title: Melanoma targeting with alpha-melanocyte stimulating hormone analogs labeled with fac-[99mTc(CO)3]+: effect of cyclization on tumor-seeking properties.
abstract: 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 99mTc-labeled cyclic peptide betaAla-Nle-cyclo[Asp-His-D-Phe-Arg-Trp-Lys]-NH2 with that of the linear analog betaAla-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 betaAla, and the resulting pz-peptide conjugates were reacted with the fac-[99mTc(CO)3]+ moiety. The 99mTc(CO)3-labeled conjugates were obtained in high yield, high specific activity, and high radiochemical purity. The cyclic 99mTc(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 99mTc(CO)3-pz-peptide presented lower values for both cellular internalization and tumor uptake. Receptor blocking studies with the potent (Nle4,DPhe7)-alphaMSH 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).
authors.affiliation: Departamento de Química, ITN, Estrada Nacional 10, 2686-953, Sacavém, Portugal.
authors.forename: Paula D
authors.initials: PD
authors.lastname: Raposinho
authors.forename: Catarina
authors.initials: C
authors.lastname: Xavier
authors.forename: João D G
authors.initials: JD
authors.lastname: Correia
authors.forename: Soraia
authors.initials: S
authors.lastname: Falcão
authors.forename: Paula
authors.initials: P
authors.lastname: Gomes
authors.forename: Isabel
authors.initials: I
authors.lastname: Santos
mesh_terms: D000818:Animals; D045744:Cell Line, Tumor; D002851:Chromatography, High Pressure Liquid; D003500:Cyclization; D008546:Melanoma, Experimental / Q000473:pathology*; D051379:Mice; D015609:Organotechnetium Compounds / Q000737:chemistry / Q000493:pharmacokinetics / Q000494:pharmacology*; D014018:Tissue Distribution; D000521:alpha-MSH / Q000031:analogs & derivatives* / Q000737:chemistry / Q000493:pharmacokinetics / Q000494:pharmacology
chemical_list: D015609:Organotechnetium Compounds; D000521:alpha-MSH