title: Methoxinine - an alternative stable amino acid substitute for oxidation-sensitive methionine in radiolabelled peptide conjugates.
abstract: Radiolabelled peptides with high specificity and affinity towards receptors that are overexpressed by tumour cells are used in nuclear medicine for the diagnosis (imaging) and therapy of cancer. In some cases, the sequences of peptides under investigations contain methionine (Met), an amino acid prone to oxidation during radiolabelling procedures. The formation of oxidative side products can affect the purity of the final radiopharmaceutical product and/or impair its specificity and affinity towards the corresponding receptor. The replacement of Met with oxidation resistant amino acid analogues, for example, norleucine (Nle), can provide a solution. While this approach has been applied successfully to different radiolabelled peptides, a Met → Nle switch only preserves the length of the amino acid side chain important for hydrophobic interactions but not its hydrogen-bonding properties. We report here the use of methoxinine (Mox), a non-canonical amino acid that resembles more closely the electronic properties of Met in comparison to Nle. Specifically, we replaced Met15by Mox15and Nle15in the binding sequence of a radiometal-labelled human gastrin derivative [d-Glu10]HG(10-17), named MG11 (d-Glu-Ala-Tyr-Gly-Trp-Met-Asp-Phe-NH2). A comparison of the physicochemical properties of177Lu-DOTA[X15]MG11 (X = Met, Nle, Mox) in vitro (cell internalization/externalization properties, receptor affinity (IC50), blood plasma stability and logD) showed that Mox indeed represents a suitable, oxidation-stable amino acid substitute of Met in radiolabelled peptide conjugates. Copyright © 2017 European Peptide Society and John Wiley & Sons, Ltd.
authors.affiliation: Center for Radiopharmaceutical Sciences ETH-PSI-USZ, Institute of Pharmaceutical Sciences ETH, Vladimir-Prelog Weg 4, CH-8093, Zurich, Switzerland.
authors.forename: Nathalie M
authors.initials: NM
authors.lastname: Grob
authors.affiliation: Center for Radiopharmaceutical Sciences ETH-PSI-USZ, Paul Scherrer Institute, CH-5232, Villigen, Switzerland.
authors.forename: Martin
authors.initials: M
authors.lastname: Behe
authors.affiliation: Department of Nuclear Medicine, Medical University of Innsbruck, A-6020, Innsbruck, Austria.
authors.forename: Elisabeth
authors.initials: E
authors.lastname: von Guggenberg
authors.affiliation: Center for Radiopharmaceutical Sciences ETH-PSI-USZ, Institute of Pharmaceutical Sciences ETH, Vladimir-Prelog Weg 4, CH-8093, Zurich, Switzerland.|Center for Radiopharmaceutical Sciences ETH-PSI-USZ, Paul Scherrer Institute, CH-5232, Villigen, Switzerland.
authors.forename: Roger
authors.initials: R
authors.lastname: Schibli
authors.affiliation: Center for Radiopharmaceutical Sciences ETH-PSI-USZ, Institute of Pharmaceutical Sciences ETH, Vladimir-Prelog Weg 4, CH-8093, Zurich, Switzerland.|Ludwig Boltzmann Institute Applied Diagnostics, General Hospital of Vienna, Währinger Gürtel 18-20, A-1090, Vienna, Austria.
authors.forename: Thomas L
authors.initials: TL
authors.lastname: Mindt
mesh_terms: D019943:Amino Acid Substitution; D045744:Cell Line, Tumor; D004847:Epithelial Cells / Q000166:cytology / Q000187:drug effects / Q000378:metabolism; D005755:Gastrins / Q000138:chemical synthesis* / Q000378:metabolism / Q000494:pharmacology; D006573:Heterocyclic Compounds, 1-Ring / Q000737:chemistry* / Q000378:metabolism / Q000494:pharmacology; D006714:Homoserine / Q000031:analogs & derivatives* / Q000737:chemistry; D006801:Humans; D006860:Hydrogen Bonding; D057927:Hydrophobic and Hydrophilic Interactions; D007553:Isotope Labeling; D008187:Lutetium / Q000737:chemistry*; D008715:Methionine / Q000737:chemistry; D009646:Norleucine / Q000737:chemistry; D009842:Oligopeptides / Q000138:chemical synthesis* / Q000378:metabolism / Q000494:pharmacology; D010084:Oxidation-Reduction; D011868:Radioisotopes / Q000737:chemistry*; D019275:Radiopharmaceuticals / Q000138:chemical synthesis* / Q000378:metabolism / Q000494:pharmacology; D013961:Thyroid Gland / Q000166:cytology / Q000187:drug effects / Q000378:metabolism
chemical_list: D005755:Gastrins; D006573:Heterocyclic Compounds, 1-Ring; D009842:Oligopeptides; D011868:Radioisotopes; D019275:Radiopharmaceuticals; C071349:1,4,7,10-tetraazacyclododecane- 1,4,7,10-tetraacetic acid; C008530:minigastrin; C037855:methoxine; D008187:Lutetium; D006714:Homoserine; D009646:Norleucine; D008715:Methionine
keywords: CCK2 receptor; Gastrin; Lu-177; non-natural amino acids; radiolabelled peptides; substitution of methionine