title: A tyrosine kinase inhibitor-based high-affinity PET radiopharmaceutical targets vascular endothelial growth factor receptor.
abstract: Tyrosine kinase receptors including vascular endothelial growth factor receptor (VEGFR) have gained significant attention as pharmacologic targets. However, clinical evaluation of small-molecule drugs or biologics that target these pathways has so far yielded mixed results in a variety of solid tumors. The reasons for response variability remain unknown, including the temporal and spatial patterns of receptor tyrosine kinase expression. Methods to detect and quantify the presence of such cellular receptors would greatly facilitate drug development and therapy response assessment. We aimed to generate specific imaging agents as potential companion diagnostics that could also be used for targeted radionuclide therapy. Here, we report on the synthesis and initial preclinical performance of (64)Cu-labeled probes that were based on the kinase inhibitor already in clinical use, vandetanib (ZD6474), as a VEGFR-selective theranostic radiopharmaceutical.
  
  
  METHODS
  A monomeric (ZD-G1) and a dimeric (ZD-G2) derivative of ZD6474 were synthesized and conjugated with DOTA for chelation with (64)Cu to produce the probes (64)Cu-DOTA-ZD-G1 and (64)Cu-DOTA-ZD-G2. The binding affinity and specificity to VEGFR were measured using U-87 MG cells known to overexpress VEGFR. Small-animal PET and biodistribution studies were performed with (64)Cu-labeled probes (3-4 MBq) intravenously administered in U-87 MG tumor-bearing mice with or without coinjection of unlabeled ZD-G2 for up to 24 h after injection.
  
  
  RESULTS
  Receptor-binding assays yielded a mean equilibrium dissociation constant of 44.7 and 0.45 nM for monomeric and dimeric forms, respectively, indicating a synergistic effect in VEGFR affinity by multivalency. Small-animal PET/CT imaging showed rapid tumor accumulation of (64)Cu-DOTA-ZD-G2, with excellent tumor-to-normal tissue contrast by 24 h. Coinjection of the (64)Cu-DOTA-ZD-G2 with 50 nmol (60 μg) of nonradioactive ZD-G2 effectively blocked tumor uptake.
  
  
  CONCLUSIONS
  A (64)Cu-labeled probe derived from an approved oncologic drug selective for VEGFR demonstrates excellent tumor targeting, particularly for the dimeric form. The multivalent probe yielded a 100-fold improvement in receptor affinity while maintaining pharmacokinetic and biodistribution properties well suited for PET imaging in our preclinical model. These results indicate that a clinically relevant theranostic platform can be rapidly developed from known small molecules that target key cellular receptors.
authors: Department of Translational Imaging, Houston Methodist Research Institute, Weill Cornell Medical College, Houston, Texas zli@houstonmethodist.org.
authors: Feng
authors: F
authors: Li
authors: Laboratory of Medicinal Chemistry, Guangzhou Institute of Biomedicine and Health, Chinese Academy of Sciences, Guangzhou, China; and.
authors: Sheng
authors: S
authors: Jiang
authors: Department of Pathology and Genomic Medicine, Houston Methodist Research Institute, Weill Cornell Medical College, Houston, Texas.
authors: Youli
authors: Y
authors: Zu
authors: Department of Translational Imaging, Houston Methodist Research Institute, Weill Cornell Medical College, Houston, Texas.
authors: Daniel Y
authors: DY
authors: Lee
authors: Department of Translational Imaging, Houston Methodist Research Institute, Weill Cornell Medical College, Houston, Texas zli@houstonmethodist.org.
authors: Zheng
authors: Z
authors: Li
mesh_terms: D000818:Animals; D045744:Cell Line, Tumor; D003301:Copper Radioisotopes; D005260:Female; D006801:Humans; D007553:Isotope Labeling; D051379:Mice; D049268:Positron-Emission Tomography / Q000379:methods*; D047428:Protein Kinase Inhibitors / Q000378:metabolism*; D055503:Protein Multimerization; D011505:Protein-Tyrosine Kinases / Q000037:antagonists & inhibitors*; D019275:Radiopharmaceuticals / Q000378:metabolism; D040262:Receptors, Vascular Endothelial Growth Factor / Q000378:metabolism*; D014018:Tissue Distribution
chemical_list: D003301:Copper Radioisotopes; D047428:Protein Kinase Inhibitors; D019275:Radiopharmaceuticals; D011505:Protein-Tyrosine Kinases; D040262:Receptors, Vascular Endothelial Growth Factor
keywords: 64Cu; theranostic; tumor angiogenesis; vascular endothelial growth factor receptor (VEGFR)