Photoradiolabeling of onartuzumab with Tc and Re-tricarbonyl for radiotheranostics of gastric cancer.

The clinically relevant nuclear isomer of technetium-99 (Tc) and the radionuclides rhenium-186/188 (Re and Re) represent an almost ideal match for the development of radiotracers for applications in diagnostic imaging and molecularly targeted radionuclide therapy. Although the chemistry of Tc and Re is similar, important differences arise in both the synthesis and properties of their complexes. Here, we report the synthesis and characterization of Tc- and Re-onartuzumab by labeling of the cancer-specific mAb onartuzumab (MetMAb) with the corresponding metal-tricarbonyl complexes derived from a novel photoactivatable ligand. The acyclic -amine ligand L1, featuring a photoactive aryl azide (ArN) group, was synthesized from -(2-aminoethyl)ethane-1,2-diamine in 5 steps with an overall yield of 32%. Radiosynthesis of the [M(CO)L1] (M = Tc or Re) photoactivatable complexes was accomplished reduction of the [MO] species to give the intermediate Tc- and Re-tricarbonyl-triaquo followed by ligand substitution with L1. The light-induced photoradiosynthesis of [M(CO)L1-azepin]-onartuzumab (M-onartuzumab; M = Tc or Re) was achieved by irradiating the [M(CO)L1] complexes in the presence of onartuzumab (formulated as MetMAb), with 395 nm light for 15 minutes at room temperature. Photoradiolabeling reactions produced M-onartuzumab radioimmunoconjugates in decay-corrected radiochemical yields of 20-30%, high radiochemical purities (RCP > 95%), and in molar activities of 1.026-4.146 MBq nmol. Cellular binding assays confirmed the specificity of radiotracer binding toward human hepatocyte growth-factor receptor (c-MET) expression on the surface of MNK-45 gastric adenocarcinoma cells. Subsequent planar γ-ray scintigraphy imaging and biodistribution experiments in mouse models bearing subcutaneous MKN-45 xenografts revealed specific tumor targeting compared against competitive inhibition (blocking) controls performed at 24 hours (Tc and Re) and 72 hours (Re). Tumor uptake reached 20.20 ± 4.05 %ID g for Tc-onartuzumab and 22.13 ± 3.11 %ID g for Re-onartuzumab after 24 hours. Blocking experiments confirmed tumor specificity, with a reduction in tumor uptake of approximately 70% for both Tc-onartuzumab and Re-onartuzumab. Experimental data also revealed the biochemical equivalence of Tc-onartuzumab and Re-onartuzumab in terms of stability and pharmacokinetics . For Re-onartuzumab, activity was retained in the tumor for over 72 hours, with uptake levels at 20.21 ± 1.47 %ID g. Overall, the experiments demonstrated that photoradiosynthesis can be employed to develop a variety of rhenium based radioimmunoconjugates for future applications in tumor targeted radioimmunotherapy. Furthermore, these results underline the high potential of rhenium and technetium radioconjugates as theranostic platforms.