Preclinical evaluation of novel HER3-targeting radioconjugates for the imaging and treatment of HER3-expressing cancers.

e15114 Background: HER3 overexpression in tumors is associated with poor survival and is implicated in drug resistance mechanisms of HER1 (EGFR) and HER2 targeted therapies. Although antibodies against HER3 have been developed and evaluated in clinical trials, no HER3 targeted therapy has been approved, underscoring the unmet need for novel HER3 therapeutic approaches. We hypothesized that HER3 radioligand targeted therapies could provide enhanced therapeutic efficacy against HER3-positive cancers through the energy deposited by alpha or beta emitting radionuclides in tumor cells. Here we describe the development and preclinical evaluation of novel anti-HER3 antibody radioconjugates (ARCs), 225 Ac-HER3 and 177 Lu-HER3, using the alpha-emitting Actinium-225 ( 225 Ac) and beta-emitting Lutetium-177 ( 177 Lu) radioisotopes, respectively. Methods: ARCs were prepared by radiolabeling HER3 antibody with 225 Ac or 177 Lu using S-2-(4-Isothiocyanatobenzyl)-1,4,7,10-tetraazacyclododecane tetraacetic acid (p-SCN-Bn-DOTA) to yield 225 Ac or 177 Lu-HER3-ARCs. DOTA modified antibodies were characterized by HPLC and MALDI-TOF and HER3-ARCs were fully characterized by instant thin-layer chromatography (iTLC) and HPLC. The in vitro binding properties of the HER3-ARCs were evaluated using HER3 recombinant proteins and cells expressing HER3 in ELISA and flow cytometry. The in vivo pharmacological properties of the HER3-ARCs were evaluated by molecular imaging using Zirconium-89 ( 89 Zr) Positron Emission Tomography (PET), biodistribution and antitumor efficacy studies. Results: HER3 antibody was successfully modified with p-SCN-Bn-DOTA to yield a conjugate with approximately 8 DOTAs per antibody molecule. Results from the in vitro binding assays of ARCs to either HER3 human recombinant protein or cells expressing HER3 demonstrated similar binding properties of 225 Ac-HER3 and 177 Lu-HER3 to unmodified HER3 antibody. The biodistribution study in NCI-H1975 tumor bearing mice showed that both 225 Ac-HER3 and 177 Lu-HER3-ARCs specifically accumulate in HER3-expressing tumors relative to other tissues. In agreement with the biodistribution studies, PET images revealed specific uptake of 89 Zr-HER3 in NCI-H1975 tumors. Furthermore, 225 Ac and 177 Lu-HER3-ARCs showed significantly more potent NCI-H1975 tumor growth inhibition compared to controls. Conclusions: Our findings suggest that a HER3 targeted radiotherapy approach can enhance therapeutic response and overcome the limitations of previously explored HER3 targeted drug modalities. Thus, further preclinical evaluation of 225 Ac-HER3 and 177 Lu-HER3 is warranted, including in settings of tumor drug resistance, which can provide the basis for investigating HER3-ARCs as an effective treatment approach in future clinical trials.