Biologically stable [(18)F]-labeled benzylfluoride derivatives

Development of a Novel 18F-Labeled Probe for PET Imaging of Estrogen Receptor β

Estrogen receptor beta (ERβ) is an important ER subtype that plays crucial roles in many physiological and pathological disorders. Herein, we developed the probe [¹⁸F]PVBO for in vivo ERβ targeted PET imaging and obtained promising results. The nonradioactive PVBO showed a 12.5-fold stronger binding affinity to ERβ than to ERα in vitro. In vitro assays revealed the specific uptake of [¹⁸F]PVBO by DU145 cells. The uptake of [¹⁸F]PVBO by DU145 xenografts increased during the 120 min dynamic scanning, with a maximum uptake of 2.80 ± 0.30% ID/g. Based on time activity curves (TACs), the injection of [¹⁸F]PVBO with unlabeled PVBO or ERB-041 resulted in a significant signal reduction with the tumor/muscle (T/M) ratio <1 at 30, 60, 75, and 120 min post-injection (p < 0.05). [¹⁸F]PVBO demonstrates the feasibility of noninvasively imaging ERβ-positive tumors by small-animal PET and provides a new strategy for visualizing ERβ in vivo.

Translating a radiolabeled imaging agent to the clinic

Molecular Imaging is entering the most fruitful, exciting period in its history with many new agents under development, and several reaching the clinic in recent years. While it is unusual for just one laboratory to take an agent from initial discovery through to full clinical approval the steps along the way are important to understand for all interested participants even if one is not involved in the entire process. Here, we provide an overview of these processes beginning at discovery and preclinical validation of a new molecular imaging agent and using as an exemplar a low molecular weight disease-specific targeted positron emission tomography (PET) agent. Compared to standard drug development requirements, molecular imaging agents may benefit from a regulatory standpoint from their low mass administered doses, they nonetheless still need to go through a series of well-defined steps before they can be considered for Phase 1 human testing. After outlining the discovery and preclinical validation approaches, we will also discuss the nuances of Phase 1, Phase 2 and Phase 3 studies that may culminate in an FDA general use approval. Finally, some post-approval aspects of novel molecular imaging agents are considered.

Methods to Enhance the Metabolic Stability of Peptide-Based PET Radiopharmaceuticals

The high affinity and specificity of peptides towards biological targets, in addition to their favorable pharmacological properties, has encouraged the development of many peptide-based pharmaceuticals, including peptide-based positron emission tomography (PET) radiopharmaceuticals. However, the poor in vivo stability of unmodified peptides against proteolysis is a major challenge that must be overcome, as it can result in an impractically short in vivo biological half-life and a subsequently poor bioavailability when used in imaging and therapeutic applications. Consequently, many biologically and pharmacologically interesting peptide-based drugs may never see application. A potential way to overcome this is using peptide analogues designed to mimic the pharmacophore of a native peptide while also containing unnatural modifications that act to maintain or improve the pharmacological properties. This review explores strategies that have been developed to increase the metabolic stability of peptide-based pharmaceuticals. It includes modifications of the C- and/or N-termini, introduction of d- or other unnatural amino acids, backbone modification, PEGylation and alkyl chain incorporation, cyclization and peptide bond substitution, and where those strategies have been, or could be, applied to PET peptide-based radiopharmaceuticals.

The Dark Side of Fluorine

Despite the perceived stability of the C-F bond, chemical instability and drug-metabolizing enzymes can lead to its cleavage. The resulting release of fluoride and formation of certain metabolites may cause safety issues and warrant the medicinal chemists' attention.

Radiosynthesis and 'click' conjugation of ethynyl-4-[(18)F]fluorobenzene--an improved [(18)F]synthon for indirect radiolabeling

Reproducible methods for [(18)F]radiolabeling of biological vectors are essential for the development of new [(18)F]radiopharmaceuticals. Molecules such as carbohydrates, peptides and proteins are challenging substrates that often require multi-step indirect radiolabeling methods. With the goal of developing more robust, time saving, and less expensive procedures for indirect [(18)F]radiolabeling of such molecules, our group has synthesized ethynyl-4-[(18)F]fluorobenzene ([(18)F]2, [(18)F]EYFB) in a single step (14 ± 2% non-decay corrected radiochemical yield (ndc RCY)) from a readily synthesized, shelf stable, inexpensive precursor. The alkyne-functionalized synthon [(18)F]2 was then conjugated to two azido-functionalized vector molecules via CuAAC reactions. The first 'proof of principle' conjugation of [(18)F]2 to 1-azido-1-deoxy-β-D-glucopyranoside (3) gave the desired radiolabeled product [(18)F]4 in excellent radiochemical yield (76 ± 4% ndc RCY (11% overall)). As a second example, the conjugation of [(18)F]2 to matrix-metalloproteinase inhibitor (5), which has potential in tumor imaging, gave the radiolabeled product [(18)F]6 in very good radiochemical yield (56 ± 12% ndc RCY (8% overall)). Total preparation time for [(18)F]4 and [(18)F]6 including [(18)F]F(-) drying, two-step reaction (nucleophilic substitution and CuAAC conjugation), two HPLC purifications, and two solid phase extractions did not exceed 70 min. The radiochemical purity of synthon [(18)F]2 and the conjugated products, [(18)F]4 and [(18)F]6, were all greater than 98%. The specific activities of [(18)F]2 and [(18)F]6 were low, 5.97 and 0.17 MBq nmol(-1), respectively.

Ortho-Stabilized (18) F-Azido Click Agents and their Application in PET Imaging with Single-Stranded DNA Aptamers

Azido (18) F-arenes are important and versatile building blocks for the radiolabeling of biomolecules via Huisgen cycloaddition ("click chemistry") for positron emission tomography (PET). However, routine access to such clickable agents is challenged by inefficient and/or poorly defined multistep radiochemical approaches. A high-yielding direct radiofluorination for azido (18) F-arenes was achieved through the development of an ortho-oxygen-stabilized iodonium derivative (OID). This OID strategy addresses an unmet need for a reliable azido (18) F-arene clickable agent for bioconjugation reactions. A ssDNA aptamer was radiolabeled with this agent and visualized in a xenograft mouse model of human colon cancer by PET, which demonstrates that this OID approach is a convenient and highly efficient way of labeling and tracking biomolecules.

Quantitative evaluation of the improvement in the pharmacokinetics of a nucleic acid drug delivery system by dynamic PET imaging with (18)F-incorporated oligodeoxynucleotides

Recently, we demonstrated the utility of positron emission tomography (PET) imaging-based pharmacokinetic evaluation studies for preclinical experiments and microdose clinical trials, mainly focused on low molecular weight compounds. In order to investigate the pharmacokinetics of nucleic acid drugs and their drug delivery systems (DDSs) in vivo by using PET imaging, we developed a novel and efficient method for radiolabeling oligodeoxynucleotides with the positron-emitting radionuclide (18)F (stoichiometry-focused Huisgen-type (18)F labeling). By using this method, we succeeded in synthesizing a variety of (18)F-labeled oligodeoxynucleotides with not only phosphodiesters (PO) in natural forms, but also phosphorothioate (PS) and bridged nucleic acid (BNA) in artificial forms, and then performed PET studies and radioactive metabolite analyses of these (18)F-labeled oligodeoxynucleotides. The tissue-distribution and dynamic changes in radioactivity showed significantly different profiles between these antisense oligodeoxynucleotides. The radioactivity of (18)F-labeled PO-DNA and PO-BNA rapidly accumulated in the kidneys and liver and then moved to the renal medulla, ureter, bladder, and intestine. However, the radioactivity of (18)F-labeled PS-DNA and PS-BNA, possessing PS backbone structures, was retained in the blood for relatively long periods and then gradually accumulated in the liver and kidneys. The metabolite analysis showed that (18)F-labeled PO-DNA rapidly degraded by 5min and (18)F-labeled PO-BNA gradually degraded over time by 60min. Conversely, (18)F-labeled PS-DNA and PS-BNA were shown to be much more stable. To demonstrate the usefulness of the PET imaging technique for evaluating the improved targeting potential of the DDS, we designed and synthesized a cholesterol-modified oligodeoxynucleotide, that we developed as an antisense nucleic acid drug against proprotein convertase subtilisin/kexin type 9 (PCSK9) for hypercholesterolemia therapy, and evaluated its pharmacokinetics using PET imaging. As expected, the (18)F-labeled cholesterol-modified PS-BNA-type oligodeoxynucleotide showed much higher and more rapid accumulation in the delivery target organ, that is, the liver, which encourages us to develop this drug. These results suggest that dynamic PET studies using (18)F-incorporated oligodeoxynucleotide synthesized by stoichiometry-focused Huisgen-type labeling is useful for quantitative pharmacokinetic evaluation of nucleic acid drugs and their delivery systems.

Single-step Radiosyntheses of '18F-Labeled Click Synthons' from Azide-functionalized Diaryliodonium Salts

Positron emission tomography (PET) is an increasingly important biomedical imaging technique that relies on the development of radiotracers labeled with positron-emitters to achieve biochemical specificity. Fluorine-18 (t_1/2 = 109.7 min) is an attractive positron-emitting radiolabel for organic radiotracers, primarily because of its longer half-life and greater availability relative to those for the main alternative, carbon-11 (t_1/2 = 20.4 min). Rapid simple methods are sought for labeling prospective PET radiotracers with fluorine-18 from cyclotron-produced aqueous [¹⁸F]fluoride ion, which must often be converted first into a suitably reactive labeling synthon for use in a subsequent labelling reaction. Use of ¹⁸F-labeled synthons in 'click chemistry' attracts increasing attention for labeling PE Tradiotracers. Here we describe rapid single-step radiosyntheses of azido- or azidomethyl-bearing [¹⁸F]fluoroarenes from the reactions of diaryliodonium salts with no-carrier-added [¹⁸F]fluoride ion within a microfluidic apparatus to provide previously poorly accessible ¹⁸F-labeled click synthons in radiochemical yields of 15% for [¹⁸F]4-fluorophenyl azide and about 40% for each of the [¹⁸F](azidomethyl)-fluorobenzenes. The radiosyntheses of the latter synthons was possible under 'wet conditions', so obviating the need to dry the cyclotron-produced [¹⁸F]fluoride ion and greatly enhancing the practicality of the method.

A comparative study on the effect of solvent on nucleophilic fluorination with [18F]fluoride: protic solvents as co-solvents in SN2 and SNAr reactions

The effect of solvent on nucleophilic substitution with cyclotron-produced [18F]fluoride was studied in polar aprotic (CH3CN and DMF) and protic solvent (t-BuOH and t-amyl alcohol) mixtures (CH3CN/co-solvent, 2:8) in a series of model compounds, 4-(R1-methyl)benzyl R2-benzoates, using a K2.2.2/[ 18F]KF phase transfer system (R1=–Cl, –OMs or –OH; R2=–Cl, –I or –NO2). 18F-fluorination of compounds 1–3, with chloride or mesylate as a leaving group in the benzylic position (R1), afforded the desired 4-([ 18F]fluoromethyl)benzyl analogues in all solvents during 15ߙmin reaction time. The highest radiochemical yields (RCY) in all the studied reaction temperatures (80, 120 and 160ºC) were achieved in CH3CN. Radiochemical yields in protic solvents were comparable to RCY in CH3CN only with the sulfonate ester 3 as a starting material. 18F-Fluorination of the benzylic halides 1 and 2 was not promoted in the same extent; in addition, labelled side-products were detected at higher reaction temperatures. Radiofluorination in tert-alcohols was also studied using [18F]CsF with and without added phase transfer catalyst, resulting in both conditions lower RCY when compared to K2.2.2/[18F]KF system. Protic solvents were not able to promote aromatic 18F-fluorination. 18F-Fluorination of compound 5, having para-activated nitro group in the aromatic position (R2), failed in tert-alcohols even at the highest temperature, but it was labelled successfully in DMF and to some extent in CH3CN.

Synthesis of 4-([18F]fluoromethyl)-2-chlorophenylisothiocyanate: A novel bifunctional 18F-labelling agent

The one-step radiosynthesis of 4-([18F]fluoromethyl)-2-chlorophenylisothiocyanate 18F-7 as a novel bifunctional 18F-labelling agent is described. Optimised reaction conditions in a remotely controlled synthesis module gave isothiocyanate 18F-7 in radiochemical yields of 45% (decay-corrected) within 40 min and high radiochemical purity of >95% after solid-phase-extraction. Coupling of compound 18F-7 with the primary amine benzylamine as a model reaction afforded the corresponding ((4-[18F]fluoromethyl)-2-chloro-phenyl)-benzyl thiourea 18F-8 in a high radiochemical yield of >90%. Stability studies of thiourea 18F-8 in terms of radiodefluorination showed appreciable buffer stability at pH 7.4, whereas significant radiodefluorination was observed when 18F-8 was incubated in buffers at pH 3.6 and pH 9.4. Preliminary dynamic PET studies with thiourea 18F-8 in male Wistar rats showed high bone accumulation, indicative of high in vivo radiodefluorination.

New strategy for the preparation of clickable peptides and labeling with 1-(azidomethyl)-4-[(18)F]-fluorobenzene for PET

The alkyne-azide Cu(I)-catalyzed Huisgen cycloaddition, a click type reaction was used to label a peptide with fluorine-18. A novel solid phase synthesis approach for the preparation of clickable peptides has been developed and has also permitted the straightforward preparation of reference compounds. A complementary azide labeling agent (1-(azidomethyl)-4-[(18)F]-fluorobenzene) has been produced in a four step procedure in 75 min with a 34% radiochemical yield (decay corrected). Conjugation of [(18)F]fluoroazide with a model alkyne-neuropeptide produced the desired (18)F-radiolabeled peptide in less than 15 min with a yield of 90% and excellent radiochemical purity.

Radiodefluorination of 3-fluoro-5-(2-(2-[18F](fluoromethyl)-thiazol-4-yl)ethynyl)benzonitrile ([18F]SP203), a radioligand for imaging brain metabotropic glutamate subtype-5 receptors with positron emission tomography, occurs by glutathionylation in rat brain

Metabotropic glutamate subtype-5 receptors (mGluR5) are implicated in several neuropsychiatric disorders. Positron emission tomography (PET) with a suitable radioligand may enable monitoring of regional brain mGluR5 density before and during treatments. We have developed a new radioligand, 3-fluoro-5-(2-(2-[(18)F](fluoromethyl)thiazol-4-yl)ethynyl)benzonitrile ([(18)F]SP203), for imaging brain mGluR5 in monkey and human. In monkey, radioactivity was observed in bone, showing release of [(18)F]-fluoride ion from [(18)F]SP203. This defluorination was not inhibited by disulfiram, a potent inhibitor of CYP2E1. PET confirmed bone uptake of radioactivity and therefore defluorination of [(18)F]SP203 in rats. To understand the biochemical basis for defluorination, we administered [(18)F]SP203 plus SP203 in rats for ex vivo analysis of metabolites. Radio-high-performance liquid chromatography detected [(18)F]fluoride ion as a major radiometabolite in both brain extract and urine. Incubation of [(18)F]SP203 with brain homogenate also generated this radiometabolite, whereas no metabolism was detected in whole blood in vitro. Liquid chromatography-mass spectrometry analysis of the brain extract detected m/z 548 and 404 ions, assignable to the [M + H](+) of S-glutathione (SP203Glu) and N-acetyl-S-l-cysteine (SP203Nac) conjugates of SP203, respectively. In urine, only the [M + H](+) of SP203Nac was detected. Mass spectrometry/mass spectrometry and multi-stage mass spectrometry analyses of each metabolite yielded product ions consistent with its proposed structure, including the former fluoromethyl group as the site of conjugation. Metabolite structures were confirmed by similar analyses of SP203Glu and SP203Nac, prepared by glutathione S-transferase reaction and chemical synthesis, respectively. Thus, glutathionylation at the 2-fluoromethyl group is responsible for the radiodefluorination of [(18)F]SP203 in rat. This study provides the first demonstration of glutathione-promoted radiodefluorination of a PET radioligand.

Method for stabilizing non carrier added 2-[(18)F]fluoromethyl-l-phenylalanine, a new tumour tracer, during radiosynthesis and radiopharmaceutical formulation

INTRODUCTION

Non carrier added (NCA) 2-[(18)F]fluoromethyl-l-phenylalanine is currently used in a Phase I study. Improvement of the stability of the fluorobenzyl analogue, very sensitive to defluorination and hydrolysis, during the synthetic route and in the radiopharmaceutical formulation was devised.

METHODS

The protected brominated precursor was synthesised in three steps. The labelling with [(18)F(-)] occurred in acetonitrile using K(2)CO(3)/K(2.2.2) (120 degrees C, 5 min). NCA 3-(2-[(18)F(-)]Fluoromethyl-phenyl)-2-tert-butoxycarbonylamino-propionic acid tert-butyl ester recovered from HPLC was submitted to deprotection in TFA/CH(2)Cl(2) in the presence of CaCl(2). After evaporation and adsorption on a mini C18 column, the tracer was recovered in 4 ml of H(2)O. Appropriate amounts of CaCl(2) and NaCl solutions were added for isotonic formulation, and this solution was sterilised by a 0.2-microm Cathivex filter. Shelf-life stability in the presence of Ca(2+) and Mg(2+) ions was studied. Stereoisomeric purity was checked by chiral HPLC.

RESULTS

The labelling showed a reproducible labelling yield of at least 90%. The followed strategy and the presence of Ca(2+) ions during deprotection, minimizing the loss of [(18)F(-)] from the labile fluorobenzyl group, allowed to obtain a decay-corrected yield of 75%. No racemisation was observed. The radiopharmaceutical formulation containing 0.04 M CaCl(2) allows a shelf-life of about 6 h without significant radiodefluorination.

CONCLUSION

The described synthetic route yields 40% of radiochemical pure NCA 2-[(18)F]fluoromethyl-l-phenylalanine within 105 min. A solution was found to reduce considerably the radiodefluorination. Addition of CaCl(2) prior to deprotection limits the loss of radiofluoride to less than 10%. The calcium ions present in the final radiopharmaceutical formulation (0.04 M) assure a shelf-life of at least 6 h.

Quinazoline derivatives as MC-I inhibitors: evaluation of myocardial uptake using Positron Emission Tomography in rat and non-human primate

Several quinazoline derivatives were made as mitochondrial complex 1 inhibitors. Compound 4 showed an IC(50) of 11.3 nM and was the most potent compound of this series. The (18)F analog of 4, [(18)F] 4, was injected in the rat and showed high and rapid heart uptake, fast liver clearance, and low blood uptake. Images obtained using a microPET showed clear delineation of the myocardium in normal rats and perfusion deficit in ischemic rats. In the non-human primate, [(18)F] 4 showed rapid uptake and clearance from the myocardium and high liver uptake.

Fluorine-18 labeling of peptides and proteins

The pool of promising peptides worthy of investigation and evaluation for clinical use is continuously filled from different sources. Driven by the promising results obtained with peptides addressing somatostatin-2 receptor positive (sst2+) neuroendocrine tumours, other peptides targeting further receptor systems are being studied and evaluated. Progress in profiling the density and incidence of peptide hormone receptors in human cancer has initiated and will further promote research on the corresponding peptidic binders. In addition, industrial pharmaceutical research will be another significant source of peptides in the future. A recent prognosis revealed that about 50% of the drugs entering clinical trials in the next years will be peptides. The extensive research activities in genomics and proteomics will point out and quantify new and already known target structures upregulated in specific diseases. Based on the knowledge of their endogenous ligands or via selection of suitable candidates by phage display, suitable peptide ligands for e.g. membrane associated receptors can be identified and thus allow targeting of such binding sites. Thus, bioactive peptides specifically addressing relevant molecular targets are expected to become an important class of tracers, also due to the possibility of bridging imaging with therapeutic approaches. In this brief overview a summary of methods and strategies for the 18F-labeling of peptides and proteins is given.

Chemoselective hydrazone formation between HYNIC-functionalized peptides and (18)F-fluorinated aldehydes

INTRODUCTION

Since the demand for (18)F-fluorinated peptides for quantitative in vivo receptor imaging using PET has increased, a new chemoselective two-step (18)F-labeling strategy based on hydrazone formation between an unprotected hydrazine-functionalized peptide and an (18)F-labeled aldehyde was developed.

METHODS

First, 4-[(18)F]fluorobenzaldehyde ([(18)F]FB-CHO) was prepared from 4-formyl-N,N,N-trimethylanilinium triflate via direct no-carrier-added (18)F-fluorination (dimethyl sulfoxide, 90 degrees C, 5 min) and purified by RP-HPLC. Hydrazone formation between [(18)F]FB-CHO and 6-hydrazinonicotinic acid (HYNIC) and the unprotected HYNIC-functionalized peptides (HYNIC-d-Phe(1))-Tyr(3)-Thr(8)-octreotide and (HYNIC-Arg(1))-substance P was evaluated with respect to the dependence of radiochemical yield on pH, precursor concentration and temperature. The stability of [(18)F]FB-CH=N-HYNIC-Tyr(3)-Thr(8)(NH(2))-octreotide in aqueous solution at various pH (4.0, 5.5 and 7.5) as well as the in vivo stability of [(18)F]FB-CH=N-HYNIC-Tyr(3)-Thr(8)-octreotide in mouse blood (30 min p.i.) was investigated.

RESULTS

Yields of the hydrazone formation were independent of pH between pH 0.5 and 5.5. Optimal labeling yields of 85% were obtained with a precursor concentration of 2.1 mM at 70 degrees C for 10 min. The labeling products were stable at pH 7.5 at 37 degrees C, while in more acidic media (pH 4.0) the product slowly decomposed to form up to 31+/-2% [(18)F]FB-CHO within 5 h. Metabolite studies showed no detectable degradation of [(18)F]FB-CH=N-HYNIC-Tyr(3)-Thr(8)-octreotide in mouse blood (30 min p.i.).

CONCLUSIONS

In conclusion, chemoselective hydrazone formation between unprotected HYNIC-functionalized peptides and [(18)F]FB-CHO is a fast and straightforward radiolabeling method leading to high yields under mild acidic conditions. In addition, it represents a powerful and versatile radiolabeling strategy that is applicable to a variety of radionuclides and peptide precursors already available for (99m)Tc labeling.

Synthesis and in vivo biodistribution of F-18 labeled 3-cis-, 3-trans-, 4-cis-, and 4-trans-fluorocyclohexane derivatives of WAY 100635

Radioligands that are specific for the serotonin 5-HT(1A) receptor will be useful in characterizing the physiological action of this receptor subtype. With radioligands of varying pharmacokinetic properties, investigators can measure not only receptor density, but also the effect of endogenous serotonin concentration. To this end, three additional fluorinated analogs of WAY 100635 were prepared and evaluated as 5-HT(1A) receptor ligands of varying pharmacokinetic properties based on our previous studies. These four compounds are cis-4-fluoro-, trans-4-fluoro-, cis-3-fluoro-, and trans-3-fluoro-N-{2-[4-(2-methoxyphenyl)piperazin-1-yl]ethyl}-N-(pyridin-2-yl)cyclohexanecarboxamides (FCWAYs). All four compounds were characterized and radiolabeled with fluorine-18, a 109.7 min half-life radionuclide used in positron emission tomography. We then determined in vitro inhibition constants at the 5-HT(1A) receptor; in vitro metabolic profile, using rat hepatocytes and liquid chromatography/mass spectroscopy (LC/MS); and the rate of defluorination and hippocampus to cerebellum ratio ex vivo. This led to the conclusion that high affinity 4-trans-F-18 FCWAY had the best properties for measuring receptor density given its high hippocampus to cerebellum ratio and 3-cis-F-18 FCWAY had the best properties for measuring dynamic change in receptors, with lower affinity and faster pharmacokinetics.

In vitro metabolism studies of 18F-labeled 1-phenylpiperazine using mouse liver S9 fraction

The in vitro metabolism of 1-(4-[(18)F]fluoromethylbenzyl)-4-phenylpiperazine ([(18)F]1) and 1-(4-[(18)F]fluorobenzyl)-4-phenylpiperazine ([(18)F]2) was investigated using mouse liver S9 fraction. Results were compared to those of in vivo metabolism using mouse blood and bone and to in vitro metabolism using mouse liver microsomes. Defluorination was the main metabolic pathway for [(18)F]1 in vitro and in vivo. Based on TLC, HPLC and LC-MS data, [(18)F]fluoride ion and less polar radioactive metabolites derived from aromatic ring oxidation were detected in vitro, and the latter metabolites were rapidly converted into the former with time, whereas only the [(18)F]fluoride ion was detected in vivo. Similarly, the in vitro metabolism of [(18)F]2 using either S9 fraction or microsomes showed the same pattern as the in vivo method using blood; however, the radioactive metabolites derived from aromatic ring oxidation were not detected in vivo. These results demonstrate that liver S9 fraction can be widely used to investigate the intermediate radioactive metabolites and to predict the in vivo metabolism of radiotracers.

Liquid chromatography-tandem mass spectrometry identification of metabolites of two 5-HT1A antagonists, N-[2-[4-(2-methoxylphenyl)piperazino]ethyl]-N-(2-pyridyl) trans- and cis-4-fluorocyclohexanecarboxamide, produced by human and rat hepatocytes

Two 5-HT1A antagonists, t-FCWAY and c-FCWAY, were developed as imaging agents for positron emission tomography (PET). In order to evaluate these compounds, hepatocytes from both human and rat were utilized to produce metabolites and LC-MS-MS was used to identify metabolites. These in vitro metabolism studies indicate that hydrolysis of the amide linkage is the major metabolism pathway for humans, whereas aromatic ring-oxidation is the major metabolism pathway for rat. The rat hepatocyte results correlate well with in vivo rat metabolism studies. Based on the structures of the metabolites, we have developed an extraction procedure to determine the concentration of the parent compound in plasma.

A simple and efficient in vitro method for metabolism studies of radiotracers

In vitro metabolism of acetylcholinesterase inhibitors containing 3-[(18)F]fluoromethylbenzyl- ([(18)F]1) and 4-[(18)F]fluorobenzyl-piperidine moieties ([(18)F]2) was studied and compared with the in vivo metabolism. Defluorination of the [(18)F]1 mainly occurred to generate [(18)F]fluoride ion both in vitro and in vivo. In contrast, the [(18)F]2 was converted into an unknown polar metabolite in both metabolism methods and another metabolite, 4-[(18)F]fluorobenzoic acid in vitro. These results demonstrated that the in vitro method can be used to predict the in vivo metabolism of both radiotracers.