[Tc-99m]O-2-AMD3100 as a SPECT tracer for CXCR4 receptor imaging

Purpose: CXCR4 plays an important role in HIV infection, tumor progression, neurogenesis, and inflammation. In-vivo imaging of CXCR4 could provide more insight in the role of this receptor in health and disease. The aim of this study was to investigate [Tc-99m]O-2-AMD3100 as a potential SPECT tracer for imaging of CXCR4. Method: AMD3100 was labelled with [Tc-99m]pertechnetate. A cysteine challenge assay was performed to test the tracer stability. Heterologous and homologous receptor binding assay and internalization assay were performed in CXCR4 expressing Jurkat-T cells. Ex vivo biodistribution was studied in healthy mice at 30, 60, and 120 min after tracer injection. Tumor uptake of the tracer was determined by microSPECT imaging in nude mice xenografted with human PC-3 prostate tumor. Specificity of tracer uptake was determined by blocking studies using an excess of unlabelled AMD3100. Results: AMD3100 was labelled with technetium-99 m with a radiochemical yield of >98%. The tracer was stable in PBS and mouse plasma for at least 6 h at 37 degrees C. Heterologous and homologous binding assays with AMD3100 showed IC50 values of 240 +/- 10 mu M, and 92 +/- 5 mu M for [I-125]SDF-1 alpha and [Tc-99m]O-2-AMD3100 respectively, with negligible receptor internalisation. The tracer showed high uptake in liver, lungs, spleen, thymus, intestine and bone. Blocking dose of AMD3100.8HCl (20 mg/kg) decreased the uptake in these organs (p <0.05). [Tc-99m]O-2-AMD3100 showed specific tumor accumulation in mice bearing PC-3 xenografts model. Time activity curves (TAC) in AMD3100 pre-treated animals tracer showed 1.7 times less tumor uptake as compared to control animals (p