A new melanoma seeker for possible clinical use: selective accumulation of radiolabelled thiouracil.

In a previous report we have shown that a few substances, especially thiouracil, are incorporated as false precursors into melanin during its synthesis. In the present investigation, we have intensified our studies on the incorporation of thiouracil into melanotic melanomas. Firstly, the distribution and retention of both 14C- and 35S-labelled thiouracil in mice with transplanted melanomas were studied. A high and selective accumulation was found in the melanotic tumours. The concentration in the rest of the body was low, with the exception of the thyroid gland. Secondly, melanoma-bearing mice were given increasing doses of thiouracil, and cultured melanoma cells were exposed to different concentrations of thiouracil, to investigate the relation between dose and uptake in melanomas and melanoma cells, respectively. A relatively linear increase in uptake with dose was found, indicating that the melanin incorporation of thiouracil is non-saturable up to subtoxic levels.