Melanoma targeting with alpha-melanocyte stimulating hormone analogs labeled with fac-[Tc-99m(CO)(3)](+): effect of cyclization on tumor-seeking properties

• Autor(a) principal: Paula D Raposinho
• Data de Publicação: 2008
• Outros Autores: Catarina Xavier, Joao D G Correia, Soraia Falcao, Paula Gomes, Isabel Santos
• Tipo de documento: Artigo
• Idioma: eng
• Título da fonte: Repositório Científico de Acesso Aberto de Portugal (Repositórios Cientìficos)
• Texto Completo: https://hdl.handle.net/10216/82033
• topic: Química; Chemical sciences

Early detection of primary melanoma tumors is essential because there is no effective treatment for metastatic melanoma. Several linear and cyclic radiolabeled alpha-melanocyte stimulating hormone (alpha-MSH) analogs have been proposed to target the melanocortin type 1 receptor (MC1R) overexpressed in melanoma. The compact structure of a rhenium-cyclized alpha-MSH analog (Re-CCMSH) significantly enhanced its in vivo tumor uptake and retention. Melanotan II (MT-II), a cyclic lactam analog of alpha-MSH (Ac-Nle-cyclo[Asp-His-DPhe-Arg-Trp-Lys]-NH2]), is a very potent and stable agonist peptide largely used in the characterization of melanocortin receptors. Taking advantage of the superior biological features associated with the MT-II cyclic peptide, we assessed the effect of lactam-based cyclization on the tumor-seeking properties of alpha-MSH analogs by comparing the pharmacokinetics profile of the (99)mTc-labeled cyclic peptide beta Ala-Nle-cyclo[Asp-His-DPhe-Arg-Trp-Lys]-NH2 with that of the linear analog beta Ala-Nle-Asp-His-DPhe-Arg-Trp-Lys-NH2 in melanoma-bearing mice. We have synthesized and coupled the linear and cyclic peptides to a bifunctional chelator containing a pyrazolyl-diamine backbone (pz) through the amino group of beta Ala, and the resulting pz-peptide conjugates were reacted with the fac-[Tc-99m(CO)(3)](+) moiety. The Tc-99m(CO)(3)-labeled conjugates were obtained in high yield, high specific activity, and high radiochemical purity. The cyclic Tc-99m(CO)(3)-labeled conjugate presents a remarkable internalization (87.1% of receptor-bound tracer and 50.5% of total applied activity, after 6 h at 37 degrees C) and cellular retention (only 24.7% released from the cells after 5 h) in murine melanoma B16F1 cells. A significant tumor uptake and retention was obtained in melanoma-bearing C57BL6 mice for the cyclic radioconjugate [9.26 +/- 0.83 and 11.31 +/- 1.83% ID/g at 1 and 4 h after injection, respectively]. The linear Tc-99m(CO)(3)-pz-peptide presented lower values for both cellular internalization and tumor uptake. Receptor blocking studies with the potent (Nle(4),DPhe(7))-alpha MSH agonist demonstrated the specificity of the radioconjugates to MC1R (74.8 and 44.5% reduction of tumor uptake at 4 h after injection for cyclic and linear radioconjugates, respectively).