Preclinical Melanoma Imaging with 68Ga-Labeled α-Melanocyte-Stimulating Hormone Derivatives Using PET

It is estimated that melanoma accounted for 76,380 new cases and 10,130 deaths in the United States in 2016.The melanocortin 1 receptor (MC1R) is highly expressed in the vast majority of melanomas, which makes it an attractive target for molecular imaging and radionuclide therapy.Lactam bridge-cyclized α-melanocyte-stimulating hormone (Ac-Nle 4 -cyclo[Asp 5 -His-D-Phe 7 -Arg-Trp-Lys 10 ]-NH2, or Nle-CycMSHhex) analogues have been successfully developed and studied for MC1R-targeted imaging, predominantly with single-photon emission computed tomography (SPECT).The goal of this study was to design and evaluate novel peptides for melanoma imaging with positron emission tomography (PET).We designed and synthesized three peptides, DOTA-PEG2-Nle-CycMSHhex (CCZ01047), DOTA-4-amino-(1-carboxymethyl) piperidine (Pip)-Nle-CycMSHhex (CCZ01048), and DOTA-Pip-Pip-Nle-CycMSHhex (CCZ01056).All three peptides exhibited high binding affinity to MC1R with sub-nanomolar Ki values, rapid internalization into B16F10 melanoma cells and high in vivo stability with more than 93% remaining intact at 15 min post-injection (p.i.) in blood plasma.All three 68 Ga-labeled tracers produced high contrast PET images in C57BL/6J mice bearing B16F10 tumors, and their respective tumor uptakes were 8.0 ± 3.0, 12.3 ± 3.3, and 6.5 ± 1.4 %ID/g at 1 h p.i. Minimal normal organ activity was observed at 1 h p.i., except for kidneys (5.1 ± 1.4, 4.7 ± 0.5, and 6.2 ± 2.0 %ID/g, respectively), and thyroid (4.1 ± 0.6 %ID/g for CCZ01047 and 2.4 ± 0.6 %ID/g for CCZ01048).Due to high accumulation at tumor sites and rapid background clearance of 68 Ga-CCZ01048, we further evaluated it at 2 h p.i., and a tumor uptake of 21.9 ± 4.6 %ID/g was observed, with background activity further decreased.Exceptional image contrast was also achieved, i.e. tumor-to-blood, tumor-to-muscle, tumor-to-bone and tumor-to-kidney ratios were 96.4 ± 13.9, 210.9 ± 20.9, 39.6 ± 11.9 and 4.0 ± 0.9, respectively.A blocking study was also performed by co-injection of excess amount of non-radioactive Ga-coupled of CCZ01048, which confirmed that the tumor uptake was MC1R mediated.In conclusion, the introduction of a cationic Pip linker to Nle-CycMSHhex, CCZ01048, not only improved tumor uptake, but also generated high tumor-to-normal tissue contrast with PET imaging in a preclinical melanoma model.Therefore, CCZ01048 is a promising candidate for PET imaging of melanoma, and potentially as a theranostic agent for radionuclide therapy of melanoma when labeled with α or β emitters.