Selective PET imaging of CXCR4 using the Al¹⁸F-labeled antagonist LY2510924

Background: [⁶⁸Ga]PentixaFor detects C-X-C chemokine receptor type 4 (CXCR4) overexpression in various malignancies, such as multiple myeloma and non-Hodgkin lymphomas, as well as in endocrine and inflammatory disorders. This study aimed to develop an Al¹⁸F-labeled radiotracer derived from LY2510924 for CXCR4-targeted imaging, leveraging the physical and logistical advantages of fluorine-18.

Methods: We designed a CXCR4-specific radioprobe, [¹⁸F]AlF-NOTA-SC, based on LY2510924 by incorporating a triglutamate linker and NOTA chelator to enable Al¹⁸F-labeling. The in vitro CXCR4 affinity was assessed using cell-based binding assays. Subsequently, in vivo pharmacokinetics and tumor uptake of [¹⁸F]AlF-NOTA-SC were assessed in naïve mice and mice with xenografts derived from U87.CD4/U87.CD4.CXCR4 and MM.1 S cells. Finally, biodistribution was determined in a non-human primate using PET-MR.

Results: Compared to Ga-PentixaFor, AlF-NOTA-SC demonstrated similar in vitro affinity for human CXCR4. [¹⁸F]AlF-NOTA-SC was produced with a decay-corrected radiochemical yield of 21.0 ± 7.1% and an apparent molar activity of 16.4 ± 3.6 GBq/µmol. In [¹⁸F]AlF-NOTA-SC binding assays on U87.CD4.CXCR4 cells, the total bound fraction was 7.1 ± 0.5% (58% blocking by AMD3100). In naïve mice, the radiotracer did not accumulate in any organs; however, it showed a significant CXCR4-specific uptake in xenografted tumors (SUVmean_U87.CD4 = 0.04 ± 0.00 (n = 3); SUVmean_{U87.CD4.CXCR4} = 3.04 ± 0.65 (n = 3); SUVmean_{MM.1 S} = 1.95 ± 0.11 (n = 3)). In a non-human primate, [¹⁸F]AlF-NOTA-SC accumulated in CXCR4 expressing organs, such as the spleen and bone marrow.

Conclusion: [¹⁸F]AlF-NOTA-SC exhibited CXCR4-specific uptake in vitro and in vivo, with fast and persistent tumor accumulation, making it a strong candidate for clinical translation as an ¹⁸F-alternative to [⁶⁸Ga]PentixaFor.