64Cu-AMD3100: A potential radiotracer for in vivo visualization of the chemokine receptor CXCR4

176 Objectives: The CXCR4 receptor is highly expressed on certain types of white blood cells (WBC) and hematopoietic stem cells (HSC), providing a homing mechanism for such cells to tissues and organs that excrete the chemokine CXCL12. CXCR4/CXCL12 also has a central role in tumor metastasis. We labeled the CXCR4 antagonist AMD3100 (mol wt 794) with 64Cu and used it for in vivo imaging and biodistribution measurements in mice. Methods: AMD3100 in ammonium acetate was added to 64Cu copper chloride and incubated at 37oC for 30 min, after which non-radioactive copper (2:1 molar ratio) was added. Reverse-phase chromatography (C18 column) demonstrated a single radioactive peak eluting at the same time as [Cu2]AMD3100. Immune-competent C57Bl mice were injected i.v. with either 0.06 (“low-dose”; n=2) or 0.46 (“high-dose”; n=2) μg/g of 64Cu-AMD3100 (130-180 kBq/g). Scans were acquired at 0.5, 1, 2 and 4 h using an R4 microPETTM (Siemens/CTMI), after which the mice were euthanized and biodistribution of 64Cu was measured by direct assay. Results: Radiolabel was rapidly cleared into urine (>50% by 4 h) and liver (35-41% injected dose/g between 1 and 4 h in the low-dose group). Uptake was highest in tissues and organs known to constitutively express CXCL12 and/or harbor large concentrations of WBC or HSC, namely liver, bone, spleen, and lung. Uptake in each of these was reduced in the high-dose relative to the low-dose group, indicating a saturable uptake mechanism. Percentage of whole blood 64Cu in the cellular fraction at 4 h was also lower in the high-dose compared with the low-dose group (33±13% compared with 52±1%). Conclusions: The bicyclam AMD3100, a natural chelating agent, is readily labeled with 64Cu. The biodistribution in immune competent mice was consistent with specific binding of the radiotracer to CXCR4.