Clinical and preclinical research with modulators at the -methyl-d-aspartate (NMDA) receptor GluN2B N-terminal domain (NTD) aims for the treatment of various neurologic diseases. The interpretation of the results is hampered by the lack of a suitable NMDA PET tracer for assessing the receptor occupancy of potential drugs. We have developedC-Me-NB1 as a PET tracer for imaging GluN1/GluN2B-containing NMDA receptors and used it to investigate in rats the dose-dependent receptor occupancy of eliprodil, a GluN2B NTD modulator. C-Me-NB1 was synthesized and characterized by in vitro displacement binding experiments with rat brain membranes, in vitro autoradiography, and blocking and displacement experiments by PET and PET kinetic modeling. Receptor occupancy by eliprodil was studied by PET withC-Me-NB1. C-Me-NB1 was synthesized at 290 ± 90 GBq/μmol molar activity, 7.4 ± 1.9 GBq total activity at the end of synthesis ( = 17), and more than 99% radiochemical purity. C-Me-NB1 binding in rat brain was blocked in vitro and in vivo by the NTD modulators Ro-25-6981 and eliprodil. Half-maximal receptor occupancy by eliprodil occurred at 1.5 μg/kg. At 1 mg/kg of eliprodil, a dose with reported neuroprotective effects, more than 99.5% of binding sites were occupied. In vitro,C-Me-NB1 binding was independent of the σ-1 receptor (Sigma1R), and the Sigma1R agonist (+)-pentazocine did not compete for high-affinity binding. In vivo, a 2.5 mg/kg dose of (+)-pentazocine abolished C-Me-NB1-specific binding, indicating an indirect effect of Sigma1R onC-Me-NB1 binding. C-Me-NB1 is suitable for the in vivo imaging of NMDA GluN1/GluN2B receptors and the assessment of receptor occupancy by NTD modulators. GluN1/GluN2B NMDA receptors are fully occupied at neuroprotective doses of eliprodil. Furthermore,C-Me-NB1 enables imaging of GluN1/GluN2B NMDA receptor cross talk.

The folate receptor (FR) has been identified as a valuable target for the imaging of cancer and activated macrophages, involved in inflammatory and autoimmune diseases via positron emission tomography (PET). Therefore, conjugates of folic acid have been synthesized by coupling of a radiolabeled prosthetic group to the glutamate part of folic acid (pendent approach). In this work, we present a novel class of folates, where the phenyl ring of folic acid was isosterically replaced by a pyridine moiety for direct labeling with [(18)F]fluoride (integrated approach). 3'-Azafolic acid and its 2'-halogenated derivatives (2'-chloro and 2'-fluoro) were evaluated in vitro to determine their binding affinity. 3'-Aza-2'-[(18)F]fluorofolic acid ([(18)F]6) was obtained, starting from N(2)-acetyl-3'-aza-2'-chlorofolic acid di-tert-butylester (2), in a maximum decay corrected radiochemical yield of about 9% in ≥98% radiochemical purity and high specific activities of 35-127 GBq/μmol. Binding affinity to the FR was high (IC(50) = 0.8 ± 0.2 nM), and the radiotracer was stable in human plasma over 4 h at 37 °C. No degradation or defluorination was detected after incubation of the radiotracer for 1 h at 37 °C with human and murine liver microsomes and human S9-fraction. In vivo PET imaging and biodistribution studies with mice demonstrated a high and specific uptake in FR-positive KB tumor xenografts (12.59 ± 1.77% ID/g, 90 min p.i.). A high and specific accumulation of radioactivity was observed in the kidneys (57.33 ± 8.40% ID/g, 90 min p.i.) and salivary glands (14.09 ± 0.93% ID/g, 90 min p.i.), which are known to express the FR and nonspecific uptake found in the liver (10.31 ± 2.37% ID/g, 90 min p.i.). Preinjection of folic acid resulted in a >85% reduced uptake of [(18)F]6 in FR-positive tissues (xenografts, kidneys, and salivary glands). Furthermore, no radioactive metabolites were detected in the blood, urine, or tumor tissue, 30 min p.i. These characteristics indicate that this new (18)F-labeled 3'-azafolate is an appropriate tool for imaging FR-positive (malignant) tissue.

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.

Folate receptor β (FR-β) is overexpressed on activated, but not resting, macrophages involved in a variety of inflammatory and autoimmune diseases. A pivotal step in atherogenesis is the subendothelial accumulation of macrophages. In nascent lesions, they coordinate the scavenging of lipids and cellular debris to define the likelihood of plaque inflammation and eventually rupture. In this study, we determined the presence of FR-β-expressing macrophages in atherosclerotic lesions by the use of a fluorine-18-labeled folate-based radiotracer. Human endarterectomized specimens were used to measure gene expression levels of FR-β and CD68. Increased FR-β and CD68 levels were found in atherosclerotic plaques compared to normal artery walls by quantitative real-time polymerase chain reaction. Western blotting and immunohistochemistry demonstrated prominent FR-β protein levels in plaques. FR-β-positive cells colocalized with activated macrophages (CD68) in plaque tissue. Carotid sections incubated with 3'-aza-2'-[18F]fluorofolic acid displayed increased accumulation in atherosclerotic plaques through in vitro autoradiography. Specific binding of the radiotracer correlated with FR-β-expressing macrophages. These results demonstrate high FR-β expression in atherosclerotic lesions of human carotid tissue correlating with CD68-positive macrophages. Areas of high 3'-aza-2'-[18F]fluorofolic acid binding within the lesions represented FR-β-expressing macrophages. Selectively targeting FR-β-positive macrophages through folate-based radiopharmaceuticals may be useful for noninvasive imaging of plaque inflammation.