Development of an Optimized CXCR4-Targeting Theranostic Pair

We developed a new C-X-C Chemokine Receptor 4 (CXCR4)-targeting radiolabeled peptide, [⁶⁸Ga]Ga/[¹⁷⁷Lu]Lu-BL34, using a novel and potent cyclic peptide based on structure-activity relationship studies of LY2510924. Methods: Candidate inhibitors were designed on the basis of structure-activity studies and synthesized using solid-phase techniques, and their CXCR4 binding was assessed using a cell-based assay. An optimized cyclic peptide sequence was modified with a Lys-cysteic acid linker and DOTA chelator to make BL34. Other analogs possessing ornithine or diaminopimelic acid linkers were also synthesized and assessed. All radiotracers were assessed in mice engrafted with a mantle cell lymphoma model (Z138) via PET/SPECT imaging and biodistribution studies. Therapeutic efficacy of [¹⁷⁷Lu]Lu-BL34 was assessed in Z138-engrafted mice with groups of 30 and 60 MBq and a control. Results: The optimized cyclic peptide showed a 3-fold improvement in CXCR4 binding compared with that of LY2510924. [⁶⁸Ga]Ga-BL34 showed high imaging contrast for the tumor at 1 and 2 h after injection. Biodistribution studies confirmed these results, with an uptake of 15.1 ± 3.1 %ID/g in the tumor at 1 h after injection and primarily renal excretion with a kidney uptake of 2.4 ± 0.4 %ID/g. SPECT imaging of [¹⁷⁷Lu]Lu-BL34 showed similar results, with rapid renal excretion of [¹⁷⁷Lu]Lu-BL34 from nontarget organs and relatively high uptake in tumors up to 72 h after injection. Biodistribution studies confirmed high tumor uptake at all time points, with low uptake across all nontarget organs. Blocking studies with LY2510924 further confirmed specificity. Therapy studies showed dose-dependent survival benefit with [¹⁷⁷Lu]Lu-BL34 treatment, with metastatic recurrence in the treatment groups. Changing the side chain length at the linker attachment site did not affect the biodistribution or tumor uptake. Conclusion: We report a new CXCR4-targeting pharmacophore that can be used as a radiotheranostic. [⁶⁸Ga]Ga-BL34 and [¹⁷⁷Lu]Lu-BL34 showed excellent imaging and therapeutic properties in preclinical studies and are promising candidates for clinical translation.