Selective Chemistry Enables Simultaneous ImmunoPET and Radioimmunotherapy with a Dual-Labeled Probe

Theranostics is an emerging paradigm in oncology predicated on using nuclear imaging agents to inform and track treatment with radiotherapeutics. Theranostic workflows typically rely upon the administration of two different radiopharmaceuticals, but this approach requires the development of separate probes and depends upon the assumption that the pharmacokinetic profiles of the agents are identical. Herein, we report the creation of a modular platform that enables simultaneous positron emission tomography (PET) and radioimmunotherapy using a vector labeled with two radionuclides. To this end, we synthesized and validated a trifunctional reagentTzAz-PODSthat facilitates site-selective bioconjugation and allows for the attachment of cargoes via both strain-promoted azide-alkyne and inverse electron-demand Diels-Alder click chemistry. This tool was used to construct an immunoconjugate of the A33 antigen-targeting antibody huA33, which was then labeled in high purity and specific activity with both ⁸⁹Zr and ¹⁷⁷Lu. ⁸⁹Zr-immunoPET and dual-isotope biodistribution experiments in a murine model of colorectal cancer confirmed the tumor tropism of the radioimmunoconjugate and, more critically, revealed that the ¹⁷⁷Lu dosimetry derived from the PET data accurately reflected the ¹⁷⁷Lu dosimetry derived directly from biodistribution studies. Finally, longitudinal radioimmunotherapy studies with the ⁸⁹Zr/¹⁷⁷Lu-labeled radioimmunoconjugate suggested that PET imaging can help predict response to therapy.