Thrombus Imaging Using Technetium-99m-Labeled High-Potency GPIIb/IIIa Receptor Antagonists. Chemistry and Initial Biological Studies

Abstract

Platelet-specific compounds which are radiolabeled with gamma-emitting radionuclides may be particularly useful for the noninvasive in vivo detection of thrombi. The synthesis of peptides which are potent inhibitors of platelet aggregation and which contain a chelator for the radionuclide technetium-99m are described. The target compounds were designed such that stable, oxotechnetium(V) species could be prepared where the site of metal coordination was well defined. A strategy was employed where the pharmacophore-Arg-Gly-Asp-(RGD), or RGD mimetic, was constrained in a ring which was formed by the S-alkylation of a cysteine residue with an N-terminal chloroacetyl group. Binding affinities were enhanced by the replacement of arginine with the arginine mimetics S-(3-aminopropyl)cysteine and 4-amidinophenylalanine. Further enhancements could be obtained by the synthesis of oligomers which contained two or more rings containing receptor binding regions. The increase in binding affinity seen was more than that expected from a simple stoichiometric increase of pharmacophore. The most potent compounds described had IC50s of approximately 0.03 microM for the inhibition of human platelet aggregation. Two of the more potent peptides (P280 and P748) were labeled with technetium-99m and assessed in a canine thrombosis model. The 99m Tc complexes of the peptides prepared in this work hold promise as thrombus imaging agents due to their high receptor binding affinity, ease of preparation, and expected rapid pharmacokinetics.

Diagnostic Nuclear Medicine

“…Lister-James and co-workers (1996; Pearson et al 1996) at Diatide Inc. developed a 99m Tc-labeled GPIIb/IIIa receptor antagonist for scintigraphic detection of deep venous thrombosis. P280 (now called bibapcitide) is a small oligopeptide, consisting of two identical, linked, cyclic 13-amino acid monomers (Fig.…”

Angiogenesis in Gliomas: Imaging and Experimental Therapeutics

“…A mean cerebral blood oxygen saturation of 58.4 ± 1.8% was obtained in the brain parenchyma that correlates with known cerebral blood oxygen saturation under normal conditions (6). Although BOLD MRI shows promise in quantifying tumor oxygenation, it is limited by its low contrast to noise ratio and clinical studies with carbogen vasomodulation are technically challenging (152).…”

“…Paramagnetic chelates that change magnetic properties at enzymatic hydrolysis ("smart contrast agents") have recently been used in the MRI imaging of angiogenesis. There are numerous MRI and non-MRI techniques in development for identifying molecular events involved with angiogenesis, some of which include targeting of endothelial integrin αvβ3 by using paramagnetic liposomes for MR imaging (197), targeting of the angiogenesis-associated fibronectin isoform by using optical probes (144), radiolabeled peptidomimetics of a uPAR antagonist (29), peptides adhering to the glycoprotein IIb/IIIa receptor on the surface of activated platelets, a major component of active thrombus formation (152), and antibodies against the transforming growth factor β receptor (26). These techniques are beyond the scope of this review, and interested readers are referred to the literature (43, 220).…”

99mTc Labeling of Highly Potent Small Peptides

“…The BFCA can be attached to the C terminus or N terminus via a linker, if needed. It can also be attached to the side chain of the peptide or incorporated in the peptide backbone (31)(32)(33)(34) provided that incorporation of the BFCA does not significantly affect the receptor binding affinity of the peptide. The radiolabeling can be achieved either by direct reduction of 99m TcO 4 -in the presence of a BFCA-peptide conjugate 6 ] + and [ 99m Tc]-bicisate, [ 99m TcO(ECD)], the ligand (MIBI and ECD, respectively) is always present in large excess.…”