“Click”-Cyclized 68Ga-Labeled Peptides for Molecular Imaging and Therapy: Synthesis and Preliminary In Vitro and In Vivo Evaluation in a Melanoma Model System
Cyclization techniques are used often to impart higher in vivo stability and binding affinity to peptide targeting vectors for molecular imaging and therapy. The two most often used techniques to impart these qualities are lactam bridge construction and disulfide bond formation. While these techniques have been demonstrated to be effective, orthogonal protection/deprotection steps can limit achievable product yields. In the work described in this chapter, new α-melanocyte stimulating hormone (α-MSH) peptide analogs were synthesized and cyclized by copper-catalyzed terminal azide-alkyne cycloaddition “click” chemistry techniques. The α-MSH peptide and its cognate receptor (melanocortin receptor subtype 1, MC1R) represent a well-characterized model system to examine the effect of the triazole linkage for peptide cyclization on receptor binding in vitro and in vivo. Four new DOTA-conjugated α-MSH analogs were cyclized and evaluated by in vitro competitive binding assays, serum stability testing, and in vivo imaging by positron emission tomography (PET) of tumor-bearing mice. These new DOTA-conjugated click-cyclized analogs exhibited selective high binding affinity (<2 nM) for MC1R on melanoma cells in vitro, high stability in human serum, and produced high-contrast PET/CT images of tumor xenografts. 68Ga-labeled DOTA bioconjugates displayed rapid pharmacokinetics with receptor-mediated tumor accumulation of up to 16 ± 5% ID/g. The results indicate that the triazole ring is an effective bioisosteric replacement for the standard lactam bridge assemblage for peptide cyclization. Radiolabeling results confirm that Cu catalyst is sufficiently removed prior to DOTA chelator addition to enable insertion of radio metals or stable metals for molecular imaging and therapy. Thus, these click-chemistry-cyclized variants show promise as agents for melanocortin receptor-targeted imaging and radionuclide therapy.
KeywordsMurine Melanoma Cell Competitive Binding Assay Semipreparative HPLC Serum Protease Rink Amide Resin
Support for this work was provided by the American Cancer Society (IRG-77004-31; M.K.S.), the Holden Comprehensive Cancer Center (M.E.M., M.K.S.), Neuroendocrine Tumor Fund (M.S.O.), and University of Iowa Dance Marathon (M.E.M.). M.E.M. is supported by T32 University of Iowa Institutional Training Grant in Hematologic and Oncologic Childhood Diseases (HL080070). The authors thank Dr. Kevin Rice, Dr. Lynn Teesch, and Vic Parcell for spirited assistance with mass spectral analyses.
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