Human melanoma imaging with 68Ga- and 18F-labeled alpha-melanocyte-stimulating hormone derivatives using positron emission tomography (2018)

609 Objectives: Melanoma imaging targeting the melanocortin 1 receptor (MC1R) has been extensively studied using mouse B16F1 or B16F10 melanoma models, due to high receptor density. Imaging with human melanoma models may be challenging due to 5-10 times lower reported MC1R expression. Previously, we successfully developed 68Ga- and 18F-labeled alpha-melanocyte-stimulating hormone (αMSH) derivatives and showed excellent PET imaging and biodistribution results using the mouse B16F10 melanoma model. We aim to further characterize these radioligands in a human melanoma model with positron emission tomography (PET). Methods: The expression level of MC1R on SK-MEL-1 cells was evaluated using Western blotting. DOTA-Pip-Nle-CycMSHhex (CCZ01048) and BF3-Pip-Nle-CycMSHhex (CCZ01064) were prepared via Fmoc chemistry on solid phase and HPLC purification. CCZ01048 and CCZ01064 were radiolabeled with 68Ga and 18F, respectively. PET imaging and biodistribution were performed using NODSCID mice bearing human SK-MEL-1 tumor. The biodistribution of 68Ga-CCZ01048 (approximately 6 pmol) was further evaluated with co-injection of 20, 50 and 100 pmol of non-radioactive Ga-CCZ01048. Results: Western blotting showed high expression of human MC1R in the SK-MEL-1 cell line. High radiochemical purity (> 95%) was achieved for both 68Ga-CCZ01048 and 18F-CCZ01064. High specific activity (> 270 MBq/nmol) was obtained with 68Ga-CCZ01048, which was separated from the cold precursor using HPLC purification. A lower specific activity (> 60 MBq/nmol) was achieved for 18F-CCZ01064. PET imaging showed clear tumor visualization and tumor-to-background contrast for both radioligands at 2 h post-injection (p.i.), and tumor uptake was abolished with co-injection of excess amount of non-radioactive Ga-CCZ01048 or CCZ01064 (> 500 pmol, blocking). Biodistribution data showed consistent results, for 68Ga-CCZ01048, tumor uptake was 6.42 ± 0.63 and 6.59 ± 1.31 percent injected dose per gram of tissue (%ID/g) at 1 h and 2 h p.i. (n = 4), respectively. Minimal background tissue activity accumulation was observed, except for kidneys (5.75 ± 1.21 %ID/g) and thyroid (4.22 ± 1.52 %ID/g) at 2 h p.i. With co-injection of excess amount of non-radioactive Ga-CCZ01048, tumor uptake significantly decreased to 0.89 ± 0.34 %ID/g. For 18F-CCZ01064, lower tumor uptake was observed compared to the 68Ga-labeled radioligand, i.e. 2.71 ± 0.55 and 3.05 ± 0.47 %ID/g at 1 h and 2 h p.i. (n = 5), respectively. Low background tissue activity accumulation was observed, except for kidneys (5.34 ± 1.38 %ID/g), thyroid (1.82 ± 0.33 %ID/g) and intestine (2.29 ± 1.24 %ID/g) at 2 h p.i. Tumor uptake also significantly decreased (0.27 ± 0.02 %ID/g) in the blocking study. The difference in tumor uptake was likely due to the specific activity of the two radioligands, suggesting receptor saturation in vivo. When co-injected with 20, 50 and 100 pmol of non-radioactive Ga-CCZ01048 to approximately 6 pmol of 68Ga-CCZ01048, tumor uptake significantly decreased to 2.66 ± 0.22, 1.31 ± 0.13 and 1.04 ± 0.08 %ID/g at 1 h p.i. (n = 4), respectively. This confirmed that the MC1R was easily saturated and high specific activity was necessary to achieve high tumor uptake. Conclusion: The SK-MEL-1 cell line expresses the MC1R receptor and is a good human model for MC1R targeted imaging and therapy. Clear tumor visualization and good image contrast was achieved with both 68Ga-CCZ01048 and 18F-CCZ01064 using PET imaging. Due to relatively low expression of MC1R in human melanoma, low mass amount of injected radiotracer was required for high tumor uptake.