Small-animal PET of melanocortin 1 receptor expression using a 18F-labeled alpha-melanocyte-stimulating hormone analog

Abstract

(18)F-Labeled small synthetic peptides have emerged as attractive probes for imaging various molecular targets with PET. The alpha-melanocyte-stimulating hormone (alpha-MSH) receptor (melanocortin type 1 receptor [MC1R]) is overexpressed in most murine and human melanomas. It is a promising molecular target for diagnosis and therapy of melanomas. However, (18)F compounds have not been successfully developed for imaging the MC1R.

Methods: In this study, an alpha-MSH analog, Ac-Nle-Asp-His-D-Phe-Arg-Trp-Gly-Lys-NH(2) (NAPamide), was radiolabeled with N-succinimidyl-4-(18)F-fluorobenzoate ((18)F-SFB). The resulting radiopeptide was evaluated as a potential molecular probe for small-animal PET of melanoma and MC1R expression in melanoma xenografted mouse models.

Results: The binding affinity of (19)F-SFB-conjugated NAPamide, (19)F-FB-NAPamide, was determined to be 7.2 +/- 1.2 nM (mean +/- SD) using B16/F10 cells and (125)I-(Tyr(2))-[Nle(4),D-Phe(7)]-alpha-MSH [(125)I-(Tyr(2))-NDP] as a radioligand. The biodistribution of (18)F-FB-NAPamide was then investigated in C57BL/6 mice bearing subcutaneous murine B16/F10 melanoma tumors with high expression of MC1Rs and Fox Chase Scid mice bearing human A375M melanoma with a relatively low number of MC1R receptors. Biodistribution experiments showed that tumor uptake values (percentage injected dose per gram of tumor [%ID/g]) of (18)F-FB-NAPamide were 1.19 +/- 0.11 %ID/g and 0.46 +/- 0.11 %ID/g, in B16/F10 and A375M xenografted melanoma at 1 h after injection, respectively. Furthermore, the B16/F10 tumor uptake was significantly inhibited by coinjection with excess alpha-MSH peptide (P < 0.05), indicating that (18)F-FB-NAPamide specifically recognizes the MC1R in living mice. Small-animal PET of (18)F-FB-NAPamide in mice bearing B16/F10 and A375M tumors at 1 h after tail vein injection revealed good B16/F10 tumor-to-background contrast and low A375M tumor-to-background ratios.

Conclusion: (18)F-FB-NAPamide is a promising molecular probe for alpha-MSH receptor-positive melanoma PET and warrants further study.

FIGURE 1

Schematic structure of ^18/19F-FB-NAPamide.

FIGURE 2

HPLC radiochromatogram of purified ¹⁸F-FB-NAPamide.

FIGURE 3

In vitro inhibition of ¹²⁵I-(Tyr²)-NDP binding to MC1R on murine B16/F10 melanoma cell line by FB-NAPamide (IC₅₀ = 7.2 ± 1.2 nM). Results expressed as percentage of binding are mean ± SD of triplicate measurements.

FIGURE 4

Decay-corrected coronal (top) and transaxial (bottom) microPET images of C57BL/6 mouse bearing B16/ F10 tumor (left and middle images) and Fox Chase Scid mouse bearing A375M tumor (right images). Images were acquired 1 h after tail vein injection of ¹⁸F-FB-NAPamide (3.7 MBq [100 μCi]) with or without coinjection of 200 μg NDP peptide. Arrows indicate location of tumors.