Dual Radionuclide Theranostic Pretargeting.

Recent years have played witness to the advent of nuclear theranostics: the synergistic use of 'matched pair' radiopharmaceuticals for diagnostic imaging and targeted radiotherapy. In this investigation, we report the extension of this concept to in vivo pretargeting based on the rapid and bioorthogonal inverse electron demand Diels-Alder reaction between tetrazine (Tz) and trans-cyclooctene (TCO). We demonstrate that a single injection of a TCO-modified immunoconjugate can be used as a platform for pretargeted PET imaging and radioimmunotherapy via the sequential administration of a pair of Tz-bearing radioligands labeled with the positron-emitting radiometal copper-64 (t1/2 ~ 12.7 h) and the beta-emitting radiometal lutetium-177 (t1/2 ~ 6.7 d). More specifically, a mouse model of human colorectal carcinoma received a dose of the A33 antigen-targeting immunoconjugate huA33-TCO, followed 24 and 48 h later by injections of [64Cu]Cu-SarAr-Tz and [177Lu]Lu-DOTA-PEG7-Tz, respectively. This approach produces high activity concentrations of both radioligands in tumor tissue (16.4 ± 2.7 %ID/g for [64Cu]Cu-SarAr-Tz at 48 h post-injection and 18.1 ± 2.1 %ID/g for [177Lu]Lu-DOTA-PEG7-Tz at 120 h post-injection) as well as promising tumor-to-healthy organ activity concentration ratios. Ultimately, we believe that this work could not only have important implications in nuclear theranostics - most excitingly with isotopologue-based radioligand pairs such as [64Cu]Cu-SarAr-Tz and [67Cu]Cu-SarAr-Tz - but also in the delivery of fractionated doses of radiation during pretargeted radioimmunotherapy.