MO tripeptide diastereomers (M=99/99mTc, Re): models to identify the structure of 99mTc peptide targeted radiopharmaceuticals.
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.
Cantorias
Melchor V
MV
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.
Howell
Robertha C
RC
Todaro
Louis
L
Cyr
John E
JE
Berndorff
Dietmar
D
Rogers
Robin D
RD
Francesconi
Lynn C
LC
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*
D009842:Oligopeptides; D019275:Radiopharmaceuticals; D012211:Rhenium; D013667:Technetium; D010100:Oxygen
Inorg Chem. 2004 May 3;43(9):2867-75
15106974
Inorg Chem. 2001 Jul 2;40(14):3555-61
11421705
Nucl Med Biol. 1997 Jul;24(5):425-32
9290078
Inorg Chem. 2001 Dec 17;40(26):6767-73
11735489
Int J Rad Appl Instrum A. 1987;38(8):661-4
2822624
Nucl Med Biol. 1999 Aug;26(6):687-93
10587108
J Nucl Med. 2001 Jan;42(1):154-61
11197967
J Med Chem. 2007 Mar 22;50(6):1354-64
17315859
PLoS Med. 2006 Nov;3(11):e427
17090209
J Med Chem. 1994 Sep 30;37(20):3282-8
7932555
J Nucl Med. 2001 Nov;42(11):1722-7
11696645
J Med Chem. 1993 Apr 16;36(8):1032-40
8478903
J Med Chem. 1997 Jan 3;40(1):9-17
9016323
J Med Chem. 1989 Feb;32(2):433-7
2913304
J Nucl Med. 1985 Nov;26(11):1287-94
4056926
J Inorg Biochem. 2005 Oct;99(10):1952-62
16051366
Bioconjug Chem. 2000 May-Jun;11(3):401-7
10821657
Inorg Chem. 1997 Dec 3;36(25):5799-5808
11670202
Cancer Biother Radiopharm. 2006 Apr;21(2):117-29
16706632
Int J Rad Appl Instrum B. 1990;17(5):499-506
2118128
J Med Chem. 1992 Jan;35(1):157-62
1732524
Proc Natl Acad Sci U S A. 2004 Oct 12;101(41):14865-70
15469923
Anticancer Res. 1997 May-Jun;17(3B):1783-95
9179235
J Nucl Med. 1986 Feb;27(2):171-7
3712035
Inorg Chem. 2001 Oct 22;40(22):5695-700
11599972
Bioconjug Chem. 1999 May-Jun;10(3):470-9
10346880
J Nucl Med. 1996 May;37(5):775-81
8965144
J Med Chem. 2007 Sep 6;50(18):4295-303
17691760
Bioconjug Chem. 1990 Mar-Apr;1(2):132-7
2095211