Synthesis and characterization of boron fenbufen and its F-18 labeled homolog for boron neutron capture therapy of COX-2 overexpressed cholangiocarcinoma

Fluorine-18 labeling PEGylated 6-boronotryptophan for PET scanning of mice for assessing the pharmacokinetics for boron neutron capture therapy of brain tumors

Two tryptophan compound classes 5- and 6-borono PEGylated boronotryptophan derivatives have been prepared for assessing their aqueous solubility as formulation of injections for boron neutron capture therapy (BNCT). The PEGylation has improved their aqueous solubility thereby increasing their test concentration in 1 mM without suffering from toxicity. In-vitro uptake assay of PEGylated 5- and 6-boronotryptophan showed that the B-10 concentration can reach 15-50 ppm in U87 cell whereas the uptake in LN229 cell varies. Shorter PEG compound 6-boronotryptophanPEG200[18F] was obtained in 1.7 % radiochemical yield and the PET-derived radioradioactivity percentage in 18 % was taken up by U87 tumor at the limb of xenograft mouse. As high as tumor to normal uptake ratio in 170 (T/N) was obtained while an inferior radioactivity uptake of 3 % and T/N of 8 was observed in LN229 xenografted mouse.

Recent Development of Radiofluorination of Boron Agents for Boron Neutron Capture Therapy of Tumor: Creation of 18F-Labeled C-F and B-F Linkages

Boron neutron capture therapy (BNCT) is a binary therapeutic technique employing a boron agent to be delivered to the tumor site followed by the irradiation of neutrons. Biofunctional molecules/nanoparticles labeled with F-18 can provide an initial pharmacokinetic profile of patients to guide the subsequent treatment planning procedure of BNCT. Borono phenylalanine (BPA), recognized by the l-type amino acid transporter, can cross the blood-brain barrier and be accumulated in gliomas. The radiofluoro BNCT agents are reviewed by considering (1) less cytotoxicity, (2) diagnosing and therapeutic purposes, (3) aqueous solubility and extraction route, as well as (4), the trifluoroborate effect. A trifluoroborate-containing amino acid such as fluoroboronotyrosine (FBY) represents an example with both functionalities of imaging and therapeutics. Comparing with the insignificant cytotoxicity of clinical BPA with IC50 > 500 μM, FBY also shows minute toxicity with IC50 > 500 μM. [18F]FBY is a potential diagnostic agent for its tumor to normal accumulation (T/N) ratio, which ranges from 2.3 to 24.5 from positron emission tomography, whereas the T/N ratio of FBPA is greater than 2.5. Additionally, in serving as a BNCT therapeutic agent, the boron concentration of FBY accumulated in gliomas remains uncertain. The solubility of 3-BPA is better than that of BPA, as evidenced by the cerebral dose of 3.4%ID/g vs. 2.2%ID/g, respectively. While the extraction route of d-BPA differs from that of BPA, an impressive T/N ratio of 6.9 vs. 1.5 is noted. [18F]FBPA, the most common clinical boron agent, facilitates the application of BPA in clinical BNCT. In addition to [18F]FBY, [18F] trifluoroborated nucleoside analog obtained through 1,3-dipolar cycloaddition shows marked tumoral uptake of 1.5%ID/g. Other examples using electrophilic and nucleophilic fluorination on the boron compounds are also reviewed, including diboronopinacolone phenylalanine and nonsteroidal anti-inflammatory agents.

Fluorine-18 Labelled Radioligands for PET Imaging of Cyclooxygenase-2

Molecular imaging probes enable the early and accurate detection of disease-specific biomarkers and facilitate personalized treatment of many chronic diseases, including cancer. Among current clinically used functional imaging modalities, positron emission tomography (PET) plays a significant role in cancer detection and in monitoring the response to therapeutic interventions. Several preclinical and clinical studies have demonstrated the crucial involvement of cyclooxygenase-2 (COX-2) isozyme in cancer development and progression, making COX-2 a promising cancer biomarker. A variety of COX-2-targeting PET radioligands has been developed based on anti-inflammatory drugs and selective COX-2 inhibitors. However, many of those suffer from non-specific binding and insufficient metabolic stability. This article highlights examples of COX-2-targeting PET radioligands labelled with the short-lived positron emitter ¹⁸F, including radiosynthesis and PET imaging studies published in the last decade (2012–2021).

Antiinflammation Derived Suzuki-Coupled Fenbufens as COX-2 Inhibitors: Minilibrary Construction and Bioassay

A small fenbufen library comprising 18 compounds was prepared via Suzuki Miyara coupling. The five-step preparations deliver 9–17% biphenyl compounds in total yield. These fenbufen analogs exert insignificant activity against the IL-1 release as well as inhibiting cyclooxygenase 2 considerably. Both the para-amino and para-hydroxy mono substituents display the most substantial COX-2 inhibition, particularly the latter one showing a comparable activity as celecoxib. The most COX-2 selective and bioactive disubstituted compound encompasses one electron-withdrawing methyl and one electron-donating fluoro groups in one arene. COX-2 is selective but not COX-2 to bioactive compounds that contain both two electron-withdrawing groups; disubstituted analogs with both resonance-formable electron-donating dihydroxy groups display high COX-2 activity but inferior COX-2 selectivity. In silico simulation and modeling for three COX-2 active—p-fluoro, p-hydroxy and p-amino—fenbufens show a preferable docking to COX-2 than COX-1. The most stabilization by the p-hydroxy fenbufen with COX-2 predicted by theoretical simulation is consistent with its prominent COX-2 inhibition resulting from experiments.

Synthesis and evaluation of ortho-[18F] fluorocelecoxib for COX-2 cholangiocarcinoma imaging

Background

An ¹⁸F-tagged NSAID analog was prepared for use as a probe for COX-2 expression, which is associated with tumor development.

Methods

The in vivo uptake of celecoxib was monitored with ortho-[¹⁸F]fluorocelecoxib using positron emission tomography (PET). The binding affinity of ortho-[¹⁸F]fluorocelecoxib to COX-1 and COX-2 enzymes were assessed using the competitor celecoxib.

Results

The IC₅₀ values were 0.039 μM and 0.024 μM, respectively. A selectivity index of 1.63 was obtained (COX-2 vs COX-1). COX-2 overexpressed cholangiocarcinoma (CCA) murine cells took up more ortho-[¹⁸F]fluorocelecoxib than that by usual CCA cells from 10 to 60 minutes post incubation. Competitive inhibition (blocking) of the tracer uptake of ortho-[¹⁸F]fluorocelecoxib in the presence of celecoxib by the COX-2 overexpressed CCA cells and the usual CCA cells gave the IC₅₀ values of 0.5 μM and 46.5 μM, respectively. Based on the in vitro accumulation data and in vivo metabolism half-life (30 min), PET scanning was performed 30–60 min after the administration of ortho-[¹⁸F]fluorocelecoxib through the tail vein. Study of ortho-[¹⁸F]F-celecoxib in the CCA rats showed a tumor to normal ratio (T/N) of 1.38±0.23 and uptake dose of 1.14±0.25 (%ID/g).

Conclusion

The inferior in vivo blocking results of 1.48±0.20 (T/N) and 1.18±0.22 (%ID/g) suggests that the nonspecificity is associated with the complex role of peroxidase or the binding to carbonic anhydrase.