Dual-Nuclide Radiopharmaceuticals for Positron Emission Tomography Based Dosimetry in Radiotherapy

Abstract

Improvement of the accuracy of dosimetry in radionuclide therapy has the potential to increase patient safety and therapeutic outcomes. Although positron emission tomography (PET) is ideally suited for acquisition of dosimetric data because PET is inherently quantitative and offers high sensitivity and spatial resolution, it is not directly applicable for this purpose because common therapeutic radionuclides lack the necessary positron emission. This work reports on the synthesis of dual-nuclide labeled radiopharmaceuticals with therapeutic and PET functionality, which are based on common and widely available metal radionuclides. Dual-chelator conjugates, featuring interlinked cyclen- and triazacyclononane-based polyphosphinates DOTPI and TRAP, allow for strictly regioselective complexation of therapeutic (e.g., ¹⁷⁷ Lu, ⁹⁰ Y, or ²¹³ Bi) and PET (e.g., ⁶⁸ Ga) radiometals in the same molecular framework by exploiting the orthogonal metal ion selectivity of these chelators (DOTPI: large cations, such as lanthanide(III) ions; TRAP: small trivalent ions, such as Ga^III ). Such DOTPI-TRAP conjugates were decorated with 3 Gly-urea-Lys (KuE) motifs for targeting prostate-specific membrane antigen (PSMA), employing Cu-catalyzed (CuAAC) as well as strain-promoted (SPAAC) click chemistry. These were labeled with ¹⁷⁷ Lu or ²¹³ Bi and ⁶⁸ Ga and used for in vivo imaging of LNCaP (human prostate carcinoma) tumor xenografts in SCID mice by PET, thus proving practical applicability of the concept.

Keywords: chelate ligands; click chemistry; molecular imaging; molecular radiotherapy; radionuclides.