Bioorthogonal Fluorophore Linked DFO-Technology Enabling Facile Chelator Quantification and Multimodal Imaging of Antibodies.

Herein we describe the development and application of a bioorthogonal fluorogenic chelate linker that can be used for facile creation of labeled imaging agents. The chelate linker is based on the trans-cyclooctene(TCO)-tetrazine(Tz) chemistry platform and incorporates deferoxamine (DFO) as a (89)Zr PET tracer and a BODIPY fluorophore for multimodal imaging. The rapid (<3 min) ligation between mAb-TCO and Tz-BODIPY-DFO chelator is monitored using fluorescence and allows for determination of labeling completion. Utilizing BODIPY as the linker between mAb and DFO facilitates in chelator quantification using spectrophotometry, allowing for an alternative to traditional methods (mass and isotope dilution assay). Radiolabeling with (89)Zr to form (89)Zr-DFO-BODIPY-trastuzumab was found to be quantitative after incubation at room temperature for 1 h (1.5 mCi/mg specific activity). The cell binding assay using HER2+ (BT474) and HER2- (BT20) cell lines showed significant binding to (89)Zr-DFO-BODIPY-trastuzumab (6.45 ± 1.87% in BT474 versus 1.47 ± 0.39% in BT20). In vivo PET imaging of mice bearing BT20 or BT474 xenografts with (89)Zr-DFO-BODIPY-trastuzumab showed high tumor conspicuity, and biodistribution confirmed excellent, specific probe uptake of 237.3 ± 14.5% ID/g in BT474 xenografts compared to low, nonspecific probe uptake in BT20 xenografts (16.4 ± 5.6% ID/g) 96 h p.i. . Ex vivo fluorescence (465ex/520em) of selected tissues confirmed superb target localization and persistence of the fluorescence of (89)Zr-DFO-BODIPY-trastuzumab. The described platform is universally adaptable for simple antibody labeling.