title: Novel, cysteine-modified chelation strategy for the incorporation of [M(I)(CO)(3)](+) (M = Re, (99m)Tc) in an α-MSH peptide.
abstract: Engineering peptide-based targeting agents with residues for site-specific and stable complexation of radionuclides is a highly desirable strategy for producing diagnostic and therapeutic agents for cancer and other diseases. In this report, a model N-S-N(Py) ligand (3) and a cysteine-derived α-melanocyte stimulating hormone (α-MSH) peptide (6) were used as novel demonstrations of a widely applicable chelation strategy for incorporation of the [M(I)(CO)(3)](+) (M = Re, (99m)Tc) core into peptide-based molecules for radiopharmaceutical applications. The structural details of the core ligand-metal complexes as model systems were demonstrated by full chemical characterization of fac-[Re(I)(CO)(3)(N,S,N(Py)-3)](+) (4) and comparative high-performance liquid chromatography (HPLC) analysis between 4 and [(99m)Tc(I)(CO)(3)(N,S,N(Py)-3)](+) (4a). The α-MSH analogue bearing the N-S-N(Py) chelate on a modified cysteine residue (6) was generated and complexed with [M(I)(CO)(3)](+) to confirm the chelation strategy's utility when applied in a peptide-based targeting agent. Characterization of the Re(I)(CO)(3)-6 peptide conjugate (7) confirmed the efficient incorporation of the metal center, and the (99m)Tc(I)(CO)(3)-6 analogue (7a) was explored as a potential single photon emission computed tomography (SPECT) compound for imaging the melanocortin 1 receptor (MC1R) in melanoma. Peptide 7a showed excellent radiolabeling yields and in vitro stability during amino acid challenge and serum stability assays. In vitro B16F10 melanoma cell uptake of 7a reached a modest value of 2.3 ± 0.08% of applied activity at 2 h at 37 °C, while this uptake was significantly reduced by coincubation with a nonlabeled α-MSH analogue, NAPamide (3.2 μM) (P < 0.05). In vivo SPECT/X-ray computed tomography (SPECT/CT) imaging and biodistribution of 7a were evaluated in a B16F10 melanoma xenografted mouse model. SPECT/CT imaging clearly visualized the tumor at 1 h post injection (p.i.) with high tumor-to-background contrast. Blocking studies with coinjected NAPamide (10 mg per kg of mouse body weight) confirmed the in vivo specificity of 7a for MC1R-positive tumors. Biodistribution results with 7a yielded a moderate tumor uptake of 1.20 ± 0.09 percentage of the injected radioactive dose per gram of tissue (% ID/g) at 1 h p.i. Relatively high uptake of 7a was also seen in the kidneys and liver at 1 h p.i. (6.55 ± 0.36% ID/g and 4.44 ± 0.17% ID/g, respectively), although reduced kidney uptake was seen at 4 h p.i. (3.20 ± 0.48% ID/g). These results demonstrate the utility of the novel [M(I)(CO)(3)](+) chelation strategy when applied in a targeting peptide.
authors.affiliation: Department of Nuclear Medicine, Second Affiliated Hospital of Zhejiang University School of Medicine, Institute of Nuclear Medicine and Molecular Imaging of Zhejiang University, Center of Excellence in Medical Molecular Imaging of Zhejiang State, Hangzhou, 310009, China.
authors.forename: Han
authors.initials: H
authors.lastname: Jiang
authors.forename: Benjamin B
authors.initials: BB
authors.lastname: Kasten
authors.forename: Hongguang
authors.initials: H
authors.lastname: Liu
authors.forename: Shibo
authors.initials: S
authors.lastname: Qi
authors.forename: Yang
authors.initials: Y
authors.lastname: Liu
authors.forename: Mei
authors.initials: M
authors.lastname: Tian
authors.forename: Charles L
authors.initials: CL
authors.lastname: Barnes
authors.forename: Hong
authors.initials: H
authors.lastname: Zhang
authors.forename: Zhen
authors.initials: Z
authors.lastname: Cheng
authors.forename: Paul D
authors.initials: PD
authors.lastname: Benny
mesh_terms: D000818:Animals; D002614:Chelating Agents / Q000737:chemistry / Q000493:pharmacokinetics*; D003545:Cysteine / Q000737:chemistry*; D008546:Melanoma, Experimental / Q000175:diagnosis*; D051379:Mice; D008810:Mice, Inbred C57BL; D008958:Models, Molecular; D015394:Molecular Structure; D009942:Organometallic Compounds / Q000737:chemistry / Q000493:pharmacokinetics*; D010455:Peptides / Q000737:chemistry / Q000493:pharmacokinetics*; D019275:Radiopharmaceuticals / Q000737:chemistry / Q000493:pharmacokinetics*; D012211:Rhenium / Q000737:chemistry; D013667:Technetium / Q000737:chemistry; D014018:Tissue Distribution; D015899:Tomography, Emission-Computed, Single-Photon; D014057:Tomography, X-Ray Computed; D014407:Tumor Cells, Cultured; D000521:alpha-MSH / Q000737:chemistry / Q000493:pharmacokinetics*
chemical_list: D002614:Chelating Agents; D009942:Organometallic Compounds; D010455:Peptides; D019275:Radiopharmaceuticals; D000521:alpha-MSH; D012211:Rhenium; D013667:Technetium; D003545:Cysteine