DESIGN AND CHARACTERIZATION OF ALPHA-MELANOTROPIN PEPTIDE ANALOGS CYCLIZED THROUGH RHENIUM AND TECHNETIUM METAL COORDINATION

alpha-Melanocyte stimulating hormone (alpha-MSH) analogs, cyclized thr ough 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-bearin g mice. Results from receptor-binding assays conducted with B16 F1 mur ine melanoma cells indicated that receptor-blinding affinity was reduc ed to approximately 1% of its original levels after Re incorporation i nto the cyclic Cys(4,10), D-Phe(7)-alpha-MSH4-13 analog. Structural an alysis 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 t hat metal complexation dramatically altered the structure of the recep tor-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 R e-cu-MSH analog. Characterization of the second-generation Re-peptide complex indicated that the peptide was still cyclized through Re coord ination, but the structure of the receptor-binding sequence was no lon ger constrained, The corresponding Tc-99m. and (ReCCMSH)-Re-188 comple xes were synthesized and shown to be stable in phosphate-buffered sali ne and to challenges from diethylenetriaminepentaacetic acid (DTPA) an d free cysteine, In vivo, the (TcCCMSH)-Tc-99m complex exhibited signi ficant tumor uptake and retention and was effective in imaging melanom a in a murine-tumor model system. Cyclization of alpha-MSH analogs via Tc-99m and Re-188 yields chemically stable and biologically active mo lecules with potential melanoma-imaging and therapeutic properties.