Efficient purification and metabolite analysis of radiotracers using high-performance liquid chromatography and on-line solid-phase extraction

PET Radiopharmaceuticals for Alzheimer's Disease and Parkinson's Disease Diagnosis, the Current and Future Landscape

Ironically, population aging which is considered a public health success has been accompanied by a myriad of new health challenges, which include neurodegenerative disorders (NDDs), the incidence of which increases proportionally to age. Among them, Alzheimer's disease (AD) and Parkinson's disease (PD) are the most common, with the misfolding and the aggregation of proteins being common and causal in the pathogenesis of both diseases. AD is characterized by the presence of hyperphosphorylated τ protein (tau), which is the main component of neurofibrillary tangles (NFTs), and senile plaques the main component of which is β-amyloid peptide aggregates (Aβ). The neuropathological hallmark of PD is α-synuclein aggregates (α-syn), which are present as insoluble fibrils, the primary structural component of Lewy body (LB) and neurites (LN). An increasing number of non-invasive PET examinations have been used for AD, to monitor the pathological progress (hallmarks) of disease. Notwithstanding, still the need for the development of novel detection tools for other proteinopathies still remains. This review, although not exhaustively, looks at the timeline of the development of existing tracers used in the imaging of Aβ and important moments that led to the development of these tracers.

Radiosynthesis and in vivo evaluation of 11C-labeled BMS-193885 and its desmethyl analog as PET tracers for neuropeptide Y1 receptors

BACKGROUND

Neuropeptide Y (NPY) has been implicated in a wide variety of physiological processes, including feeding, learning, memory, emotion, cardiovascular homeostasis, hormone secretion, and circadian rhythms. NPY Yl receptor (NPY Y1-R) is the most widely studied NPY receptor, and is involved in many of these processes. BMS-193885 (1) was previously developed as a potent and selective NPY Y1-R antagonist, which has good systemic bioavailability and brain penetration. To evaluate the characteristics of 1 in vivo, we developed ¹¹C-labeled BMS-193885 ([¹¹C]1) and its desmethyl analog ([¹¹C]2) for potential use as two new positron emission tomography (PET) tracers.

RESULTS

[¹¹C]1 was synthesized from [¹¹C]methyl iodide using 2. [¹¹C]2 was synthesized from [¹¹C]phosgene using its aniline and amine derivatives. The mean ± S.D. decay-corrected radiochemical yields of [¹¹C]1 and [¹¹C]2 from ¹¹CO₂ at the end of radionuclide production were 23 ± 3.2% (n = 6) and 24 ± 1.5% (n = 4), respectively. In biodistribution on mice, radioactivity levels for both tracers were relatively high in the kidney, small intestine, and liver at 60 min post-injection. The radioactivity levels in the kidney, lung, and spleen of mice at 30 min post-injection with [¹¹C]1 were significantly reduced by pretreatment with 1 (10 mg/kg), and levels of [¹¹C]1 in the brain of mice were significantly increased by pretreatment with the P-glycoprotein and breast cancer resistance protein inhibitor elacridar (10 mg/kg). In metabolite analysis using mouse plasma, [¹¹C]1 and [¹¹C]2 were rapidly metabolized within 30 min post-injection, and [¹¹C]1 was mainly metabolized into unlabeled 2 and radiolabeled components.

CONCLUSION

[¹¹C]1 and [¹¹C]2 were successfully synthesized with sufficient amount of radioactivity and high quality for use in vivo. Our study of [¹¹C]1 and its desmethyl analog [¹¹C]2 was useful in that it helped to elucidate the in vivo characteristics of 1.

A practical synthesis of [(18) F]FtRGD: an angiogenesis biomarker for PET

Integrins have become increasingly attractive targets for molecular imaging of angiogenesis with positron emission tomography or single-photon emission computed tomography, but the reliable production of radiopharmaceuticals remains challenging. A strategy for chemoselective labeling of the integrin ligand-c(RGDyK) peptide-has been developed on the basis of the Cu(I)-catalyzed conjugation reaction. Recently, we reported a nucleophilic detagging and fluorous solid-phase extraction method providing an easy way to implement an approach for obtaining 2-[(18) F]fluoroethyl azide. In this work, we report the practical use of this method for the preparation of the 2-[(18) F]fluoroethyl-triazolyl conjugated c(RGDyK) peptide: [(18) F]FtRGD. The two-step, two-pot synthesis, HPLC purification, and reformulation could be readily performed with a standard nucleophilic radiofluorination synthesizer (GE TRACERlab FXFN ), with minimal modifications. [(18) F]FtRGD was obtained in a solution for injection (>500 MBq/mL) in 10-30% nondecay-corrected radiochemical yield, excellent radiochemical purity (>98%), and 28 ± 13 GBq/µmol specific activity. [(18) F]FtRGD (Ki  = 54 ± 14 nM for αV β3 and 1.7 ± 0.2 nM for αV β5 ) was evaluated in mice and showed good stability in vivo, good tumor-to-background ratio (1.6 ± 0.3 %ID/g at 1.5 h post-injection in U87-MG tumors), and rapid urinary excretion. Therefore, [(18) F]FtRGD proved valuable for preclinical positron emission tomography imaging of integrin expression.

Preliminary validation of varicella zoster virus thymidine kinase as a novel reporter gene for PET

INTRODUCTION

Imaging of gene expression with positron emission tomography (PET) has emerged as a powerful tool for biomedical research during the last decade. The prototypical herpes simplex virus type 1 thymidine kinase (HSV1-TK) PET reporter gene (PRG) is widely used and many other PRGs have also been validated. We investigated varicella zoster virus thymidine kinase (VZV-tk) as new PRG with radiolabeled bicyclic nucleoside analogues (BCNAs) as PET tracers.

METHODS

The uptake and washout of four different radiolabeled BCNAs was evaluated in cells expressing VZV-tk after lentiviral vector (LV) transduction and in control cells. Metabolism of the tracers was assayed by high pressure liquid chromatography (HPLC). Mice bearing VZV-TK expressing xenografts were imaged with PET.

RESULTS

High uptake in VZV-tk expressing cells was seen for 3 of the 4 tracers tested. The uptake of the tracers could be blocked by the presence of excess thymidine in the incubation solution. Cellular retention was variable, with one tracer showing an acceptable half-life of ~1 hour. The amount of intracellular tracer correlated with the titer of LV used to transduce the cells. VZV-TK dependent conversion into metabolites was shown by HPLC. No specific accumulation was observed in cells expressing a fusion protein containing an HSV1-TK moiety. VZV-tk expression in xenografts resulted in a 60% increase in uptake in vivo as measured with PET.

CONCLUSIONS

We have validated the combination of VZV-tk and radiolabeled BCNAs as new PRG/PRP system. Further optimization of the PRPs and the PRG are warranted to increase the signal.

Direct plasma metabolite analysis of positron emission tomography radioligands by micellar liquid chromatography with radiometric detection

Determination of radio-metabolites in plasma samples taken during a positron emission tomography (PET) study is an important component in the pharmacokinetic evaluation of PET radioligands. We have developed and validated a new analytical procedure for the plasma metabolite analysis of PET radioligands based on micellar liquid chromatography using an anionic surfactant mobile phase. Chromatographic separation was performed on an octadecyl semipreparative column (10 mm I.D. × 160 mm, 10 μm) using 100 mM sodium dodecyl sulfate (SDS) and 1-butanol in 10 mM sodium-phosphate (pH 7.2) at a flow rate of 5 mL/min. The samples taken from monkey or human plasma during PET measurements were directly injected into a liquid chromatographic (LC) system coupled to an online radiometric detector under micellar conditions using 1-2% (v/v) 1-butanol mobile phase to remove plasma proteins and concentrate the analytes at the column head. At 2 min, mobile phase was changed to elute and separate PET radioligand and its radiometabolites with high peak capacity under high submicellar conditions (10-25% 1-butanol). This procedure allowed direct plasma injection (up to 2 mL) into the LC column without any pretreatment with a short analysis-time of 8-10 min. Satisfactory reproducibility, linearity, sensitivity, accuracy and recovery were obtained in the validation study. The developed method was successfully applied to study the metabolism for diverse groups of PET radioligands and provided reliable determination of PET radioligands in human and monkey plasma. This method is advantageous in terms of simplifying and shortening the processes required to analyze short-lived radioligands as well as in providing a more accurate estimation of the metabolite corrected input function, especially for the radioligands with lower recoveries or degradation potential during the deproteination process in a conventional procedure.

Synthesis, in vitro and in vivo evaluation of fluorine-18 labelled FE-GW405833 as a PET tracer for type 2 cannabinoid receptor imaging

The type 2 cannabinoid receptor (CB₂R) is part of the endocannabinoid system and is expressed in tissues related to the immune system. As the CB₂R has a very low brain expression in non-pathological conditions, but is upregulated in activated microglia, it is an interesting target for visualization of neuroinflammation using positron emission tomography with a suitable radiolabeled CB₂R ligand. In this study, we radiolabelled a fluoroethyl derivative of GW405833, a well known CB₂R partial agonist, with fluorine-18 (half-life 109.8 min) by alkylation of the phenol precursor with 1-bromo-2-[¹⁸F]fluoroethane. In vitro studies showed that FE-GW405833 behaved as a selective high affinity (27 nM) inverse agonist for hCB₂R. [¹⁸F]FE-GW405833 showed moderate initial brain uptake in mice and rats, but a slow washout from brain and plasma due to retention of a radiometabolite. Specific binding of the tracer to human CB₂R was demonstrated in vivo in a rat model with local CB₂R overexpression in the brain. Optimized derivatives of GW405833 that are less susceptible to metabolism will need to be developed in order to provide a useful tracer for CB₂R quantification with PET.

A PET brain reporter gene system based on type 2 cannabinoid receptors

PET of gene expression in the brain may greatly facilitate neuroscience research and potential clinical implementation of gene or cell therapy of central nervous system diseases. To date, no adequate PET reporter system is available for the central nervous system because available tracers either do not cross the intact blood-brain barrier or have high background signals. Here we report the first, to our knowledge, PET reporter system for imaging gene expression in the intact brain.

METHODS

We selected the human type 2 cannabinoid receptor (hCB(2)) as a reporter because of its low basal expression in the brain. An inactive mutant (D80N) was chosen so as not to interfere with signal transduction. As a reporter probe we used the (11)C-labeled CB(2) ligand, (11)C-GW405833, which readily crosses the blood-brain barrier. Dual-modality imaging lentiviral and adeno-associated viral vectors encoding both hCB(2)(D80N) and firefly luciferase or enhanced green fluorescent protein were engineered and validated in cell culture. Next, hCB(2)(D80N) was locoregionally overexpressed in rat striatum by stereotactic injection of lentiviral and adeno-associated viral vectors.

RESULTS

Kinetic PET revealed specific and reversible CB(2) binding of (11)C-GW405833 in the transduced rat striatum. hCB(2) and firefly luciferase expression was followed until 9 mo and showed similar kinetics. Both hCB(2) expression and enhanced green fluorescent protein expression were confirmed by immunohistochemistry.

CONCLUSION

Dual-modality imaging viral vectors encoding hCB(2)(D80N) were engineered, and the reporter system was validated in different animal species. The results support the potential future clinical use of CB(2) as a PET reporter in the intact brain.

A rapid solid-phase extraction method for measurement of non-metabolised peripheral benzodiazepine receptor ligands, [(18)F]PBR102 and [(18)F]PBR111, in rat and primate plasma

OBJECTIVES

To develop a rapid and reliable method for estimating non-metabolised PBR ligands fluoroethoxy ([(18)F]PBR102)- and fluoropropoxy ([(18)F]PBR111)-substituted 2-(6-chloro-2-phenyl)imidazo[1,2-a]pyridine-3-yl)-N,N-diethylacetamides in plasma.

METHODS

Rats and baboons were imaged with PET up to 2 h postinjection of [(18)F]PBR102 and [(18)F]PBR111 under baseline conditions, after pre-blocking or displacement with PK11195. Arterial plasma samples were directly analysed by reverse-phase solid-phase extraction (RP-SPE) and RP-HPLC and by normal-phase TLC. SPE cartridges were successively washed with acetonitrile/water mixtures. SPE eluant radioactivity was measured in a γ-counter to determine the parent compound fraction and then analysed by HPLC and TLC for validation.

RESULTS

In SPE, hydrophilic and lipophilic radiolabelled metabolites were eluted in water and 20% acetonitrile/water. All non-metabolised [(18)F]PBR102 and [(18)F]PBR111 were in SPE acetonitrile fraction as confirmed by HPLC and TLC analysis. Unchanged (%) [(18)F]PBR102 and [(18)F]PBR111 from SPE analysis in rat and baboon plasma agreed with those from HPLC and TLC analysis. In rats and baboons, the fraction of unchanged tracer followed a bi-exponential decrease, with half-lives of 7 to 10 min for the fast component and >80 min for the slow component for both tracers.

CONCLUSIONS

Direct plasma SPE analysis of [(18)F]PBR102 and [(18)F]PBR111 can reliably estimate parent compound fraction. SPE was superior to HPLC for samples with low activity; it allows rapid and accurate metabolite analysis of a large number of plasma samples for improved estimation of metabolite-corrected input function during quantitative PET imaging studies.

Radiolabeling and preliminary biological evaluation of a (99m)Tc(CO)(3) labeled 3,3'-(benzylidene)-bis-(1H-indole-2-carbohydrazide) derivative as a potential SPECT tracer for in vivo visualization of necrosis

N,N'-bis(diethylenetriamine pentaacetic acid)-3,3'-(benzylidene)-bis-(1H-indole-2-carbohydrazide) (bis-DTPA-BI) was radiolabeled with (99m)Tc(CO)(3). The resulting (99m)Tc(CO)(3)-bis-DTPA-BI was characterized (LC-MS) and evaluated as a potential SPECT tracer for imaging of necrosis in Wistar rats with a reperfused partial liver infarction and Wistar rats with ethanol induced muscular necrosis. To study the specificity, uptake of (99m)Tc(CO)(3)-bis-DTPA-BI was also studied in a mouse model of Fas-mediated hepatic apoptosis. The obtained results indicate that (99m)Tc(CO)(3)-bis-DTPA-BI displays selective uptake in necrotic tissue and can be used for in vivo visualization of necrosis by SPECT.

Development and evaluation of a 68Ga labeled pamoic acid derivative for in vivo visualization of necrosis using positron emission tomography

In this study, we labeled N,N'-bis(diethylenetriamine pentaacetic acid)-pamoic acid bis-hydrazide (bis-DTPA-PA) with the generator produced PET radionuclide gallium-68 and evaluated 68Ga-bis-DTPA-PA as a potential tracer for in vivo visualization of necrosis by positron emission tomography (PET). Radiolabeling was achieved with a decay-corrected radiochemical yield of 63%. Biodistribution and in vivo stability studies in normal mice showed that 68Ga-bis-DTPA-PA is cleared faster from normal tissue than the previously reported 99mTc(CO)3 complex with bis-DTPA-PA which on the other hand is more stable in vivo. 68Ga-bis-DTPA-PA showed a 3.5-5 times higher binding to necrotic tissue than to viable tissue as shown by in vitro autoradiography while no statistically significant increased hepatic uptake was found in a biodistribution study in a mouse model of hepatic apoptosis. Specificity and avidity for necrosis was further evaluated in rats with a reperfused partial liver infarction and ethanol induced muscular necrosis. Dynamic microPET images showed a fast and prolonged uptake of 68Ga-bis-DTPA-PA in necrotic tissue with in vivo and ex vivo images correlating well with histochemical stainings. With necrotic to viable tissue activity ratios of 8-15 on ex vivo autoradiography, depending on the necrosis model, 68Ga-bis-DTPA-PA showed a faster and higher uptake in necrotic tissue than the 99mTc(CO)3 analog. These results show that 68Ga-bis-DTPA-PA specifically binds to necrotic tissue and is a promising tracer for in vivo visualization of necrosis using PET.

Synthesis and biological evaluation of carbon-11- and fluorine-18-labeled 2-oxoquinoline derivatives for type 2 cannabinoid receptor positron emission tomography imaging

INTRODUCTION

The type 2 cannabinoid (CB(2)) receptor is part of the endocannabinoid system and has been suggested as a mediator of several central and peripheral inflammatory processes. Imaging of the CB(2) receptor has been unsuccessful so far. We synthesized and evaluated a carbon-11- and a fluorine-18-labeled 2-oxoquinoline derivative as new PET tracers with high specificity and affinity for the CB(2) receptor.

METHODS

Two 2-oxoquinoline derivatives were synthesized and radiolabeled with either carbon-11 or fluorine-18. Their affinity and selectivity for the human CB(2) receptor were determined. Biological evaluation was done by biodistribution, radiometabolite and autoradiography studies in mice.

RESULTS

In vitro studies showed that both compounds are high affinity CB(2)-specific inverse agonists. Biodistribution study of the tracers in mice showed a high in vivo initial brain uptake and fast brain washout, in accordance with the low CB(2) receptor expression levels in normal brain. A persistently high in vivo binding to the spleen was observed, which was inhibited by pretreatment with two structurally unrelated CB(2) selective inverse agonists. In vitro autoradiography studies with the radioligands confirmed CB(2)-specific binding to the mouse spleen.

CONCLUSION

We synthesized two novel CB(2) receptor PET tracers that show high affinity/selectivity for CB(2) receptors. Both tracers show favourable characteristics as radioligands for central and peripheral in vivo visualization of the CB(2) receptor and are promising candidates for primate and human CB(2) PET imaging.

Developing new molecular imaging probes for PET

Positron emission tomography (PET) is a fully translational molecular imaging technique that requires specific probes radiolabelled with short-lived positron emitting radionuclides. This review discusses relevant methods which are applied throughout the different steps in the development of new PET probes for in vivo visualization of specific molecular targets related to diagnosis or important for drug development.

Synthesis of 18F-labelled 2-(4'-fluorophenyl)-1,3-benzothiazole and evaluation as amyloid imaging agent in comparison with [11C]PIB

2-(4'-[(18)F]fluorophenyl)-1,3-benzothiazole was synthesized as a fluorine-18 labelled derivative of the Pittsburg Compound-B (PIB), which has known affinity for amyloid beta and promising characteristics as tracer for in vivo visualisation of amyloid deposits in patients suffering from Alzheimer's disease (AD). Both the nitro-precursor 2-(4'-nitrophenyl)-1,3-benzothiazole and the non-radioactive reference compound were synthesized using a 1-step synthesis pathway. Labelling was achieved by direct aromatic nucleophilic substitution of the nitro-precursor using [(18)F]fluoride by heating for 20 min at 150 degrees C and with a radiochemical yield of 38%. The reference compound showed high affinity for amyloid in an in vitro competition binding study using human AD brain homogenates (K(i)=9.0 nM) and fluorescence imaging of incubated transgenic APP mouse brain slices confirmed binding to amyloid plaques. A biodistribution study in normal mice showed a high brain uptake at 2 min pi (3.20%ID/g) followed by a fast washout (60 min pi: 0.21%ID/g). A dynamic microPET study was performed in a transgenic APP and normal WT mouse, but, similar to [(11)C]PIB, no difference was seen in tracer retention between both kind of mice. The new (18)F-labelled 2-phenylbenzothiazole showed excellent preclinical characteristics comparable with those of the (11)C-labelled PIB.

Labelling and biological evaluation of [(11)C]methoxy-Sch225336: a radioligand for the cannabinoid-type 2 receptor

INTRODUCTION

The cannabinoid type 2 receptor (CB(2) receptor) is part of the endocannabinoid system and has been suggested as mediator of a number of central and peripheral inflammatory processes. In the present study, we have synthesized N-[(1s)-1-[4-[[4-methoxy-2-[(4-[(11)C]methoxyphenyl)sulfonyl)-phenyl]sulfonyl] phenyl]ethyl]methanesulfonamide ([(11)C]methoxy-Sch225336) and evaluated this new tracer agent as a potential positron emission tomography radioligand for the in vivo visualization of CB(2) receptors.

METHODS

Sch225336 was demethylated and the resulting phenol precursor was radiolabelled with a carbon-11 methyl group by methylation using [(11)C]methyl iodide, followed by purification by high-performance liquid chromatography. The log P of [(11)C]methoxy-Sch225336 and its biodistribution in normal mice were determined. Enhancement of brain uptake by inhibition of blood-brain barrier (BBB) efflux transporters was studied. Mouse plasma was analysed to quantify the formation of radiometabolites. The affinity of Sch225336 for the human cannabinoid type 1 and type 2 receptor was determined.

RESULTS

[(11)C]methoxy-Sch225336 was obtained with a decay corrected radiochemical yield of about 30% and a specific activity of 88.8 GBq/mumol (end of synthesis). After intravenous injection in mice, the compound is rapidly cleared from the blood through the hepatobiliary pathway and does not show particular retention in any of the major organs. Polar metabolites were found in mouse plasma. Brain uptake was low despite the favourable log P value of 2.15, which is partly due to efflux by BBB pumps.

CONCLUSION

[(11)C]methoxy-Sch225336 is a good candidate for in vivo imaging of the CB(2) receptor, although the low blood-brain barrier penetration limits its potential for central nervous system imaging.