A gallium-labeled DOTA-α-melanocyte-stimulating hormone analog for PET Imaging of melanoma metastases

Although F-18-FDG PET is widely used for metastatic melanoma diagnosis, it is less accurate than desirable, particularly for small foci. Since both melanotic and amelanotic melanomas overexpress receptors for a-melanocyte-stimulating hormone (alpha-MSH; receptor name, melanocortin type 1 receptor [MC1 R]), radiolabeled alpha-MSH analogs are potential candidates for melanoma diagnosis. The aim of this study was to develop a positron emitter-labeled a-MSH analog suitable for PET imaging of melanoma metastases. Methods: A short linear alpha-MSH analog, [NIe(4),Asp(5),D-Phe(7)]-alpha-MSH4-11 (NAPamide), was newly designed and conjugated to the metal chelator DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) to enable radiometal incorporation. Compared with our previously reported DOTA-alpha-MSH analog, DOTA-MSHoct ([DOTA-betaAla(3),NIe(4),Asp(5),DPhe(7),Lys(10)]-alpha-MSH3-10), the major modification lies in the conjugation of DOTA to the C-terminal end of the peptide via the E-amino group of Lys(11), as opposed to the N-terminal alpha-amino group. After labeling with In-111, Ga-67, and the short-lived positron emitter Ga-68, DOTA-NAPamide was characterized in vitro and in vivo using the mouse melanoma B16F1cell line. Results: DOTA-NAPamide exhibited an almost 7-fold higher MC1R binding potency as compared with DOTA-MSHoct. In B16F1 melanoma-bearing mice, both (111)in-DOTA-NAPamide and Ga-67-DOTA-NAPamide behaved more favorably than (111)InDOTA-MSHoct. Both radiopeptides exhibited higher tumor and lower kidney uptake leading to tumor-to-kidney ratios of the 4-to 48-h area under the curve that were 4.6 times (In-111) and 7.5 times (Ga-67) greater than that obtained with In-111-DOTA-MSHoct. In addition, the 4-h kidney uptake of Ga-67-DOTA-NAPamide could be reduced by 64% by coinjection of 15 Mg L-lysine, without affecting tumor uptake. Skin primary melanoma as well as lung and liver melanoma metastases could be easily visualized on tissue section autoradiographs after systemic injection of 67Ga-DOTA-NAPamide. The melanoma selectivity of DOTA-NAPamide was confirmed by PET imaging studies using (68)GaDOTA-NAPamide. Tumor uptake was found to be highest when the smallest amount of peptide was administered. Conclusion: DOTA-NAPamide labeled with either In-111 or Ga-67/Ga-68 is in every way superior to In-111-DOTA-MSHoct in murine models of