- Comparative Studies of Three Pairs of α- And γ-Conjugated Folic Acid Derivatives Labeled with Fluorine-18
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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 18F-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 18F-based radiofolates, but not on their in vitro FR-binding affinity. These findings may potentially stimulate new directions for the design of novel 18F-labeled folate-based radiotracers.
- Boss, Silvan D.,Betzel, Thomas,Müller, Cristina,Fischer, Cindy R.,Haller, Stephanie,Reber, Josefine,Groehn, Viola,Schibli, Roger,Ametamey, Simon M.
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- 18F-SACCHARIDE-FOLATES
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The present invention is directed towards new 18F-folate radiopharmaceuticals, wherein the 18F isotope is linked via a prosthetic group, more specifically via a prosthetic group having a saccharide group, such as a cyclic mono- or oligosaccharide, preferably based on a pyranoside or furanoside, which is covalently linked to the glutamate portion of a folate or derivative thereof, a method of their preparation, as well as their use in diagnosis and monitoring of cancer and inflammatory and autoimmune diseases and therapy thereof.
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