Chemosorption of radiometals of interest to nuclear medicine by synthetic melanins.

INTRODUCTION Melanins are high-molecular-weight pigments that are ubiquitous in nature and can also be synthesized in the laboratory from a variety of precursors. Melanins possess numerous interesting physicochemical characteristics, including electromagnetic radiation absorption properties and ability to chelate metals. We have recently reported that melanin has remarkable ionizing-radiation-shielding properties, possibly because it can interact with photons via Compton scattering. We hypothesized that, if administered internally, melanin could play a beneficial role by scavenging various radionuclides, in addition to radiation shielding. METHODS Three melanins were synthesized from dopamine, 3,4-dihydroxyphenylalanine (l-Dopa) and a combination of l-cysteine and l-Dopa. For control, synthetic melanin made from tyrosine polymerization (Sigma) was used. Melanins were characterized by elemental analysis. The chemosorption of 111In, 225Ac and 213Bi by melanins was studied at 37 degrees C for up to 48 h. RESULTS The C-to-N molar ratios for dopamine, l-Dopa and tyrosine melanins were very close at 7.92, 8.39 and 8.48, respectively, while in mixed l-cysteine/l-Dopa melanin, that ratio was much lower at 3.63. This mixed melanin also contained 22.33% sulfur, thus confirming incorporation of S-containing motifs into its structure. Dopamine, l-Dopa and tyrosine melanins were very similar in their abilities to decrease the activity of 111In, 225Ac and 213Bi and their radioactive daughters in supernatants by >10-fold in comparison with the starting levels, while mixed l-cysteine/l-Dopa melanin was able to chemosorb only 111In. CONCLUSIONS We have demonstrated that synthetic melanins made of diverse precursors can chemosorb 111In, 213Bi and 225Ac, with dopamine, l-Dopa and tyrosine melanins being the most efficient towards all three of these radionuclides. Such properties of synthetic melanins can contribute to the development of the novel melanin-based radioprotective materials.