Difficulties in dopamine transporter radioligand PET analysis: the example of LBT-999 using [18F] and [11C] labelling

Translational Preclinical PET Imaging and Metabolic Evaluation of a New Cannabinoid 2 Receptor (CB2R) Radioligand, (Z)-N-(3-(2-(2-[18F]Fluoroethoxy)ethyl)-4,5-dimethylthiazol-2(3H)-ylidene)-2,2,3,3-tetramethylcyclopropane-1-carboxamide

We have previously developed seven fluorinated analogues of A-836339 as new PET tracers for cannabinoid type 2 receptor (CB2R) imaging, among which (Z)-N-(3-(2-(2-[18F]fluoroethoxy)ethyl)-4,5-dimethylthiazol-2(3H)-ylidene)-2,2,3,3-tetramethylcyclopropane-1-carboxamide ([18F]FC0324) displayed high affinity and selectivity for CB2R in healthy rats. In the present study, we have further evaluated the imaging and metabolic properties of [18F]FC0324 in a rat model of human CB2R overexpression in the brain (AAV-hCB2) and in non-human primates (NHPs). Autoradiography with AAV-hCB2 rat brain sections exhibited a signal of [18F]FC0324 8-fold higher in the ipsilateral region than in the contralateral region. Blocking with NE40, a CB2R-specific agonist, resulted in a 91% decrease in the radioactivity. PET experiments showed a signal 7-fold higher in the ipsilateral region, and the specificity of [18F]FC0324 for hCB2R in vivo was confirmed by the 80% decrease after blocking with NE40. In NHPs, brain time-activity curves displayed a fast and homogeneous distribution followed by a rapid washout, in accordance with the low amount of CB2Rs in healthy brain. Whole-body PET-CT suggested a high and specific uptake of the radiotracer in the spleen, a CB2R-rich organ, and in the organs involved in metabolism and excretion, with a low bone uptake. In vitro metabolism with monkey liver microsomes (MLMs) led to the formation of six main hydroxylated metabolites of FC0324. Five of them were produced by human liver microsomes, being much less active than MLMs. In vivo, in NHPs, the main radiometabolite was likely to result from further oxidation of hydroxylated compounds, and parent [18F]FC0324 accounted for 8 ± 3% of plasma radioactivity (at 120 min p.i.) with a low level of potential interfering radiometabolites. Furthermore, this metabolism should be significantly reduced in humans due to species differences. In conclusion, [18F]FC0324 appears to be a promising candidate for further human studies with suitable kinetics, selectivity, and metabolic profile for CB2R PET imaging.

Developing Low Molecular Weight PET and SPECT Imaging Agents

Imaging agents for positron emission tomography (PET) and single-photon emission computerized tomography (SPECT) have shown their utility in many situations, answering clinical questions related to drug development and medical considerations. The discovery and development of imaging agents follow a well-understood process, with variations related to available starting points and to the envisaged imaging application. This article describes the general development path leading from the expression of an imaging need and project initiation to a clinically usable imaging agent. The definition of the project rationale, the design and optimization of early leads, and the assessment of the imaging potential of an imaging agent candidate are followed by preclinical and clinical development activities that differ from those required for therapeutic agents. These include radiolabeling with a positron emitter and first-in-human clinical studies, to rapidly evaluate the ability of a new imaging agent to address the questions it was designed to answer.

The Oxidation Cascade of a Rare Multifunctional P450 Enzyme Involved in Asperterpenoid A Biosynthesis

The cytochrome P450 enzymes (P450s or CYPs) are heme-containing enzymes which catalyze a wide range of oxidation reactions in nature. In our previous study, a rare multifunctional P450 AstB was found, which can dually oxidize two methyl groups (C-19 and C-21) of preasperterpenoid A to asperterpenoid A with 3-carboxyl and 11-hydroxymethyl groups. However, the oxidation order of C-19 and C-21 catalyzed by AstB is unclear. In order to reveal this oxidation order, probable pathways catalyzed by AstB were proposed, and the oxidation order of C-19 and C-21 was obtained by quantum chemistry calculations. The potential intermediates (three new asperterpenoids D–F, 1–3) were obtained through the chemical investigation on the extract of the transformant strain and chemical conversions, which were used as the standards to detect their existences in the extract of the transformant strain with HPLC-MS. Combined with the quantum chemistry calculation and the HPLC-MS analysis, the catalyzed order of AstB in asperterpenoid A biosynthesis was revealed. Furthermore, the mPTPB inhibition of obtained asperterpenoids was evaluated, and the results showed that 3-carboxyl and the oxidation station of C-21 would be the key factors for mPTPB inhibition of asperterpenoids.

Kinetic isotope effects and synthetic strategies for deuterated carbon-11 and fluorine-18 labelled PET radiopharmaceuticals

The deuterium labelling of pharmaceuticals is a useful strategy for altering pharmacokinetic properties, particularly for improving metabolic resistance. The pharmacological effects of such metabolites are often assumed to be negligible during standard drug discovery and are factored in later at the clinical phases of development, where the risks and benefits of the treatment and side-effects can be wholly assessed. This paradigm does not translate to the discovery of radiopharmaceuticals, however, as the confounding effects of radiometabolites can inevitably show in preliminary positron emission tomography (PET) scans and thus complicate interpretation. Consequently, the formation of radiometabolites is crucial to take into consideration, compared to non-radioactive metabolites, and the application of deuterium labelling is a particularly attractive approach to minimise radiometabolite formation. Herein, we provide a comprehensive overview of the deuterated carbon-11 and fluorine-18 radiopharmaceuticals employed in PET imaging experiments. Specifically, we explore six categories of deuterated radiopharmaceuticals used to investigate the activities of monoamine oxygenase (MAO), choline, translocator protein (TSPO), vesicular monoamine transporter 2 (VMAT2), neurotransmission and the diagnosis of Alzheimer's disease; from which we derive four prominent deuteration strategies giving rise to a kinetic isotope effect (KIE) for reducing the rate of metabolism. Synthetic approaches for over thirty of these deuterated radiopharmaceuticals are discussed from the perspective of deuterium and radioisotope incorporation, alongside an evaluation of the deuterium labelling and radiolabelling efficacies across these independent studies. Clinical and manufacturing implications are also discussed to provide a more comprehensive overview of how deuterated radiopharmaceuticals may be introduced to routine practice.

Synthesis and Biological Evaluation of [18F]FECNT-d4 as a Novel PET Agent for Dopamine Transporter Imaging

PURPOSE

The dopamine transporter (DAT) is a marker of the occurrence and development of Parkinson's disease (PD) and other diseases with nigrostriatal degeneration. 2β-Carbomethoxy-3β-(4-chlorophenyl)-8-(2-[¹⁸F]-fluoroethyl)nortropane ([¹⁸F]FECNT), an ¹⁸F-labelled tropane derivative, was reported to be a useful positron-emitting probe for DAT. However, the rapid formation of brain-penetrating radioactive metabolites is an impediment to the proper quantitation of DAT in PET studies with [¹⁸F]FECNT. Deuterium-substituted analogues have presented better in vivo stability to reduce metabolites. This study aimed to synthesize a deuterium-substituted DAT radiotracer, [¹⁸F]FECNT-d₄, and to make a preliminary investigation of its properties as a DAT tracer in vivo.

PROCEDURES

The ligand [¹⁸F]FECNT-d₄ was obtained by one-step radiolabelling reaction. The lipophilicity was measured by the shake-flask method. Binding properties of [¹⁸F]FECNT-d₄ were estimated by in vitro binding assay, biodistribution, and microPET imaging in rats. In vivo stability of [¹⁸F]FECNT-d₄ was estimated by radio-HPLC.

RESULTS

[¹⁸F]FECNT-d₄ was synthesized at an average activity yield of 46 ± 17 % (n = 15) and the molar activity was 67 ± 12 GBq/μmol. The deuterated tracer showed suitable lipophilicity and the ability to penetrate the blood-brain barrier (brain uptake of 1.72 % ID at 5 min). [¹⁸F]FECNT-d₄ displayed a high binding affinity for DAT comparable to that of [¹⁸F]FECNT in rat striatum homogenates. Biodistribution results in normal rats showed that [¹⁸F]FECNT-d₄ exhibited a higher ratio of the target to non-target (striatum/cerebellum) at 15 min post administration (5.00 ± 0.44 vs 3.84 ± 0.24 for [¹⁸F]FECNT-d₄ vs [¹⁸F]FECNT). MicroPET imaging studies of [¹⁸F]FECNT-d₄ in normal rats showed that the ligand selectively localized to DAT-rich striatal regions and the accumulation could be blocked with DAT inhibitor. Furthermore, in the unilateral PD model rat, a significant reduction of the signal was found in the lesioned side relative to the unlesioned side. Striatal standardized uptake value of [¹⁸F]FECNT-d₄ remained ~4.02 in the striatum between 5 and 20 min, whereas that of [¹⁸F]FECNT fell rapidly from 4.11 to 2.95. Radio-HPLC analysis of the plasma demonstrated better in vivo stability of [¹⁸F]FECNT-d₄ than [¹⁸F]FECNT.

CONCLUSION

The deuterated compound [¹⁸F]FECNT-d₄ may serve as a promising PET imaging agent to assess DAT-related disorders.

An original radio-biomimetic approach to synthesize radiometabolites for PET imaging

To fully exploit the potential of positron emission tomography (PET) imaging to assess drug distribution and pharmacokinetics in the central nervous system, the contribution of radiometabolites to the PET signal has to be determined for correct interpretation of data. However, radiosynthesis and extensive study of radiometabolites are rarely investigated and very challenging for complex drugs. Therefore, an original radio-biomimetic (RBM) approach was developed to rapidly synthesize radiometabolites and non-invasively investigate their kinetics with PET imaging. This method enabled the challenging radiosynthesis of [11C]nor-buprenorphine ([11C]nor-BUP), the main metabolite of buprenorphine (BUP) which has been identified as a substrate of the P-glycoprotein (P-gp) transport function at the blood-brain barrier (BBB). Biomimetic conditions using cytochromes P450 3A4 to convert BUP into nor-BUP were optimized taking into account the short half-life of carbon-11 (t1/2 = 20.4 min). Those conditions afforded 32% of conversion within 20 min and were applied to the biomimetic radiosynthesis of [11C]nor-BUP from [11C]BUP. Automated radiosynthesis of [11C]BUP according to a procedure described in the literature followed by optimized RBM conditions afforded [11C]nor-BUP in 1.5% decay-corrected radiochemical yield within 90 min and 90 ± 15 GBq/μmol molar activity. HPLC quality control showed chemical and radiochemical purities above 98%. To demonstrate the applicability of the RBM approach to preclinical studies, brain PET images in rats showed a drastic lower uptake of [11C]nor-BUP (0.067 ± 0.023%ID/cm-3) compared to [11C]BUP (0.436 ± 0.054%ID/cm-3). P-gp inhibition using Tariquidar increased the brain uptake of [11C]nor-BUP (0.557 ± 0.077%ID/cm-3).

Dealing with PET radiometabolites

Abstract

Positron emission tomography (PET) offers the study of biochemical, physiological, and pharmacological functions at a cellular and molecular level. The performance of a PET study mostly depends on the used radiotracer of interest. However, the development of a novel PET tracer is very difficult, as it is required to fulfill a lot of important criteria. PET radiotracers usually encounter different chemical modifications including redox reaction, hydrolysis, decarboxylation, and various conjugation processes within living organisms. Due to this biotransformation, different chemical entities are produced, and the amount of the parent radiotracer is declined. Consequently, the signal measured by the PET scanner indicates the entire amount of radioactivity deposited in the tissue; however, it does not offer any indication about the chemical disposition of the parent radiotracer itself. From a radiopharmaceutical perspective, it is necessary to quantify the parent radiotracer’s fraction present in the tissue. Hence, the identification of radiometabolites of the radiotracers is vital for PET imaging. There are mainly two reasons for the chemical identification of PET radiometabolites: firstly, to determine the amount of parent radiotracers in plasma, and secondly, to rule out (if a radiometabolite enters the brain) or correct any radiometabolite accumulation in peripheral tissue. Besides, radiometabolite formations of the tracer might be of concern for the PET study, as the radiometabolic products may display considerably contrasting distribution patterns inside the body when compared with the radiotracer itself. Therefore, necessary information is needed about these biochemical transformations to understand the distribution of radioactivity throughout the body. Various published review articles on PET radiometabolites mainly focus on the sample preparation techniques and recently available technology to improve the radiometabolite analysis process. This article essentially summarizes the chemical and structural identity of the radiometabolites of various radiotracers including [¹¹C]PBB3, [¹¹C]flumazenil, [¹⁸F]FEPE2I, [¹¹C]PBR28, [¹¹C]MADAM, and (+)[¹⁸F]flubatine. Besides, the importance of radiometabolite analysis in PET imaging is also briefly summarized. Moreover, this review also highlights how a slight chemical modification could reduce the formation of radiometabolites, which could interfere with the results of PET imaging.

Graphical abstract

Usefulness of PET With [18F]LBT-999 for the Evaluation of Presynaptic Dopaminergic Neuronal Loss in a Clinical Environment

Purpose: The density of the neuronal dopamine transporter (DAT) is directly correlated with the presynaptic dopaminergic system injury. In a first study, we evaluated the brain distribution and kinetics of [¹⁸F]LBT-999, a DAT PET radioligand, in a group of eight healthy subjects. Taking into account the results obtained in healthy volunteers, we wanted to evaluate whether the loss of presynaptic striatal dopaminergic fibers could be estimated, under routine clinical conditions, using [¹⁸F]LBT-999 and a short PET acquisition.

Materials and methods: Six patients with Parkinson's disease (PD) were compared with eight controls. Eighty-nine minutes of dynamic PET following an intravenous injection of [¹⁸F]LBT-999 were acquired. Using regions of interest for striatal nuclei, substantia nigra (SN), cerebellum, and occipital cortex, defined over each T1 3D MRI, time–activity curves (TACs) were obtained. From TACs, binding potential (BP_ND) using the simplified reference tissue model and distribution volume ratios (DVRs) using Logan graphical analysis were calculated. Ratios obtained for a 10-min image, acquired between 30 and 40 min post-injection, were also calculated. Cerebellum activity was used as non-specific reference region.

Results: In PD patients and as expected, striatal uptake was lower than in controls which is confirmed by BP_ND, DVR, and ratios calculated for both striatal nuclei and SN, significantly inferior in PD patients compared with controls (p < 0.001).

Conclusions: PET with [¹⁸F]LBT-999 could be an alternative to assess dopaminergic presynaptic injury in a clinical environment using a single 10 min acquisition.

PET imaging of Dopamine Transporter with [18F]LBT-999: initial evaluation in healthy volunteers

BACKGROUND

To evaluate in healthy human brain the distribution, uptake, and kinetics of [18F]LBT-999, a PET ligand targeting the dopamine transporter, to assess its ability to explore dopaminergic innervation, using a shorter protocol, more convenient for patients than currently with [123I]ioflupane.

METHODS

After intravenous injection of [18F]LBT-999, 8 healthy subjects (53-80y) underwent a dynamic PET-scan. Venous samples were concomitantly obtained for metabolites analysis. Time activity curves (TACs) were generated for several ROIs (caudate, putamen, occipital cortex, substantia nigra and cerebellum). Cerebellum was used as reference region to calculate binding potentials (BPND).

RESULTS

No adverse events or detectable pharmacological effects were reported. [18F]LBT-999 PET revealed a good cerebral distribution, with an intense and symmetric uptake in both putamen and caudate (BPND of 6.75±1.17 and 6.30±1.17, respectively), without other brain abnormal tracer accumulation. Regional TACs showed a plateau from the maximal uptake, 20min pi, to the end of the acquisition for both caudate and putamen, whereas uptake in substantia nigra decreased progressively. A faster clearance and lowest BPND values were observed in both cortex and cerebellum. Ratios to the cerebellum exhibit value of about 3 in substantia nigra, close to 10 for both caudate and putamen, and remained around the value of 1 in cortex. The parent fraction of [18F]LBT-999 in plasma was 80%, 60% and 45% at 15, 30 and 45 min pi, respectively.

CONCLUSIONS

These findings support the usefulness of [18F]LBT-999 for a quantitative clinical evaluation of presynaptic dopaminergic innervation.

Preclinical in vivo and in vitro comparison of the translocator protein PET ligands [18F]PBR102 and [18F]PBR111

PURPOSE

To determine the metabolic profiles of the translocator protein ligands PBR102 and PBR111 in rat and human microsomes and compare their in vivo binding and metabolite uptake in the brain of non-human primates (Papio hamadryas) using PET-CT.

METHODS

In vitro metabolic profiles of PBR102 and PBR111 in rat and human liver microsomes were assessed by liquid chromatography-tandem mass spectrometry. [¹⁸F]PBR102 and [¹⁸F]PBR111 were prepared by nucleophilic substitution of their corresponding p-toluenesulfonyl precursors with [¹⁸F]fluoride. List mode PET-CT brain imaging with arterial blood sampling was performed in non-human primates. Blood plasma measurements and metabolite analysis, using solid-phase extraction, provided the metabolite profile and metabolite-corrected input functions for kinetic model fitting. Blocking and displacement PET-CT scans, using PK11195, were performed.

RESULTS

Microsomal analyses identified the O-de-alkylated, hydroxylated and N-de-ethyl derivatives of PBR102 and PBR111 as the main metabolites. The O-de-alkylated compounds were the major metabolites in both species; human liver microsomes were less active than those from rat. Metabolic profiles in vivo in non-human primates and previously published rat experiments were consistent with the microsomal results. PET-CT studies showed that K₁ was similar for baseline and blocking studies for both radiotracers; V_T was reduced during the blocking study, suggesting low non-specific binding and lack of appreciable metabolite uptake in the brain.

CONCLUSIONS

[¹⁸F]PBR102 and [¹⁸F]PBR111 have distinct metabolic profiles in rat and non-human primates. Radiometabolites contributed to non-specific binding and confounded in vivo brain analysis of [¹⁸F]PBR102 in rodents; the impact in primates was less pronounced. Both [¹⁸F]PBR102 and [¹⁸F]PBR111 are suitable for PET imaging of TSPO in vivo. In vitro metabolite studies can be used to predict in vivo radioligand metabolism and can assist in the design and development of better radioligands.

LC-MS Supported Studies on the in Vitro Metabolism of both Enantiomers of Flubatine and the in Vivo Metabolism of (+)-[(18)F]Flubatine-A Positron Emission Tomography Radioligand for Imaging α4β2 Nicotinic Acetylcholine Receptors

Both enantiomers of [¹⁸F]flubatine are promising radioligands for neuroimaging of α4β2 nicotinic acetylcholine receptors (nAChRs) by positron emission tomography (PET). To support clinical studies in patients with early Alzheimer’s disease, a detailed examination of the metabolism in vitro and in vivo has been performed. (+)- and (−)-flubatine, respectively, were incubated with liver microsomes from mouse and human in the presence of NADPH (β-nicotinamide adenine dinucleotide 2′-phosphate reduced tetrasodium salt). Phase I in vitro metabolites were detected and their structures elucidated by LC-MS/MS (liquid chromatography-tandem mass spectrometry). Selected metabolite candidates were synthesized and investigated for structural confirmation. Besides a high level of in vitro stability, the microsomal incubations revealed some species differences as well as enantiomer discrimination with regard to the formation of monohydroxylated products, which was identified as the main metabolic pathway in this assay. Furthermore, after injection of 250 MBq (+)-[¹⁸F]flubatine (specific activity > 350 GBq/μmol) into mouse, samples were prepared from brain, liver, plasma, and urine after 30 min and investigated by radio-HPLC (high performance liquid chromatography with radioactivity detection). For structure elucidation of the radiometabolites of (+)-[¹⁸F]flubatine formed in vivo, identical chromatographic conditions were applied to LC-MS/MS and radio-HPLC to compare samples obtained in vitro and in vivo. By this correlation approach, we assigned three of four main in vivo radiometabolites to products that are exclusively C- or N-hydroxylated at the azabicyclic ring system of the parent molecule.

Efficient radiosynthesis and non-clinical safety tests of the TSPO radioprobe [(18)F]FEDAC: Prerequisites for clinical application

INTRODUCTION

[(18)F]FEDAC ([(18)F]1) has potent binding affinity and selectivity for translocator protein (18kDa, TSPO), and has been used to noninvasively visualize neuroinflammation, lung inflammation, acute liver damage, nonalcoholic fatty liver disease, and liver fibrosis. We had previously synthesized [(18)F]1 in two steps: (i) preparation of [(18)F]fluoroethyl bromide and (ii) coupling of [(18)F]fluoroethyl bromide with the appropriate precursor (2) for labeling. In this study, to clinically utilize [(18)F]1 as a PET radiopharmaceutical and to transfer the production technique of [(18)F]1 to other PET centers, we simplified its preparation by using a direct, one-step, tosyloxy-for-fluorine substitution. We also performed an acute toxicity study as a major non-clinical safety test, and determined radiometabolites using human liver microsomes.

METHODS

[(18)F]1 was prepared via direct (18)F-fluorination by heating the corresponding tosylated derivative (3) with [(18)F]fluoride as its Kryptofix 222 complex in dimethyl sulfoxide at 110°C for 15min, following by HPLC purification. Non-clinical safety tests were performed for the extended single-dose toxicity study in rats, and for the in vitro metabolite analysis with human liver microsomal incubation.

RESULTS

High quality batches of [(18)F]1, compatible with clinical applications, were obtained. At the end of irradiation, the decay-corrected radiochemical yield of [(18)F]1 using 1 and 5mg of precursor based on [(18)F]fluoride was 18.5±7.9% (n=10) and 52.0±5.8% (n=3), respectively. A single-dose of [(18)F]1 did not show toxicological effects for 14 days after the injection in male and female rats. In human liver microsomal incubations, [(18)F]1 was easily metabolized to [(18)F]desbenzyl-FEDAC ([(18)F]10) by CYPs (4.2% of parent compound left 60min after incubation).

CONCLUSION

We successfully synthesized clinical grade batches of [(18)F]1 and verified the absence of innocuity of this radiotracer. [(18)F]1 will be used to first-in-human studies in our facility.

Considerations in the Development of Reversibly Binding PET Radioligands for Brain Imaging

The development of reversibly binding radioligands for imaging brain proteins in vivo, such as enzymes, neurotransmitter transporters, receptors and ion channels, with positron emission tomography (PET) is keenly sought for biomedical studies of neuropsychiatric disorders and for drug discovery and development, but is recognized as being highly challenging at the medicinal chemistry level. This article aims to compile and discuss the main considerations to be taken into account by chemists embarking on programs of radioligand development for PET imaging of brain protein targets.

Identification of a major radiometabolite of [11C]PBB3

INTRODUCTION

[(11)C]PBB3 is a clinically used positron emission tomography (PET) probe for in vivo imaging of tau pathology in the brain. Our previous study showed that [(11)C]PBB3 was rapidly decomposed to a polar radiometabolite in the plasma of mice. For the pharmacokinetic evaluation of [(11)C]PBB3 it is important to elucidate the characteristics of radiometabolites. In this study, we identified the chemical structure of a major radiometabolite of [(11)C]PBB3 and proposed the metabolic pathway of [(11)C]PBB3.

METHODS

Carrier-added [(11)C]PBB3 was injected into a mouse for in vivo metabolite analysis. The chemical structure of a major radiometabolite was identified using LC-MS. Mouse and human liver microsomes and liver S9 samples were incubated with [(11)C]PBB3 in vitro. In silico prediction software was used to assist in the determination of the metabolite and metabolic pathway of [(11)C]PBB3.

RESULTS

In vivo analysis showed that the molecular weight of a major radiometabolite of [(11)C]PBB3, which was called as [(11)C]M2, was m/z 390 [M+H(+)]. In vitro analysis assisted by in silico prediction showed that [(11)C]M2, which was not generated by cytochrome P450 enzymes (CYPs), was generated by sulfated conjugation mediated by a sulfotransferase.

CONCLUSION

The major radiometabolite, [(11)C]M2, was identified as a sulfated conjugate of [(11)C]PBB3. [(11)C]PBB3 was metabolized mainly by a sulfotransferase and subsidiarily by CYPs.

Development of (18)F-labeled radiotracers for neuroreceptor imaging with positron emission tomography

Positron emission tomography (PET) is an in vivo molecular imaging tool which is widely used in nuclear medicine for early diagnosis and treatment follow-up of many brain diseases. PET uses biomolecules as probes which are labeled with radionuclides of short half-lives, synthesized prior to the imaging studies. These probes are called radiotracers. Fluorine-18 is a radionuclide routinely used in the radiolabeling of neuroreceptor ligands for PET because of its favorable half-life of 109.8 min. The delivery of such radiotracers into the brain provides images of transport, metabolic, and neurotransmission processes on the molecular level. After a short introduction into the principles of PET, this review mainly focuses on the strategy of radiotracer development bridging from basic science to biomedical application. Successful radiotracer design as described here provides molecular probes which not only are useful for imaging of human brain diseases, but also allow molecular neuroreceptor imaging studies in various small-animal models of disease, including genetically-engineered animals. Furthermore, they provide a powerful tool for in vivo pharmacology during the process of pre-clinical drug development to identify new drug targets, to investigate pathophysiology, to discover potential drug candidates, and to evaluate the pharmacokinetics and pharmacodynamics of drugs in vivo.

Tropane-derived (11) C-labelled and (18) F-labelled DAT ligands

Radiolabelling of cocaine-derived 3-phenyltropanes for dopamine transporter positron emission tomography with (18) F and (11) C is reviewed.

Radiosynthesis and validation of ¹⁸F-FP-CMT, a phenyltropane with superior properties for imaging the dopamine transporter in living brain

To date there is no validated, (18)F-labeled dopamine transporter (DAT) radiotracer with a rapid kinetic profile suitable for preclinical small-animal positron emission tomography (PET) studies in rodent models of human basal ganglia disease. Herein we report radiosynthesis and validation of the phenyltropane (18)F-FP-CMT. Dynamic PET recordings were obtained for (18)F-FP-CMT in six untreated rats, and six rats pretreated with the high-affinity DAT ligand GBR 12909; mean parametric maps of binding potential (BPND) relative to the cerebellum reference region, and maps of total distribution volume (VT) relative to the metabolite-corrected arterial input were produced. (18)F-FP-CMT BPND maps showed peak values of ∼4 in the striatum, versus ∼0.4 in the vicinity of the substantia nigra. Successive truncation of the PET recordings indicated that stable BPND estimates could be obtained with recordings lasting only 45 minutes, reflecting rapid kinetics of (18)F-FP-CMT. Pretreatment with GBR 12909 reduced the striatal binding by 72% to 76%. High-performance liquid chromatography analysis revealed rapid metabolism of (18)F-FP-CMT to a single, non-brain penetrant hydrophilic metabolite. Total distribution of volume calculated relative to the metabolite-corrected arterial input was 4.4 mL/g in the cerebellum. The pharmacological selectivity of (18)F-FP-CMT, rapid kinetic profile, and lack of problematic metabolites constitute optimal properties for quantitation of DAT in rat, and may also predict applicability in human PET studies.

[18F]DPA-C5yne, a novel fluorine-18-labelled analogue of DPA-714: radiosynthesis and preliminary evaluation as a radiotracer for imaging neuroinflammation with PET

DPA-C5yne, the lead compound of a novel series of DPA-714 derivatives in which the fluoroethoxy chain linked to the phenylpyrazolopyrimidine scaffold has been replaced by a fluoroalkyn-1-yl moiety, is a high affinity (Ki : 0.35 nM) and selective ligand targeting the translocator protein 18 kDa. In the present work, DPA-C5yne was labelled with no-carrier-added [(18)F]fluoride based on a one-step tosyloxy-for-fluorine nucleophilic substitution reaction, purified by cartridge and HPLC, and formulated as an i.v. injectable solution using a TRACERLab FX N Pro synthesizer. Typically, 4.3-5.2 GBq of [(18)F]DPA-C5yne, ready-to-use, chemically and radiochemically pure (> 95%), was obtained with specific radioactivities ranging from 55 to 110 GBq/µmol within 50-60 min, starting from a 30 GBq [(18)F]fluoride batch (14-17%). LogP and LogD of [(18)F]DPA-C5yne were measured using the shake-flask method and values of 2.39 and 2.51 were found, respectively. Autoradiography studies performed on slices of ((R,S)-α-amino-3-hydroxy-5-methyl-4-isoxazolopropionique (AMPA)-lesioned rat brains showed a high target-to-background ratio (1.9 ± 0.3). Selectivity and specificity of the binding for the translocator protein was demonstrated using DPA-C5yne (unlabelled), PK11195 and Flumazenil (central benzodiazepine receptor ligand) as competitors. Furthermore, DPA-C5yne proved to be stable in plasma at 37°C for at least 90 min.

[(18)F]Fallypride: metabolism studies and quantification of the radiotracer and its radiometabolites in plasma using a simple and rapid solid-phase extraction method

INTRODUCTION

[(18)F]Fallypride, a fluorinated and substituted benzamide with high affinity for D2/D3 receptors, is a useful PET radioligand for the study of striatal/extrastriatal areas. Since [(18)F]fallypride is extensively metabolized in vivo and since PET examinations are long lasting in humans, the rapid measurement of the unchanged radiotracer in plasma is essential for the quantification of images. The present study aims: i) to evaluate if the radiometabolites of [(18)F]fallypride cross the blood-brain barrier in rodents, ii) to identify these radiometabolites in baboon plasma and iii) to develop a rapid solid phase extraction method (SPE) suitable for human applications to quantify both [(18)F]fallypride and its radiometabolites in plasma.

METHODS

The metabolites P450-dependant in rat and human liver microsomes were characterized by LC-MS-MS and compared to those detected in vivo. Sequential solvent elution on Oasis®-MCX-SPE cartridges was used to quantify [(18)F]fallypride and its radiometabolites.

RESULT

In rat microsomal incubations, five metabolites generated upon N/O-dealkylation or hydroxylation at the pyrrolidine and/or at the benzamide moiety were identified. No radiometabolite was detected in the rat brain. N-dealkylated and hydroxylated derivatives were detected in human microsomal incubations as well as in baboon plasma. The use of SPE (total recovery 100.2%± 2.8%, extraction yield 95.5%± 0.3%) allowed a complete separation of [(18)F]fallypride from its radiometabolites in plasma and evaluate [(18)F]fallypride at 150 min pi to be 22%± 5% of plasma radioactivity.

CONCLUSIONS

The major in vivo radiometabolites of [(18)F]fallypride were produced by N-dealkylation and hydroxylation. Allowing the rapid analysis of multiple plasma samples, SPE is a method of choice for the determination of [(18)F]fallypride until late images required for quantitative PET imaging in humans.

Synthesis and evaluation of novel tropane derivatives as potential PET imaging agents for the dopamine transporter

A novel series of tropane derivatives containing a fluorinated tertiary amino or amide at the 2β position was synthesized, labeled with the positron-emitter fluorine-18 (t(1/2)=109.8 min), and tested as potential in vivo dopamine transporter (DAT) imaging agents. The corresponding chlorinated analogs were prepared and employed as precursors for radiolabeling leading to the fluorine-18-labeled derivatives via a one-step nucleophilic aliphatic substitution reaction. In vitro binding results showed that the 2β-amino compounds 6b, 6d and 7b displayed moderately high affinities to DAT (K(i)<10nM). Biodistribution studies of [(18)F]6b and [(18)F]6d showed that the brain uptakes in rats were low. This is likely due to their low lipophilicities. Further structural modifications of these tropane derivatives will be needed to improve their in vivo properties as DAT imaging agents.

Difficulties in dopamine transporter radioligand PET analysis: the example of LBT-999 using [18F] and [11C] labelling Part I: PET studies

BACKGROUND

LBT-999 (E)-N-(4-fluorobut-2-enyl)-2β-carbomethoxy-3β-(4'-tolyl)nortropane is a dopamine transporter (DAT) ligand. [(18)F]LBT-999 was first labelled with carbon-11; we will now describe its in vivo behaviour in comparison to that of [(11)C]LBT-999.

METHODS/RESULTS

Positron emission tomography (PET) experiments (baboons) confirmed the high affinity/specificity of [(18)F]LBT-999 for DAT. The brain regional distribution was in accordance with that of DAT. Pre-treatment with LBT-999 (1 mg/kg iv), but not with desipramine, a norepinephrine (NET) antagonist, reduced the striatum-to-cerebellum ratio by 96%, confirming the specificity for DAT vs. NET. The parent compound decreased rapidly and represented 24.3 ± 5.0% of plasma radioactivity at 30 min pi. Whole-body scans showed an important bone uptake of free fluorine following metabolism of [(18)F]LBT-999. In the cerebellum and striatum, distribution volumes increased by 30-40% between 80 and 230 min, suggesting the polluting role of a radiometabolite(s). [(11)C]LBT-999 exhibited a 40% higher standardized uptake value in the striata. This difference is likely due to N-dealkylation followed by [(18)F]fluoride release. 2β-Carbomethoxy-3β-(4'-tolyl) nortropane is then formed, while [(11)C]2β-carbomethoxy-3β-(4'-tolyl) nortropane is formed following injection of [(11)C]LBT-999. This metabolite has high affinity for the DAT. In one specific PET experiment, intravenous injection of this metabolite induced a strong displacement of [(18)F]LBT-999 in the striata, confirming that this metabolite readily crosses the blood-brain barrier (BBB) and binds to DAT.

CONCLUSIONS

[(18)F]LBT-999 is N-dealkylated in vivo to yield (1) a nonradioactive metabolite that crosses the BBB and has a high affinity for the DAT and (2) a [(18)F]fluoro-alkyl chain which is further defluorinated. The temporal changes in distribution volumes are consistent with the accumulation of a radiometabolite(s) in the brain. Therefore, the quantification of DAT density with [(18)F]LBT-999 is rather difficult.