title: MO tripeptide diastereomers (M=99/99mTc, Re): models to identify the structure of 99mTc peptide targeted radiopharmaceuticals.
abstract: Biologically active molecules, such as many peptides, serve as targeting vectors for radiopharmaceuticals based on 99mTc. Tripeptides can be suitable chelates and are easily and conveniently synthesized and linked to peptide targeting vectors through solid-phase peptide synthesis and form stable TcVO complexes. Upon complexation with [TcO]3+, two products form; these are syn and anti diastereomers, and they often have different biological behavior. This is the case with the approved radiopharmaceutical [99mTcO]depreotide ([99mTcO]P829, NeoTect) that is used to image lung cancer. [99mTcO]depreotide indeed exhibits two product peaks in its HPLC profile, but assignment of the product peaks to the diastereomers has proven to be difficult because the metal peptide complex is difficult to crystallize for structural analysis. In this study, we isolated diastereomers of [99TcO] and [ReO] complexes of several tripeptide ligands that model the metal chelator region of [99mTcO]depreotide. Using X-ray crystallography, we observed that the early eluting peak (A) corresponds to the anti diastereomer, where the Tc=O group is on the opposite side of the plane formed by the ligand backbone relative to the pendant groups of the tripeptide ligand, and the later eluting peak (B) corresponds to the syn diastereomer, where the Tc=O group is on the same side of the plane as the residues of the tripeptide. 1H NMR and circular dichroism (CD) spectroscopy report on the metal environment and prove to be diagnostic for syn or anti diastereomers, and we identified characteristic features from these techniques that can be used to assign the diastereomer profile in 99mTc peptide radiopharmaceuticals like [99mTcO]depreotide and in 188Re peptide radiotherapeutic agents. Crystallography, potentiometric titration, and NMR results presented insights into the chemistry occurring under physiological conditions. The tripeptide complexes where lysine is the second amino acid crystallized in a deprotonated metallo-amide form, possessing a short N1-M bond. The pKa measurements of the N1 amine (pKa approximately 5.6) suggested that this amine is rendered more acidic by both metal complexation and the presence of the lysine residue. Furthermore, peptide chelators incorporating a lysine (like the chelator of [TcO]depreotide) likely exist in the deprotonated form in vivo, comprising a neutral metal center. Deprotonation possibly mediates the interconversion process between the syn and anti diastereomers. The N1 amine group on non-lysine-containing metallopeptides is not as acidic (pKa approximately 6.8) and does not deprotonate and crystallize as do the metallo-amide species. Three of the tripeptide ligands (FGC, FSC, and FKC) were radiolabeled with 99mTc, and the individual syn and anti isomers were isolated for biodistribution studies in normal female nude mice. The main organs of uptake were the liver, intestines, and kidneys, with the FGC compounds exhibiting the highest liver uptake. In comparing the diastereomers, the syn compounds had substantially higher organ uptake and slower blood clearance than the anti compounds.
authors: Department of Chemistry, Hunter College and the Graduate Center of the City University of New York, 695 Park Avenue, New York, New York 10021, USA.
authors: Melchor V
authors: MV
authors: Cantorias
authors: Robertha C
authors: RC
authors: Howell
authors: Louis
authors: L
authors: Todaro
authors: John E
authors: JE
authors: Cyr
authors: Dietmar
authors: D
authors: Berndorff
authors: Robin D
authors: RD
authors: Rogers
authors: Lynn C
authors: LC
authors: Francesconi
mesh_terms: D000818:Animals; D018360:Crystallography, X-Ray; D005260:Female; D051379:Mice; D008954:Models, Biological; D008958:Models, Molecular; D015394:Molecular Structure; D009842:Oligopeptides / Q000737:chemistry* / Q000494:pharmacology*; D010100:Oxygen / Q000737:chemistry*; D019275:Radiopharmaceuticals / Q000737:chemistry* / Q000494:pharmacology*; D012211:Rhenium / Q000737:chemistry*; D013237:Stereoisomerism; D013667:Technetium / Q000737:chemistry*
chemical_list: D009842:Oligopeptides; D019275:Radiopharmaceuticals; D012211:Rhenium; D013667:Technetium; D010100:Oxygen
references: Inorg Chem. 2004 May 3;43(9):2867-75
references: 15106974
references: Inorg Chem. 2001 Jul 2;40(14):3555-61
references: 11421705
references: Nucl Med Biol. 1997 Jul;24(5):425-32
references: 9290078
references: Inorg Chem. 2001 Dec 17;40(26):6767-73
references: 11735489
references: Int J Rad Appl Instrum A. 1987;38(8):661-4
references: 2822624
references: Nucl Med Biol. 1999 Aug;26(6):687-93
references: 10587108
references: J Nucl Med. 2001 Jan;42(1):154-61
references: 11197967
references: J Med Chem. 2007 Mar 22;50(6):1354-64
references: 17315859
references: PLoS Med. 2006 Nov;3(11):e427
references: 17090209
references: J Med Chem. 1994 Sep 30;37(20):3282-8
references: 7932555
references: J Nucl Med. 2001 Nov;42(11):1722-7
references: 11696645
references: J Med Chem. 1993 Apr 16;36(8):1032-40
references: 8478903
references: J Med Chem. 1997 Jan 3;40(1):9-17
references: 9016323
references: J Med Chem. 1989 Feb;32(2):433-7
references: 2913304
references: J Nucl Med. 1985 Nov;26(11):1287-94
references: 4056926
references: J Inorg Biochem. 2005 Oct;99(10):1952-62
references: 16051366
references: Bioconjug Chem. 2000 May-Jun;11(3):401-7
references: 10821657
references: Inorg Chem. 1997 Dec 3;36(25):5799-5808
references: 11670202
references: Cancer Biother Radiopharm. 2006 Apr;21(2):117-29
references: 16706632
references: Int J Rad Appl Instrum B. 1990;17(5):499-506
references: 2118128
references: J Med Chem. 1992 Jan;35(1):157-62
references: 1732524
references: Proc Natl Acad Sci U S A. 2004 Oct 12;101(41):14865-70
references: 15469923
references: Anticancer Res. 1997 May-Jun;17(3B):1783-95
references: 9179235
references: J Nucl Med. 1986 Feb;27(2):171-7
references: 3712035
references: Inorg Chem. 2001 Oct 22;40(22):5695-700
references: 11599972
references: Bioconjug Chem. 1999 May-Jun;10(3):470-9
references: 10346880
references: J Nucl Med. 1996 May;37(5):775-81
references: 8965144
references: J Med Chem. 2007 Sep 6;50(18):4295-303
references: 17691760
references: Bioconjug Chem. 1990 Mar-Apr;1(2):132-7
references: 2095211