MSH receptors on mouse and human melanoma cells: receptor identification, analysis and quantification

Biomedical applications of radioiodinated peptides

The overexpression of peptide receptors in certain tumors as compared to endogeneous expression levels represents the molecular basis for the design of peptide-based tools for targeted nuclear imaging and therapy. Receptor targeting with radiolabelled peptides became a very important imaging and/or therapeutic approach in nuclear medicine and oncology. A great variety of peptides has been radiolabelled with clinical relevant radionuclides, such as radiometals and radiohalogens. However, to the best of our knowledge concise and updated reviews providing information about the biomedical application of radioiodinated peptides are still missing. This review outlines the synthetic efforts in the preparation of radioiodinated peptides highlighting the importance of radioiodine in nuclear medicine, giving an overview of the most relevant radioiodination strategies that have been employed and describes relevant examples of their use in the biomedical field.

Radiolabeled alpha-melanocyte-stimulating hormone analogs for receptor-mediated targeting of melanoma: from tritium to indium

Following the first synthesis of tritiated alpha-melanocyte-stimulating hormone (alpha-MSH, alpha-melanotropin) in 1974 by Medzihradszky et al., several alpha-MSH analogs were designed containing between 2 and 12 tritium atoms, the latter of which displayed a specific radioactivity of 12.21 GBq/micromol (330 Ci/mmol). Similarly, radioiodinated alpha-MSH analogs of high purity, full biological activity and a specific radioactivity of approximately 140 GBq/micromol were obtained. Although tritiated and radioiodinated alpha-MSH became indispensable tools as tracer molecules for numerous in vitro and in vivo studies, above all for receptor identification and characterization as well as for structure-activity studies, they did not fulfill the criteria required for therapeutic in vivo targeting of metastatic melanoma. Therefore, we recently developed alpha-MSH analogs containing the universal metal chelator 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) in different positions of the molecule. As DOTA can equally well incorporate diagnostic (e.g. (111)In, (67,68)Ga) and therapeutic (e.g. (90)Y, (67)Cu) radionuclides, DOTA-MSH compounds may serve for both melanoma scintigraphy and therapy. The analog DOTA-[betaAla(3), Nle(4), Asp(5), D-Phe(7), Lys(10)]-alpha-MSH(3-10) (DOTA-MSH(OCT)), which contains the metal chelator at its N-terminal end, displayed good in vitro MC1R affinity (IC(50) 9.21 nm). In vivo, [(111)In]DOTA-MSH(OCT) exhibited a favorable biodistribution profile after injection in B16-F1 tumor-bearing mice. The radiopeptide was rapidly cleared from blood through the kidneys and, most importantly, accumulated preferentially in the melanoma lesions. Lung and liver melanoma metastases could be clearly imaged on tissue section autoradiographs 4 h after injection of [(111)In]DOTA-MSH(OCT). A comparative study of [(111)In]DOTA-MSH(OCT) with [(111)In]DOTA-[Nle(4), D-Phe(7)]-alpha-MSH ([(111)In]DOTA-NDP-MSH) demonstrated the superiority of the DOTA-MSH(OCT) peptide, particularly with respect to the amount of radioactivity taken up by non-malignant organs, including bone, the most radiosensitive tissue. These results demonstrate that [(111)In]DOTA-MSH(OCT) specifically targets melanoma metastases and represents a lead compound for the development of therapeutic DOTA-MSH analogs.

DOTA alpha-melanocyte-stimulating hormone analogues for imaging metastatic melanoma lesions

Scintigraphic imaging of metastatic melanoma lesions requires highly tumor-specific radiopharmaceuticals. Because both melanotic and amelanotic melanomas overexpress melanocortin-1 receptors (MC1R), radiolabeled analogues of alpha-melanocyte-stimulating hormone (alpha-MSH) are potential candidates for melanoma diagnosis. Here, we report the in vivo performance of a newly designed octapeptide analogue, [betaAla(3), Nle(4), Asp(5), D-Phe(7), Lys(10)]-alpha-MSH(3-10) (MSH(OCT)), which was conjugated through its N-terminal amino group to the metal chelator 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) to enable incorporation of radiometals (e.g., indium-111) into the peptide. DOTA-MSH(OCT) displayed high in vitro MC1R affinity (IC(50) 9.21 nM). In vivo [(111)In]DOTA-MSH(OCT) exhibited a favorable biodistribution profile after injection in B16-F1 tumorbearing mice. The radiopeptide was rapidly cleared from blood through the kidneys and, most importantly, accumulated preferentially in the melanoma lesions. Lung and liver melanoma metastases could be clearly imaged on tissue section autoradiographs 4 h after injection of [(111)In]DOTA-MSH(OCT). A comparative study of [(111)In]DOTA-MSH(OCT) with [(111)In]DOTA-[Nle(4), D-Phe(7)]-alpha-MSH ([(111)In]-DOTA-NDP-MSH) demonstrated the superiority of the DOTA-MSH(OCT) peptide, particularly for the amount of radioactivity taken up by nonmalignant organs, including bone, the most radiosensitive tissue. These results demonstrate that [(111)In]DOTA-MSH(OCT) is a promising melanoma imaging agent.

Immunohistochemical evidence of alpha-, beta- and gamma 3-melanocyte stimulating hormone expression in cutaneous malignant melanoma of nodular type

Opiomelanocortins are formed after cleavage of the larger precursor molecule, proopiomelanocortin (POMC), which contains several peptide residues, sharing certain amino acid homology, including adrenocorticotrophic hormone (ACTH) and alpha-, beta- and gamma-melanocyte stimulating hormone (MSH). The expressions of alpha-, beta- and gamma 3-MSH in human cutaneous malignant melanoma of nodular type are demonstrated. For the MSHs, the immunolabelling was concentrated mostly in the tumour cellular cytoplasm, with occasional cells displaying a nuclear staining. Labelled tumour cells were dispersed throughout the epidermis and dermis as individual cells or in so-called 'pearl-like nests', most of which consisted mainly of round or oval shaped cells as well as a few pleomorphic or spindle-shaped cells. The fluorescence intensity seemed to increase in accordance with the development of the tumours. All cases examined were clearly stained with protein S-100, which provided us with a definite diagnosis. Considering the overall MSHs-related staining intensity of each section, the general perception we got was that the closer to the centre of the tumour parenchyma, the stronger was the staining and, furthermore the larger/more poorly differentiated the cells, the stronger was the staining. We also found the MSHs expressions to appear in the peripheral part of the tumour and the perilesional tissues including epidermis, sweat glands, sebaceous glands as well as hair follicles. Neurohypertrophic features were encountered including increases in both the number of nerve fibres and their diameter. Our results presented here strongly support the viewpoint earlier proposed that MSH peptides, by an autocrine and/or paracrine production from melanoma cells, are engaged in the regulation of melanogenesis, growth and proliferation of the tumour cells. We also conclude that, although alpha-, beta- and gamma 3-MSH peptides do not provide as high a sensitivity for diagnosis as protein S-100, they appear as useful markers for supportive diagnosis and assessment of malignant melanoma.

Melanocortins and their implication in melanoma

The melanocortins (MCs), that is, the melanocyte-stimulating hormones (MSHs) and ACTH, are a group of related peptides containing the typical melanotropin core sequence, His-Phe-Arg-Trp, and are derived from a common precursor, pro-opiomelanocortin. They are pleiotropic molecules that occur in the pituitary, some brain regions, and also in several peripheral tissues, and they exert a variety of physiologic functions. Their effect on melanogenesis in the skin is well established, but their role in melanocyte and melanoma cell proliferation and metastasis is less clear. The recent cloning of five types of MC receptors (MC1-5), new studies on the regulation of these receptors, the discovery of a naturally occurring MSH antagonist, the agouti protein, and the finding that melanocytes and melanoma cells exclusively express MC1 receptors have laid the basis for the future development of specific MC ligands, which may become useful for melanoma diagnosis and eventually therapy.

[111In]-DTPA-labeled analogues of alpha-melanocyte-stimulating hormone for melanoma targeting: receptor binding in vitro and in vivo

Six alpha-MSH(4-10) [Nle-Asp-His-D-Phe-Arg-Trp-Lys-amide] derivatives carrying 2 or 1 or no 2,3-dihydroxy-(2S)-propyl (DHP) groups on the Lys10 amino side chain were coupled to diethylene-triaminopentaacetic acid (DTPA, a chelator for 111In) in monomeric and dimeric forms and tested for their binding activity and bioactivity in vitro with mouse and human melanoma cell lines and by receptor autoradiography to tumor sections, as well as in vivo with normal and melanoma-bearing mice: DTPA-[Nle4,Asp5,D-Phe7,Lys(bis-DHP)10]-alpha-MSH(4-10),DTPA-[Nle4, Asp5, D-Phe7,Lys(mono-DHP)10]-alpha-MSH(4-10), DTPA[Nle4,Asp5,D-Phe7,Lys10]-alpha-MSH(4-10), DTPA-bis-([Nle4,Asp5,D-Phe7,Lys(bis-DHP)10]-alpha-MSH(4-10)), DTPA-bis[([Nle4,Asp5,D-Phe7,Lys(mono-DHP)10]-alpha-MSH(4-10)) and DTPA-bis-([Nle4,Asp5,D-Phe7,Lys10]-alpha-MSH(4-10)). In the receptor-binding assays with B16-F1 mouse and D10 human melanoma cells, the KD values ranged between 0.76 and 31.17 nM and in the melanin bioassay the results were similar (EC50 values between 0.15 and 4.40 nM). The tissue distribution of the 111In-labeled compounds in C57Bl/6J mice showed that the dimeric [111In]-DTPA-bis([Nle4,Asp5,D-Phe7,Lys10]-alpha-MSH(4-10)) and the monomeric [111In]-DTPA-[Nle4,Asp5,D-Phe7,Lys(bis-DHP)10]-alpha-MSH(4-10) exhibited the lowest non-specific binding. In mice carrying B16-F1 melanoma tumors, the monomeric compound displayed 2-fold higher 111In uptake by the tumor and a much lower non-specific uptake by the liver (12-fold) and the kidneys (2.5-fold) than the dimeric derivative. This demonstrates that modification of the Lys10 side chain by DHP is a promising lead for new MSH radiopharmaceuticals for melanoma targeting.