Targeted radionuclide therapy is a revolutionary tool for the treatment of highly spread metastatic cancers. Most current approaches rely on the use of vectors to deliver radionuclides to tumor cells, targeting membrane‐bound cancer‐specific moieties. Here, we report the embryonic navigation cue netrin‐1 as an unanticipated target for vectorized radiotherapy. While netrin‐1, known to be re‐expressed in tumoral cells to promote cancer progression, is usually characterized as a diffusible ligand, we demonstrate here that netrin‐1 is actually poorly diffusible and bound to the extracellular matrix. A therapeutic anti‐netrin‐1 monoclonal antibody (NP137) has been preclinically developed and was tested in various clinical trials showing an excellent safety profile. In order to provide a companion test detecting netrin‐1 in solid tumors and allowing the selection of therapy‐eligible patients, we used the clinical‐grade NP137 agent and developed an indium‐111‐NODAGA‐NP137 single photon emission computed tomography (SPECT) contrast agent. NP137‐111In provided specific detection of netrin‐1‐positive tumors with an excellent signal‐to‐noise ratio using SPECT/CT imaging in different mouse models. The high specificity and strong affinity of NP137 paved the way for the generation of lutetium‐177‐DOTA‐NP137, a novel vectorized radiotherapy, which specifically accumulated in netrin‐1‐positive tumors. We demonstrate here, using tumor cell‐engrafted mouse models and a genetically engineered mouse model, that a single systemic injection of NP137‐177Lu provides important antitumor effects and prolonged mouse survival. Together, these data support the view that NP137‐111In and NP137‐177Lu may represent original and unexplored imaging and therapeutic tools against advanced solid cancers.

Purpose Chondrosarcomas (CHSs), which represent 20% of primary bone tumors in adults, are mostly resistant to radio- and chemotherapy. It is therefore essential that new therapeutic approaches, targeted to the tumour, be developed to improve the prognosis of patients. The effectiveness, as a radiosensitizing agent, of gadolinium oxide nanoparticles (GdoNP, AGuIX ® ) nanoparticles in CHS was evaluated in vitro, in spheroid CHS models allowing to reproduce cell-cell extracellular matrix interactions, and, in vivo, in a nude mouse model with heterotopic tumour xenograft. Methods Spheroids from SW1353 and HEMC-SS cells were characterized by confocal microscopy with or without GdoNP treatment. Real-time microscopy enabled quantification of cell viability, cell migration and invasion. In vivo, the efficacy of the association of GdoNP combined with a single (4Gy) or fractionated (4x1Gy) irradiation was evaluated in HEMC-SS tumor-bearing mice by monitoring tumor growth, mouse survival and gene expression profile. Results The expression of proteoglycans in the extra-cellular matrix (ECM) of spheroids demonstrated the relevance of the 3-D model. The combination of GdoNP with single or fractionated irradiation increased the lethal effects of irradiation on 2-D- and 3-D-cultured cells. In vivo, a single or a fractionated dose of 4 Gy associated with IT or IV injection of GdoNP decreased tumor growth significantly. Only IT injection increased mice survival. Unexpectedly, the radiosensitizing effect of GdoNP was associated, in vitro, with a significant decrease in invasion-migration capacities and, in vivo, with the decreased expression of PTX3, a protein involved in the epithelial-to-mesenchymal transition process, suggesting a potential impact of GdoNP on metastasis formation. Conclusion These results provide the first proof of concept of the radiosensitizing effect of GdoNP in CHSs and opened the way for a multicentre, randomized Phase 2 trial evaluating the association of GdoNP with radiotherapy for the therapeutic management of patients with symptomatic inoperable musculoskeletal tumor lesions.