PET of malignant melanoma using 18F-labeled metallopeptides.
Melanocortin type 1 receptor (MC1R), also known as alpha-melanocyte-stimulating hormone (alpha-MSH) receptor, is an attractive molecular target for melanoma imaging and therapy. An (18)F-labeled linear alpha-MSH peptide ((18)F-FB-Ac-Nle-Asp-His-d-Phe-Arg-Trp-Gly-Lys-NH(2) [NAPamide]) shows promising melanoma imaging properties but with only moderate tumor uptake and retention. A transition metal rhenium-cyclized alpha-MSH peptide, ReO[Cys(3,4,10),d-Phe(7),Arg(11)]alpha-MSH(3-13) (ReCCMSH(Arg(11))), has shown high in vitro binding affinity to MC1R and excellent in vivo melanoma-targeting profiles when labeled with radiometals. Therefore, we hypothesized that ReCCMSH(Arg(11)) could be a good platform for the further development of an (18)F-labeled probe for PET of MC1R-positive malignant melanoma.


METHODS
In this study, the metallopeptide Ac-d-Lys-ReCCMSH(Arg(11)) was synthesized using conventional solid-phase peptide synthesis chemistry and a rhenium cyclization reaction. The resulting peptides were then labeled with N-succinimidyl-4-(18)F-fluorobenzoate ((18)F-SFB). The (18)F-labeled metallopeptides were further tested for their in vitro receptor binding affinities, in vivo biodistribution, and PET imaging properties.


RESULTS
Both isomers of Ac-d-Lys-ReCCMSH(Arg(11)), named as RMSH-1 and RMSH-2, were purified and identified by high-performance liquid chromatography. The binding affinities of RMSH-1 and RMSH-2 and their respective (19)F-SFB-conjugated peptides ((19)F-FB-RMSH-1 and (19)F-FB-RMSH-2) were all determined to be within nanomolar range. Both (18)F-labeled metallopeptides showed good tumor uptake in the B16F10 murine model, with high MC1R expression, but much lower uptake in the A375M human melanoma xenografted in mice, indicating low MC1R expression. (18)F-FB-RMSH-1, when compared with (18)F-FB-RMSH-2, displayed more favorable in vivo performance in terms of slightly higher tumor uptakes and much lower accumulations in the kidney and liver at 2 h after injection. Small-animal PET of (18)F-FB-RMSH-1 and -2 in mice bearing B16F10 tumors at 1 and 2 h showed good tumor imaging quality. As expected, much lower tumor uptakes and poorer tumor-to-normal organ contrasts were observed for the A375M model than for the B16F10 model. (18)F-FB-RMSH-1 and -2 showed higher tumor uptake and better tumor retention than did (18)F-FB-NAPamide.


CONCLUSIONS
Specific in vivo targeting of (18)F-FB-RMSH-1 to malignant melanoma was successfully achieved in preclinical models with high MC1R expression. Thus, the radiofluorinated metallopeptide (18)F-FB-RMSH-1 is a promising molecular probe for PET of MC1R-positive tumors.
Ren
Gang
G
Molecular Imaging Program at Stanford, Department of Radiology and Bio-X Program, Stanford University, Stanford, California 94305-5484, USA.
Liu
Zhe
Z
Miao
Zheng
Z
Liu
Hongguang
H
Subbarayan
Murugesan
M
Chin
Frederick T
FT
Zhang
Lan
L
Gambhir
Sanjiv S
SS
Cheng
Zhen
Z
D000595:Amino Acid Sequence; D000818:Animals; D001565:Benzoates / Q000737:chemistry; D015153:Blotting, Western; D045744:Cell Line, Tumor; D005462:Fluorine Radioisotopes / Q000737:chemistry*; D015972:Gene Expression Regulation, Neoplastic; D006801:Humans; D020128:Inhibitory Concentration 50; D007553:Isotope Labeling; D008297:Male; D008545:Melanoma / Q000000981:diagnostic imaging* / Q000235:genetics / Q000473:pathology*; D008670:Metals / Q000737:chemistry*; D051379:Mice; D010446:Peptide Fragments / Q000138:chemical synthesis / Q000737:chemistry* / Q000493:pharmacokinetics; D049268:Positron-Emission Tomography; D044102:Receptor, Melanocortin, Type 1 / Q000737:chemistry / Q000378:metabolism; D013388:Succinimides / Q000737:chemistry; D014018:Tissue Distribution
D001565:Benzoates; D005462:Fluorine Radioisotopes; D008670:Metals; D010446:Peptide Fragments; D044102:Receptor, Melanocortin, Type 1; D013388:Succinimides; C075713:N-succinimidyl-4-fluorobenzoate