DARPins are superior to cetuximab and may form the basis of new opportunities in tumor targeting and tumor therapy.

Antibodies specifically targeting a locally misfolded region of tumor associated EGFR

Epidermal Growth Factor Receptor (EGFR) is involved in stimulating the growth of many human tumors, but the success of therapeutic agents has been limited in part by interference from the EGFR on normal tissues. Previously, we reported an antibody (mab806) against a truncated form of EGFR found commonly in gliomas. Remarkably, it also recognizes full-length EGFR on tumor cells but not on normal cells. However, the mechanism for this activity was unclear. Crystallographic structures for Fab:EGFR287–302 complexes of mAb806 (and a second, related antibody, mAb175) show that this peptide epitope adopts conformations similar to those found in the wtEGFR. However, in both conformations observed for wtEGFR, tethered and untethered, antibody binding would be prohibited by significant steric clashes with the CR1 domain. Thus, these antibodies must recognize a cryptic epitope in EGFR. Structurally, it appeared that breaking the disulfide bond preceding the epitope might allow the CR1 domain to open up sufficiently for antibody binding. The EGFRC271A/C283A mutant not only binds mAb806, but binds with 1:1 stoichiometry, which is significantly greater than wtEGFR binding. Although mAb806 and mAb175 decrease tumor growth in xenografts displaying mutant, overexpressed, or autocrine stimulated EGFR, neither antibody inhibits the in vitro growth of cells expressing wtEGFR. In contrast, mAb806 completely inhibits the ligand-associated stimulation of cells expressing EGFRC271A/C283A. Clearly, the binding of mAb806 and mAb175 to the wtEGFR requires the epitope to be exposed either during receptor activation, mutation, or overexpression. This mechanism suggests the possibility of generating antibodies to target other wild-type receptors on tumor cells.