Exploratory clinical trials using therapeutic anti-HER3 antibodies strongly suggest that neuregulin (NRG1; HER3 ligand) expression at tumor sites is a predictive biomarker of anti-HER3 antibody efficacy in cancer. We hypothesized that in NRG1-expressing tumors, where the ligand is present before antibody treatment, anti-HER3 antibodies that do not compete with NRG1 for receptor binding have a higher receptor-neutralizing action than antibodies competing with the ligand for binding to HER3. Using time resolved-fluorescence energy transfer (TR-FRET), we demonstrated that in the presence of recombinant NRG1, binding of 9F7-F11 (a non-ligand competing anti-HER3 antibody) to HER3 is increased, whereas that of ligand-competing anti-HER3 antibodies (H4B-121, U3-1287, Ab#6, Mab205.10.2 and MOR09825) is decreased. Moreover, 9F7-F11 showed higher efficacy than antibodies that compete with the ligand for binding to HER3. Specifically, 9F7-F11 inhibition of cell proliferation and of HER3/AKT/ERK1/2 phosphorylation as well as 9F7-F11-dependent cell- mediated cytotoxicity were higher in cancer cells pre-incubated with recombinant NRG1 compared with cells directly exposed to the anti-HER3 antibody. This translated in vivo into enhanced growth inhibition of NRG1-expressing BxPC3 pancreatic, A549 lung and HCC-1806 breast cell tumor xenografts in mice treated with 9F7-F11 compared with H4B-121. Conversely, both antibodies had similar anti-tumor effect in NRG1-negative HPAC pancreatic carcinoma cells. In conclusion, the allosteric modulator 9F7-F11 shows increased anti-cancer effectiveness in the presence of NRG1 and thus represents a novel treatment strategy for NRG1-addicted tumors.
- Received December 20, 2016.
- Revision received March 16, 2017.
- Accepted April 18, 2017.
- Copyright ©2017, American Association for Cancer Research.