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 front 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 In-111, Ac-225 and Bi-213 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 In-111, Ac-225 and Bi-213 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 In-111. Conclusions: We have demonstrated that synthetic melanins made of diverse precursors can chemosorb In-111, Bi-213 and Ac-225, 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. (C) 2008 Elsevier Inc. All rights reserved.