Pre- and postnatal ontogeny of [125I]Nle4,D-Phe7-alpha-MSH binding sites in rat brain

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.

Stereochemical studies of the monocyclic agouti-related protein (103-122) Arg-Phe-Phe residues: conversion of a melanocortin-4 receptor antagonist into an agonist and results in the discovery of a potent and selective melanocortin-1 agonist

The agouti-related protein (AGRP) is an endogenous antagonist of the centrally expressed melanocortin receptors. The melanocortin-4 receptor (MC4R) is involved in energy homeostasis, food intake, sexual function, and obesity. The endogenous hAGRP protein is 132 amino acids in length, possesses five disulfide bridges at the C-terminus of the molecule, and is expressed in the hypothalamus of the brain. We have previously reported that a monocyclic hAGRP(103-122) peptide is an antagonist at the melanocortin receptors expressed in the brain. Stereochemical inversion from the endogenous l- to d-isomers of single or multiple amino acid modifications in this monocyclic truncated hAGRP sequence resulted in molecules that are converted from melanocortin receptor antagonists into melanocortin receptor agonists. The Asp-Pro-Ala-Ala-Thr-Ala-Tyr-cyclo[Cys-Arg-DPhe-DPhe-Asn-Ala-Phe-Cys]-Tyr-Ala-Arg-Lys-Leu peptide resulted in a 60 nM melanocortin-1 receptor agonist that is 100-fold selective versus the mMC4R, 1000-fold selective versus the mMC3R, and ca. 180-fold selective versus the mMC5R. In attempts to identify putative ligand-receptor interactions that may be participating in the agonist induced stimulation of the MC4R, selected ligands were docked into a homology molecular model of the mMC4R. These modeling studies have putatively identified hAGRP ligand DArg111-mMC4RAsn115 (TM3) and the hAGRP DPhe113-mMC4RPhe176 (TM4) interactions as important for agonist activity.

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.

Characterisation of the melanocortin 4 receptor by radioligand binding

The DNA encoding the human melanocortin 4 receptor was expressed in COS (CV-1 origin, SV 40) cells and its radioligand binding properties was tested by using the [125I][Nle4, D-Phe7] alpha-melanocyte stimulating hormone (MSH). The radioligand was found to bind to a single saturable site with a Kd of 3.84 +/- 0.57 nmol/l in the MC4 receptor expressing cells. The order of potency of a number of substance competing for the [125I][Nle4, D-Phe7] alpha-MSH binding was the following; [Nle4, D-Phe7] alpha-MSH > [Nle4]-alpha-MSH > beta-MSH > desacetyl-alpha-MSH > alpha-MSH > ACTH (1-39) > ACTH (4-10) > gamma 1-MSH > gamma 2-MSH. This order of potency is unique for the melanocortin 4 receptor when compared to our previously published data for the other melanocortin receptor subtypes. Most notably the melanocortin 4 receptor shows highest affinity for beta-MSH, among the endogenous MSH-peptides. Furthermore the melanocortin 4 receptor shows very low affinity for the gamma-MSH peptides. This distinguishes the melanocortin 4 receptor from the melanocortin 3 receptor, which is the other major central nervous system melanocortin-receptor, as melanocortin 3 receptor shows high affinity for gamma-MSH. Our finding might indicate a specific role for beta-MSH for the melanocortin 4 receptor.

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.

Receptors for melanocortin peptides in the hypothalamic-pituitary-adrenal axis and skin

The syndromes of hereditary IGD and triple A syndrome are potentially life threatening and severely disabling diseases. Clinical awareness of these syndromes is of considerable prognostic and therapeutic importance. The defects in the ACTH receptor causing IGD help illuminate the mechanisms of ligand binding and signal transduction by this receptor. Identification of the molecular defect(s) responsible for IGD cases with a normal ACTH receptor structural gene and for the triple A syndrome remains a challenge, which will hopefully eventually provide further insight into the mechanisms of adrenocortical function. The cloning of the melanocortin receptors has been a giant step towards a better insight into the physiological role of the POMC-derived peptides. It is now becoming apparent that the differential processing of POMC in the brain, the pituitary, and peripheral tissues, yielding a number of different biologically active melanocortin peptides, combined with the distinct tissue distribution and pharmacological profile of the melanocortin receptors will help elucidate the molecular basis of these functions.

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.