2-Thiouracil and some related thioureas are receiving growing interest as selective
melanoma seekers. They are incorporated into growing
melanin, apparently due to covalent binding to
dopaquinone, and the adduct is gradually trapped in the
melanin polymer during its formation. To be clinically useful in
melanoma scanning,
thiouracil has been radioiodinated, and
5-iodo-2-thiouracil (ITU) was found to be localized in melanotic
melanoma as selectively as
thiouracil. Clinical trials with ITU, for the detection of
malignant melanoma, are in progress, and the results so far are promising. Treatment with [35S]
thiouracil has been performed on
melanoma-bearing mice. The radiodoses needed for cure, however, were very high, which makes clinical application hazardous.
Boron neutron capture therapy, on the other hand, might be a better approach. The technique is based on the irradiation of tumours with slow neutrons from an external source after the accumulation of
boron in tumour tissue and clearance from normal tissues.
Boron-10 undergoes instantaneous nuclear fission through the reaction 10B(n,alpha)7Li, and the emitted particles are efficient in cell killing. Boronated thioureas have been synthesized in various laboratories, and data from experiments on
melanoma-bearing mice indicate that some of these compounds accumulate in the tumours in concentrations necessary for successful treatment.