Melanin-related metabolites in urine of B16 melanoma-bearing mice.

5-S-Cysteinyldopa (5-S-CD), a pheomelanin precursor, has been used as a biochemical marker of melanoma metastasis. Recently, 5-hydroxy-6-methoxyindole-2-carboxylic acid (5H6MI2C), a eumelanin-related metabolite, was shown to reflect well the degree of skin pigmentation. We measured the urinary excretion of 5H6MI2C and 5-S-CD in mice bearing B16 melanoma to determine which of the two markers better reflects the progression of melanoma. The urinary excretion of both 5H6MI2C and 5-S-CD increased rapidly in parallel with the tumour volume. The highest values for the two metabolites in melanoma-bearing mice were three orders of magnitude higher than those in control mice. However, 5H6MI2C had a higher excretion level at the early stage of melanoma progression, while 5-S-CD had a higher excretion level at the later stage.