Positron emission tomography of experimental melanoma with [76Br]5-bromo-2-thiouracil

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 [76Br]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 [76Br]5-bromo-2-thiouracil was retained in the tumors though the overall radioactivity concentration in the body had declined. Accumulation of 76Br was also seen in bladder, liver, and kidney. In addition, the rats were simultaneously injected with [125I] 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 125I-radioactivity was about three-fold higher than that in the liver and lungs. These results show the possibility of using [76Br]5-bromo-2-thiouracil for PET diagnostics of melanoma, including dosimetry, prior to targeted therapy using [131I]5-iodo-2thiouracil or [211At]5-astato-2-thiouracil.