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, beta- or alpha-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 (Re-188)-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 Lu-177, Ho-166, and Sm-153 via a DO3A chelate. The stability studies of Ln*-DO3A-4B4 in phosphate-buffered saline, serum, and a hydroxyapatite assay demonstrated that Lu-177-labeled peptide was more stable than Ho-166- and Sm-153-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 Lu-177-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 Lu-177-DO3A- 4B4 could be its decreased ability to bind to melanin during in vitro binding studies in comparison with Re-188-HYNIC-4B4, exacerbated by the very fast clearance from the blood and the kidneys sink effect.