Metastatic colorectal cancer (mCRC) and metastatic gastric cancer (mGC) are among the most aggressive cancers for which current therapeutic strategies remain insufficient. The administration of radiopharmaceuticals to specifically deliver radioactive isotopes to a target associated with tumors, appears to be a promising approach combining diagnosis and (theranostic) therapy of this type of disseminated cancer. The COMETE project aims to develop theranostic molecules by chemical modification of proteins capable of specifically recognizing tumor targets, by grafting them with a chelating agent of radionuclides for therapeutic (vectored internal radiotherapy, RIV) and diagnostic (diagnostic companion) purposes. Our project is structured around four objectives: 1/ The identification and validation of tumor targets, 2/ the development of new RIV molecules to treat mCRC and mGC, 3/ The evaluation of the antitumor efficacy and toxicity of RIV molecules according to the radionuclide used in preclinical models and 4/ The development of companion imaging agents to select responder patients, determine the optimal dose, and monitor the effectiveness of RIV. This project paves the way for the development of new approaches to personalized patient care to improve the effectiveness of therapies while limiting potential side effects.