Imaging COX-2 expression in cancer using PET/SPECT radioligands: current status and future directions

[Development of Novel COX-2 Imaging Agents for the Diagnosis of Brain Lesions]

Cyclooxygenase-2 (COX-2) has attracted attention as a biomarker for neurodegenerative brain diseases. The aim of this study was to develop a COX-2 imaging agent for positron emission tomography (PET) that binds to and emits radiation from COX-2 in the central nervous system to diagnose brain lesions related to COX-2. To this end, the development of PET imaging probes by derivatizing non-steroidal anti-inflammatory drugs that bind to COX-2 was investigated. Herein, we present the findings of a series of studies on indomethacin and nimesulide derivatives. All five 11C-labeled indomethacin derivatives showed low brain uptake and were rapidly metabolized in vivo, indicating that they are inadequate COX-2 imaging agents. However, the evaluation of 11C-labeled indomethacin derivatives revealed an inverse relationship between the amount taken up by the brain and the lipophilicity of the compound, and that P-glycoprotein (P-gp) may be responsible for the low brain uptake of 11C-labeled indomethacin derivatives. To overcome the problems associated with 11C-labeled indomethacin derivatives, nimesulide was selected as a novel COX-2 imaging agent. Although the nimesulide derivatives were less lipophilic and unaffected by P-gp, all three 11C-labeled nimesulide derivatives showed low brain uptake and were rapidly metabolized. However, the 11C-labeled nimesulide derivatives were partially useful as brain-targeted COX-2 imaging agents because they bound specifically to COX-2 in the brain of mice and successfully imaged the regional brain distribution associated with COX-2. In the development of COX-2 imaging agents, in vivo stability of the compounds is a future objective.

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).

Cyclooxygenases as Potential PET Imaging Biomarkers to Explore Neuroinflammation in Dementia

The most frequently studied target of neuroinflammation using PET is 18-kDa translocator protein, but its limitations have spurred the molecular imaging community to find more promising targets. This article reviews the development of PET radioligands for cyclooxygenase (COX) subtypes 1 and 2, enzymes that catalyze the production of inflammatory prostanoids in the periphery and brain. Although both isozymes produce the same precursor compound, prostaglandin H₂, they have distinct functions based on their differential cellular localization in the periphery and brain. For example, COX-1 is located primarily in microglia, a resident inflammatory cell in the brain whose role in producing inflammatory cytokines is well documented. In contrast, COX-2 is located primarily in neurons and can be markedly upregulated by inflammatory and excitatory stimuli, but its functions are poorly understood. This article reviews these 2 isozymes as biomarkers of neuroinflammation, as well as the radioligands that have recently been developed to image them in animals and humans. To place this work into context, the properties of COX-1 and COX-2 are compared with 18-kDa translocator protein, with special consideration of their application in Alzheimer disease as a representative neurodegenerative disorder.

[18F]Fluoro-DCP, a first generation PET radiotracer for monitoring protein sulfenylation in vivo

Redox metabolism plays essential functions in the pathology of cancer and many other diseases. While several radiotracers for imaging redox metabolism have been developed, there are no reports of radiotracers for in vivo imaging of protein oxidation. Here we take the first step towards this goal and describe the synthesis and kinetic properties of a new positron emission tomography (PET) [18F]Fluoro-DCP radiotracer for in vivo imaging of protein sulfenylation. Time course biodistribution and PET/CT studies using xenograft animal models of Head and Neck Squamous Cell Cancer (HNSCC) demonstrate its capability to distinguish between tumors with radiation sensitive and resistant phenotypes consistent with previous reports of decreased protein sulfenylation in clinical specimens of radiation resistant HNSCC. We envision further development of this technology to aid research efforts towards improving diagnosis of patients with radiation resistant tumors.

Radiosynthesis and in Vivo and ex Vivo Evaluation of Isomeric [11C]methoxy Analogs of Nimesulide as Brain Cyclooxygenase-2-Targeted Imaging Agents

Our previous studies identified that nimesulide analogs which bear a methoxy substituent at the para-position of the phenyl ring could be potential radiotracer candidates for detecting disorders related to cyclooxygenase-2 (COX-2) expression and activity in vivo using positron emission tomography (PET) in the brain. The present study was conducted to evaluate the in vivo characteristics of ¹¹C-labeled para-methoxy nimesulide ([¹¹C]1d) as a brain COX-2-targeted imaging agent compared to other isomeric methoxy analogs of nimesulide ([¹¹C]1b and [¹¹C]1c). [¹¹C]1b-d were synthesized with reasonable yield and purity by the methylation of the O-desmethyl precursor with [¹¹C]methyl triflate in the presence of NaOH at room temperature. We performed in vivo biodistribution analysis, brain PET imaging, ex vivo autoradiography, and metabolite analysis in mice. The uptake of [¹¹C]1b-d was lower in the brain than in other tissues, including in the blood, and both [¹¹C]1c and [¹¹C]1d were rapidly metabolized. However, [¹¹C]1d showed a small, but significant, specific signal and heterogeneous distribution in the brain. In vivo evaluation suggested that [¹¹C]1d might correlate with COX-2 expression in the brain. Given its instability in vivo, [¹¹C]1d seems unsuitable as a brain-COX-2 radioimaging agent. Further structural refinement of these radiotracers is necessary to enhance their uptake in the brain and to achieve sufficient metabolic stability.

Development of PET Radioligands Targeting COX-2 for Colorectal Cancer Staging, a Review of in vitro and Preclinical Imaging Studies

Colorectal cancer (CRC) is the second most common cause of cancer death, making early diagnosis a major public health challenge. The role of inflammation in tumorigenesis has been extensively explored, and among the identified markers of inflammation, cyclooxygenase-2 (COX-2) expression seems to be linked to lesions with a poor prognosis. Until now, COX-2 expression could only be accessed by invasive methods, mainly by biopsy. Imaging techniques such as functional Positron Emission Tomography (PET) could give access to in vivo COX-2 expression. This could make the staging of the disease more accurate and would be of particular interest in the exploration of the first metastatic stages. In this paper, we review recent progress in the development of COX-2 specific PET tracers by comparing the radioligands' characteristics and highlighting the obstacles that remain to be overcome in order to achieve the clinical development of such a radiotracer, and its evaluation in the management of CRC.

Deuteration versus ethylation - strategies to improve the metabolic fate of an 18F-labeled celecoxib derivative

The inducible isoenzyme cyclooxygenase-2 (COX-2) is closely associated with chemo-/radioresistance and poor prognosis of solid tumors. Therefore, COX-2 represents an attractive target for functional characterization of tumors by positron emission tomography (PET). In this study, the celecoxib derivative 3-([¹⁸F]fluoromethyl)-1-[4-(methylsulfonyl)phenyl]-5-(p-tolyl)-1H-pyrazole ([¹⁸F]5a) was chosen as a lead compound having a reported high COX-2 inhibitory potency and a potentially low carbonic anhydrase binding tendency. The respective deuterated analog [D₂,¹⁸F]5a and the fluoroethyl-substituted derivative [¹⁸F]5b were selected to study the influence of these modifications with respect to COX inhibition potency in vitro and metabolic stability of the radiolabeled tracers in vivo. COX-2 inhibitory potency was found to be influenced by elongation of the side chain but, as expected, not by deuteration. An automated radiosynthesis comprising ¹⁸F-fluorination and purification under comparable conditions provided the radiotracers [¹⁸F]5a,b and [D₂,¹⁸F]5a in good radiochemical yields (RCY) and high radiochemical purity (RCP). Biodistribution and PET studies comparing all three compounds revealed bone accumulation of ¹⁸F-activity to be lowest for the ethyl derivative [¹⁸F]5b. However, the deuterated analog [D₂,¹⁸F]5a turned out to be the most stable compound of the three derivatives studied here. Time-dependent degradation of [¹⁸F]5a,b and [D₂,¹⁸F]5a after incubation in murine liver microsomes was in accordance with the data on metabolism in vivo. Furthermore, metabolites were identified based on UPLC-MS/MS.

The aim of this study is to investigate the influence of deuteration and elongation on an ¹⁸F-labeled COX-2 inhibitor with focus on metabolic stability to develop suitable COX-2 targeting radiotracers.

Radiosynthesis and evaluation of [18F]FMTP, a COX-2 PET ligand

BACKGROUND

The upregulation of cyclooxygenase-2 (COX-2) is involved in neuroinflammation associated with many neurological diseases as well as cancers of the brain. Outside the brain, inflammation and COX-2 induction contribute to the pathogenesis of pain, arthritis, acute allograft rejection, and in response to infections, tumors, autoimmune disorders, and injuries. Herein, we report the radiochemical synthesis and evaluation of [¹⁸F]6-fluoro-2-(4-(methylsulfonyl)phenyl)-N-(thiophen-2-ylmethyl)pyrimidin-4-amine ([¹⁸F]FMTP), a high-affinity COX-2 inhibitor, by cell uptake and PET imaging studies.

METHODS

The radiochemical synthesis of [¹⁸F]FMTP was optimized using chlorine to fluorine displacement method, by reacting [¹⁸F]fluoride/K222/K₂CO₃ with the precursor molecule. Cellular uptake studies of [¹⁸F]FMTP was performed in COX-2 positive BxPC3 and COX-2 negative PANC-1 cell lines with unlabeled FMTP as well as celecoxib to define specific binding agents. Dynamic microPET image acquisitionwas performed in anesthetized nude mice (n = 3), lipopolysaccharide (LPS) induced neuroinflammation mice (n = 4), and phosphate-buffered saline (PBS) administered control mice (n = 4) using a Trifoil microPET/CT for a scan period of 60 min.

RESULTS

A twofold higher binding of [¹⁸F]FMTP was found in COX-2 positive BxPC3 cells compared with COX-2 negative PANC-1 cells. The radioligand did not show specific binding to COX-2 negative PANC-1 cells. MicroPET imaging in wild-type mice indicated blood-brain barrier (BBB) penetration and fast washout of [¹⁸F]FMTP in the brain, likely due to the low constitutive COX-2 expression in the normal brain. In contrast, a ~ twofold higher uptake of the radioligand was found in LPS-induced mice brain than PBS treated control mice.

CONCLUSIONS

Specific binding to COX-2 in BxPC3 cell lines, BBB permeability, and increased brain uptake in neuroinflammation mice qualifies [¹⁸F]FMTP as a potential PET tracer for studying inflammation.

PET measurement of cyclooxygenase-2 using a novel radioligand: upregulation in primate neuroinflammation and first-in-human study

Background

Cyclooxygenase-2 (COX-2), which is rapidly upregulated by inflammation, is a key enzyme catalyzing the rate-limiting step in the synthesis of several inflammatory prostanoids. Successful positron emission tomography (PET) radioligand imaging of COX-2 in vivo could be a potentially powerful tool for assessing inflammatory response in the brain and periphery. To date, however, the development of PET radioligands for COX-2 has had limited success.

Methods

The novel PET tracer [¹¹C]MC1 was used to examine COX-2 expression [1] in the brains of four rhesus macaques at baseline and after injection of the inflammogen lipopolysaccharide (LPS) into the right putamen, and [2] in the joints of two human participants with rheumatoid arthritis and two healthy individuals. In the primate study, two monkeys had one LPS injection, and two monkeys had a second injection 33 and 44 days, respectively, after the first LPS injection. As a comparator, COX-1 expression was measured using [¹¹C]PS13.

Results

COX-2 binding, expressed as the ratio of specific to nondisplaceable uptake (BP_ND) of [¹¹C]MC1, increased on day 1 post-LPS injection; no such increase in COX-1 expression, measured using [¹¹C]PS13, was observed. The day after the second LPS injection, a brain lesion (~ 0.5 cm in diameter) with high COX-2 density and high BP_ND (1.8) was observed. Postmortem brain analysis at the gene transcript or protein level confirmed in vivo PET results. An incidental finding in an unrelated monkey found a line of COX-2 positivity along an incision in skull muscle, demonstrating that [¹¹C]MC1 can localize inflammation peripheral to the brain. In patients with rheumatoid arthritis, [¹¹C]MC1 successfully imaged upregulated COX-2 in the arthritic hand and shoulder and apparently in the brain. Uptake was blocked by celecoxib, a COX-2 preferential inhibitor.

Conclusions

Taken together, these results indicate that [¹¹C]MC1 can image and quantify COX-2 upregulation in both monkey brain after LPS-induced neuroinflammation and in human peripheral tissue with inflammation.

Trial registration

ClinicalTrials.gov NCT03912428. Registered April 11, 2019.

Synthesis and Cyclooxygenase Inhibition of Sulfonamide-Substituted (Dihydro)Pyrrolo[3,2,1-hi]indoles and Their Potential Prodrugs

Non-invasive imaging of cyclooxygenase-2 (COX-2) by radiolabeled ligands is attractive for the diagnosis of cancer, and novel highly affine leads with optimized pharmacokinetic profile are of great interest for future developments. Recent findings have shown that methylsulfonyl-substituted (dihydro)pyrrolo[3,2,1-hi]indoles represent highly potent and selective COX-2 inhibitors but possess unsuitable pharmacokinetic properties for radiotracer applications. Based on these results, we herein present the development and evaluation of a second series of sulfonamide-substituted (dihydro)pyrrolo[3,2,1-hi]indoles and their conversion into the respective more hydrophilic N-propionamide-substituted analogs. In comparison to the methylsulfonyl-substituted leads, COX inhibition potency and selectivity was retained in the sulfonamide-substituted compounds; however, the high lipophilicity might hinder their future use. The N-propionamide-substituted analogs showed a significantly decreased lipophilicity and, as expected, lower or no COX-inhibition potency. Hence, the N-(sulfonyl)propionamides can be regarded as potential prodrugs, which represents a potential approach for more sophisticated radiotracer developments.

Radiosynthesis and Preclinical Evaluation of 11C-VA426, a Cyclooxygenase-2 Selective Ligand

Cyclooxygenase-2 (COX-2) is involved in the inflammatory response, and its recurrent overexpression in cancers as well as in neurodegenerative disorders has made it an important target for therapy. For this reason, noninvasive imaging of COX-2 expression may represent an important diagnostic tool. In this work, a COX-2 inhibitor analogue, VA426 [1-(4-fluorophenyl)-3-(2-methoxyethyl)-2-methyl-5-(4-(methylsulfonil)phenyl)-1H-pyrrole], was synthesized and radiolabelled with the 11C radioisotope. The ex vivo biodistribution profile of 11C-VA426 was evaluated in the brain and periphery of healthy rats and mice and in brain and periphery of inflammation models, based on the administration of LPS. 11C-VA426 synthesis with the tBuOK base showed optimal radiochemical yield (15 ± 2%) based on triflate activity, molar activity (range 37-148 GBq/μmol), and radiochemical purity (>95%). Ex vivo biodistribution studies showed a fast uptake of radioactivity but a rapid washout, except in regions expressing COX-2 (lungs, liver, and kidney) both in rats and in mice, with maximum values at 30 and 10 minutes p.i., respectively. LPS administration did not show significant effect on radioactivity accumulation. Celecoxib competition experiments performed in rats and mice treated with LPS produced a general target unrelated reduction of radioactivity concentration in all peripheral tissues and brain areas examined. Finally, in agreement with the negative results obtained from biodistribution experiments, radiometabolites analysis revealed that 11C-VA426 is highly unstable in vivo. This study indicates that the compound 11C-VA426 is not currently suitable to be used as radiopharmaceutical for PET imaging. This family of compounds needs further implementation in order to improve in vivo stability.

Initial biological evaluations of 18F-KS1, a novel ascorbate derivative to image oxidative stress in cancer

Background

Reactive oxygen species (ROS)-induced oxidative stress damages many cellular components such as fatty acids, DNA, and proteins. This damage is implicated in many disease pathologies including cancer and neurodegenerative and cardiovascular diseases. Antioxidants like ascorbate (vitamin C, ascorbic acid) have been shown to protect against the deleterious effects of oxidative stress in patients with cancer. In contrast, other data indicate potential tumor-promoting activity of antioxidants, demonstrating a potential temporal benefit of ROS. However, quantifying real-time tumor ROS is currently not feasible, since there is no way to directly probe global tumor ROS. In order to study this ROS-induced damage and design novel therapeutics to prevent its sequelae, the quantitative nature of positron emission tomography (PET) can be harnessed to measure in vivo concentrations of ROS. Therefore, our goal is to develop a novel translational ascorbate-based probe to image ROS in cancer in vivo using noninvasive PET imaging of tumor tissue. The real-time evaluations of ROS state can prove critical in developing new therapies and stratifying patients to therapies that are affected by tumor ROS.

Methods

We designed, synthesized, and characterized a novel ascorbate derivative (E)-5-(2-chloroethylidene)-3-((4-(2-fluoroethoxy)benzyl)oxy)-4-hydroxyfuran-2(5H)-one (KS1). We used KS1 in an in vitro ROS MitoSOX-based assay in two different head and neck squamous cancer cells (HNSCC) that express different ROS levels, with ascorbate as reference standard. We radiolabeled ¹⁸F-KS1 following ¹⁸F-based nucleophilic substitution reactions and determined in vitro reactivity and specificity of ¹⁸F-KS1 in HNSCC and prostate cancer (PCa) cells. MicroPET imaging and standard biodistribution studies of ¹⁸F-KS1 were performed in mice bearing PCa cells. To further demonstrate specificity, we performed microPET blocking experiments using nonradioactive KS1 as a blocker.

Results

KS1 was synthesized and characterized using ¹H NMR spectra. MitoSOX assay demonstrated good correlations between increasing concentrations of KS1 and ascorbate and increased reactivity in SCC-61 cells (with high ROS levels) versus rSCC-61cells (with low ROS levels). ¹⁸F-KS1 was radiolabeled with high radiochemical purity (> 94%) and specific activity (~ 100 GBq/μmol) at end of synthesis (EOS). Cell uptake of ¹⁸F-KS1 was high in both types of cancer cells, and the uptake was significantly blocked by nonradioactive KS1, and the ROS blocker, superoxide dismutase (SOD) demonstrating specificity. Furthermore, ¹⁸F-KS1 uptake was increased in PCa cells under hypoxic conditions, which have been shown to generate high ROS. Initial in vivo tumor uptake studies in PCa tumor-bearing mice demonstrated that ¹⁸F-KS1 specifically bound to tumor, which was significantly blocked (threefold) by pre-injecting unlabeled KS1. Furthermore, biodistribution studies in the same tumor-bearing mice showed high tumor to muscle (target to nontarget) ratios.

Conclusion

This work demonstrates the strong preliminary support of ¹⁸F-KS1, both in vitro and in vivo for imaging ROS in cancer. If successful, this work will provide a new paradigm to directly probe real-time oxidative stress levels in vivo. Our work could enhance precision medicine approaches to treat cancer, as well as neurodegenerative and cardiovascular diseases affected by ROS.

Electronic supplementary material

The online version of this article (10.1186/s13550-019-0513-x) contains supplementary material, which is available to authorized users.

Pyrimidine-based fluorescent COX-2 inhibitors: synthesis and biological evaluation

The cyclooxygenase-2 (COX-2) enzyme is overexpressed in a variety of cancers and mediates inflammatory processes that aid the growth and progression of malignancies. Three novel and selective fluorescent COX-2 inhibitors have been designed and synthesized on the basis of previously reported pyrimidine-based COX-2 inhibitors and the 7-nitrobenzofurazan fluorophore. In vitro evaluation of COX-1/COX-2 isozyme inhibition identified N-(2-((7-nitro-benzo[c][1,2,5]oxadiazol-4-yl)amino)propyl)-4-[4-(methylsulfonyl)phenyl]-6-(trifluoro-methyl)-pyrimidin-2-amine (6) as a novel potent and selective COX-2 inhibitor (IC50 = 1.8 μM). Lead compound (6) was further evaluated for its ability to selectively visualize COX-2 isozyme in COX-2 expressing human colon cancer cell line HCA-7 using confocal microscopy experiments.

Design of selective COX-2 inhibitors in the (aza)indazole series. Chemistry, in vitro studies, radiochemistry and evaluations in rats of a [18F] PET tracer

A series of novel derivatives exhibiting high affinity and selectivity towards the COX-2 enzyme in the (aza) indazole series was developed. A short synthetic route involving a bromination/arylation sequence under microwave irradiation and direct C–H activation were established in the indazole and azaindazole series respectively. In vitro assays were conducted and structural modifications were carried out on these scaffolds to furnish compound 16 which exhibited effective COX-2 inhibitory activity, with IC₅₀ values of 0.409 µM and an excellent selectivity versus COX-1. Radiolabeling of this most potent derivative [¹⁸F]16 was achieved after boron ester release and the tracer was evaluated in vivo in a rat model of neuroinflammation. All chemistry, radiochemistry and biological experimental data are discussed.

In vivo evaluation of [11C]TMI, a COX-2 selective PET tracer, in baboons

Overexpression of Cyclooxygenase-2 (COX-2) enzyme is associated with the pathogenesis of inflammation, cancers, stroke, arthritis, and neurological disorders. Because of the involvement of COX-2 in these diseases, quantification of COX-2 expression using Positron Emission Tomography (PET) may be a biological marker for early diagnosis, monitoring of disease progression, and an indicator of effective treatment. At present there is no target-specific or validated PET tracer available for in vivo quantification of COX-2. The objective of this study is to evaluate [¹¹C]TMI, a selective COX-2 inhibitor (Ki ≤ 1 nM) in nonhuman primates using PET imaging. PET imaging in baboons showed that [¹¹C]TMI penetrates the blood brain barrier (BBB) and accumulates in brain in a somewhat heterogeneous pattern. Metabolite analyses indicated that [¹¹C]TMI undergoes no significant metabolism of parent tracer in the plasma for baseline scans, however a relative faster metabolism was found for blocking scan. All the tested quantification approaches provide comparable tracer total distribution volume (V_T) estimates in the range of 3.2-7 (mL/cm³). We observed about 25% lower V_T values in blocking studies with meloxicam, a nonselective COX-2 inhibitor, compared to baseline [¹¹C]TMI binding. Our findings indicate that [¹¹C]TMI may be a suitable PET tracer for the quantification of COX-2 in vivo. Further experiments are needed to confirm the potential of this tracer in COX-2 overexpressing models for brain diseases.

Design, synthesis and biological evaluation of novel ferrocene-pyrazole derivatives containing nitric oxide donors as COX-2 inhibitors for cancer therapy

A series of novel ferrocene-pyrazole derivatives containing nitric oxide donors as COX-2 inhibitors for cancer therapy were designed, synthesized and biologically evaluated. Among them, compound 7l displayed the most potent inhibitory against COX-2 (IC₅₀ = 0.82 μM) and antiproliferative activities against Hela cells (IC₅₀ = 0.34 μM) compared with Celecoxib (IC₅₀ = 0.38 and 7.91 μM). The further mechanistic studies revealed that 7l could induce apoptosis of Hela cells by mitochondrial depolarization and the antiproliferative activities of 7l were positively correlated with the levels of intracellular NO release in Hela cells. Most notably, 7l could dramatically suppress tumor growth in Hela cells xenografted mouse model. In summary, compound 7l may be promising candidates for cancer therapy.

Radiosynthesis and in vivo evaluation of [11C]MOV as a PET imaging agent for COX-2

Radiosynthesis and in vivo evaluation of [¹¹C]4-[5-(4-methylphenyl)-3-(trifluoromethyl)-1H-pyrazol-1-yl]benzenesulfonamide (methoxy analogue of valdecoxib, [¹¹C]MOV), a COX-2 inhibitor, was conducted in rat and baboon. Synthesis of the reference standard MOV (3), and its desmethyl precursor 2 for radiolabeling were performed using 1,2-diphenylethan-1-one as the starting material in five steps with 15% overall yield. Radiosynthesis of [¹¹C]MOV was accomplished in 40 ± 10% yield and >99% radiochemical purity by reacting the precursor 2 in dimethyl formamide (DMF) with [¹¹C]CH₃I followed by removal of the dimethoxytrityl (DMT) protective group using trifluroacetic acid. PET studies in anesthetized baboon showed very low uptake and homogeneous distribution of [¹¹C]MOV in brain. The radioligand underwent rapid metabolism in baboon plasma. MicroPET studies in male Sprague Dawley rats revealed [¹¹C]MOV binding in lower thorax. The tracer binding in rats was partially blocked in heart and duodenum by the administration of 1 mg/kg oral dose of COX-2 inhibitor valdecoxib.

Impact of structural alterations on the radiopharmacological profile of 18F-labeled pyrimidines as cyclooxygenase-2 (COX-2) imaging agents

INTRODUCTION

Non-invasive imaging of COX-2 in cancer represents a powerful tool for assessing COX-2-mediated effects on chemoprevention and radiosensitization using potent and selective COX-2 inhibitors as an emerging class of anticancer drugs. Careful assessment of the pharmacokinetic profile of radiolabeled COX-2 inhibitors is of crucial importance for the development of suitable radiotracers for COX-2 imaging in vivo. The delicate balance between the selection of typical COX-2 pharmacophores and the resulting physicochemical characteristics of the COX-2 inhibitor represents a formidable challenge for the search of radiolabeled COX-2 imaging agents. Several pyrimidine-based COX-2 inhibitors demonstrated favorable in vitro and in vivo COX-2 imaging properties in various COX-2 expressing cancer cell lines. Here, we describe a comparative radiopharmacological study of three ¹⁸F-labeled COX-2 inhibitors based on a pyrimidine scaffold. The objective of this study was to investigate how subtle structural alterations influence the pharmacokinetic profile of lead compound [¹⁸F]1a ([¹⁸F]Pyricoxib) to afford ¹⁸F-labeled pyrimidine-based COX-2 inhibitors with improved COX-2 imaging properties in vivo.

METHODS

Radiosynthesis of radiotracers was accomplished through reaction with 4-[¹⁸F]fluorobenzyl amine on a methyl-sulfone labeling precursor ([¹⁸F]1a and [¹⁸F]2a) or late-stage radiofluorination using a iodyl-containing labeling precursor ([¹⁸F]3a). Radiopharmacological profile of ¹⁸F-labeled pyrimidine-based COX-2 inhibitors [¹⁸F]1a, [¹⁸F]2a and [¹⁸F]3a was studied in COX-2-expressing human HCA-7 colorectal cancer cell line, including cellular uptake studies in HCA-7 cells and dynamic PET imaging studies in HCA-7 xenografts.

RESULTS

Cellular uptake of radiotracers [¹⁸F]2a and [¹⁸F]3a in HCA-7 cells was 450% and 300% radioactivity/mg protein, respectively, after 90 min incubation, compared to 600% radioactivity/mg protein for radiotracer [¹⁸F]1a. Dynamic PET imaging studies revealed a tumor SUV of 0.53 ([¹⁸F]2a) and 0.54 ([¹⁸F]3a) after 60 min p.i. with a tumor-to-muscle ratio of ~1. Tumor SUV for [¹⁸F]1a (60 min p.i.) was 0.76 and a tumor-to-muscle ratio of ~1.5. Pyricoxib analogues [¹⁸F]2a and [¹⁸F]3a showed distinct pharmacokinetic profiles in comparison to lead compound [¹⁸F]1a with a significantly improved lung clearance pattern. Replacing the 4-[¹⁸F]fluorobenzyl amine motif in radiotracer [¹⁸F]1a with a 4-[¹⁸F]fluorobenzyl alcohol motif in radiotracer [¹⁸F]3a resulted in re-routing of the metabolic pathway as demonstrated by a more rapid liver clearance and higher initial kidney uptake and more rapid kidney clearance compared to radiotracers [¹⁸F]1a and [¹⁸F]2a. Moreover, radiotracer [¹⁸F]3a displayed favorable rapid brain uptake and retention.

CONCLUSION

The radiopharmacological profile of three ¹⁸F-labeled COX-2 inhibitors based on a pyrimidine scaffold were evaluated in COX-2 expressing human colorectal cancer cell line HCA-7 and HCA-7 xenografts in mice. Despite the overall structural similarity and comparable COX-2 inhibitory potency of all three radiotracers, subtle structural alterations led to significantly different in vitro and in vivo metabolic profiles.

ADVANCES IN KNOWLEDGE

Among all tested pyrimidine-based ¹⁸F-labeled COX-2 inhibitors, lead compound [¹⁸F]1a remains the most suitable radiotracer for assessing COX-2 expression in vivo. Radiotracer [¹⁸F]3a showed significantly improved first pass pulmonary passage in comparison to radiotracer [¹⁸F]1a and might represents a promising lead compound for the development of radiotracers for PET imaging of COX-2 in neuroinflammation.

PET Imaging of Microglial Activation-Beyond Targeting TSPO

Neuroinflammation, which involves microglial activation, is thought to play a key role in the development and progression of neurodegenerative diseases and other brain pathologies. Positron emission tomography is an ideal imaging technique for studying biochemical processes in vivo, and particularly for studying the living brain. Neuroinflammation has been traditionally studied using radiotracers targeting the translocator protein 18 kDa, but this comes with certain limitations. The current review describes alternative biological targets that have gained interest for the imaging of microglial activation over recent years, such as the cannabinoid receptor type 2, cyclooxygenase-2, the P2X₇ receptor and reactive oxygen species, and some promising radiotracers for these targets. Although many advances have been made in the field of neuroinflammation imaging, current radiotracers all target the pro-inflammatory (M1) phenotype of activated microglia, since the number of known biological targets specific for the anti-inflammatory (M2) phenotype that are also suited as a target for radiotracer development is still limited. Next to proceeding the currently available tracers for M1 microglia into the clinic, the development of a suitable radiotracer for M2 microglia would mean a great advance in the field, as this would allow for imaging of the dynamics of microglial activation in different diseases.

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

Boron neutron capture therapy (BNCT) is a binary therapy that employs neutron irradiation on the boron agents to release high-energy helium and alpha particles to kill cancer cells. An optimal response to BNCT depends critically on the time point of maximal ¹⁰B accumulation and highest tumor to normal ratio (T/N) for performing the neutron irradiation. The aggressive cholangiocarcinoma (CCA) representing a liver cancer that overexpresses COX-2 enzyme is aimed to be targeted by COX-2 selective boron carrier, fenbufen boronopinacol (FBPin). Two main works were performed including: 1) chemical synthesis of FBPin as the boron carrier and 2) radiochemical labeling with F-18 to provide the radiofluoro congener, m-[¹⁸F]fluorofenbufen ester boronopinacol (m-[¹⁸F]FFBPin), to assess the binding affinity, cellular accumulation level and distribution profile in CCA rats. FBPin was prepared from bromofenbufen via 3 steps with 82% yield. The binding assay employed [¹⁸F]FFBPin to compete FBPin for binding to COX-1 (IC₅₀=0.91±0.68μM) and COX-2 (IC₅₀=0.33±0.24μM). [¹⁸F]FFBPin-derived 60-min dynamic PET scans predict the ¹⁰B-accumulation of 0.8-1.2ppm in liver and 1.2-1.8ppm in tumor and tumor to normal ratio=1.38±0.12. BNCT was performed 40-55min post intravenous administration of FBPin (20-30mg) in the CCA rats. CCA rats treated with BNCT display more tumor reduction than that by NCT with respect of 2-[¹⁸F]fluoro-2-deoxy glucose uptake in the tumor region of interest, 20.83±3.00% (n=12) vs. 12.83±3.79% (n=10), P=0.05. The visualizing agent [¹⁸F]FFBPin resembles FBPin to generate the time-dependent boron concentration profile. Optimal neutron irradiation period is thus determinable for BNCT. A boron-substituted agent based on COX-2-binding features has been prepared. The moderate COX-2/COX-1 selectivity index of 2.78 allows a fair tumor selectivity index of 1.38 with a mild cardiovascular effect. The therapeutic effect from FBPin with BNCT warrants a proper COX-2 targeting of boron NSAIDs.

PET imaging of cyclooxygenase-2 (COX-2) in a pre-clinical colorectal cancer model

Background

Cyclooxygenase-2 (COX-2) is the inducible isoform of the cyclooxygenase enzyme family. COX-2 is involved in tumor development and progression, and frequent overexpression of COX-2 in a variety of human cancers has made COX-2 an important drug target for cancer treatment. Non-invasive imaging of COX-2 expression in cancer would be useful for assessing COX-2-mediated effects on chemoprevention and radiosensitization using COX-2 inhibitors as an emerging class of anti-cancer drugs, especially for colorectal cancer. Herein, we describe the radiopharmacological analysis of [¹⁸F]Pyricoxib, a novel radiolabeled COX-2 inhibitor, for specific PET imaging of COX-2 in colorectal cancer.

Methods

Uptake of [¹⁸F]Pyricoxib was assessed in human colorectal cancer cell lines HCA-7 (COX-2 positive) and HCT-116 (COX-2 negative). Standard COX-2 inhibitors were used to test for specificity of [¹⁸F]Pyricoxib for COX-2 binding in vitro and in vivo. PET imaging, biodistribution, and radiometabolite analyses were included into radiopharmacological evaluation of [¹⁸F]Pyricoxib.

Results

Radiotracer uptake in COX-2 positive HCA-7 cells was significantly higher than in COX-2 negative HCT-116 cells (P < 0.05). COX-2 inhibitors, celecoxib, rofecoxib, and SC58125, blocked uptake of [¹⁸F]Pyricoxib in HCA-7 cells in a concentration-dependent manner. The radiotracer was slowly metabolized in mice, with approximately 60 % of intact compound after 2 h post-injection. Selective COX-2-mediated tumor uptake of [¹⁸F]Pyricoxib in HCA-7 xenografts was confirmed in vivo. Celecoxib (100 mg/kg) selectively blocked tumor uptake by 16 % (PET image analysis; P < 0.05) and by 51 % (biodistribution studies; P < 0.01).

Conclusions

The novel PET radiotracer [¹⁸F]Pyricoxib displays a promising radiopharmacological profile to study COX-2 expression in cancer in vivo.

Electronic supplementary material

The online version of this article (doi:10.1186/s13550-016-0192-9) contains supplementary material, which is available to authorized users.

Synthesis of 2-(methylsulfonyl)-5-(4-(methylsulfonyl) phenyl)-4-phenyl-1H-[5-(14)C]imidazole, a selective COX-2 inhibitor, via asymmetrical benzoins

4,5-Diarylimidazoles labeled with carbon-14 in the 5-position of the imidazole ring were prepared as a part of three-step sequence from 2-hydroxy-1-(4-(methylthio)phenyl)-2-phenyl[1-(14) C]ethanone as a key synthetic intermediate which has been synthesized from potassium [(14) C]cyanide.

Synthesis of dihydropyrazole sulphonamide derivatives that act as anti-cancer agents through COX-2 inhibition

COX-2 has long been exploited in the treatment of inflammation and relief of pain; however, research increasingly suggests COX-2 inhibitors might possess potential benefits to thwart tumour processes. In the present study, we designed a series of novel COX-2 inhibitors based on analysis of known inhibitors combined with an in silico scaffold modification strategy. A docking simulation combined with a primary screen in vitro were performed to filter for the lead compound, which was then substituted, synthesized and evaluated by a variety of bioassays. Derivative 4d was identified as a potent COX-2 enzyme inhibitor and exerted an anticancer effect through COX-2 inhibition. Further investigation confirmed that 4d could induce A549 cell apoptosis and arrest the cell cycle at the G2/M phase. Moreover, treatment with 4d reduced A549 cell adhesive ability and COX-2 expression. The morphological variation of treated cells was also visualized by confocal microscopy. Overall, the biological profile of 4d suggests that this compound may be developed as a potential anticancer agent.