title: Evaluation of novel bifunctional chelates for the development of Cu-64-based radiopharmaceuticals.
abstract: BACKGROUND
  Currently available bifunctional chelates (BFCs) for attaching Cu-64 to a targeting molecule are limited by either their radiolabeling conditions or in vivo stability. With the goal of identifying highly effective BFCs, we compared the properties of two novel BFCs, 1-oxa-4,7,10-triazacyclododecane-S-5-(4-nitrobenzyl)-4,7,10-triacetic acid (p-NO(2)-Bn-Oxo) and 3,6,9,15-tetraazabicyclo[9.3.1]pentadeca-1(15),11,13-triene-S-4-(4-nitrobenzyl)-3,6,9-triacetic acid (p-NO(2)-Bn-PCTA), with the commonly used S-2-(4-nitrobenzyl)-1,4,7,10-tetraazacyclododecanetetraacetic acid (p-NO(2)-Bn-DOTA).
  
  
  METHODS
  p-NO(2)-Bn-DOTA, p-NO(2)-Bn-Oxo and p-NO(2)-Bn-PCTA were each radiolabeled with Cu-64 under various conditions to assess the reaction kinetics and robustness of the radiolabeling. Stability of each Cu-64 BFC complex was evaluated at low pH and in serum. Small animal positron emission tomography imaging and biodistribution studies in mice were undertaken.
  
  
  RESULTS
  p-NO(2)-Bn-Oxo and p-NO(2)-Bn-PCTA possessed superior reaction kinetics compared to p-NO(2)-Bn-DOTA under all radiolabeling conditions; >98% radiochemical yields were achieved in <5 min at room temperature even when using near stoichiometric amounts of BFC. Under nonideal conditions, such as low or high pH, high radiochemical yields were still achievable with the novel BFCs. The radiolabeled compounds were stable in serum and at pH 2 for 48 h. The imaging and biodistribution of the Cu-64-radiolabeled BFCs illustrated differences between the BFCs, including preferential clearance via the kidneys for the p-NO(2)-Bn-PCTA Cu-64 complex.
  
  
  CONCLUSIONS
  The novel BFCs facilitated efficient Cu-64 radiolabeling under mild conditions to produce stable complexes at potentially high specific activities. These BFCs may find wide utility in the development of Cu-64-based radiopharmaceuticals.
authors.affiliation: MDS Nordion, 4004 Wesbrook Mall, Vancouver, BC, Canada V6T 2A3. cara.ferreira@mdsinc.com
authors.forename: Cara L
authors.initials: CL
authors.lastname: Ferreira
authors.forename: Donald T
authors.initials: DT
authors.lastname: Yapp
authors.forename: Eric
authors.initials: E
authors.lastname: Lamsa
authors.forename: Martin
authors.initials: M
authors.lastname: Gleave
authors.forename: Corinne
authors.initials: C
authors.lastname: Bensimon
authors.forename: Paul
authors.initials: P
authors.lastname: Jurek
authors.forename: Garry E
authors.initials: GE
authors.lastname: Kiefer
mesh_terms: D000818:Animals; D002614:Chelating Agents / Q000378:metabolism*; D003301:Copper Radioisotopes / Q000378:metabolism*; D004355:Drug Stability; D006863:Hydrogen-Ion Concentration; D007553:Isotope Labeling; D051379:Mice; D049268:Positron-Emission Tomography; D019275:Radiopharmaceuticals / Q000737:chemistry / Q000378:metabolism*; D014018:Tissue Distribution
chemical_list: D002614:Chelating Agents; D003301:Copper Radioisotopes; D019275:Radiopharmaceuticals