Investigating the feasibility of EPI-X4 analogs for targeting CXCR4-expressing tumors in vivo

The overexpression of the C-X-C motif chemokine receptor 4 (CXCR4) in more than 23 types of human cancer, followed by its role in tumor growth, angiogenesis, and metastasis designates it as an attractive target in oncology. The identification of an endogenous peptide antagonist of CXCR4, termed EPI-X4 [1] and its optimized derivative EPI-X4 JM#21 [2], opens the space for the development of radiotracers for non-invasive molecular imaging and treatment (theranostics) of CXCR4-expressing cancers using the EPI-X4 as platform. One drawback of EPI-X4 and EPI-X4 JM#21, however, is their low stability in human plasma with t1/2 of 17 min and 6 min, respectively. Since the N-terminus of these peptides is metabolically vulnerable in plasma, different modification were introduced at the N-terminus of EPI-X4 leding to derivatives with significantly increased stability (t1/2 > 8 h). In addition, amidation at the C-terminus enabled further truncations of EPI-X4. A series of these derivatives was selected for conjugation to the DOTA chelator and labeling with Lutetium-177 (177Lu) in order to assess for first time the CXCR4 targeting ability of EPI-X4-based radiotracers in vitro and in vivo.