Light-Induced Radiosynthesis of Zr-89-DFO-Azepin-Onartuzumab for Imaging the Hepatocyte Growth Factor Receptor

Methods that provide rapid access to radiolabeled antibodies are vital in the development of diagnostic and radiotherapeutic agents for PET or radioimmunotherapy. The human hepatocyte growth factor receptor (c-MET) signaling pathway is dysregulated in several malignancies, including gastric cancer, and is an important biomarker in drug discovery. Here, we used a photoradiochemical approach to produce Zr-89-radiolabeled onartuzumab (a monovalent, antihuman c-MET antibody), starting directly from the fully formulated drug (MetMAb). Methods: Simultaneous Zr-89-radiolabeling and protein conjugation was performed in one-pot reactions containing Zr-89-oxalate, the photoactive chelate desferrioxamine B (DFO)-aryl azide (DFO-ArN3), and MetMAb to give Zr-89-DFO-azepin-onartuzumab. As a control, Zr-89-DFO-benzyl Bn-isothiocyanate Bn-NCS-onartuzumab was prepared via a conventional two-step process using prepurified onartuzumab and DFO-Bn-NCS. Radiotracers were purified by using size-exclusion methods and evaluated by radiochromatography. Radiochemical stability was studied in human serum, and immunoreactivity was determined by cellular binding assays using MKN-45 gastric carcinoma cells. PET imaging at multiple time points (0-72 h) was performed on female athymic nude mice bearing subcutaneous MKN-45 xenografts. Biodistribution experiments were performed after the final image was obtained. The tumor specificity of Zr-89-DFO-azepin-onartuzumab was assessed in vivo by competitive inhibition (blocking) studies. Results: Initial photoradiosynthesis experiments produced Zr-89-DFO-azepinonartuzumab in less than 15 min, with an isolated decay-corrected radiochemical yield (RCY) of 24.8%, a radiochemical purity of approximately 90%, and a molar activity of approximately 1.5 MBq nmol(-1). Reaction optimization improved the radiochemical conversion of Zr-89-DFO-azepin-onartuzumab to 56.9% +/- 4.1% (n = 3), with isolated RCYs of 41.2% +/- 10.6% (n = 3) and radiochemical purity of more than 90%. Conventional methods produced Zr-89-DFO-Bn-NCS-onartuzumab with an isolated RCY of more than 97%, radiochemical purity of more than 97% and molar activity of approximately 14.0 MBq nmol(-)(1). Both radiotracers were immunoreactive and stable in human serum. PET imaging and biodistribution studies showed high tumor uptake for both radiotracers. By 72 h, tumor and liver uptake (percentage injected dose [%ID]) reached 15.37 +/- 5.21 %ID g(-1) and 6.56 +/- 4.03 % ID g(-1), respectively, for Zr-89-DFO-azepin-onartuzumab (n = 4) and 21.38 +/- 11.57 %ID g(-1) and 18.84 +/- 6.03 %ID g(-1), respectively, for Zr-89-DFO-Bn-NCS-onartuzumab (n = 4). Blocking experiments gave a statistically significant reduction in tumor uptake 6.34 +/- 0.47 %ID g(-1)) of Zr-89-DFO-azepin-onartuzumab (n = 4). Conclusion: The experiments demonstrated that photoradiosynthesis is a viable alternative approach for producing Zr-89-radiolabeled antibodies directly in protein formulation buffer, reducing protein aggregation and liver uptake.