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Evaluation of the novel folate receptor ligand [18F]fluoro-PEG-folate for macrophage targeting in a rat model of arthritis.

Gent YY, Weijers K, Molthoff CF, Windhorst AD, Huisman MC, Smith DE, Kularatne SA, Jansen G, Low PS, Lammertsma AA, van der Laken CJ - Arthritis Res. Ther. (2013)

Bottom Line: Images, however, also showed significant peri-articular background activity.In the rat model, [18F]fluoro-PEG-folate uptake in arthritic knees was increased compared with both contralateral knees and knees of normal rats.Arthritic knee-to-bone and arthritic knee-to-blood ratios of [18F]fluoro-PEG-folate were increased compared with those of (R)-[11C]PK11195.

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ABSTRACT

Introduction: Detection of (subclinical) synovitis is relevant for both early diagnosis and monitoring of therapy of rheumatoid arthritis (RA). Previously, the potential of imaging (sub)clinical arthritis was demonstrated by targeting the translocator protein in activated macrophages using (R)-[11C]PK11195 and positron emission tomography (PET). Images, however, also showed significant peri-articular background activity. The folate receptor (FR)-β is a potential alternative target for imaging activated macrophages. Therefore, the PET tracer [18F]fluoro-PEG-folate was synthesized and evaluated in both in vitro and ex vivo studies using a methylated BSA induced arthritis model.

Methods: [18F]fluoro-PEG-folate was synthesized in a two-step procedure. Relative binding affinities of non-radioactive fluoro-PEG-folate, folic acid and naturally circulating 5-methyltetrahydrofolate (5-Me-THF) to FR were determined using KB cells with high expression of FR. Both in vivo [18F]fluoro-PEG-folate PET and ex vivo tissue distribution studies were performed in arthritic and normal rats and results were compared with those of the established macrophage tracer (R)-[11C]PK11195.

Results: [18F]fluoro-PEG-folate was synthesized with a purity >97%, a yield of 300 to 1,700 MBq and a specific activity between 40 and 70 GBq/µmol. Relative in vitro binding affinity for FR of F-PEG-folate was 1.8-fold lower than that of folic acid, but 3-fold higher than that of 5-Me-THF. In the rat model, [18F]fluoro-PEG-folate uptake in arthritic knees was increased compared with both contralateral knees and knees of normal rats. Uptake in arthritic knees could be blocked by an excess of glucosamine-folate, consistent with [18F]fluoro-PEG-folate being specifically bound to FR. Arthritic knee-to-bone and arthritic knee-to-blood ratios of [18F]fluoro-PEG-folate were increased compared with those of (R)-[11C]PK11195. Reduction of 5-Me-THF levels in rat plasma to those mimicking human levels increased absolute [18F]fluoro-PEG-folate uptake in arthritic joints, but without improving target-to-background ratios.

Conclusions: The novel PET tracer [18F]fluoro-PEG-folate, designed to target FR on activated macrophages provided improved contrast in a rat model of arthritis compared with the accepted macrophage tracer (R)-[11C]PK11195. These results warrant further exploration of [18F]fluoro-PEG-folate as a putative PET tracer for imaging (sub)clinical arthritis in RA patients.

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FR-binding affinities. (A) Displacement of folate-FITC binding from human KB cells (10 nM, 4ºC) with increasing concentrations of unlabelled folic acid, fluoro-PEG-folate and 5-Me-THF. (B) Relative binding affinities to FR of folic acid (set at 1) versus fluoro-PEG-folate and 5-Me-THF. Results are presented in mean ± SEM of seven separate experiments.
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Figure 2: FR-binding affinities. (A) Displacement of folate-FITC binding from human KB cells (10 nM, 4ºC) with increasing concentrations of unlabelled folic acid, fluoro-PEG-folate and 5-Me-THF. (B) Relative binding affinities to FR of folic acid (set at 1) versus fluoro-PEG-folate and 5-Me-THF. Results are presented in mean ± SEM of seven separate experiments.

Mentions: In vitro binding competition assays with the FR expressing KB cell line were performed to compare relative binding affinities of fluoro-PEG-folate with that of folic acid and the biologically active and circulating plasma form of folate, 5-Me-THF. Figure 2A shows displacement of folate-FITC from the receptor in the presence of increasing concentrations of folic acid, unlabelled fluoro-PEG-folate and 5-Me-THF. Relative binding affinities were calculated from these studies indicating that binding affinity for FR of fluoro-PEG-folate is 1.8-fold lower than that of folic acid, but 3-fold higher than that of 5-Me-THF (Figure 2B).


Evaluation of the novel folate receptor ligand [18F]fluoro-PEG-folate for macrophage targeting in a rat model of arthritis.

Gent YY, Weijers K, Molthoff CF, Windhorst AD, Huisman MC, Smith DE, Kularatne SA, Jansen G, Low PS, Lammertsma AA, van der Laken CJ - Arthritis Res. Ther. (2013)

FR-binding affinities. (A) Displacement of folate-FITC binding from human KB cells (10 nM, 4ºC) with increasing concentrations of unlabelled folic acid, fluoro-PEG-folate and 5-Me-THF. (B) Relative binding affinities to FR of folic acid (set at 1) versus fluoro-PEG-folate and 5-Me-THF. Results are presented in mean ± SEM of seven separate experiments.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3672671&req=5

Figure 2: FR-binding affinities. (A) Displacement of folate-FITC binding from human KB cells (10 nM, 4ºC) with increasing concentrations of unlabelled folic acid, fluoro-PEG-folate and 5-Me-THF. (B) Relative binding affinities to FR of folic acid (set at 1) versus fluoro-PEG-folate and 5-Me-THF. Results are presented in mean ± SEM of seven separate experiments.
Mentions: In vitro binding competition assays with the FR expressing KB cell line were performed to compare relative binding affinities of fluoro-PEG-folate with that of folic acid and the biologically active and circulating plasma form of folate, 5-Me-THF. Figure 2A shows displacement of folate-FITC from the receptor in the presence of increasing concentrations of folic acid, unlabelled fluoro-PEG-folate and 5-Me-THF. Relative binding affinities were calculated from these studies indicating that binding affinity for FR of fluoro-PEG-folate is 1.8-fold lower than that of folic acid, but 3-fold higher than that of 5-Me-THF (Figure 2B).

Bottom Line: Images, however, also showed significant peri-articular background activity.In the rat model, [18F]fluoro-PEG-folate uptake in arthritic knees was increased compared with both contralateral knees and knees of normal rats.Arthritic knee-to-bone and arthritic knee-to-blood ratios of [18F]fluoro-PEG-folate were increased compared with those of (R)-[11C]PK11195.

View Article: PubMed Central - HTML - PubMed

ABSTRACT

Introduction: Detection of (subclinical) synovitis is relevant for both early diagnosis and monitoring of therapy of rheumatoid arthritis (RA). Previously, the potential of imaging (sub)clinical arthritis was demonstrated by targeting the translocator protein in activated macrophages using (R)-[11C]PK11195 and positron emission tomography (PET). Images, however, also showed significant peri-articular background activity. The folate receptor (FR)-β is a potential alternative target for imaging activated macrophages. Therefore, the PET tracer [18F]fluoro-PEG-folate was synthesized and evaluated in both in vitro and ex vivo studies using a methylated BSA induced arthritis model.

Methods: [18F]fluoro-PEG-folate was synthesized in a two-step procedure. Relative binding affinities of non-radioactive fluoro-PEG-folate, folic acid and naturally circulating 5-methyltetrahydrofolate (5-Me-THF) to FR were determined using KB cells with high expression of FR. Both in vivo [18F]fluoro-PEG-folate PET and ex vivo tissue distribution studies were performed in arthritic and normal rats and results were compared with those of the established macrophage tracer (R)-[11C]PK11195.

Results: [18F]fluoro-PEG-folate was synthesized with a purity >97%, a yield of 300 to 1,700 MBq and a specific activity between 40 and 70 GBq/µmol. Relative in vitro binding affinity for FR of F-PEG-folate was 1.8-fold lower than that of folic acid, but 3-fold higher than that of 5-Me-THF. In the rat model, [18F]fluoro-PEG-folate uptake in arthritic knees was increased compared with both contralateral knees and knees of normal rats. Uptake in arthritic knees could be blocked by an excess of glucosamine-folate, consistent with [18F]fluoro-PEG-folate being specifically bound to FR. Arthritic knee-to-bone and arthritic knee-to-blood ratios of [18F]fluoro-PEG-folate were increased compared with those of (R)-[11C]PK11195. Reduction of 5-Me-THF levels in rat plasma to those mimicking human levels increased absolute [18F]fluoro-PEG-folate uptake in arthritic joints, but without improving target-to-background ratios.

Conclusions: The novel PET tracer [18F]fluoro-PEG-folate, designed to target FR on activated macrophages provided improved contrast in a rat model of arthritis compared with the accepted macrophage tracer (R)-[11C]PK11195. These results warrant further exploration of [18F]fluoro-PEG-folate as a putative PET tracer for imaging (sub)clinical arthritis in RA patients.

Show MeSH
Related in: MedlinePlus