Preparation and Biological Evaluation of Copper-64–Labeled Tyr3-Octreotate Using a Cross-Bridged Macrocyclic Chelator

Abstract

Experimental Design: CB-TE2A and TETA were conjugated to the somatostatin analogue tyrosine-3-octreotate (Y3-TATE) for evaluation of CB-TE2A as a bifunctional chelator of 64 Cu. The in vitro affinity of each compound for SSTr was determined using a homologous competitive binding assay. In vivo characteristics of both radiolabeled compounds were examined in biodistribution and microPET studies of AR42J tumor-bearing rats.Results: Cu-CB-TE2A-Y3-TATE (K d ‫؍‬ 1.7 nmol/L) and Cu-TETA-Y3-TATE (K d ‫؍‬ 0.7 nmol/L) showed similar affinities for AR42J derived SSTr. In biodistribution studies, nonspecific uptake in blood and liver was lower for 64 Cu-CB-TE2A-Y3-TATE. Differences increased with time such that, at 4 hours, blood uptake was 4.3-fold higher and liver uptake was 2.4-fold higher for 64 Cu-TETA-Y3-TATE than for 64 Cu-CB-TE2A-Y3-TATE. In addition, 4.4-times greater tumor uptake was detected with 64 Cu-CB-TE2A-Y3-TATE than with 64 Cu-TETA-Y3-TATE at 4 hours postinjection. MicroPET imaging yielded similar results.Conclusions: CB-TE2A appears to be a superior in vivo bifunctional chelator of 64 Cu for use in molecular imaging by PET or targeted radiotherapy due to both improved nontarget organ clearance and higher target organ uptake of 64 Cu-CB-TE2A-Y3-TATE compared with 64 Cu-TETA-Y3-TATE.

A New Synthesis of TE2A—a Potential Bifunctional Chelator for 64Cu

“…In the X-ray crystallographic studies of CU(II)-CB-TEAMA, the amide pendant arm was found to be coordinated via the carbonyl oxygen and the distorted octahedral coordination geometry is maintained, as in the Cu-CB-TE2A complex [34]. The biodistribution studies also showed the good in vivo stability of the 64 Cu-crossbridged monoamides, supporting the hypothesis that CB-TE2A is able to be used as a BFC without any structural modifications of the macrocycle backbone [35]. When considering the structural similarity between TE2A and CB-TE2A in terms of the two carboxylate groups located in the trans-positions, the carbonyl oxygen of the amide bond of the TE2A-c(RGDyK) conjugate is expected to coordinate to the Cu(II) ion, and the octahedral coordination geometry around the Cu(II) ions would also be maintained in the 64 Cu-TE2A-c(RGDyK) complex (Scheme 3).…”

[ 64 Cu-NOTA-8-Aoc-BBN(7-14)NH 2 ] targeting vector for positron-emission tomography imaging of gastrin-releasing peptide receptor-expressing tissues

“…Recently, investigators have begun to overcome many of the inherent difficulties of DOTA and TETA chelators for producing in vivo stable conjugates with 64 Cu radionuclide by introduction of a new class of cross-bridged chelating ligand framework for this radiometal (27)(28)(29). These new chelators have shown marked promise to be used as integral components of site-directed targeting vectors based on octreotide (27). However, targeting vectors of this specific type suffer from retention in normal kidney, presumably because of the overall ϩ1 charge of the conjugate (27).…”

“…However, 64 Cu 2ϩ complexes of these specific chelating ligands are only moderately stable under in vivo conditions, resulting in demetallation and subsequent accumulation in nontarget tissues such as liver. Cross-bridged, cyclam-based ligand frameworks appended to specific biologically active targeting vectors offer improved kinetic stability to in vivo transmetallation reactions with various proteins in comparison with DOTA and TETA (27)(28)(29). However, ligands of this general type still suffer from difficult synthetic protocols and renal accumulation and retention of 64 Cu radionuclide (27)(28)(29).…”

“…Cross-bridged, cyclam-based ligand frameworks appended to specific biologically active targeting vectors offer improved kinetic stability to in vivo transmetallation reactions with various proteins in comparison with DOTA and TETA (27)(28)(29). However, ligands of this general type still suffer from difficult synthetic protocols and renal accumulation and retention of 64 Cu radionuclide (27)(28)(29). Thus, there is some impetus to improve the in vivo kinetic stability of 64 Cu-macrocyclic bioconjugates to reduce accumulation in collateral tissues such as liver (22)(23)(24)(25)(26).…”

“…Thus, there is some impetus to improve the in vivo kinetic stability of 64 Cu-macrocyclic bioconjugates to reduce accumulation in collateral tissues such as liver (22)(23)(24)(25)(26). Furthermore, reduction of uptake in normal kidney would do much to improve the inherent renal toxicities of many peptide-based therapeutic agents (27)(28)(29). Previous studies have described the potential use of NOTA (1,4,7-triazacyclononane-1,4,7-triacetic acid) as a bifunctional chelating agent for divalent copper (30,31).…”