Generation and evaluation of bispecific affibody molecules for simultaneous targeting of EGFR and HER2.

Coexpression of several ErbB receptors has been found in many cancers and has been linked with increased aggressiveness of tumors and a worse patient prognosis. This makes the simultaneous targeting of two surface receptors by using bispecific constructs an increasingly appreciated strategy. Here, we have generated six such bispecific targeting proteins, each comprising two monomeric affibody molecules with specific binding to either of the two human epidermal growth factor receptors, EGFR and HER2, respectively. The bispecific constructs were designed with (i) alternative positioning (N- or C-terminal) of the different affibody molecules, (ii) two alternative peptide linkers (Gly(4)Ser)(3) or (Ser(4)Gly)(3), and (iii) affibody molecules with different affinity (nanomolar or picomolar) for HER2. Using both Biacore technology and cell binding assays, it was demonstrated that all six constructs could bind simultaneously to both their target proteins. N-terminal positioning of the inherent monomeric affibody molecules was favorable to promote the binding to the respective target. Interestingly, bispecific constructs containing the novel (Ser(4)Gly)(3) linker displayed a higher affinity in cell binding, as compared to constructs containing the more conventional linker, (Gly(4)Ser)(3). It could further be concluded that bispecific constructs (but not the monomeric affibody molecules) induced dimer formation and phosphorylation of EGFR in SKBR3 cells, which express fairly high levels of both receptors. It was also investigated whether the bispecific binding would influence cell growth or sensitize cells for ionizing radiation, but no such effects were observed.

Engineering and characterization of a bispecific HER2 x EGFR-binding affibody molecule.

HER2 (human epidermal-growth-factor receptor-2; ErbB2) and EGFR (epidermal-growth-factor receptor) are overexpressed in various forms of cancer, and the co-expression of both HER2 and EGFR has been reported in a number of studies. The simultaneous targeting of HER2 and EGFR has been discussed as a strategy with which to potentially increase efficiency and selectivity in molecular imaging and therapy of certain cancers. In an effort to generate a molecule capable of bispecifically targeting HER2 and EGFR, a gene fragment encoding a bivalent HER2-binding affibody molecule was genetically fused in-frame with a bivalent EGFR-binding affibody molecule via a (G4S)3 [(Gly4-Ser)3]-encoding gene fragment. The encoded 30 kDa affibody construct (ZHER2)2-(G4S)3-(ZEGFR)2, with potential for bs (bispecific) binding to HER2 and EGFR, was expressed in Escherichia coli and characterized in terms of its binding capabilities. The retained ability to bind HER2 and EGFR separately was demonstrated using both biosensor technology and flow-cytometric analysis, the latter using HER2- and EGFR-overexpressing cells. Furthermore, simultaneous binding to HER2 and EGFR was demonstrated in: (i) a sandwich format employing real-time biospecific interaction analysis where the bs affibody molecule bound immobilized EGFR and soluble HER2; (ii) immunofluorescence microscopy, where the bs affibody molecule bound EGFR-overexpressing cells and soluble HER2; and (iii) a cell-cell interaction analysis where the bs affibody molecule bound HER2-overexpressing SKBR-3 cells and EGFR-overexpressing A-431 cells. This is, to our knowledge, the first reported bs affinity protein with potential ability for the simultaneous targeting of HER2 and EGFR. The potential future use of this and similar constructs, capable of bs targeting of receptors to increase the efficacy and selectivity in imaging and therapy, is discussed.

Dimeric HER2-specific affibody molecules inhibit proliferation of the SKBR-3 breast cancer cell line.

HER2-specific affibody molecules in different formats have previously been shown to be useful tumor targeting agents for radionuclide-based imaging and therapy applications, but their biological effect on tumor cells is not well known. In this study, two dimeric ((Z(HER2:4))(2) and (Z(HER2:342))(2)) and one monomeric (Z(HER2:342)) HER2-specific affibody molecules are investigated with respect to biological activity. Both (Z(HER2:4))(2) and (Z(HER2:342))(2) were found to decrease the growth rate of SKBR-3 cells to the same extent as the antibody trastuzumab. When the substances were removed, the cells treated with the dimeric affibody molecules continued to be growth suppressed while the cells treated with trastuzumab immediately resumed normal proliferation. The effects of Z(HER2:342) were minor on both proliferation and cell signaling. The dimeric (Z(HER2:4))(2) and (Z(HER2:342))(2) both reduced growth of SKBR-3 cells and may prove therapeutically useful either by themselves or as carriers of radionuclides or other cytotoxic agents.

The HER2-binding affibody molecule (Z(HER2∶342))2 increases radiosensitivity in SKBR-3 cells.

We have previously shown that the HER2-specific affibody molecule (Z(HER2∶342))2 inhibits proliferation of SKBR-3 cells. Here, we continue to investigate its biological effects in vitro by studying receptor dimerization and clonogenic survival following irradiation. We found that (Z(HER2∶342))2 sensitizes the HER2-overexpressing cell line SKBR-3 to ionizing radiation. The survival after exposure to (Z(HER2∶342))2 and 8 Gy (S(8Gy) 0.006) was decreased by a factor four compared to the untreated (S(8Gy) 0.023). The low HER2-expressing cell line MCF-7 was more radiosensitive than SKBR-3 but did not respond to (Z(HER2∶342))2. Treatment by (Z(HER2∶342))2 strongly increased the levels of dimerized and phosphorylated HER2 even after 5 minutes of stimulation. The monomeric Z(HER2∶342) does not seem to be able to induce receptor phosphorylation and dimerization or sensitize cells to irradiation.

Effects of HER2-binding affibody molecules on intracellular signaling pathways.

BACKGROUND: HER2, which is overexpressed in 25-30% of human breast cancers, is a tyrosine kinase receptor critical for the signal transduction network that regulates proliferation, migration and apoptosis of cells. METHOD: We report the effects of two novel HER2-binding affibody molecules (Affibody), (ZHER2:4)2 and ZHER2:342, on intracellular signal transduction pathways (Erk1/2, Akt and PLCgamma1) using quantitative immunoblotting techniques and their biological effects in cell culture. The clinically approved antibody trastuzumab (Herceptin) was used as reference substance. RESULTS: Our data showed that, although all substances target HER2, the effects on the receptor and signaling molecules differed. For example, HER2 phosphorylation was induced by trastuzumab and (ZHER2:4)2 but inhibited by ZHER2:342. The effects these substances had on signal transduction correlated to some degree with changes in growth and migration, e.g. (ZHER2:4)2 stimulated phosphorylation of Erk1/2 and PLCgamma1, as well as growth and migration, while ZHER2:342 did not. ZHER2:342 even inhibited phosphorylation of PLCgamma1 and migration. CONCLUSION: Our data suggest that ZHER2:342 is a promising small agent (7 kDa) that may be used as an alternative, or complement, to trastuzumab. If radiolabelled, it can hopefully also be used for HER2 imaging and radionuclide therapy.