Comparative Studies of Three Pairs of α- and γ-Conjugated Folic Acid Derivatives Labeled with Fluorine-18

Bioconjug Chem. 2016 Jan 20;27(1):74-86. doi: 10.1021/acs.bioconjchem.5b00644. Epub 2015 Dec 11.

Abstract

The folate receptor (FR) is upregulated in various epithelial cancer types (FR α-isoform), while healthy tissues show only restricted expression. FR-targeted imaging using folate radiopharmaceuticals is therefore a promising approach for the detection of FR-positive cancer tissue. Almost all folate-based radiopharmaceuticals have been prepared by conjugation at the γ-carboxylic functionality of the glutamate moiety of folic acid. In this work, three pairs of fluorinated α- and γ-conjugated folate derivatives were synthesized and their in vitro and in vivo properties compared. The syntheses of all six regioisomers were obtained in good chemical yields using a multistep synthetic approach including the highly selective Cu(I)-catalyzed 1,3-dipolar cycloaddition. The radiosyntheses of the α- and γ-conjugated (18)F-labeled folate derivatives were accomplished in moderate to good radiochemical yields, high radiochemical purities (>95%), and specific activities ranging from 25 to 196 GBq/μmol. In vitro, all folate derivatives showed high binding affinity to the FR-α (IC50 = 1.4-2.2 nM). In vivo PET imaging and biodistribution studies in FR-positive KB tumor-bearing mice demonstrated similar FR-specific tumor uptake for both regioisomers of each pair of compounds. However, FR-unspecific liver uptake was significantly lower for the α-regioisomers compared to the corresponding γ-regioisomers. In contrast, kidney uptake was up to 50% lower for the γ-regioisomers than for the α-regioisomers. These results show that the site of conjugation in the glutamyl moiety of folic acid has a significant impact on the in vivo behavior of (18)F-based radiofolates, but not on their in vitro FR-binding affinity. These findings may potentially stimulate new directions for the design of novel (18)F-labeled folate-based radiotracers.

Publication types

  • Comparative Study
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Catalysis
  • Copper
  • Female
  • Fluorine Radioisotopes / chemistry*
  • Fluorine Radioisotopes / pharmacokinetics
  • Folate Receptors, GPI-Anchored / metabolism
  • Folic Acid / chemistry*
  • Humans
  • Isomerism
  • Isotope Labeling
  • KB Cells
  • Mice, Nude
  • Molecular Structure
  • Positron-Emission Tomography / methods*
  • Radiochemistry / methods
  • Radiopharmaceuticals / chemical synthesis
  • Radiopharmaceuticals / chemistry*
  • Radiopharmaceuticals / pharmacokinetics
  • Tissue Distribution

Substances

  • Fluorine Radioisotopes
  • Folate Receptors, GPI-Anchored
  • Radiopharmaceuticals
  • Copper
  • Folic Acid