Pilot Study of ⁶⁴Cu(I) for PET Imaging of Melanoma

At present, ⁶⁴Cu(II) labeled tracers including ⁶⁴CuCl₂ have been widely applied in the research of molecular imaging and therapy. Human copper transporter 1 (hCTR1) is the major high affinity copper influx transporter in mammalian cells, and specially responsible for the transportation of Cu(I) not Cu(II). Thus, we investigated the feasible application of ⁶⁴Cu(I) for PET imaging. ⁶⁴Cu(II) was reduced to ⁶⁴Cu(I) with the existence of sodium L-ascorbate, DL-Dithiothreitol or cysteine. Cell uptake and efflux assay was investigated using B16F10 and A375 cell lines, respectively. Small animal PET and biodistribution studies were performed in both B16F10 and A375 tumor-bearing mice. Compared with ⁶⁴Cu(II), ⁶⁴Cu(I) exhibited higher cellular uptake by melanoma, which testified CTR1 specially influx of Cu(I). However, due to oxidation reaction in vivo, no significant difference between ⁶⁴Cu(I) and ⁶⁴Cu(II) was observed through PET images and biodistribution. Additionally, radiation absorbed doses for major tissues of human were calculated based on the mouse biodistribution. Radiodosimetry calculations for ^64/67Cu(I) and ^64/67Cu(II) were similar, which suggested that although melanoma were with high radiation absorbed doses, high radioactivity accumulation by liver and kidney should be noticed for the further application. Thus, ⁶⁴Cu(I) should be further studied to evaluate it as a PET imaging radiotracer.