Evaluation of 64Cu-ATSM in vitro and in vivo in a hypoxic tumor model

We have evaluated Cu-diacetyl-bis(N4-methylthiosemicarbazone) (Cu-ATSM), an effective marker for the delineation of hypoxic but viable tissue, in vitro in the EMT6 carcinoma cell line under varying degrees of hypoxia and compared it with the flow tracer 64Cu-pyruvaldehyde-bis(N4-methylthiosemicarbazone) (Cu-PTSM) and the hypoxic tracer 18F-fluoromisonidazole (MISO). We have also compared the uptake of Cu-ATSM and Cu-PTSM in vivo and ex vivo in a murine animal model bearing the EMT6 tumor. METHODS: Uptake of 64Cu-ATSM, 64Cu-PTSM and 18F-MISO in vitro into EMT6 cells was investigated at the dissolved oxygen concentrations of 0, 1 x 10(3), 5 x 10(3), 5 x 10(4) and 2 x 10(5) ppm. Biodistribution performed at 1, 5, 10, 20 and 40 min compared 64Cu-ATSM with 64Cu-PTSM in BALB/c mice bearing EMT6 tumors. To determine long-term retention of 64Cu-ATSM, biodistribution was also performed at 1, 2 and 4 h. Ex vivo autoradiography of tumor slices after co-injection of 60Cu-PTSM (60Cu, T1/2 = 23.7 min) and 64Cu-ATSM (64Cu, t1/2 = 12.7 h) into the same animal was performed. RESULTS: After 1 h, 64Cu-ATSM was taken up by EMT6 cells: 90% at 0 ppm, 77% at 1 x 10(3) ppm, 38% at 5 x 10(3) ppm, 35% at 5 x 10(4) ppm and 31% at 2 x 10(5) ppm. 18F-MISO also showed oxygen concentration dependent uptake, but with lower percentages than 64Cu-ATSM. 64Cu-PTSM showed 83%-85% uptake into the cells after 1 h, independent of oxygen concentration. Biodistribution data of 64Cu-ATSM and 64Cu-PTSM showed optimal tumor uptake after 5 and 10 min, respectively (0.76% injected dose (ID)/organ for 64Cu-ATSM and 1.11%ID/organ for 64Cu-PTSM). Ex vivo imaging experiments showed 60Cu-PTSM uniform throughout the EMT6 tumor, but heterogeneous uptake of 64Cu-ATSM, indicative of selective trapping of 64Cu-ATSM into the hypoxic tumor cells. CONCLUSION: Cu-ATSM exhibits selectivity for hypoxic tumor tissue both in vivo and in vitro and may provide a successful diagnostic modality for the detection of tumor ischemia.