title: Development of a New Folate-Derived Ga-68-Based PET Imaging Agent.
abstract: OBJECTIVE
  The folate receptor (FR) has emerged as an interesting diagnostic and therapeutic drug target with many potential applications in oncologic and inflammatory disorders. It was therefore the aim of this study to develop a folate-derived Ga-68-based positron emission tomography (PET) imaging tracer that is straightforward to radiolabel and could be broadly used in clinical studies. We validated its target binding affinity and specificity and compared it to [99mTc]EC20, the folate single-photon emission computed tomography (SPECT) imaging tracer that has been most extensively studied clinically so far.
  
  
  METHODS
  The new folic acid-derived PET imaging agent is linked via a polyethyleneglycol linker to the chelator 1,4,7-triazacyclononane-1,4,7-trisacetic acid (NOTA). This new compound, NOTA-folate, was labeled with gallium-68. We tested the probe's stability in human plasma and its selectivity in vitro, using the FR-positive KB cell line as well as the FR-negative A549 cell line. The pharmacokinetic profile of [68Ga]NOTA-folate was evaluated in FR-positive KB mouse xenografts. Following intravenous injection of [68Ga]NOTA-folate (383 ± 53 μCi), PET/computed tomography (CT) imaging studies as well as biodistribution studies were performed using KB tumor-bearing mice (n = 3). In vitro as well as in vivo studies were performed in parallel with the SPECT imaging tracer [99mTc]EC20.
  
  
  RESULTS
  In comparison to [99mTc]EC20 (radiochemical yield (RCY) = 82.0 ± 2.9 %, 91.8 ± 2.0 % purity), similar radiochemical yield (87.2 ± 6.9 %) and radiochemical purity (95.6 ± 1.8 %) could be achieved for [68Ga]NOTA-folate. For both tracers, we observed high affinity for FR-positive cells in vitro and high plasma stability. In PET/CT and biodistribution studies, [68Ga]NOTA-folate appeared to display slightly superior in vivo performance in comparison to [99mTc]EC20. In detail,68Ga-NOTA-folate showed very good tumor uptake and retention (6.6 ± 1.1 %ID/g), relatively low kidney uptake (21.7 ± 1.1 %ID/g), and very low liver uptake (0.38 ± 0.08 %ID/g). In vivo blocking studies using a fivefold excess of EC20 reduced the tumor uptake to 2.5 ± 0.7 %ID/g, confirming receptor specific binding of [68Ga]NOTA-folate in vivo.
  
  
  CONCLUSIONS
  We validated a new Ga-68 folate-based PET imaging agent with excellent pharmacokinetics and tumor uptake. Based on a head-to-head comparison between both tracers, [68Ga]NOTA-folate is a suitable imaging probe for the delineation of FR-positive tumors and a promising candidate for clinical translation.
authors.affiliation: Department of Radiology, Memorial Sloan Kettering Cancer Center, 1275 York Avenue, New York, NY, 10065, USA.
authors.forename: Christian
authors.initials: C
authors.lastname: Brand
authors.affiliation: Small-Animal Imaging Core Facility, Memorial Sloan Kettering Cancer Center, New York, NY, 10065, USA.
authors.forename: Valerie A
authors.initials: VA
authors.lastname: Longo
authors.affiliation: Endocyte, Inc., West Lafayette, IN, 47906, USA.
authors.forename: Mike
authors.initials: M
authors.lastname: Groaning
authors.affiliation: Department of Radiology, Memorial Sloan Kettering Cancer Center, 1275 York Avenue, New York, NY, 10065, USA. weberw@mskcc.org.|Molecular Pharmacology and Chemistry Program, Memorial Sloan Kettering Cancer Center, New York, NY, 10065, USA. weberw@mskcc.org.|Weill Cornell Medical College, Cornell University, New York, NY, 10065, USA. weberw@mskcc.org.
authors.forename: Wolfgang A
authors.initials: WA
authors.lastname: Weber
authors.affiliation: Department of Radiology, Memorial Sloan Kettering Cancer Center, 1275 York Avenue, New York, NY, 10065, USA. reinert@mskcc.org.|Weill Cornell Medical College, Cornell University, New York, NY, 10065, USA. reinert@mskcc.org.
authors.forename: Thomas
authors.identifier: http://orcid.org/0000-0002-7819-5480
authors.initials: T
authors.lastname: Reiner
mesh_terms: D000818:Animals; D045744:Cell Line, Tumor; D005260:Female; D005492:Folic Acid / Q000737:chemistry*; D005710:Gallium Radioisotopes / Q000737:chemistry*; D006801:Humans; D008819:Mice, Nude; D000072078:Positron Emission Tomography Computed Tomography; D049268:Positron-Emission Tomography / Q000379:methods*; D011485:Protein Binding; D044967:Serum / Q000378:metabolism; D014018:Tissue Distribution; D015899:Tomography, Emission-Computed, Single-Photon
chemical_list: D005710:Gallium Radioisotopes; D005492:Folic Acid; C000615430:Gallium-68
keywords: Folate receptor expression; PET imaging; SPECT; Small molecule