title: Design and characterization of alpha-melanotropin peptide analogs cyclized through rhenium and technetium metal coordination.
abstract: alpha-Melanocyte stimulating hormone (alpha-MSH) analogs, cyclized through site-specific rhenium (Re) and technetium (Tc) metal coordination, were structurally characterized and analyzed for their abilities to bind alpha-MSH receptors present on melanoma cells and in tumor-bearing mice. Results from receptor-binding assays conducted with B16 F1 murine melanoma cells indicated that receptor-binding affinity was reduced to approximately 1% of its original levels after Re incorporation into the cyclic Cys4,10, D-Phe7-alpha-MSH4-13 analog. Structural analysis of the Re-peptide complex showed that the disulfide bond of the original peptide was replaced by thiolate-metal-thiolate cyclization. A comparison of the metal-bound and metal-free structures indicated that metal complexation dramatically altered the structure of the receptor-binding core sequence. Redesign of the metal binding site resulted in a second-generation Re-peptide complex (ReCCMSH) that displayed a receptor-binding affinity of 2.9 nM, 25-fold higher than the initial Re-alpha-MSH analog. Characterization of the second-generation Re-peptide complex indicated that the peptide was still cyclized through Re coordination, but the structure of the receptor-binding sequence was no longer constrained. The corresponding 99mTc- and 188ReCCMSH complexes were synthesized and shown to be stable in phosphate-buffered saline and to challenges from diethylenetriaminepentaacetic acid (DTPA) and free cysteine. In vivo, the 99mTcCCMSH complex exhibited significant tumor uptake and retention and was effective in imaging melanoma in a murine-tumor model system. Cyclization of alpha-MSH analogs via 99mTc and 188Re yields chemically stable and biologically active molecules with potential melanoma-imaging and therapeutic properties.
authors.affiliation: Department of Biochemistry, University of Missouri, Columbia, MO 65211, USA.
authors.forename: M F
authors.initials: MF
authors.lastname: Giblin
authors.forename: N
authors.initials: N
authors.lastname: Wang
authors.forename: T J
authors.initials: TJ
authors.lastname: Hoffman
authors.forename: S S
authors.initials: SS
authors.lastname: Jurisson
authors.forename: T P
authors.initials: TP
authors.lastname: Quinn
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D001665:Binding Sites; D015195:Drug Design; D008546:Melanoma, Experimental / Q000000981:diagnostic imaging / Q000378:metabolism*; D051379:Mice; D008810:Mice, Inbred C57BL; D008958:Models, Molecular; D008969:Molecular Sequence Data; D019906:Nuclear Magnetic Resonance, Biomolecular; D010456:Peptides, Cyclic / Q000138:chemical synthesis / Q000737:chemistry / Q000493:pharmacokinetics*; D011487:Protein Conformation; D011877:Radionuclide Imaging; D011979:Receptors, Pituitary Hormone / Q000378:metabolism*; D012211:Rhenium / Q000737:chemistry / Q000493:pharmacokinetics*; D013667:Technetium / Q000737:chemistry / Q000493:pharmacokinetics*; D014018:Tissue Distribution; D000521:alpha-MSH / Q000031:analogs & derivatives* / Q000138:chemical synthesis / Q000737:chemistry / Q000493:pharmacokinetics*
chemical_list: D010456:Peptides, Cyclic; D011979:Receptors, Pituitary Hormone; D000521:alpha-MSH; D012211:Rhenium; D013667:Technetium; C029446:MSH receptor