Selective incorporation of thiouracil into murine metastatic melanomas

The uptake and retention of 14C-thiouracil and 125I-thiouracil in small lung metastases of B16 murine melanoma was studied in beige mice injected intravenously with melanoma cells. By impulse counting of excised tumor and organ pieces, a high concentration of radioactivity was found in the lung metastases, as compared to normal tissues. The highest tumor/organ concentration ratios appeared 24 h after injection of the radiolabeled thiouracil. A separate autoradiographic study on the disposition of 14C-thiouracil in mice with melanoma metastases confirmed the impulse counting results and also showed the absence of any other site of retention of radioactivity except for hair follicles and to some extent the thyroid. The selective uptake of 14C- and 125I-thiouracil in melanomas depends on their acceptance as false melanin precursors, making them specific markers for growing melanin. The results indicate that radiolabeled thiouracil may be useful for clinical diagnosis and, possibly, therapy of malignant melanotic melanomas.