In vitro and in vivo evaluation of melanin-binding decapeptide 4B4 radiolabeled with ¹⁷⁷Lu, ¹⁶⁶Ho, and ¹⁵³Sm radiolanthanides for the purpose of targeted radionuclide therapy of melanoma

Melanoma is a malignancy with increasing incidence. Although primary tumors that are localized to the skin can be successfully treated by surgical removal, there is no satisfactory treatment for metastatic melanoma, a condition that has currently an estimated 5-year survival of just 6%. During the last decade, β-or α-emitter-radiolabeled peptides that bind to different receptors on a variety of tumors have been investigated as potential therapeutic agents in both the preclinical and clinical settings with encouraging results. A recent study demonstrated that 188-Rhenium ( ¹⁸⁸Re)-labeled, via HYNIC ligand, fungal melanin-binding decapeptide 4B4 was effective against experimental MNT1 human melanoma and was safe to normal melanized tissues. The availability of radiolanthanides with diverse nuclear emission schemes and half-lives provides an opportunity to expand the repertoire of peptides for radionuclide therapy of melanoma. The melanin-binding decapeptide 4B4 was radiolabeled with ¹⁷⁷Lu, ¹⁶⁶Ho, and ¹⁵³Sm via a DO3A chelate. The stability studies of Ln*-DO3A-4B4 in phosphate-buffered saline, serum, and a hydroxyapatite assay demonstrated that ¹⁷⁷Lu-labeled peptide was more stable than ¹⁶⁶Ho-and ¹⁵³Sm-labeled peptides, most likely because of the smallest ionic radius of the former allowing for better complexation with DO3A. Binding of Ln*-DO3A-4B4 to the lysed highly melanized MNT1 melanoma cells demonstrated the specificity of peptides binding to melanin. In vivo biodistribution data for ¹⁷⁷Lu-DO3A-4B4 given by intraperitoneal administration to lightly pigmented human metastatic A2058 melanoma-bearing mice demonstrated very high uptake in the kidneys and low tumor uptake. Intravenous administration did not improve the tumor uptake. The plausible explanation of low tumor uptake of ¹⁷⁷Lu-DO3A-4B4 could be its decreased ability to bind to melanin during in vitro binding studies in comparison with ¹⁸⁸Re-HYNIC-4B4, exacerbated by the very fast clearance from the blood and the kidneys "sink" effect.