Positron emission tomography (PET) has evolved as a new diagnostic modality in
cancer patients. Thioureylenes, such as
thiouracil and
methimazole, are known to be incorporated into growing
melanin and selectively retained in melanotic
melanoma. In the present study we used [(76)Br]5-bromo-2-
thiouracil as tracer for PET imaging of human and murine melanotic
melanoma transplanted subcutaneously into rats. The
melanomas were clearly depicted 1 day after the injection, when [(76)Br]5-bromo-2-
thiouracil was retained in the
tumors though the overall radioactivity concentration in the body had declined. Accumulation of (76)Br was also seen in bladder, liver, and kidney. In addition, the rats were simultaneously injected with [(125)I]5-iodo-2-
thiouracil and the tissue distribution of radioactivity was mapped by whole-body autoradiography. The results confirmed the selective uptake of
thiouracil in the
melanoma where the concentration of (125)I-radioactivity was about three-fold higher than that in the liver and lungs. These results show the possibility of using [(76)Br]5-bromo-2-
thiouracil for PET diagnostics of
melanoma, including dosimetry, prior to targeted
therapy using [(131)I]5-iodo-2-
thiouracil or [(211)At]5-astato-2-
thiouracil.