Evaluation of <i>N</i>‑[¹¹C]Methyl-AMD3465 as a PET Tracer for Imaging of CXCR4 Receptor Expression in a C6 Glioma Tumor Model

The chemokine receptor CXCR4 and its ligand CXCL12 play an important role in tumor progression and metastasis. CXCR4 receptors are expressed by many cancer types and provide a potential target for treatment. Noninvasive detection of CXCR4 may aid diagnosis and improve therapy selection. It has been demonstrated in preclinical studies that positron emission tomography (PET) with a radiolabeled small molecule could enable noninvasive monitoring of CXCR4 expression. Here, we prepared <i>N</i>-[¹¹C]­methyl-AMD3465 as a new PET tracer for CXCR4. <i>N</i>-[¹¹C]­Methyl-AMD3465 was readily prepared by <i>N</i>-methylation with [¹¹C]­CH₃OTf. The tracer was obtained in a 60 ± 2% yield (decay corrected), the purity of the tracer was >99%, and specific activity was 47 ± 14 GBq/μmol. Tracer stability was tested <i>in vitro</i> using liver microsomes and rat plasma; excellent stability was observed. The tracer was evaluated in rat C6 glioma and human PC-3 cell lines. <i>In vitro</i> cellular uptake of <i>N</i>-[¹¹C]­methyl-AMD3465 was receptor mediated. The effect of transition metal ions (Cu²⁺, Ni²⁺, and Zn²⁺) on cellular binding was examined in C6 cells, and the presence of these ions increased the cellular binding of the tracer 9-, 7-, and 3-fold, respectively. <i>Ex vivo</i> biodistribution and PET imaging of <i>N</i>-[¹¹C]­methyl-AMD3465 were performed in rats with C6 tumor xenografts. Both PET and biodistribution studies demonstrated specific accumulation of the tracer in the tumor (SUV 0.6 ± 0.2) and other CXCR4 expressing organs, such as lymph node (1.5 ± 0.2), liver (8.9 ± 1.0), bone marrow (1.0 ± 0.3), and spleen (1.0 ± 0.1). Tumor uptake was significantly reduced (66%, <i>p</i> < 0.01) after pretreatment with Plerixafor (AMD3100). Biodistribution data indicates a tumor-to-muscle ratio of 7.85 and tumor-to-plasma ratio of 1.14, at 60 min after tracer injection. Our data demonstrated that <i>N</i>-[¹¹C]­methyl-AMD3465 is capable of detecting physiologic CXCR4 expression in tumors and other CXCR4 expressing tissues. These results warrant further evaluation of <i>N</i>-[¹¹C]­methyl-AMD3465 as a potential PET tracer for CXCR4 receptor imaging