title: DOTA conjugate with an albumin-binding entity enables the first folic acid-targeted 177Lu-radionuclide tumor therapy in mice.
abstract: The folate receptor (FR) has proven a valuable target for nuclear imaging using folic acid radioconjugates. However, using folate-based radiopharmaceuticals for therapy has long been regarded as an unattainable goal because of their considerable renal accumulation. Herein, we present a novel strategy in which a DOTA-folate conjugate with an albumin-binding entity (cm09) was designed with the aim of prolonging circulation in the blood and therewith potentially improving tumor-to-kidney ratios.
  
  
  METHODS
  The folate conjugate cm09 was radiolabeled with (177)LuCl(3), and stability experiments were performed in plasma. Cell uptake studies were performed on FR-positive KB tumor cells, and an ultrafiltration assay was used to determine the plasma protein-binding properties of (177)Lu-cm09. In vivo, (177)Lu-cm09 was tested in KB tumor-bearing mice using SPECT/CT. The therapeutic anticancer effect of (177)Lu-cm09 (20 MBq) applied as a single injection or as fractionated injections was investigated in different groups of mice (n = 5) by monitoring tumor size and the survival time of treated mice, compared with untreated controls.
  
  
  RESULTS
  Compound cm09 was radiolabeled at a specific activity of 40 MBq/nmol, a radiochemical yield of more than 98%, and a stability of more than 99% over 5 d in plasma. Ultrafiltration revealed significant binding of (177)Lu-cm09 to serum proteins (∼91%) in plasma, compared with folate radioconjugate without an albumin-binding entity. Cell uptake and internalization of (177)Lu-cm09 was FR-specific and comparable to other folate radioconjugates. In vivo studies resulted in high tumor uptake (17.56 percentage injected dose per gram [%ID/g] at 4 h after injection), which was almost completely retained for at least 72 h. Renal accumulation was significantly reduced (28 %ID/g at 4 h after injection), compared with folate conjugates that lack an albumin-binding entity (∼70 %ID/g at 4 h after injection). These circumstances enabled SPECT imaging of excellent quality. Radionuclide therapy (1 × 20 MBq) revealed complete remission of tumors in 4 of 5 cases and a significantly prolonged survival time, compared with untreated controls.
  
  
  CONCLUSIONS
  The modification of a folate radioconjugate with an albumin-binding entity resulted in a significant increase of the tumor-to-kidney ratio of radioactivity, enabling for the first time, to our knowledge, the preclinical application of folic acid-targeted radionuclide therapy in mice.
authors.affiliation: Center for Radiopharmaceutical Sciences ETH-PSI-USZ, Paul Scherrer Institute, Villigen-PSI, Switzerland. cristina.mueller@psi.ch
authors.forename: Cristina
authors.initials: C
authors.lastname: Müller
authors.forename: Harriet
authors.initials: H
authors.lastname: Struthers
authors.forename: Christian
authors.initials: C
authors.lastname: Winiger
authors.forename: Konstantin
authors.initials: K
authors.lastname: Zhernosekov
authors.forename: Roger
authors.initials: R
authors.lastname: Schibli
mesh_terms: D000418:Albumins / Q000378:metabolism*; D000818:Animals; D049109:Cell Proliferation; D002471:Cell Transformation, Neoplastic; D005492:Folic Acid / Q000737:chemistry* / Q000378:metabolism / Q000627:therapeutic use*; D006573:Heterocyclic Compounds, 1-Ring / Q000737:chemistry*; D006801:Humans; D007624:KB Cells; D008187:Lutetium / Q000627:therapeutic use*; D051379:Mice; D058990:Molecular Targeted Therapy / Q000379:methods*; D064847:Multimodal Imaging; D049268:Positron-Emission Tomography; D011868:Radioisotopes / Q000627:therapeutic use*; D014057:Tomography, X-Ray Computed
chemical_list: D000418:Albumins; D006573:Heterocyclic Compounds, 1-Ring; D011868:Radioisotopes; C071349:1,4,7,10-tetraazacyclododecane- 1,4,7,10-tetraacetic acid; D008187:Lutetium; D005492:Folic Acid