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°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.