In Vitro and In Vivo Evaluations of a Hydrophilic 64Cu-Bis(Thiosemicarbazonato)-Glucose Conjugate for Hypoxia Imaging

A water-soluble glucose conjugate of the hypoxia tracer Cu-64-diacetyl-bis(N-4-methylthiosemicarbazone) (Cu-64-ATSM) was synthesized and radiolabeled (Cu-64-ATSE/A-G). Here we report our initial biological experiments with Cu-64-ATSE/A-G and compare the results with those obtained for Cu-64-ATSM and F-18-FDG. Methods: The uptake of Cu-64-ATSE/A-G and Cu-64-ATSM into HeLa cells in vitro was investigated at a range of dissolved oxygen concentrations representing normoxia, hypoxia, and anoxia. Small-animal PET with Cu-64-ATSE/A-G was performed in male BDIX rats implanted with P22 syngeneic carcinosarcomas. Images of Cu-64-ATSM and F-18-FDG were obtained in the same model for comparison. Results: (64)CuATSE/A-G showed oxygen concentration-dependent uptake in vitro and, under anoxic conditions, showed slightly lower levels of cellular uptake than Cu-64-ATSM; uptake levels under hypoxic conditions were also lower. Whereas the normoxic uptake of Cu-64-ATSM increased linearly over time, Cu-64-ATSE/A-G uptake remained at low levels over the entire time course. In the PET study, (64)CuATSE/A-G showed good tumor uptake and a biodistribution pattern substantially different from that of each of the controls. In marked contrast to the findings for Cu-64-ATSM, renal clearance and accumulation in the bladder were observed. Cu-64-ATSE/A-G did not display the characteristic brain and heart uptake of F-18-FDG. Conclusion: The in vitro cell uptake studies demonstrated that Cu-64-ATSE/A-G retained hypoxia selectivity and had improved characteristics when compared with Cu-64-ATSM. The in vivo PET results indicated a difference in the excretion pathways, with a shift from primarily hepatointestinal for Cu-64-ATSM to partially renal with Cu-64-ATSE/A-G. This finding is consistent with the hydrophilic nature of the glucose conjugate. A comparison with F-18-FDG PET results revealed that Cu-64-ATSE/A-G was not a surrogate for glucose metabolism. We have demonstrated that our method for the modification of Cu-bis(thiosemicarbazonato) complexes allows their biodistribution to be modified without negating their hypoxia selectivity or tumor uptake properties.