Biologically active monoiodinated alpha-MSH derivatives for receptor binding studies using human melanoma cells

Radiochemical and radiopharmacological characterization of a 64 Cu-labeled α-MSH analog conjugated with different chelators

Radiolabeled α-melanocyte-stimulating hormone (α-MSH) derivatives have a high potential for diagnosis and treatment of melanoma, because of high specificity and binding affinity to the melanocortin-1 receptor (MC1R). Hence, the α-MSH-derived peptide NAP-NS1 with a β-Ala linker (ε-Ahx-β-Ala-Nle-Asp-His-D-Phe-Arg-Trp-Gly-NH₂ ) was conjugated to different chelators: either to NOTA (p-SCN-Bn-1,4,7-triazacyclononane-1,4,7-triacetic acid), to a hexadentate bispidine carbonate derivative (dimethyl-9-(((4-nitrophenoxy)carbonyl)oxy)-2,4-di(pyridin-2-yl)-3,7-bis(pyridin-2-ylmethyl)-3,7-diazabicyclo[3.3.1]nonane-1,5-dicarboxylate), or to DMPTACN (p-SCN-Ph-bis(2-pyridyl-methyl)-1,4,7-triaza-cyclononane), labeled with ⁶⁴ Cu, and investigated in terms of radiochemical and radiopharmacological properties. For the three ⁶⁴ Cu-labeled conjugates negligible transchelation, suitable buffer and serum stability, as well as appropriate water solubility, was determined. The three conjugates exhibited high binding affinity (low nanomolar range) in murine B16F10, human MeWo, and human TXM13 cells. The B_max values of [⁶⁴ Cu]Cu-bispidine-NAP-NS1 ([⁶⁴ Cu]Cu-2) and [⁶⁴ Cu]Cu-DMPTACN-NAP-NS1 ([⁶⁴ Cu]Cu-3) were higher than those of [⁶⁴ Cu]Cu-NOTA-NAP-NS1 ([⁶⁴ Cu]Cu-1), implying that different charged chelate units might have an impact on binding capacity. Preliminary in vivo biodistribution studies suggested the main excretion pathway of [⁶⁴ Cu]Cu-1 and [⁶⁴ Cu]Cu-3 to be renal, while that of [⁶⁴ Cu]Cu-2 seemed to be both renal and hepatobiliary. An initial moderate uptake in the kidney decreased clearly after 60 minutes. All three ⁶⁴ Cu-labeled conjugates should be considered for further in vivo investigations using a suitable xenograft mouse model.

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.

Synthesis, Characterization, and Initial Biological Evaluation of [99m Tc]Tc-Tricarbonyl-labeled DPA-α-MSH Peptide Derivatives for Potential Melanoma Imaging

α-Melanocyte stimulating hormone (α-MSH) derivatives target the melanocortin-1 receptor (MC1R) specifically and selectively. In this study, the α-MSH-derived peptide NAP-NS1 (Nle-Asp-His-d-Phe-Arg-Trp-Gly-NH₂ ) with and without linkers was conjugated with 5-(bis(pyridin-2-ylmethyl)amino)pentanoic acid (DPA-COOH) and labeled with [^99m Tc]Tc-tricarbonyl by two methods. With the one-pot method the labeling was faster than with the two-pot method, while obtaining similarly high yields. Negligible trans-chelation and high stability in physiological solutions was determined for the [^99m Tc]Tc-tricarbonyl-peptide conjugates. Coupling an ethylene glycol (EG)-based linker increased the hydrophilicity. The peptide derivatives displayed high binding affinity in murine B16F10 melanoma cells as well as in human MeWo and TXM13 melanoma cell homogenates. Preliminary in vivo studies with one of the [^99m Tc]Tc-tricarbonyl-peptide conjugates showed good stability in blood and both renal and hepatobiliary excretion. Biodistribution was performed on healthy rats to gain initial insight into the potential relevance of the ^99m Tc-labeled peptides for in vivo imaging.

Radiopharmacological characterization of ⁶⁴Cu-labeled α-MSH analogs for potential use in imaging of malignant melanoma

The melanocortin-1 receptor (MC1R) plays an important role in melanoma growth, angiogenesis and metastasis, and is overexpressed in melanoma cells. α-Melanocyte stimulating hormone (α-MSH) and derivatives are known to bind with high affinity at this receptor that provides the potential for selective targeting of melanoma. In this study, one linear α-MSH-derived peptide Nle-Asp-His-D-Phe-Arg-Trp-Gly-NH2 (NAP-NS1) without linker and with εAhx-β-Ala linker, and a cyclic α-MSH derivative, [Lys-Glu-His-D-Phe-Arg-Trp-Glu]-Arg-Pro-Val-NH2 (NAP-NS2) with εAhx-β-Ala linker were conjugated with p-SCN-Bn-NOTA and labeled with (64)Cu. Radiochemical and radiopharmacological investigations were performed with regard to transchelation, stability, lipophilicity and in vitro binding assays as well as biodistribution in healthy rats. No transchelation reactions, but high metabolic stability and water solubility were demonstrated. The linear derivatives showed higher affinity than the cyclic one. [(64)Cu]Cu-NOTA-εAhx-β-Ala-NAP-NS1 ([(64)Cu]Cu-2) displayed rapid cellular association and dissociation in murine B16F10 cell homogenate. All [(64)Cu]Cu-labeled conjugates exhibited affinities in the low nanomolar range in B16F10. [(64)Cu]Cu-2 showed also high affinity in human MeWo and TXM13 cell homogenate. In vivo studies suggested that [(64)Cu]Cu-2 was stable, with about 85 % of intact peptide in rat plasma at 2 h p.i. Biodistribution confirmed the renal pathway as the major elimination route. The uptake of [(64)Cu]Cu-2 in the kidney was 5.9 % ID/g at 5 min p.i. and decreased to 2.0 % ID/g at 60 min p.i. Due to the prospective radiochemical and radiopharmacological properties of the linear α-MSH derivative [(64)Cu]Cu-2, this conjugate is a promising candidate for tracer development in human melanoma imaging.

"Click"-cyclized (68)Ga-labeled peptides for molecular imaging and therapy: synthesis and preliminary in vitro and in vivo evaluation in a melanoma model system

Cyclization techniques are used often to impart higher in vivo stability and binding affinity to peptide targeting vectors for molecular imaging and therapy. The two most often used techniques to impart these qualities are lactam bridge construction and disulfide bond formation. While these techniques have been demonstrated to be effective, orthogonal protection/deprotection steps can limit achievable product yields. In the work described in this chapter, new α-melanocyte stimulating hormone (α-MSH) peptide analogs were synthesized and cyclized by copper-catalyzed terminal azide-alkyne cycloaddition "click" chemistry techniques. The α-MSH peptide and its cognate receptor (melanocortin receptor subtype 1, MC1R) represent a well-characterized model system to examine the effect of the triazole linkage for peptide cyclization on receptor binding in vitro and in vivo. Four new DOTA-conjugated α-MSH analogs were cyclized and evaluated by in vitro competitive binding assays, serum stability testing, and in vivo imaging by positron emission tomography (PET) of tumor-bearing mice. These new DOTA-conjugated click-cyclized analogs exhibited selective high binding affinity (<2 nM) for MC1R on melanoma cells in vitro, high stability in human serum, and produced high-contrast PET/CT images of tumor xenografts. (68)Ga-labeled DOTA bioconjugates displayed rapid pharmacokinetics with receptor-mediated tumor accumulation of up to 16 ± 5% ID/g. The results indicate that the triazole ring is an effective bioisosteric replacement for the standard lactam bridge assemblage for peptide cyclization. Radiolabeling results confirm that Cu catalyst is sufficiently removed prior to DOTA chelator addition to enable insertion of radio metals or stable metals for molecular imaging and therapy. Thus, these click-chemistry-cyclized variants show promise as agents for melanocortin receptor-targeted imaging and radionuclide therapy.

Improved synthesis and biological evaluation of chelator-modified α-MSH analogs prepared by copper-free click chemistry

Radionuclide chelators (DOTA, NOTA) functionalized with a monofluorocyclooctyne group were prepared. These materials reacted rapidly and in high yield with a fully deprotected azide-modified peptide via Cu-free click chemistry under mild reaction conditions (aqueous solution, room temperature). The resulting bioconjugates bind with high affinity and specificity to their cell-surface receptor targets in vitro and appear stable to degradation in mouse serum over 3h of incubation at 37°C.

Peptide-targeted radionuclide therapy for melanoma

Melanocortin-1 receptor (MC1-R) and melanin are two attractive melanoma-specific targets for peptide-targeted radionuclide therapy for melanoma. Radiolabeled peptides targeting MC1-R/melanin can selectively and specifically target cytotoxic radiation generated from therapeutic radionuclides to melanoma cells for cell killing, while sparing the normal tissues and organs. This review highlights the recent advances of peptide-targeted radionuclide therapy of melanoma targeting MC1-R and melanin. The promising therapeutic efficacies of 188Re-(Arg(11))CCMSH (188Re-[Cys(3,4,10), D-Phe(7),Arg(11)]-alpha-MSH(3-13)), 177Lu- and 212Pb-labeled DOTA-Re(Arg(11))CCMSH (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid-[ReO-(Cys(3,4,10), D-Phe(7), Arg(11))]-alpha-MSH(3-13)) and 188Re-HYNIC-4B4 (188Re-hydrazinonicotinamide-Tyr-Glu-Arg-Lys-Phe-Trp-His-Gly-Arg-His) in preclinical melanoma-bearing models demonstrate an optimistic outlook for peptide-targeted radionuclide therapy for melanoma. Peptide-targeted radionuclide therapy for melanoma will likely contribute in an adjuvant setting, once the primary tumor has been surgically removed, to treat metastatic deposits and for treatment of end-stage disease. The lack of effective treatments for metastatic melanoma and end-stage disease underscores the necessity to develop and implement new treatment strategies, such as peptide-targeted radionuclide therapy.

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.

Antagonist and agonist activities of the mouse agouti protein fragment (91-131) at the melanocortin-1 receptor

Antagonist and agonist activities of chemically synthetized mouse agouti protein fragment (91-131) (AP91-131) at the melanocortin type-1 receptor (MC1-R) were assessed using B 16-F1 mouse melanoma cells in vitro and the following assay systems: (i) receptor binding, (ii) adenylate cyclase, (iii) tyrosinase, (iv) melanin production, and (v) cell proliferation. In competition binding studies AP91-131 was about 3-fold less potent than the natural agonist alpha-melanocyte-stimulating hormone (alpha-MSH) in displacing the radioligand [125I]-[Nle4, D-Phe7]-alpha-MSH (Ki 6.5 +/- 0.8 nmol/l). Alpha-MSH-induced tyrosinase activation and melanin production were completely inhibited by a 100-fold higher concentration of AP9 l -131; the IC50 values for AP91-131 in thetwo assay systems were 91 +/- 22 nM and 95 +/- 15 nM respectively. Basal melanin production and adenylate cyclase activity in the absence of agonist were decreased by AP91-131 with IC50 values of 9.6+/-1.8 nM and 5.0+/-2.4 nM, respectively. This indicates inverse agonist activity of AP91-131 similar to that of native AP. The presence of 10 nM melanin-concentrating hormone (MCH) slightly potentiated the inhibitory activity of AP91-131 in the adenylate cyclase and melanin assays. On the other hand, AP91-131 inhibited cell growth similar to alpha-MSH (IC50 11.0 +/- 2.1 nM; maximal inhibition 1.8-fold higher than that of alpha-MSH). Furthermore, MC1-R was down-regulated by AP91-131 with about the same potency and time-course as with alpha-MSH. These results demonstrate that AP91-131 displays both agonist and antagonist activities at the MC1-R and hence that it is the cysteine-rich region of agouti protein which inhibits and mimics the different alpha-MSH functions, most likely by simultaneous modulation of different intracellular signalling pathways.

Interaction of melanin-concentrating hormone (MCH), neuropeptide E-I (NEI), neuropeptide G-E (NGE), and alpha-MSH with melanocortin and MCH receptors on mouse B16 melanoma cells

Melanin-concentrating hormone (MCH) and alpha-melanocyte-stimulating hormone (alpha-MSH) are known to exhibit mostly functionally antagonistic, but in some cases agonistic activities, e.g., in pigment cells and in the brain. Neuropeptide E-I (NEI) displays functional MCH-antagonist and MSH-agonist activity in different behavioral paradigms; the role of neuropeptide G-E (NGE) is not known. This study addressed the question of possible molecular interactions between alpha-MSH, MCH and the MCH-precursor-derived peptides NEI and NGE at the level of the pigment cell MCH receptor subtype (MCH-Rpc) and the different melanocortin (MC) receptors. Radioreceptor assays using [125I]MCH, [125l]alpha-MSH and [125I]NEI as radioligands and bioassays were performed with MCI-R-positive and MC1-R-negative mouse B16 melanoma cells and with COS cells expressing the different MC receptors. The IC50s of alpha-MSH and NEI or NGE for [125I]MCH displacement from mouse MCH-Rpc were 80-fold and, respectively, >300-fold higher than that of MCH, and the IC50s for MCH and NEI or NGE for [125I]alpha-MSH displacement from mouse MC1-R were 50,000-fold and >200,000-fold higher than that of alpha-MSH. No high-affinity binding sites for NEI were detected on B16 melanoma cells and there was no significant displacement of [1251]alpha-MSH by MCH, NEI or NGE with MC3-R, MC4-R and MC5-R expressed in COS cells. At concentrations of 100 nM to 10 microM, however, MCH, NEI and NGE induced cAMP formation and melanin synthesis which could be blocked by agouti protein or inhibitors of adenylate cyclase or protein kinase A. This shows that mammalian MCH-precursor-derived peptides may mimic MSH signalling via MC1-R activation at relatively high, but physiologically still relevant concentrations, as e.g. found in autocrine/paracrine signalling mechanisms.

(D-(p-benzoylphenylalanine)13, tyrosine19)-melanin-concentrating hormone, a potent analogue for MCH receptor crosslinking

A photoreactive analogue of human melanin-concentrating hormone was designed, [D-Bpa13,Tyr19-MCH, containing the D-enantiomer of photolabile p-benzoylphenylalanine (Bpa) in position 13 and tyrosine for radioiodination in position 19. The linear peptide was synthesized by the continuous-flow solid-phase methodology using Fmoc-strategy and PEG-PS resins, purified to homogeneity and cyclized by iodine oxidation. Radioiodination of [D-Bpa13,Tyr19]-MCH at its Tyr19 residue was carried out enzymatically using solid-phase bound glucose oxidase/lactoperoxidase, followed by purification on a reversed-phase mini-column and HPLC. Saturation binding analysis of [125I]-[D-Bpa13,Tyr19]-MCH with G4F-7 mouse melanoma cells gave a K(D) of 2.2+/-0.2 x 10(-10) mol/l and a B(max) of 1047+/-50 receptors/cell. Competition binding analysis showed that MCH and rANF(1-28) displace [125I]-[D-Bpa13,Tyr19]-MCH from the MCH binding sites on G4F-7 cells whereas alpha-MSH has no effect. Receptor crosslinking by UV-irradiation of G4F-7 cells in the presence of [125I]-[D-Bpa13,Tyr19]-MCH followed by SDS-polyacrylamide gel electrophoresis and autoradiography yielded a band of 45-50 kDa. Identical crosslinked bands were also detected in B16-F1 and G4F mouse melanoma cells, in RE and D10 human melanoma cells as well as in COS-7 cells. Weak staining was found in rat PC12 phaeochromocytoma and Chinese hamster ovary cells. No crosslinking was detected in human MP fibroblasts. These data demonstrate that [125I]-[D-Bpa13,Tyr19]-MCH is a versatile photocrosslinking analogue of MCH suitable to identify MCH receptors in different cells and tissues; the MCH receptor in these cells appears to have the size of a G protein-coupled receptor, most likely with a varying degree of glycosylation.

A combinatorial peptoid library for the identification of novel MSH and GRP/bombesin receptor ligands

A tripeptoid library was synthesized using 69 different primary amines in initially 69 individual reactions by the mix and split approach. The resulting library consisted of 328,509 (69(3)) single compounds, divided in 69 subpools each containing 4,761 entities. The 69 subpools were tested in two binding assays, one for alpha-MSH (alpha-melanotropin) and one for GRP (gastrin-releasing peptide)/bombesin. The sublibraries with the highest affinity to the MSH receptor (i.e. melanocortin type 1 or MC1 receptor) and, respectively, the GRP-preferring bombesin receptor were identified by an iterative process. Individual tripeptoids with good binding activity were resynthesized, analyzed and their dissociation constants and biological activity determined. The KD of the most potent MC1 receptor ligand was 1.58 mumol/l and that of the GRP-preferring bombesin receptor 3.40 mumol/l. Extension of this latter tripeptoid structure whose KD value increased to 280 nmol/l. A similar increase in activity was not observed with the most potent MSH tripeptoid ligand when extended by one residue, but a compound suitable for radioiodination and lacking the N-terminal amino group had a slightly higher binding activity than the tripeptoids (KD approximately 850 nmol/l). These results demonstrate that testing a peptoid library containing 328,509 single compounds led to the successful identification of new ligands for both the MC1 receptor as well as the GRP-preferring bombesin receptor.

Synthesis and characterization of new radioligands for the mammalian melanin-concentrating hormone (MCH) receptor

Melanin-concentrating hormone (MCH) is a neuropeptide present in the brain of all vertebrates. For the characterization of MCH receptors, a monoiodinated [Phe13, Tyr19]-MCH radioligand analogue was developed. The high susceptibility of [125I]-[Phe13, Tyr19]-MCH to oxidative damage and its very lipophilic nature made it necessary to develop new MCH radioligands. To increase the stability, native methionines were replaced by non-sulphur containing amino acid residues. In one analogue, the L-enantiomer of the phenylalanine residue at position 13 was substituted by the D-enantiomer, which increased the relative affinity of the ensuing [125I]-[D-Phe13, Tyr19]-MCH about 7-fold. The different analogues were iodinated by an enzymatic reaction and used for binding studies with mouse melanoma cells. [125I]-[Met(O)4,8, Phe13, Tyr19]-MCH and [125I]-[Hse4,8, Phe13, Tyr19]-MCH showed only about 19% of total binding and [125I]-[Ser4,8, Phe13, Tyr19]-MCH displayed about 44% of total binding when compared with [125I]-[Phe13, Tyr19]-MCH. Non-specific binding for all tracers was below 11% of total binding of [125I]-[Phe13, Tyr19]-MCH binding. [125I]-[D-Phe13, Tyr19]-MCH was used for saturation binding studies and revealed a KD of 122.7 +/- 15.3 pmol/l. This radioligand was further characterized by association and dissociation binding studies.

Synthesis and receptor binding analysis of thirteen oligomeric alpha-MSH analogs

Thirteen oligomeric analogs from dimers up to a hexamer of alpha-melanocyte-stimulating hormone (alpha-MSH) were synthesized and tested on melanoma cells for their ability to bind to melanocortin type 1 (MC1) receptors and to stimulate melanin production in the cells. The peptidic oligomers were made by linking several copies of the alpha-MSH fragment analog Nle-Asp-His-[D-Phe]-Arg-Trp-Lys-NH2 to different templates through formation of oxime bonds. They were found to have binding affinities at 37 degrees C up to 8 times higher and melanogenesis-inducing activities up to 4 times higher than those of the native hormone. At 15 degrees C, one dimer showed a binding affinity 20 times higher than that of alpha-MSH. These results are discussed in terms of possible bridging of neighboring receptors which has been suggested to occur in some other systems.

A microplate binding assay for the somatostatin type-2 receptor (SSTR2)

The clinical importance of somatostatin type-2 receptors (SSTR2) and the study of novel analogues of somatostatin such as OctreoScan or [Tyr3]-octreotide containing DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) as metal chelator led us to develop a methodology to monitor the expression of SSTR2 on tumours of pancreatic origin (e.g. rat AR4-2J cancer cells). Usual binding assay protocols using the commercial [125I][Tyr1]-somatostatin radioligand failed, even in the presence of a cocktail of protease inhibitors with a broad spectrum of activity, possibly due to the high susceptibility of this tracer to proteases expressed in pancreatic cells. We prepared our own radioligand [125I][Tyr2]-octreotide which was shown to be much more resistant to degradation after incubation with AR4-2J plasma membranes. As expected, the increased stability of [125I][Tyr3]-octreotide was associated with good binding to SSTR2. Addition of appropriate protease inhibitors further increased the specific binding of [125I][Tyr3]-octreotide to AR4-2J plasma membranes without affecting the stability of the tracer, suggesting that the protease inhibitors also protect the integrity of SSTR2. Optimal conditions (time, temperature, medium) were developed for a binding assay in 96-well plates using AR4-2J plasma membranes in order to make the assay suitable for high-throughput analysis. This protocol was the basis for studying the in vivo regulation of SSTR2 expression in AR4-2J cells implanted into scid mice after exposure to different compounds.

Melanin-concentrating hormone system in the brain of the lungfish Protopterus annectens

The neurochemical anatomy of the lungfish brain is of particular interest, because many features in these animals might be representative of the common ancestor of land vertebrates. In the present study, we have investigated the localization and biochemical characteristics of melanin-concentrating hormone (MCH)-immunoreactive material in the central nervous system of the African lungfish, Protopterus annectens. The most prominent group of MCH-immunoreactive cell bodies was found in the dorsal hypothalamus. Additional groups of MCH-immunoreactive perikarya were detected in the telencephalon within the medial and dorsal pallium, the medial subpallium, and the ventral part of the lateral subpallium. Brightly immunofluorescent nerve fibers were seen in the anterior olfactory nucleus, the ventral part of the medial pallium, the medial subpallium, and the anterior preoptic area. In the diencephalon, the hypothalamus and the medial region of the dorsal thalamus exhibited a dense accumulation of fibers. MCH-immunoreactive fibers were also found in the tectum and the tegmentum of the mesencephalon and within the reticular formation of the rhombencephalon. In the pituitary, several small groups of cells of the intermediate lobe showed a bright fluorescence. Reversed-phase high-performance liquid chromatography (HPLC) analysis of diencephalon and pituitary extracts resolved a major MCH-immunoreactive peak that coeluted with synthetic salmon MCH. The distribution of MCH in the brain of P. annectens suggests that, in lungfishes, this peptide may exert neuromodulator or neurotransmitter functions. The presence of MCH-like immunoreactivity in the intermediate lobe of the pituitary indicates that, in dipnoans, MCH may also act as a typical pituitary hormone.

Fluorine-18-labeled [Nle4,D-Phe7]-alpha-MSH, an alpha-melanocyte stimulating hormone analogue

The alpha-melanocyte stimulating hormone (alpha-MSH) analogue [Nle4,D-Phe7]-alpha-MSH was labeled with 18F using N-succinimidyl 4-[18F]fluorobenzoate ([18F]SFB) in > 80% radiochemical yield. The IC50 values of [Nle4,D-Phe7]-alpha-MSH and para-fluorobenzoyl-[Nle4, D-Phe7]-alpha-MSH ([Nle4,D-Phe7, Lys 11 -(18F)PFB]-alpha-MSH) for inhibiting the binding of meta-[131I]iodobenzoyl -[Nle4,D-Phe7]-alpha-MSH ([Nle4,D-Phe7, Lys11-(131I)MIB]-alpha-MSH) to B16-F1 murine melanoma cells were 89 +/- 9 pM and 112 +/- 22 pM, respectively, suggesting that addition of 4-fluorobenzoate did not compromise alpha-MSH receptor binding affinity. Binding of [Nle4,D-Phe7,Lys11-(18F)PFB]-alpha-MSH was influenced by the specific activity of the preparation (400-1000 Ci/mmol). The normal tissue clearance of [Nle4, D-Phe7, Lys11-(18F) PFB]-alpha-MSH in mice was quite rapid, with little evidence for defluorination.

Interactions of alpha-melanotropin and agouti on B16 melanoma cells: evidence for inverse agonism of agouti

alpha-Melanocyte-stimulating hormone (alpha-MSH, alpha-melanotropin) and agouti control the switch between eumelanin and pheomelanin synthesis in mammalian melanocytes. Here we investigated interactions between alpha-MSH, agouti protein, cAMP elevating agents and phorbol ester on mouse B16 melanoma cells. Agouti (Kd 3.7 nmol/l) and alpha-MSH (Kd 2.3 nmol/l) had similar affinities to the MC1 melanocortin receptor. Both alpha-MSH and agouti induced MC1 receptor down-regulation. Agouti antagonized melanogenesis induced by alpha-MSH, forskolin, cholera toxin (CT), and pertussis toxin (PT). It also reduced the constitutive melanin formation of long-term cultures. Cell proliferation was inhibited by agouti (43% at 100 nM). This effect was reversed by alpha-MSH, forskolin, or CT. B16-G4F cells, a cell variant that lacks the MC1 receptor, did not respond to agouti. From these results we conclude that agouti shows the characteristics of an inverse agonist acting through the MC1 receptor.

Induction of constitutive melanogenesis in amelanotic mouse melanoma cells by transfection of the human melanocortin-1 receptor gene

The human melanocortin-1 (MC1) receptor was stably expressed in the amelanotic mouse melanoma cell clone B16-G4F which does not express its own (mouse) MC1 receptor and hence is unresponsive to alpha melanocyte stimulating hormone (alpha MSH). From several stable transfectant cell lines expressing the human MC1 receptor in relatively high numbers, three melanin producing clones (G4F-12, 14, and 15) and one amelanotic clone (G4F-7) were further analyzed in competition binding experiments and in cAMP and melanin assays. The dissociation constants (KD) for [Nle4, D-Phe7]-alpha MSH in all four clones ranged from 0.187 to 0.705 nmol/l, thus corresponding to the KD observed with the different human melanoma cell lines so far studied. Intracellular cAMP content was 3- to 5-fold higher than that of control cells, and alpha MSH induced an additional 1.5- to 1.7-fold increase. G4F-15 cells secreted melanin into the medium whereas the other clones did not secrete melanin. The extent of melanin secretion was similar to that of fully alpha MSH-stimulated B16-F1 mouse melanoma cells but the onset of secretion was delayed. alpha MSH induced an additional dose-related increase (up to 1.3-fold) in melanin production which could be suppressed by the addition of specific alpha MSH antibodies without altering the constitutive part of melanogenesis. Human and mouse agouti proteins, which inhibit basal and alpha MSH-induced melanogenesis in B16-F1 cells, both reduced alpha MSH-induced melanin production in G4F-15 cells but did not affect the constitutive melanogenesis. These results indicate that human MC1 receptor expressed in mouse B16-G4F cells induces constitutive activation of the signalling pathway controlling melanogenesis, most likely by tightly coupling to Gs alpha, in a similar manner to that reported for constitutively active receptor mutants in other systems.

Region- and stage-specific patterns of melanocortin receptor ontogeny in rat central nervous system, cranial nerve ganglia and sympathetic ganglia

Observations on developmental actions of melanotropic peptides in nervous system have been difficult to interpret in the absence of data on receptor ontogeny. We investigated binding of [125I]Nle4,D-Phe7-alpha-MSH ([125I]NDP) in developing Long Evans rats from gestational day (E) 13 by quantitative autoradiography. Regional [125I]NDP binding characteristics were assessed by competition experiments in early postnatal brain. The study revealed region- and stage-specific, often transient ontogenetic patterns. Sympathetic ganglia exhibit high [125I]NDP binding from E13, with a peak in superior cervical ganglion at E16-E18. The first central [125I]NDP binding sites transiently appear in parts of thalamus between E13 and E15. The early fetal period is characterized by prominent peaks of receptor density in somatosensory and viscerosensory nuclei (trigeminal sensory nuclei, solitary tract nucleus), paralleled by receptor expression in 5th, 7th, 9th and 10th cranial nerve ganglia. During late fetal life, receptor density peaks in dorsal motor nucleus of vagus and inferior olive; binding sites transiently appear in cerebellum. Caudate-putamen, nucleus accumbens, olfactory tubercle and septohippocampal nucleus show a high perinatal maximum. Starting with late fetal piriform cortex, [125I]NDP binding peaks sequentially in cerebral cortical areas, with highest levels in entorhinal cortex. Preoptic, septal, hypothalamic and amygdaloid areas known for elevated receptor densities in adulthood, exhibit a slow, peri- and postnatal receptor ontogeny. Temporal relations to regional developmental processes support the idea of a role of melanocortins during ontogeny.

Melanin-concentrating hormone binding to mouse melanoma cells in vitro

An analogue of human melanin-concentrating hormone (MCH) suitable for radioiodination was designed in which Tyr13 was replaced by Phe and Val19 by Tyr. The resulting monoiodinated [125I] [Phe13,Tyr19]-MCH radioligand was biologically active and led to the discovery of high-affinity binding sites on mouse B16-F1, G4F and G4F-7 melanoma cells. Saturation binding analysis with G4F-7 cells revealed 1090 MCH receptors per cell and a KD of 1.18 x 10(-10) mol/l. Receptors for MCH were also found on rat PC12 phaeochromocytoma cells, human RE melanoma cells and COS-7 cells. Competition binding analyses with other peptides such as alpha-MSH, NPY and PACAP demonstrated that MCH receptor binding is specific. rANF(1-28) was found to be a weak competitor of MCH, indicating topological similarities between MCH and rANF(1-28) when interacting with MCH receptors.

Synthesis and iodination of human (phenylalanine 13, tyrosine 19) melanin-concentrating hormone for radioreceptor assay

An analogue of human melanin-concentrating hormone (MCH) suitable for radioiodination was designed in which Tyr13 and Val19 of the natural peptide were replaced by phenylalanyl and tyrosyl residues: [Phe13, Tyr19]-MCH. The peptide was synthesized by the continuous-flow solid-phase methodology using Fmoc-strategy and polyhipe PA 500 and PEG-PS resins. The linear MCH peptides with either acetamidomethyl-protected or free cysteinyl residues were purified to homogeneity and cyclized by iodine oxidation, yielding the final product with the correct molecular weight of 2434.61. Radioiodination of the C-terminal tyrosine was carried out enzymatically using solid-phase bound glucose oxidase/lactoperoxidase, followed by purification on a reversed-phase mini-column and by high-pressure liquid chromatography. The resulting [125I]-[Phe13, Tyr19]-MCH tracer was the first radiolabelled MCH peptide suitable for radioreceptor assay: saturation binding analysis using mouse G4F-7 melanoma cells demonstrated the presence of 1090 MCH receptors per cell. The dissociation constant (KD) was 1.18 x 10(-10) M, indicating high-affinity MCH receptors on these cells. MCH receptors were also found in other cell lines such as mouse B16-F1 and G4F and human RE melanoma cells as well as in PC12 and COS-7 cells. Competition binding analyses with a number of other peptides such as alpha-MSH, neuropeptide Y, substance P and pituitary adenylate cyclase activating peptide, demonstrated that the binding to the MCH receptor is specific. Atrial natriuretic factor was found to be a weak competitor of MCH, indicating topological similarities between MCH and ANF when interacting with MCH receptors.

alpha-MSH receptor autoradiography on mouse and human melanoma tissue sections and biopsies

MSH receptors and their binding characteristics of [125I]-labelled derivatives of alpha-MSH have been studied extensively on various mouse and human melanoma cell lines in culture. The aim of this study was to determine the binding characteristics of alpha-MSH radioligands to MSH receptors occurring in experimental mouse and human melanoma tumours as well as in human melanoma biopsies. For this reason, solid tumours were grown on experimental animals by inoculation of murine B16-F1 and human D10 and HBL melanoma cells. After excision and cryosectioning of the tumours, frozen tissue sections were incubated with [(125I)Tyr2]-alpha-MSH or [(125I)Tyr2,Nle4,D-Phe7]-alpha-MSH and specific alpha-MSH binding sites were visualized by subsequent autoradiography. The presence of increasing concentrations of unlabelled alpha-MSH during incubation with tracer led to a dose-dependent displacement of the radioligand. Quantitative analysis of the autoradiograms produced dissociation constants which were comparable with those obtained with cell binding assays: KD = 1.87 and 1.31 nmol/l for B16 tumours and cells, respectively; 0.32 and 0.33 nmol/l for D10, and 2.24 and 1.36 nmol/l for HBL tumours and cells, respectively. This indicates similar binding properties of alpha-MSH radioligands to both cultured melanoma cells and tissue sections of melanoma tumours from experimental animals. Similar binding characteristics were also observed with human melanoma tissue sections originating from biopsies of melanoma patients.

Effects of BIM26226, a potent and specific bombesin receptor antagonist, on amylase release and binding of bombesin-like peptides to AR4-2J cells

Using AR4-2J rat pancreatic carcinoma cells, the effects of a novel bombesin (BN) receptor antagonist [D-F5Phe6, D-Ala11]BN(6-13)OMe (BIM26226) on BN- or GRP-stimulated amylase release and binding of radio-labeled bombesin-like peptides to these cells were examined and compared to [D-Phe6,Leu13 psi(CH2NH)Leu14]BN(6-14) (Psi Bn(6-14)), one of the most potent BN receptor antagonists presently known. BN and GRP both stimulated amylase release with EC50 values in the nanomolar range. Both antagonists were devoid of agonist activity when tested alone. BIM26226 was most potent, antagonizing BN- or GRP-stimulated amylase release with IC50 values in the nanomolar range, whereas Psi Bn(6-14) was approximately ten times less potent. With 125I-[Tyr15]GRP bound to these cells, the binding affinities were BIM26226 > GRP > Psi Bn(6-14) >> neuromedin B. BIM 22626 was not able to inhibit binding of radio-labeled CCK-33, gastrin-17 or VIP. These results suggest that BIM26226 is one of the most potent and specific bombesin receptor antagonists in vitro and seems to be a useful tool to define the physiologic role of GRP in vivo.

[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.

Homologous regulation of the MSH receptor in melanoma cells

We have examined the mechanism of homologous regulation of MSH receptor binding and receptor-mediated adenylate cyclase activation in three human and two mouse melanoma cell lines. Pretreatment with alpha-MSH resulted in a time- and dose-dependent up-regulation of MSH receptors in human D10 and 205 melanoma cells whereas in human HBL and in mouse B16-F1 and Cloudman S91 cells alpha-MSH induced receptor down-regulation. Up-regulation of receptors was maximal after a 24-h incubation period and an alpha-MSH concentration of 100 nM (EC50 = 2.4 nM). The increase in alpha-MSH binding was independent of adenylate cyclase activation and protein synthesis and appeared to be caused by recruitment of spare receptors. The structural requirements of the peptide for triggering this process differed from those found in receptor-binding analyses. Receptor down-regulation was maximal after 12 h and hence more rapid than up-regulation. In B16-F1 cells, 10 nM alpha-MSH caused the disappearance of 85-90% of the MSH receptors, the EC50 of 0.23 nM lying exactly between that for alpha-MSH-induced melanogenesis (0.027 nM) and the dissociation constant of receptor binding (1.31 nM). Down-regulation in B16-F1 cells appears to be the consequence of receptor internalization following MSH binding and seems to be initiated during an early step in MSH signalling, preceding the activation of adenylate cyclase and the cAMP signal. Receptor up- and down-regulation were not accompanied by an alteration in affinity to alpha-MSH, as demonstrated by Scatchard analysis of the binding curves.

Synthesis and biological properties of a biotinylated derivative of ACTH1-17 for MSH receptor studies

A biotinylated derivative of [beta-Ala1,Lys17]-ACTH1-17-NH-(CH2)4-NH2 (ACTH1-17) was synthesized and biologically characterized. The heptadecapeptide with free N-terminus and blocked side-chains was prepared by the solid-phase method using TentaGel resin and a 4-aminobutylamide linker. Biotinyl-beta-Ala-OH was then coupled to the terminal amino group and the resulting [N alpha-(biotinyl-beta-alanyl)-beta-Ala1,Lys17]-ACTH1-17-NH-(CH2)4-N H2 (Bio-ACTH1-17) cleaved from the resin, purified and analyzed. Competition binding assays with mouse B16-F1 and human D10 and HBL melanoma cells using [125I]-alpha-MSH as radioligand gave dissociation constants for Bio-ACTH1-17 of 1.67 +/- 0.07 nM (B16-F1), 0.02 +/- 0.005 nM (D10) and 0.21 +/- 0.02 nM (HBL). The EC50 for Bio-ACTH1-17 in the B16 melanin assay was 4.15 +/- 1.0 nM. Analysis of the binding characteristics of [125I]-Bio-ACTH1-17 demonstrated that in human melanoma cells this radioligand was displaced by ACTH1-17 as well as alpha-MSH whereas in B16-F1 cells the tracer was only displaced from the binding site by ACTH1-17. Studies of Bio-ACTH1-17 with streptavidin showed that the peptide is to a large extent trapped specifically through reaction with biotin. These results demonstrate that (1) the biological characteristics of Bio-ACTH1-17 are almost identical to those of ACTH1-17, (2) Bio-ACTH1-17 is bound by avidin, and (3) Bio-ACTH1-17 may become a useful tool for MSH receptor targeting.