Distributions of 137Cs, 201Tl, 203Hg, 203Pb and 57Co in a rat hepatoma model. Comparison with 67Ga.

The distribution of carrier-free 203Pb-acetate, 203HgCl2, 57 CoCl2, 137CsCl and 201TlCl was investigated in rats bearing thigh-implanted Morris 7777 hepatomas. Viable and nonviable tumor tissue was collected in order to determine the relative affinities of the radiopharmaceuticals for these tissues. The animals were sacrificed at 4, 24, 48, 72 and 96 hrs following intravenous injection. Washout of the radioisotope from the viable tumor tissue was rapid, the maximum concentration being reached on or before 4 hrs following injection. In contrast, residual activity within the nonviable tumor tissue decreased much more slowly and in some cases even increased with time. Viable tumor-to-muscle and nonviable tumor-to-muscle ratios for 203Pb, 203Hg and 57Co were comparable to the analogous ratios reported for 67Ga. However, none of these isotopes approached 67Ga as a potential tumor imaging agent because the large ratios were the result of low muscle uptake rather than high tumor uptake. Blood clearance of 67Ga was faster than any of the five cations, while viable and nonviable tumor affinity for 67Ga was greater than for any of the radiopharmaceuticals studied.