a substance being studied in the treatment of melanoma and many other types of cancer. d-1mt blocks the breakdown of the amino acid tryptophan, which is needed for t cells (a type of immune system cell) to kill tumor cells. giving d-1mt to patients who have received chemotherapy for cancer may help kill more tumor cells. it is a type of enzyme inhibitor and immunosuppressant. also called 1-methyl-d-tryptophan.