Abstract 1401: Lead optimization of radiopharmaceuticals for molecular radiotherapy and preclinical evaluation

Abstract Molecular Radiotherapy (MRT) targeting SSTR2 or PSMA have proven to be highly efficient for the treatment of neuroendocrine or metastatic prostate cancer, respectively. Beyond the leading radiopharmaceutical molecules 177Lu-DOTATATE or 177Lu-PSMA-617, a variety of vectors (small molecules, peptides, panel of biologics) have been developed on the same targets in order to improve the biodistribution within the tumor, the blood clearance, the route of elimination or the dosimetry. The labeling of the targeting ligand, whatever its nature, is a crucial step as it may affect significantly the properties of the theranostic conjugate, i.e. its binding affinity, PK and biodistribution. The addition of linkers, such as albumin binding domain or PEG, and choice of chelating agents have a major impact on the chemical and biological properties of the vectors. Random or site-specific bioconjugation, click chemistry, also have to be considered in the early stage as the choice of the selected technology will modify your development plan and manufacturing. New ligands and biological platforms are now being developed based on this historical knowledge, improved Target Product Profiles are built to conduct optimal lead optimization of MRT. Herein, we will present our lead optimization and preclinical evaluation process to select efficiently good radiolabeled molecules and list the key parameters to be checked. To date, it remains hard to predict the behavior of the modified bioconjugated molecules, and versatile synthesis strategies are needed to screen various combinations of radiometal complexes, linker and conjugation function, in order to converge rapidly to the optimized bioconjugate. As an example, we will present a study case where the conjugation of various bifunctional chelating agents on a small NTS1 receptor antagonist resulted in drastically different in vivo behavior of the resulting 68Ga-labeled compounds. Once optimal in vivo tumor uptake has been achieved, preclinical evaluation requires the selection of appropriate and relevant models, driven by target expression, radioresistance, and potentially tumor immune infiltrate for combination studies with immunotherapies. The therapeutic evaluation should take into consideration the dose and specific activity, the tolerance of a model related to ionizing radiations and the scheduling of treatments (cumulated dose, fractionation). We will present our recent results which highlight the importance to optimize these parameters to improve the efficacy of MRT. Citation Format: Emma Renard, Olivier Raguin, Victor Goncalves, Celine Mothes, Mathieu Moreau, Claire Bernhard, Peggy Provent, Frederic Boschetti, Franck Denat, Fabrice Viviani, Cyril Berthet. Lead optimization of radiopharmaceuticals for molecular radiotherapy and preclinical evaluation [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 1401.