Design and characterization of α-melanotropin peptide analogs cyclized through rhenium and technetium metal coordination

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-blinding affinity was reduced to approximately 1% of its original levels after Re incorporation into the cyclic Cys(4,10), D-Phe(7)-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-cu-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 Tc-99m. and (ReCCMSH)-Re-188 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 (TcCCMSH)-Tc-99m 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 Tc-99m and Re-188 yields chemically stable and biologically active molecules with potential melanoma-imaging and therapeutic properties.