The
alpha-melanocyte-stimulating hormone (
alpha-MSH)
receptor (melanocortin type 1 receptor, or MC1R) plays an important role in the development and growth of
melanoma cells. It was found that MC1R was overexpressed on most murine and human
melanoma, making it a promising molecular target for
melanoma imaging and
therapy. Radiolabeled
alpha-MSH peptide and its analogs that can specifically bind with MC1R have been extensively explored for developing novel agents for
melanoma detection and
radionuclide therapy. The goal of this study was to evaluate a 64Cu-labeled
alpha-MSH analog, Ac-Nle-Asp-His-D-Phe-Arg-Trp-Gly-Lys(DOTA)-NH2 (
DOTA-
NAPamide), as a potential
molecular probe for microPET imaging of
melanoma and MC1R expression in
melanoma xenografted mouse models.
1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (
DOTA) conjugated
NAPamide was synthesized and radiolabeled with 64Cu (t1/2=12 h) in NH4OAc (0.1 M; pH 5.5) buffered
solution for 60 min at 50 degrees C. Cell culture studies reveal rapid and high uptake and internalization of
64Cu-DOTA-NAPamide in B16F10 cells. Over 90% of receptor-bound tracer is internalized at 3 h incubation. A cellular retention study demonstrates that the receptor-bound
64Cu-DOTA-NAPamide is slowly released from the B16F10 cells into the medium; 66% of the radioactivity is still associated with the cells even after 3 h incubation. The biodistribution of
64Cu-DOTA-NAPamide was then investigated in C57BL/6 mice bearing subcutaneous murine B16F10
melanoma tumors with high capacity of MC1R and Fox Chase Scid mice bearing human A375M
melanoma with a relatively low number of MC1R receptors.
Tumor uptake values of
64Cu-DOTA-NAPamide are found to be 4.63 +/- 0.45% and 2.49 +/- 0.31% ID/g in B16F10 and A375M xenografted
melanoma at 2 h postinjection (pi), respectively. The B16F10
tumor uptake at 2 h pi is further inhibited to 2.29 +/- 0.24% ID/g, while A375M
tumor uptake at 2 h pi remains 2.20 +/- 0.41% ID/g with a coinjection of excess
alpha-MSH peptide. MicroPET imaging of
64Cu-DOTA-NAPamide in B16F10
tumor mice clearly shows good
tumor localization. However, low A375M
tumor uptake and poor
tumor to normal tissue contrast were observed. This study demonstrates that
64Cu-DOTA-NAPamide is a promising
molecular probe for
alpha-MSH receptor positive
melanoma PET imaging as well as MC1R expression imaging in living mice.