title: (64)Cu- and (68)Ga-Based PET Imaging of Folate Receptor-Positive Tumors: Development and Evaluation of an Albumin-Binding NODAGA-Folate.
abstract: A number of folate-based radioconjugates have been synthesized and evaluated for nuclear imaging purposes of folate receptor (FR)-positive tumors and potential therapeutic application. A common shortcoming of radiofolates is, however, a significant accumulation of radioactivity in the kidneys. This situation has been faced by modifying the folate conjugate with an albumin-binding entity to increase the circulation time of the radiofolate, which led to significantly improved tumor-to-kidney ratios. The aim of this study was to develop an albumin-binding folate conjugate with a NODAGA-chelator (rf42) for labeling with (64)Cu and (68)Ga, allowing application for PET imaging. The folate conjugate rf42 was synthesized in 8 steps, with an overall yield of 5%. Radiolabeling with (64)Cu and (68)Ga was carried out at room temperature within 10 min resulting in (64)Cu-rf42 and (68)Ga-rf42 with >95% radiochemical purity. (64)Cu-rf42 and (68)Ga-rf42 were stable (>95% intact) in phosphate-buffered saline over more than 4 half-lives of the corresponding radionuclide. In vitro, the plasma protein-bound fraction of (64)Cu-rf42 and (68)Ga-rf42 was determined to be >96%. Cell experiments proved FR-specific uptake of both radiofolates, as it was reduced to <1% when KB tumor cells were coincubated with excess folic acid. In vivo, high accumulation of (64)Cu-rf42 and (68)Ga-rf42 was found in KB tumors of mice (14.52 ± 0.99% IA/g and 11.92 ± 1.68% IA/g, respectively) at 4 h after injection. The tumor-to-kidney ratios were in the range of 0.43-0.55 over the first 4 h of investigation. At later time points (up to 72 h p.i. of (64)Cu-rf42) the tumor-to-kidney ratio increased to 0.73. High-quality PET/CT images were obtained 2 h after injection of (64)Cu-rf42 and (68)Ga-rf42, respectively, allowing distinct visualization of tumors and kidneys. Comparison of PET/CT images obtained with (64)Cu-rf42 and a (64)Cu-labeled DOTA-folate conjugate (cm10) clearly proved the superiority of NODAGA for stable coordination of (64)Cu. (64)Cu-cm10 showed high liver uptake, most probably as a consequence of released (64)Cu(2+). The data reported in this study clearly proved the promising features of (64)Cu-rf42, particularly in terms of favorable tumor-to-kidney ratios. The relatively long half-life of (64)Cu (T1/2 = 12.7 h) matches well with the enhanced circulation time of the albumin-binding NODAGA-folate, allowing PET imaging at longer time points after injection than is possible when using (68)Ga (T1/2 = 68 min).
authors.affiliation: Center for Radiopharmaceutical Sciences ETH-PSI-USZ, Paul Scherrer Institut , Villigen-PSI, Switzerland.
authors.forename: Renáta
authors.initials: R
authors.lastname: Farkas
authors.affiliation: Center for Radiopharmaceutical Sciences ETH-PSI-USZ, Paul Scherrer Institut , Villigen-PSI, Switzerland.
authors.forename: Klaudia
authors.initials: K
authors.lastname: Siwowska
authors.affiliation: Department of Chemistry and Applied Biosciences, ETH Zurich , Zurich, Switzerland.
authors.forename: Simon M
authors.initials: SM
authors.lastname: Ametamey
authors.affiliation: Center for Radiopharmaceutical Sciences ETH-PSI-USZ, Paul Scherrer Institut , Villigen-PSI, Switzerland.|Department of Chemistry and Applied Biosciences, ETH Zurich , Zurich, Switzerland.
authors.forename: Roger
authors.initials: R
authors.lastname: Schibli
authors.affiliation: Center for Radiopharmaceutical Sciences ETH-PSI-USZ, Paul Scherrer Institut , Villigen-PSI, Switzerland.|Laboratory of Radiochemistry, Paul Scherrer Institut , Villigen-PSI, Switzerland.
authors.forename: Nicholas P
authors.initials: NP
authors.lastname: van der Meulen
authors.affiliation: Center for Radiopharmaceutical Sciences ETH-PSI-USZ, Paul Scherrer Institut , Villigen-PSI, Switzerland.|Department of Chemistry and Applied Biosciences, ETH Zurich , Zurich, Switzerland.
authors.forename: Cristina
authors.initials: C
authors.lastname: Müller
mesh_terms: D000085:Acetates / Q000737:chemistry*; D000418:Albumins / Q000737:chemistry*; D000818:Animals; D002614:Chelating Agents / Q000737:chemistry; D005492:Folic Acid / Q000737:chemistry*; D005710:Gallium Radioisotopes / Q000737:chemistry*; D006207:Half-Life; D006573:Heterocyclic Compounds, 1-Ring / Q000737:chemistry*; D006801:Humans; D007624:KB Cells; D051379:Mice; D008819:Mice, Nude; D049268:Positron-Emission Tomography / Q000379:methods; D011854:Radiochemistry / Q000379:methods; D019275:Radiopharmaceuticals / Q000737:chemistry*
chemical_list: C572723:1-(1,3-carboxypropyl)-4,7-carboxymethyl-1,4,7-triazacyclononane; D000085:Acetates; D000418:Albumins; D002614:Chelating Agents; D005710:Gallium Radioisotopes; D006573:Heterocyclic Compounds, 1-Ring; D019275:Radiopharmaceuticals; D005492:Folic Acid
keywords: 64Cu; 68Ga; KB cells; NODAGA; PET; folate; folate receptor