High-Contrast CXCR4-Targeted ¹⁸F-PET Imaging Using a Potent and Selective Antagonist

C-X-C chemokine receptor 4 (CXCR4) is highly expressed in cancers, contributing to proliferation, metastasis, and a poor prognosis. The noninvasive imaging of CXCR4 can enable the detection and characterization of aggressive cancers with poor outcomes. Currently, no ¹⁸F-labeled CXCR4 positron emission tomography (PET) radiotracer has demonstrated imaging contrast comparable to [⁶⁸Ga]Ga-Pentixafor, a CXCR4-targeting radioligand. We, therefore, aimed to develop a high-contrast CXCR4-targeting radiotracer by incorporating a hydrophilic linker and trifluoroborate radioprosthesis to LY2510924, a known CXCR4 antagonist. A carboxy-ammoniomethyl-trifluoroborate (PepBF₃) moiety was conjugated to the LY2510924-derived peptide possessing a triglutamate linker via amide bond formation to obtain BL08, whereas an alkyne ammoniomethyl-trifluoroborate (AMBF₃) moiety was conjugated using the copper-catalyzed [3+2] cycloaddition click reaction to obtain BL09. BL08 and BL09 were radiolabeled with [¹⁸F]fluoride ion using ¹⁸F-¹⁹F isotope exchange. Pentixafor was radiolabeled with [⁶⁸Ga]GaCl₃. Side-by-side PET imaging and biodistribution studies were performed on immunocompromised mice bearing Daudi Burkitt lymphoma xenografts. The biodistribution of [¹⁸F]BL08 and [¹⁸F]BL09 showed tumor uptake at 2 h postinjection (p.i.) (5.67 ± 1.25%ID/g and 5.83 ± 0.92%ID/g, respectively), which were concordant with the results of PET imaging. [¹⁸F]BL08 had low background activity, providing tumor-to-blood, -muscle, and -liver ratios of 72 ± 20, 339 ± 81, and 14 ± 3 (2 h p.i.), respectively. [¹⁸F]BL09 behaved similarly, with ratios of 64 ± 20, 239 ± 72, and 17 ± 3 (2 h p.i.), respectively. This resulted in high-contrast visualization of tumors on PET imaging for both radiotracers. [¹⁸F]BL08 exhibited lower kidney uptake (2.2 ± 0.5%ID/g) compared to [¹⁸F]BL09 (7.6 ± 1.0%ID/g) at 2 h p.i. [¹⁸F]BL08 and [¹⁸F]BL09 demonstrated higher tumor-to-blood, -muscle, and -liver ratios compared to [⁶⁸Ga]Ga-Pentixafor (18.9 ± 2.7, 95.4 ± 36.7, and 5.9 ± 0.7 at 2 h p.i., respectively). In conclusion, [¹⁸F]BL08 and [¹⁸F]BL09 enable high-contrast visualization of CXCR4 expression in Daudi xenografts. Based on high tumor-to-organ ratios, [¹⁸F]BL08 may prove a valuable new tool for CXCR4-targeted PET imaging with potential for translation. The use of a PepBF₃ moiety is a new approach for the orthogonal conjugation of organotrifluoroborates for ¹⁸F-labeling of peptides.