Photoradiolabeling of onartuzumab with 99mTc and 188Re-tricarbonyl for radiotheranostics of gastric cancer

Abstract

The clinically relevant nuclear isomer of technetium-99 (^99mTc) and the radionuclides rhenium-186/188 (¹⁸⁶Re and ¹⁸⁸Re) 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 ^99mTc- and ¹⁸⁸Re-onartuzumab by labeling of the cancer-specific mAb onartuzumab (MetMAb) with the corresponding metal-tricarbonyl complexes derived from a novel photoactivatable ligand. The acyclic tris-amine ligand L1, featuring a photoactive aryl azide (ArN₃) group, was synthesized from N¹-(2-aminoethyl)ethane-1,2-diamine in 5 steps with an overall yield of 32%. Radiosynthesis of the [M(CO)₃L1]⁺ (M = ^99mTc or ¹⁸⁸Re) photoactivatable complexes was accomplished via reduction of the [M^VIIO₄]⁻ species to give the intermediate ^99mTc^I- and ¹⁸⁸Re^I-tricarbonyl-triaquo followed by ligand substitution with L1. The light-induced photoradiosynthesis of [M(CO)₃L1-azepin]-onartuzumab (M-onartuzumab; M = ^99mTc or ¹⁸⁸Re) was achieved by irradiating the [M(CO)₃L1]⁺ 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⁻¹. 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 ex vivo biodistribution experiments in mouse models bearing subcutaneous MKN-45 xenografts revealed specific tumor targeting compared against competitive inhibition (blocking) controls performed at 24 hours (^99mTc and ¹⁸⁸Re) and 72 hours (¹⁸⁸Re). Tumor uptake reached 20.20 ± 4.05 %ID g⁻¹ for ^99mTc-onartuzumab and 22.13 ± 3.11 %ID g⁻¹ for ¹⁸⁸Re-onartuzumab after 24 hours. Blocking experiments confirmed tumor specificity, with a reduction in tumor uptake of approximately 70% for both ^99mTc-onartuzumab and ¹⁸⁸Re-onartuzumab. Experimental data also revealed the biochemical equivalence of ^99mTc-onartuzumab and ¹⁸⁸Re-onartuzumab in terms of stability and pharmacokinetics in vivo. For ¹⁸⁸Re-onartuzumab, activity was retained in the tumor for over 72 hours, with uptake levels at 20.21 ± 1.47 %ID g⁻¹. 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.