Synthesis and evaluation of no-carrier-added 8-cyclopentyl-3-(3-[(18)F]fluoropropyl)-1-propylxanthine ([(18)F]CPFPX): a potent and selective A(1)-adenosine receptor antagonist for in vivo imaging

8-Bicycloalkyl-CPFPX derivatives as potent and selective tools for in vivo imaging of the A1 adenosine receptor

Imaging of the A1 adenosine receptor (A1R) by positron emission tomography (PET) with 8-cyclopentyl-3-(3-[18F]fluoropropyl)-1-propyl-xanthine ([18F]CPFPX) has been widely used in preclinical and clinical studies. However, this radioligand suffers from rapid peripheral metabolism and subsequent accumulation of radiometabolites in the vascular compartment. In the present work, we prepared four derivatives of CPFPX by replacement of the cyclopentyl group with norbornane moieties. These derivatives were evaluated by competition binding studies, microsomal stability assays and LC-MS analysis of microsomal metabolites. In addition, the 18F-labeled isotopologue of 8-(1-norbornyl)-3-(3-fluoropropyl)-1-propylxanthine (1-NBX) as the most promising candidate was prepared by radiofluorination of the corresponding tosylate precursor and the resulting radioligand ([18F]1-NBX) was evaluated by permeability assays with Caco-2 cells and in vitro autoradiography in rat brain slices. Our results demonstrate that 1-NBX exhibits significantly improved A1R affinity and selectivity when compared to CPFPX and that it does not give rise to lipophilic metabolites expected to cross the blood-brain-barrier in microsomal assays. Furthermore, [18F]1-NBX showed a high passive permeability (Pc = 6.9 ± 2.9 × 10-5 cm/s) and in vitro autoradiography with this radioligand resulted in a distribution pattern matching A1R expression in the brain. Moreover, a low degree of non-specific binding (5%) was observed. Taken together, these findings identify [18F]1-NBX as a promising candidate for further preclinical evaluation as potential PET tracer for A1R imaging.

A genetic variation in the adenosine A2A receptor gene contributes to variability in oscillatory alpha power in wake and sleep EEG and A1 adenosine receptor availability in the human brain

The EEG alpha rhythm (∼ 8-13 Hz) is one of the most salient human brain activity rhythms, modulated by the level of attention and vigilance and related to cerebral energy metabolism. Spectral power in the alpha range in wakefulness and sleep strongly varies among individuals based on genetic predisposition. Knowledge about the underlying genes is scarce, yet small studies indicated that the variant rs5751876 of the gene encoding A2A adenosine receptors (ADORA2A) may contribute to the inter-individual variation. The neuromodulator adenosine is directly linked to energy metabolism as product of adenosine tri-phosphate breakdown and acts as a sleep promoting molecule by activating A1 and A2A adenosine receptors. We performed sleep and positron emission tomography studies in 59 healthy carriers of different rs5751876 alleles, and quantified EEG oscillatory alpha power in wakefulness and sleep, as well as A1 adenosine receptor availability with 18F-CPFPX. Oscillatory alpha power was higher in homozygous C-allele carriers (n = 27, 11 females) compared to heterozygous and homozygous carriers of the T-allele (n(C/T) = 23, n(T/T) = 5, 13 females) (F(18,37) = 2.35, p = 0.014, Wilk's Λ = 0.487). Furthermore, a modulatory effect of ADORA2A genotype on A1 adenosine receptor binding potential was found across all considered brain regions (F(18,40) = 2.62, p = 0.006, Wilk's Λ = 0.459), which remained significant for circumscribed occipital region of calcarine fissures after correction for multiple comparisons. In female participants, a correlation between individual differences in oscillatory alpha power and A1 receptor availability was observed. In conclusion, we confirmed that a genetic variant of ADORA2A affects individual alpha power, while a direct modulatory effect via A1 adenosine receptors in females is suggested.

Validation of analytical HPLC with post-column injection as a method for rapid and precise quantification of radiochemical yields

Accurate assessment of isolated radiochemical yields (RCYs) is a prerequisite for efficient and reliable optimization of labeling reactions. In practice, radiochemical conversions (RCCs) determined by HPLC analysis of crude reaction mixtures are often used to estimate RCYs. However, incomplete recovery of radioactivity from the stationary phase can lead to significant inaccuracies if RCCs are calculated based on the activity eluted from the column (i.e. the summed integrals of all peaks). Here, we validate a simple and practical method that overcomes problems associated with retention of activity on the column by determination of the total activity in the sample using post-column injection. Post-column injections were carried out using an additional injection valve, which was placed between the outlet of the HPLC column and the inlet of the detectors. 2-[18F]Fluoropyridine ([18F]FPy) and 8-cyclopentyl-3-(3-[18F]fluoropropyl)-1-propylxanthine ([18F]CPFPX) were prepared with radiochemical purities of > 99.8% and mixed with [18F]fluoride at a ratio of 1:1 to simulate reaction mixtures obtained by radiolabeling reactions with an RCC of 50%. The samples were analyzed on three different C18 HPLC columns using neutral and acidic mobile phases. RCCs determined using the summed area of all peaks in the chromatograms were compared with those determined using post-column injection. Additionally, RCCs determined by post-column injection were corrected for activity losses before, during and after radiosyntheses to afford analytical RCYs, which were compared with isolated RCYs. Determination of RCCs based on the summed area of all peaks gave correct results under certain chromatographic conditions, but led to overestimation of the actual RCCs by up to 50% in other cases. In contrast, determination of RCCs using post-column injection provided precise results in all cases, and often significantly reduced analysis time. Moreover, analytical RCYs calculated from RCCs determined by post-column injection showed excellent agreement with isolated RCYs (<3% deviation). In conclusion, HPLC analysis using post-column injection enables reliable determination of RCCs independent of the chromatographic conditions and, together with a simple activity balance, rapid and accurate prediction of isolated RCYs.

Effects of electroconvulsive therapy on cerebral A1 adenosine receptor availability: a PET study in patients suffering from treatment-resistant major depressive disorder

Introduction

Sleep deprivation and electroconvulsive therapy (ECT) effectively ameliorate symptoms in major depressive disorder (MDD). In rodents, both are associated with an enhancement of cerebral adenosine levels, which in turn likely influence adenosinergic receptor expression. The aim of the current study was to investigate cerebral A1 adenosine receptor (A1AR) availability in patients with MDD as a potential mediating factor of antidepressant effects of ECT using [18F]CPFPX and positron emission tomography (PET).

Methods

Regional A1AR availability was determined before and after a series of ECT applications (mean number ± SD 10.4 ± 1.2) in 14 subjects (4 males, mean age 49.5 ± 11.8 years). Clinical outcome, measured by neuropsychological testing, and ECT parameters were correlated with changes in A1AR availability.

Results

ECT had a strong antidepressive effect (p < 0.01) while on average cerebral A1AR availability remained unaltered between pre-and post-ECT conditions (F = 0.65, p = 0.42, mean difference ± SD 3.93% ± 22.7%). There was no correlation between changes in clinical outcome parameters and regional A1AR availability, although individual patients showed striking bidirectional alterations of up to 30-40% in A1AR availability after ECT. Solely, for the mean seizure quality index of the applied ECTs a significant association with changes in A1AR availability was found (rs = -0.6, p = 0.02).

Discussion

In the present study, therapeutically effective ECT treatment did not result in coherent changes of A1AR availability after a series of ECT treatments. These findings do not exclude a potential role for cerebral A1ARs in ECT, but shift attention to rather short-termed and adaptive mechanisms during ECT-related convulsive effects.

Imaging of the calcium activated potassium channel 3.1 (KCa 3.1) in vivo using a senicapoc-derived positron emission tomography tracer

The calcium-activated potassium channel 3.1 (K_Ca 3.1) is overexpressed in many tumor entities and has predictive power concerning disease progression and outcome. Imaging of the K_Ca 3.1 channel in vivo using a radiotracer for positron emission tomography (PET) could therefore establish a potentially powerful diagnostic tool. Senicapoc shows high affinity and excellent selectivity toward the K_Ca 3.1 channel. We have successfully pursued the synthesis of the ¹⁸ F-labeled derivative [¹⁸ F]3 of senicapoc using the prosthetic group approach with 1-azido-2-[¹⁸ F]fluoroethane ([¹⁸ F]6) in a "click" reaction. The biological activity of the new PET tracer was evaluated in vitro and in vivo. Inhibition of the K_Ca 3.1 channel by 3 was demonstrated by patch clamp experiments and the binding pose was analyzed by docking studies. In mouse and human serum, [¹⁸ F]3 was stable for at least one half-life of [¹⁸ F]fluorine. Biodistribution experiments in wild-type mice were promising, showing rapid and predominantly renal excretion. An in vivo study using A549-based tumor-bearing mice was performed. The tumor signal could be delineated and image analysis showed a tumor-to-muscle ratio of 1.47 ± 0.24. The approach using 1-azido-2-[¹⁸ F]fluoroethane seems to be a good general strategy to achieve triarylacetamide-based fluorinated PET tracers for imaging of the K_Ca 3.1 channel in vivo.

Strained Ammonium Precursors for Radiofluorinations

The increasing application of positron emission tomography (PET) in nuclear medicine has stimulated the extensive development of a multitude of novel and versatile techniques to introduce fluorine‐18, especially for the radiolabelling of biologically or pharmacologically active molecules. Taking into consideration that the introduction of fluorine‐18 (t_1/2=109.8 min) mostly proceeds under harsh conditions, radiolabelling of such molecules represents a challenge and is of enormous interest. Ideally, it should proceed in a regioselective manner under mild physiological conditions, in an acceptable time span, with high yields and high specific activities. Special attention has been drawn to 2‐fluoroethyl and 3‐fluoropropyl groups, which are often the active sites of radiofluorinated compounds. Precursors containing an ammonium leaving group – such as a strained azetidinium or aziridinium moiety – can help to overcome these obstacles leading to a convenient and mild introduction of [¹⁸F]fluoride with high radiochemical yields.

Cerebral A1 adenosine receptor availability in female and male participants and its relationship to sleep

The neuromodulator adenosine and its receptors are mediators of sleep-wake regulation which is known to differ between sexes. We, therefore, investigated sex differences in A₁ adenosine receptor (A₁AR) availability in healthy human subjects under well-rested conditions using [¹⁸F]CPFPX and positron emission tomography (PET). [¹⁸F]CPFPX PET scans were acquired in 50 healthy human participants (20 females; mean age ± SD 28.0 ± 5.3 years). Mean binding potential (BP_ND; Logan's reference tissue model with cerebellum as reference region) and volume of distribution (V_T) values were calculated in 12 and 15 grey matter brain regions, respectively. [¹⁸F]CPFPX BP_ND was higher in females compared to males in all investigated brain regions (p < 0.025). The largest differences were found in the pallidum and anterior cingulate cortex, where mean BP_ND values were higher by 29% in females than in males. In females, sleep efficiency correlated positively and sleep latency negatively with BP_ND in most brain regions. V_T values did not differ between sexes. Sleep efficiency correlated positively with V_T in most brain regions in female participants. In conclusion, our analysis gives a first indication for potential sex differences in A₁AR availability even under well-rested conditions. A₁AR availability as measured by [¹⁸F]CPFPX BP_ND is higher in females compared to males. Considering the involvement of adenosine in sleep-wake control, this finding might partially explain the known sex differences in sleep efficiency and sleep latency.

Species Differences in Microsomal Metabolism of Xanthine-Derived A1 Adenosine Receptor Ligands

Tracer development for positron emission tomography (PET) requires thorough evaluation of pharmacokinetics, metabolism, and dosimetry of candidate radioligands in preclinical animal studies. Since variations in pharmacokinetics and metabolism of a compound occur in different species, careful selection of a suitable model species is mandatory to obtain valid data. This study focuses on species differences in the in vitro metabolism of three xanthine-derived ligands for the A₁ adenosine receptor (A₁AR), which, in their ¹⁸F-labeled form, can be used to image A₁AR via PET. In vitro intrinsic clearance and metabolite profiles of 8-cyclopentyl-3-(3-fluoropropyl)-1-propylxanthine (CPFPX), an established A₁AR-ligand, and two novel analogs, 8-cyclobutyl-3-(3-fluoropropyl)-1-propylxanthine (CBX) and 3-(3-fluoropropyl)-8-(1-methylcyclobutyl)-1-propylxanthine (MCBX), were determined in liver microsomes from humans and preclinical animal species. Molecular mechanisms leading to significant differences between human and animal metabolite profiles were also examined. The results revealed significant species differences regarding qualitative and quantitative aspects of microsomal metabolism. None of the tested animal species fully matched human microsomal metabolism of the three A₁AR ligands. In conclusion, preclinical evaluation of xanthine-derived A₁AR ligands should employ at least two animal species, preferably rodent and dog, to predict in vivo behavior in humans. Surprisingly, rhesus macaques appear unsuitable due to large differences in metabolic activity towards the test compounds.

Quantitative Rodent Brain Receptor Imaging

Positron emission tomography (PET) is a non-invasive imaging technology employed to describe metabolic, physiological, and biochemical processes in vivo. These include receptor availability, metabolic changes, neurotransmitter release, and alterations of gene expression in the brain. Since the introduction of dedicated small-animal PET systems along with the development of many novel PET imaging probes, the number of PET studies using rats and mice in basic biomedical research tremendously increased over the last decade. This article reviews challenges and advances of quantitative rodent brain imaging to make the readers aware of its physical limitations, as well as to inspire them for its potential applications in preclinical research. In the first section, we briefly discuss the limitations of small-animal PET systems in terms of spatial resolution and sensitivity and point to possible improvements in detector development. In addition, different acquisition and post-processing methods used in rodent PET studies are summarized. We further discuss factors influencing the test-retest variability in small-animal PET studies, e.g., different receptor quantification methodologies which have been mainly translated from human to rodent receptor studies to determine the binding potential and changes of receptor availability and radioligand affinity. We further review different kinetic modeling approaches to obtain quantitative binding data in rodents and PET studies focusing on the quantification of endogenous neurotransmitter release using pharmacological interventions. While several studies have focused on the dopamine system due to the availability of several PET tracers which are sensitive to dopamine release, other neurotransmitter systems have become more and more into focus and are described in this review, as well. We further provide an overview of latest genome engineering technologies, including the CRISPR/Cas9 and DREADD systems that may advance our understanding of brain disorders and function and how imaging has been successfully applied to animal models of human brain disorders. Finally, we review the strengths and opportunities of simultaneous PET/magnetic resonance imaging systems to study drug-receptor interactions and challenges for the translation of PET results from bench to bedside.

In vivo multimodal imaging of adenosine A1 receptors in neuroinflammation after experimental stroke

Adenosine A₁ receptors (A₁ARs) are promising imaging biomarkers and targets for the treatment of stroke. Nevertheless, the role of A₁ARs on ischemic damage and its subsequent neuroinflammatory response has been scarcely explored so far.

Methods: In this study, the expression of A₁ARs after transient middle cerebral artery occlusion (MCAO) was evaluated by positron emission tomography (PET) with [¹⁸F]CPFPX and immunohistochemistry (IHC). In addition, the role of A₁ARs on stroke inflammation using pharmacological modulation was assessed with magnetic resonance imaging (MRI), PET imaging with [¹⁸F]DPA-714 (TSPO) and [¹⁸F]FLT (cellular proliferation), as well as IHC and neurofunctional studies.

Results: In the ischemic territory, [¹⁸F]CPFPX signal and IHC showed the overexpression of A₁ARs in microglia and infiltrated leukocytes after cerebral ischemia. Ischemic rats treated with the A₁AR agonist ENBA showed a significant decrease in both [¹⁸F]DPA-714 and [¹⁸F]FLT signal intensities at day 7 after cerebral ischemia, a feature that was confirmed by IHC results. Besides, the activation of A₁ARs promoted the reduction of the brain lesion, as measured with T₂W-MRI, and the improvement of neurological outcome including motor, sensory and reflex responses. These results show for the first time the in vivo PET imaging of A₁ARs expression after cerebral ischemia in rats and the application of [¹⁸F]FLT to evaluate glial proliferation in response to treatment.

Conclusion: Notably, these data provide evidence for A₁ARs playing a key role in the control of both the activation of resident glia and the de novo proliferation of microglia and macrophages after experimental stroke in rats.

ADORA2A variation and adenosine A1 receptor availability in the human brain with a focus on anxiety-related brain regions: modulation by ADORA1 variation

Adenosine, its interacting A₁ and A_2A receptors, and particularly the variant rs5751876 in the A_2A gene ADORA2A have been shown to modulate anxiety, arousal, and sleep. In a pilot positron emission tomography (PET) study in healthy male subjects, we suggested an effect of rs5751876 on in vivo brain A₁ receptor (A₁AR) availability. As female sex and adenosinergic/dopaminergic interaction partners might have an impact on this rs5751876 effect on A₁AR availability, we aimed to (1) further investigate the pilot male-based findings in an independent, newly recruited cohort including women and (2) analyze potential modulation of this rs5751876 effect by additional adenosinergic/dopaminergic gene variation. Healthy volunteers (32/11 males/females) underwent phenotypic characterization including self-reported sleep and A₁AR-specific quantitative PET. Rs5751876 and 31 gene variants of adenosine A₁, A_2A, A_2B, and A₃ receptors, adenosine deaminase, and dopamine D₂ receptor were genotyped. Multivariate analysis revealed an rs5751876 effect on A₁AR availability (P = 0.047), post hoc confirmed in 30 of 31 brain regions (false discovery rate (FDR) corrected P values < 0.05), but statistically stronger in anxiety-related regions (e.g., amygdala, hippocampus). Additional effects of ADORA1 rs1874142 were identified; under its influence rs5751876 and rs5751876 × sleep had strengthened effects on A₁AR availability (P_both < 0.02; post hoc FDR-corrected Ps < 0.05 for 29/30 regions, respectively). Our results support the relationship between rs5751876 and A₁AR availability. Additional impact of rs1874142, together with rs5751876 and sleep, might be involved in regulating arousal and thus the development of mental disorders like anxiety disorders. The interplay of further detected suggestive ADORA2A × DRD2 interaction, however, necessitates larger future samples more comparable to magnetic resonance imaging (MRI)-based samples.

Image-Derived Input Functions for Quantification of A1 Adenosine Receptors Availability in Mice Brains Using PET and [18F]CPFPX

Purpose

In vivo imaging for the A₁ adenosine receptors (A₁ARs) with positron emission tomography (PET) using 8-cyclopentyl-3-(3-[¹⁸F]fluoropropyl)-1-propylxan- thine ([¹⁸F]CPFPX) has become an important tool for studying physiological processes quantitatively in mice. However, the measurement of arterial input functions (AIFs) on mice is a method with restricted applicability because of the small total blood volume and the related difficulties in withdrawing blood. Therefore, the aim of this study was to extract an appropriate [¹⁸F]CPFPX image-derived input function (IDIF) from dynamic PET images of mice.

Procedures

In this study, five mice were scanned with [¹⁸F]CPFPX for 60 min. Arterial blood samples (n = 7 per animal) were collected from the femoral artery and corrected for metabolites. To generate IDIFs, three different approaches were selected: (A) volume of interest (VOI) placed over the heart (cube, 10 mm); (B) VOI set over abdominal vena cava/aorta region with a cuboid (5 × 5 × 15 mm); and (C) with 1 × 1 × 1 mm voxels on five consecutive slices. A calculated scaling factor (α) was used to correct for partial volume effect; the method of obtaining the total metabolite correction of [¹⁸F]CPFPX for IDIFs was developed. Three IDIFs were validated by comparison with AIF. Validation included the following: visual performance; computing area under the curve (AUC) ratios (IDIF/AIF) of whole-blood curves and parent curves; and the mean distribution volume (V_T) ratios (IDIF/AIF) of A₁ARs calculated by Logan plot and two-tissue compartment model.

Results

Compared with the AIF, the IDIF with VOI over heart showed the best performance among the three IDIFs after scaling by 1.77 (α) in terms of visual analysis, AUC ratios (IDIF/AIF; whole-blood AUC ratio, 1.03 ± 0.06; parent curve AUC ratio, 1.01 ± 0.10) and V_T ratios (IDIF/AIF; Logan V_T ratio, 1.00 ± 0.17; two-tissue compartment model V_T ratio, 1.00 ± 0.13) evaluation. The A₁ARs distribution of average parametric images was in good accordance to autoradiography of the mouse brain.

Conclusion

The proposed study provides evidence that IDIF with VOI over heart can replace AIF effectively for quantification of A₁ARs using PET and [¹⁸F]CPFPX in mice brains.

Huntington's Disease: A Review of the Known PET Imaging Biomarkers and Targeting Radiotracers

Huntington’s disease (HD) is a fatal neurodegenerative disease caused by a CAG expansion mutation in the huntingtin gene. As a result, intranuclear inclusions of mutant huntingtin protein are formed, which damage striatal medium spiny neurons (MSNs). A review of Positron Emission Tomography (PET) studies relating to HD was performed, including clinical and preclinical data. PET is a powerful tool for visualisation of the HD pathology by non-invasive imaging of specific radiopharmaceuticals, which provide a detailed molecular snapshot of complex mechanistic pathways within the brain. Nowadays, radiochemists are equipped with an impressive arsenal of radioligands to accurately recognise particular receptors of interest. These include key biomarkers of HD: adenosine, cannabinoid, dopaminergic and glutamateric receptors, microglial activation, phosphodiesterase 10 A and synaptic vesicle proteins. This review aims to provide a radiochemical picture of the recent developments in the field of HD PET, with significant attention devoted to radiosynthetic routes towards the tracers relevant to this disease.

PET Imaging of Adenosine Receptors in Diseases

Adenosine receptors (ARs) are a class of purinergic G-protein-coupled receptors (GPCRs). Extracellular adenosine is a pivotal regulation molecule that adjusts physiological function through the interaction with four ARs: A1R, A2AR, A2BR, and A3R. Alterations of ARs function and expression have been studied in neurological diseases (epilepsy, Alzheimer's disease, and Parkinson's disease), cardiovascular diseases, cancer, and inflammation and autoimmune diseases. A series of Positron Emission Tomography (PET) probes for imaging ARs have been developed. The PET imaging probes have provided valuable information for diagnosis and therapy of diseases related to alterations of ARs expression. This review presents a concise overview of various ARs-targeted radioligands for PET imaging in diseases. The most recent advances in PET imaging studies by using ARs-targeted probes are briefly summarized.

Relevance of In Vitro Metabolism Models to PET Radiotracer Development: Prediction of In Vivo Clearance in Rats from Microsomal Stability Data

The prediction of in vivo clearance from in vitro metabolism models such as liver microsomes is an established procedure in drug discovery. The potentials and limitations of this approach have been extensively evaluated in the pharmaceutical sector; however, this is not the case for the field of positron emission tomography (PET) radiotracer development. The application of PET radiotracers and classical drugs differs greatly with regard to the amount of substance administered. In typical PET imaging sessions, subnanomolar quantities of the radiotracer are injected, resulting in body concentrations that cannot be readily simulated in analytical assays. This raises concerns regarding the predictability of radiotracer clearance from in vitro data. We assessed the accuracy of clearance prediction for three prototypical PET radiotracers developed for imaging the A₁ adenosine receptor (A₁AR). Using the half-life (t_1/2) approach and physiologically based scaling, in vivo clearance in the rat model was predicted from microsomal stability data. Actual clearance could be accurately predicted with an average fold error (AFE) of 0.78 and a root mean square error (RMSE) of 1.6. The observed slight underprediction (1.3-fold) is in accordance with the prediction accuracy reported for classical drugs. This result indicates that the prediction of radiotracer clearance is possible despite concentration differences of more than three orders of magnitude between in vitro and in vivo conditions. Consequently, in vitro metabolism models represent a valuable tool for PET radiotracer development.

Effects of Long-Term Caffeine Consumption on the Adenosine A1 Receptor in the Rat Brain: an In Vivo PET Study with [18F]CPFPX

PURPOSE

Caffeine, a nonselective antagonist of adenosine receptors, is the most popular psychostimulant worldwide. Recently, a protective role of moderate chronic caffeine consumption against neurodegenerative diseases such as Alzheimer's and Parkinson's disease has been discussed. Thus, aim of the present study was an in vivo investigation of effects of long-term caffeine consumption on the adenosine A₁ receptor (A₁AR) in the rat brain.

PROCEDURES

Sixteen adult, male rats underwent five positron emission tomography (PET) scans with the highly selective A₁AR radioligand [¹⁸F]CPFPX in order to determine A₁AR availability. After the first baseline PET scan, the animals were assigned to two groups: Caffeine treatment and control group. The caffeine-treated animals received caffeinated tap water (30 mg/kg bodyweight/day, corresponding to 4-5 cups of coffee per day in humans) for 12 weeks. Subsequently, caffeine was withdrawn and repeated PET measurements were performed on day 1, 2, 4, and 7 of caffeine withdrawal. The control animals were measured according to the same time schedule.

RESULTS

At day 1, after 4.4 h of caffeine withdrawal, a significant decrease (- 34.5%, p < 0.001) of whole brain A₁AR availability was observed. Unlike all other investigated brain regions in caffeine-treated rats, the hypothalamus and nucleus accumbens showed no significant intraindividual differences between baseline and first withdrawal PET scan. After approximately 27 h of caffeine withdrawal, the region- and group-specific effects disappeared and A₁AR availability settled around baseline.

CONCLUSIONS

The present study provides evidence that chronic caffeine consumption does not lead to persistent changes in functional availability of cerebral A₁ARs which have previously been associated with neuroprotective effects of caffeine. The acute and region-specific decrease in cerebral A₁AR availability directly after caffeine withdrawal is most likely caused by residual amounts of caffeine metabolites disguising an unchanged A₁AR expression at this early time-point.

Influence of incubation conditions on microsomal metabolism of xanthine-derived A1 adenosine receptor ligands

INTRODUCTION

In vitro metabolism models such as liver microsomes represent an important tool for the development of novel radioligands. Comparability and physiological relevance of in vitro metabolism data critically depend on the careful evaluation and optimization of assay protocols. We therefore investigated the influence of incubation conditions on the microsomal stability of xanthine-derived A₁ adenosine receptor (A₁AR) ligands which have been developed for positron emission tomography (PET).

METHODS

Substrate depletion assays using rat liver microsomes (RLM) were performed for three analogous compounds which differ with regard to the metabolically vulnerable substituent at the xanthine C8 position. Incubation conditions were varied systematically. Additionally, the stability of the cofactor NADPH during incubation was investigated.

RESULTS

Microsomal metabolism was strongly influenced by buffer pH, organic solvents and preincubation time. Substrate depletion values varied up to 5-fold depending on incubation matrix composition, however, the rank order of metabolic stability remained unchanged. Prolonged incubation periods led to drastic loss in enzyme activity which could not be prevented by addition of metal chelators or antioxidants. Cofactor NADPH was rapidly oxidized in microsomal matrix, even in the absence of cytochrome P450 substrates.

DISCUSSION

In summary, short incubation times, precise pH control and minimal concentrations of organic solvents are mandatory to obtain reliable microsomal stability data. Furthermore, in vitro metabolic stability of the tested A₁AR ligands varied largely depending on the particular C8 substituent. Consequently, structural modifications at the xanthine C8 position appear to be a promising strategy for the improvement of A₁AR PET radioligands.

Preclinical Evaluation of the First Adenosine A1 Receptor Partial Agonist Radioligand for Positron Emission Tomography Imaging

Central adenosine A₁ receptor (A₁R) is implicated in pain, sleep, substance use disorders, and neurodegenerative diseases, and is an important target for pharmaceutical development. Radiotracers for A₁R positron emission tomography (PET) would enable measurement of the dynamic interaction of endogenous adenosine and A₁R during the sleep-awake cycle. Although several human A₁R PET tracers have been developed, most are xanthine-based antagonists that failed to demonstrate competitive binding against endogenous adenosine. Herein, we explored non-nucleoside (3,5-dicyanopyridine and 5-cyanopyrimidine) templates for developing an agonist A₁R PET radiotracer. We synthesized novel analogues, including 2-amino-4-(3-methoxyphenyl)-6-(2-(6-methylpyridin-2-yl)ethyl)pyridine-3,5-dicarbonitrile (MMPD, 22b), a partial A₁R agonist of sub-nanomolar affinity. [¹¹C]22b showed suitable blood-brain barrier (BBB) permeability and test-retest reproducibility. Regional brain uptake of [¹¹C]22b was consistent with known brain A₁R distribution and was blocked significantly by A₁R but not A_2AR ligands. [¹¹C]22b is the first BBB-permeable A₁R partial agonist PET radiotracer with the promise of detecting endogenous adenosine fluctuations.

The Role of Adenosine Tone and Adenosine Receptors in Huntington's Disease

Huntington's disease (HD) is a hereditary neurodegenerative disorder caused by a mutation in the IT15 gene that encodes for the huntingtin protein. Mutated hungtingtin, although widely expressed in the brain, predominantly affects striato-pallidal neurons, particularly enriched with adenosine A2A receptors (A2AR), suggesting a possible involvement of adenosine and A2AR is the pathogenesis of HD. In fact, polymorphic variation in the ADORA2A gene influences the age at onset in HD, and A2AR dynamics is altered by mutated huntingtin. Basal levels of adenosine and adenosine receptors are involved in many processes critical for neuronal function and homeostasis, including modulation of synaptic activity and excitotoxicity, the control of neurotrophin levels and functions, and the regulation of protein degradation mechanisms. In the present review, we critically analyze the current literature involving the effect of altered adenosine tone and adenosine receptors in HD and discuss why therapeutics that modulate the adenosine system may represent a novel approach for the treatment of HD.

Synthesis and Pharmacological Evaluation of Identified and Putative Metabolites of the A1 Adenosine Receptor Antagonist 8-Cyclopentyl-3-(3-fluoropropyl)-1-propylxanthine (CPFPX)

The A₁ adenosine receptor (A₁ AR) antagonist [¹⁸ F]8-cyclopentyl-3-(3-fluoropropyl)-1-propylxanthine ([¹⁸ F]CPFPX), used in imaging human brain A₁ ARs by positron emission tomography (PET), is stable in the brain, but rapidly undergoes transformation into one major (3-(3-fluoropropyl)-8-(3-oxocyclopenten-1-yl)-1-propylxanthine, M1) and several minor metabolites in blood. This report describes the synthesis of putative metabolites of CPFPX as standards for the identification of those metabolites. Analysis by (radio)HPLC revealed that extracts of human liver microsomes incubated with no-carrier-added (n.c.a.)[¹⁸ F]CPFPX contain the major metabolite, M1, as well as radioactive metabolites corresponding to derivatives functionalized at the cyclopentyl moiety, but no N1-despropyl species or metabolites resulting from functionalization of the N3-fluoropropyl chain. The putative metabolites were found to displace the binding of [³ H]CPFPX to the A₁ AR in pig brain cortex at K_i values between 1.9 and 380 nm and the binding of [³ H]ZM241385 to the A_2A AR in pig striatum at K_i values >180 nm. One metabolite, a derivative functionalized at the ω-position of the N1-propyl chain, showed high affinity (K_i 2 nm) to and very good selectivity (>9000) for the A₁ AR.

Reproducibility of non-invasive a1 adenosine receptor quantification in the rat brain using [(18)F]CPFPX and positron emission tomography

PURPOSE

The A1AR antagonist 8-cyclopentyl-3-(3-fluoropropyl)-1-propylxanthine ([(18)F]CPFPX) has recently been shown to be a suitable radiotracer for quantitative in vivo imaging of the A1 adenosine receptor (A1AR) in rats. The present study evaluates the reproducibility of non-invasive longitudinal A1AR studies with [(18)F]CPFPX and a dedicated small animal positron emission tomography (PET) scanner.

PROCEDURES

Twelve male Sprague Dawley rats underwent four repeated dynamic PET scans with a bolus injection of [(18)F]CPFPX. A1AR availability was determined by different non-invasive approaches including simplified and multilinear reference tissue (olfactory bulb)-based models and graphical methods. The outcome parameter binding potential (BP) was evaluated in terms of variability and reproducibility.

RESULTS

Repeated estimations of [(18)F]CPFPX BP ND gave reliable results with acceptable variability (mean 12 %) and reproducibility (intraclass correlation coefficients raging from 0.57 to 0.68) in cortical and subcortical regions of the rat brain. With regard to kinetic models, test-retest stability of the simplified reference-tissue model (SRTM) was superior to multilinear and graphical approaches.

CONCLUSIONS

Non-invasive quantification of A1AR density in the rat brain is reproducible and reliable with [(18)F]CPFPX PET and allows longitudinal designs of in vivo imaging studies in rodents.

Association of adenosine receptor gene polymorphisms and in vivo adenosine A1 receptor binding in the human brain

Adenosine A1 receptors (A1ARs) and the interacting adenosine A2A receptors are implicated in neurological and psychiatric disorders. Variants within the corresponding genes ADORA1 and ADORA2A were shown associated with pathophysiologic alterations, particularly increased anxiety. It is unknown so far, if these variants might modulate the A1AR distribution and availability in different brain regions. In this pilot study, the influence of ADORA1 and ADORA2A variants on in vivo A1AR binding was assessed with the A1AR-selective positron emission tomography (PET) radioligand [(18)F]CPFPX in brains of healthy humans. Twenty-eight normal control subjects underwent PET procedures to calculate the binding potential BPND of [(18)F]CPFPX in cerebral regions and to assess ADORA1 and ADORA2A single nucleotide polymorphism (SNP) effects on regional BPND data. Our results revealed SNPs of both genes associated with [(18)F]CPFPX binding to the A1AR. The strongest effects that withstood even Bonferroni correction of multiple SNP testing were found in non-smoking subjects (N=22) for ADORA2A SNPs rs2236624 and rs5751876 (corr. Pall<0.05). SNP alleles previously identified at risk for increased anxiety like the rs5751876 T-allele corresponded to consistently higher A1AR availability in all brain regions. Our data indicate for the first time that variation of A1AR availability was associated with ADORA SNPs. The finding of increased A1AR availability in regions of the fear network, particularly in ADORA2A risk allele carriers, strongly warrants evaluation and replication in further studies including individuals with increased anxiety.

Cross sectional PET study of cerebral adenosine A₁ receptors in premanifest and manifest Huntington's disease

PURPOSE

To study cerebral adenosine receptors (AR) in premanifest and manifest stages of Huntington's disease (HD).

METHODS

We quantified the cerebral binding potential (BP ND) of the A₁AR in carriers of the HD CAG trinucleotide repeat expansion using the radioligand [(18) F]CPFPX and PET. Four groups were investigated: (i) premanifest individuals far (preHD-A; n = 7) or (ii) near (preHD-B; n = 6) to the predicted symptom onset, (iii) manifest HD patients (n = 8), and (iv) controls (n = 36).

RESULTS

Cerebral A₁AR values of preHD-A subjects were generally higher than those of controls (by up to 31%, p < .01, in the thalamus on average). Across stages a successive reduction of A₁AR BPND was observed to the levels of controls in preHD-B and undercutting controls in manifest HD by down to 25%, p < .01, in the caudatus and amygdala. There was a strong correlation between A₁AR BP ND and years to onset. Before onset of HD, the assumed annual rates of change of A₁AR density were -1.2% in the caudatus, -1.7% in the thalamus and -3.4% in the amygdala, while the corresponding volume losses amounted to 0.6%, 0.1% and 0.2%, respectively.

CONCLUSIONS

Adenosine receptors switch from supra to subnormal levels during phenoconversion of HD. This differential regulation may play a role in the pathophysiology of altered energy metabolism.

In vivo kinetic and steady-state quantification of 18F-CPFPX binding to rat cerebral A1 adenosine receptors: validation by displacement and autoradiographic experiments

In vivo imaging of the A1 adenosine receptor (A1AR) using (18)F-8-cyclopentyl-3-(3-fluoropropyl)-1-propylxanthine ((18)F-CPFPX) and PET has become an important tool for studying physiologic and pathologic states of the human brain. However, dedicated experimental settings for small-animal studies are still lacking. The aim of the present study was therefore to develop and evaluate suitable pharmacokinetic models for the quantification of the cerebral A1AR in high-resolution PET.

METHODS

On a dedicated animal PET scanner, 15 rats underwent (18)F-CPFPX PET scans of 120-min duration. In all animals, arterial blood samples were drawn and corrected for metabolites. The radioligand was injected either as a bolus or as a bolus plus constant infusion. For the definition of unspecific binding, the A1AR selective antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) was applied. After PET, the brains of 9 animals were dissected and in vitro saturation binding was performed using high-resolution (3)H-DPCPX autoradiography.

RESULTS

The kinetics of (18)F-CPFPX were well described by either compartmental or noncompartmental models based on arterial input function. The resulting distribution volume ratio correlated with a low bias toward identity with the binding potential derived from a reference region (olfactory bulb) approach. Furthermore, PET quantification correlated significantly with autoradiographic in vitro data. Blockade of the A1AR with DPCPX identified specific binding of about 45% in the reference region olfactory bulb.

CONCLUSION

The present study provides evidence that (18)F-CPFPX PET based on a reference tissue approach can be performed quantitatively in rodents in selected applications. Specific binding in the reference region needs careful consideration for quantitative investigations.

Collision-induced dissociation studies of caffeine in positive electrospray ionisation mass spectrometry using six deuterated isotopomers and one N1-ethylated homologue

RATIONALE

In order to deepen the understanding of electrospray ionisation collision-induced dissociation (ESI-CID) fragmentation reactions of xanthine derivatives for the identification of metabolites using low-resolution liquid chromatography/mass spectrometry (LC/MS) analysis, basic experiments using caffeine (1,3,7-trimethylxanthine) as model compound have been performed.

METHODS

Six deuterium isotopomers and one N1-ethylated homologue of caffeine have been synthesized and their ESI fragmentation spectra have been obtained by using LC/MS in combination with either standard or perdeuterated eluent mixtures.

RESULTS

One result of these studies is the finding that the positive charges of the ESI-CID caffeine fragments are caused by the addition of protons. Furthermore, the performed experiments allow the determination of all molecular formulae of each ESI-CID caffeine fragment.

CONCLUSIONS

As basic CID reactions of caffeine have been elucidated in this work, the developed fragmentation scheme may serve as a valuable tool for the interpretation of ESI-CID fragmentation spectra of more complex xanthine derivatives and their respective metabolites.

Caffeine occupancy of human cerebral A1 adenosine receptors: in vivo quantification with 18F-CPFPX and PET

Caffeine is the neuroactive agent in coffee and tea and is a broadly consumed stimulant. It is a nonselective antagonist of the neuromodulator adenosine and, if applied in commonly consumed doses, evokes its stimulating effects through the blockade of adenosine receptors. (18)F-8-cyclopentyl-3-(3-fluoropropyl)-1-propylxanthine ((18)F-CPFPX) has been established as a highly selective and affine PET ligand for the A(1) adenosine receptor (A(1)AR). The objective of the present study was to visualize and quantify the in vivo occupancy of the human cerebral A(1)AR by caffeine using (18)F-CPFPX and PET.

METHODS

Fifteen subjects (age range, 24-68 y) underwent a 140-min bolus-plus-constant-infusion PET experiment after at least 36 h of caffeine abstinence. Metabolite-corrected blood data were used to calculate steady-state distribution volumes (V(T)) during the baseline condition of the scan between 70 and 90 min. Subsequently, subjects received a 10-min infusion of varying concentrations (0.5-4.3 mg/kg of body weight) of caffeine at 90 min. Occupancy V(T) of the A(1)AR was thereafter estimated using data acquired between 120 and 140 min. Occupancy levels were calculated using the Lassen plot, from which the inhibitory concentrations of 50% were derived. Plasma levels of caffeine were determined at regular intervals. One subject received an intravenous vehicle as a placebo.

RESULTS

Caffeine displaced 5%-44% of (18)F-CPFPX binding in a concentration-dependent manner. There was no change of radioligand binding after the administration of placebo. Half-maximal displacement was achieved at a plasma caffeine concentration of 67 μM, which corresponds to 450 mg in a 70-kg subject or approximately 4.5 cups of coffee.

CONCLUSION

Given a biologic half-life of about 5 h, caffeine might therefore occupy up to 50% of the cerebral A(1)AR when caffeinated beverages are repeatedly consumed during a day. Furthermore, the present study provides evidence that (18)F-CPFPX PET is suitable for studying the cerebral actions of caffeine, the most popular neurostimulant worldwide.

1-(3-[18F]fluoropropyl)piperazines as model compounds for the radiofluorination of pyrido[2,3-d]pyrimidines

The visualization of cyclin-dependent kinases (CDKs), which are overexpressed in multiple tumor types, with radiolabeled CDK inhibitors by means of positron emission tomography in vivo is a promising approach for tumor imaging. Pyrido[2,3-d]pyrimidines belong to a class of inhibitors, which bind with high affinity to CDK4 and CDK6. 1-(3-[18F]Fluoropropyl)-4-(4-nitrophenyl)piperazine and 1-(3-[18F]fluoropropyl)-4-(6-nitropyridin-3-yl)piperazine represent structural elements of the appropriate CDK inhibitors and were therefore chosen as model compounds for the incorporation of fluorine-18 into pyrido[2,3-d]pyrimidines. Three methods are known for the preparation of tertiary 3-[18F]fluoropropyl-amines: 1) the direct substitution of a good leaving group, 2) the two-step reaction synthesizing a 3-[18F]fluoropropyl intermediate, and 3) the utilization of aziridinium or azetidinium salts. In general, radiofluorinations using azetidinium salts lead to excellent radiochemical yields in short periods of time. For these reasons, we developed a synthesis route to tosylated piperazine precursors and established a radiolabeling approach based on the incorporation of fluorine-18 into open-chained tosylates as well as the respective azetidinium spiro compounds to yield the desired radiofluorinated piperazines successfully.

International Union of Basic and Clinical Pharmacology. LXXXI. Nomenclature and classification of adenosine receptors--an update

In the 10 years since our previous International Union of Basic and Clinical Pharmacology report on the nomenclature and classification of adenosine receptors, no developments have led to major changes in the recommendations. However, there have been so many other developments that an update is needed. The fact that the structure of one of the adenosine receptors has recently been solved has already led to new ways of in silico screening of ligands. The evidence that adenosine receptors can form homo- and heteromultimers has accumulated, but the functional significance of such complexes remains unclear. The availability of mice with genetic modification of all the adenosine receptors has led to a clarification of the functional roles of adenosine, and to excellent means to study the specificity of drugs. There are also interesting associations between disease and structural variants in one or more of the adenosine receptors. Several new selective agonists and antagonists have become available. They provide improved possibilities for receptor classification. There are also developments hinting at the usefulness of allosteric modulators. Many drugs targeting adenosine receptors are in clinical trials, but the established therapeutic use is still very limited.

Xanthines as adenosine receptor antagonists

The natural plant alkaloids caffeine and theophylline were the first adenosine receptor (AR) antagonists described in the literature. They exhibit micromolar affinities and are non-selective. A large number of derivatives and analogues were subsequently synthesized and evaluated as AR antagonists. Very potent antagonists have thus been developed with selectivity for each of the four AR subtypes.

Adenosine A(1) receptors in human brain and transfected CHO cells: Inhibition of [(3)H]CPFPX binding by adenosine and caffeine

In vivo imaging of adenosine function has become feasible with the specific A(1) adenosine receptor ligand [(18)F]CPFPX and positron emission tomography (PET). It is, however, still an open question whether [(18)F]CPFPX is displaceable by endogenous adenosine, which would allow to detect activity-dependent adenosine release in vivo. We used the tritiated analog of [(18)F]CPFPX, [(3)H]CPFPX, to quantify A(1) adenosine receptors (A(1)AR) in grey matter tissue homogenates of four human brains and A(1)AR transfected Chinese hamster ovary cells, respectively. Saturation binding experiments in the presence of a stable GTP analog revealed a dissociation constant (K(D)) of 2.4±0.5nM. The unselective endogenous A(1)AR agonist adenosine and the antagonist caffeine displaced specific [(3)H]CPFPX binding completely at high doses. Concentrations sufficient to inhibit 50% of binding (IC(50)) were 6.9±2.7μM for adenosine and 148±15.4μM for caffeine. Respective inhibition constants (K(i)) were 2.8±0.9μM and 61.4±11.2μM.The present report supports the possibility of studying acute effects of adenosine and caffeine in vivo with [(18)F]CPFPX and PET. Pathophysiological conditions like hypoxia which increase endogenous adenosine concentrations several folds might interfere with in vivo [(18)F]CPFPX binding. Caffeine intake previous to the investigation should be considered as a confounding factor regarding the determination of receptor densities with [(18)F]CPFPX and PET.

Adenosine receptor ligands and PET imaging of the CNS

Advances in radiotracer chemistry have resulted in the development of novel molecular imaging probes for adenosine receptors (ARs). With the availability of these molecules, the function of ARs in human pathophysiology as well as the safety and efficacy of approaches to the different AR targets can now be determined. Molecular imaging is a rapidly growing field of research that allows the identification of molecular targets and functional processes in vivo. It is therefore gaining increasing interest as a tool in drug development because it permits the process of evaluating promising therapeutic targets to be stratified. Further, molecular imaging has the potential to evolve into a useful diagnostic tool, particularly for neurological and psychiatric disorders. This chapter focuses on currently available AR ligands that are suitable for molecular neuroimaging and describes first applications in healthy subjects and patients using positron emission tomography (PET).

Sleep deprivation increases A(1) adenosine receptor density in the rat brain

Adenosine, increasing after sleep deprivation and acting via the A(1) adenosine receptor (A(1)AR), is likely a key factor in the homeostatic control of sleep. This study examines the impact of sleep deprivation on A(1)AR density in different parts of the rat brain with [(3)H]CPFPX autoradiography. Binding of [(3)H]CPFPX was significantly increased in parietal cortex (PAR) (7%), thalamus (11%) and caudate-putamen (9%) after 24 h of sleep deprivation compared to a control group with an undisturbed circadian sleep-wake rhythm. Sleep deprivation of 12 h changed receptor density regionally between -5% and +9% (motor cortex (M1), statistically significant) compared to the circadian control group. These results suggest cerebral A(1)ARs are involved in effects of sleep deprivation and the regulation of sleep. The increase of A(1)AR density could serve the purpose of not only maintaining the responsiveness to increased adenosine levels but also amplifying the effect of sleep deprivation and is in line with a sleep-induced homoeostatic reorganization at the synaptic level.

Sleep deprivation increases A1 adenosine receptor binding in the human brain: a positron emission tomography study

It is currently hypothesized that adenosine is involved in the induction of sleep after prolonged wakefulness. This effect is partially reversed by the application of caffeine, which is a nonselective blocker of adenosine receptors. Here, we report that the most abundant and highly concentrated A1 subtype of cerebral adenosine receptors is upregulated after 24 h of sleep deprivation. We used the highly selective A1 adenosine receptor (A1AR) radioligand [18F]CPFPX ([18F]8-cyclopentyl-3-(3-fluoropropyl)-1-propylxanthine) and quantitative positron emission tomography to assess cerebral A1ARs before and after sleep deprivation in 12 healthy volunteers and a control group (n = 10) with regular sleep. In sleep deprived subjects, we found an increase of the apparent equilibrium total distribution volume in a region-specific pattern in all examined brain regions with a maximum increase in the orbitofrontal cortex (15.3%; p = 0.014). There were no changes in the control group with regular sleep. This is the first molecular imaging study that provides in vivo evidence for an A1AR upregulation in cortical and subcortical brain regions after prolonged wakefulness, indicating that A1AR expression is contributing to the homeostatic sleep regulation.

Test–retest stability of cerebral A1 adenosine receptor quantification using [18F]CPFPX and PET

PURPOSE

The goal of the present study was to evaluate the reproducibility of cerebral A1 adenosine receptor (A1AR) quantification using [18F]CPFPX and PET in a test-retest design.

METHODS

Eleven healthy volunteers were studied twice. Eight brain regions ranging from high to low receptor binding were examined. [18F]CPFPX was injected as a bolus with subsequent infusion over 120 min. Various outcome parameters were compared based on either metabolite-corrected venous blood sampling [e.g. apparent equilibrium total distribution volume (DVt')] or a reference region [ratio of specific to non-specific distribution volume (BP2)].

RESULTS

Test-retest variability was low in the outcome measure BP2 (on average 5.9%) and moderate in DVt' (on average 13.2%). Regarding reproducibility, the outcome parameter BP2 showed an intra-class correlation coefficient (ICC) of 0.94 +/- 0.1. For DVt' the between-subject coefficient of variation (%CV) was similar to the within-subject %CV (around 10%), resulting in a poor ICC of 0.06 +/- 0.2.

CONCLUSION

Our results suggest that quantification of [18F]CPFPX imaging is reproducible and reliable for PET studies of the cerebral A1AR. Among the outcome parameters the non-invasive measures were of superior test-retest stability over the invasive.

Metabolism of the A1 adenosine receptor PET ligand [18F]CPFPX by CYP1A2: implications for bolus/infusion PET studies

The A1 adenosine receptor positron emission tomography (PET) ligand 8-cyclopentyl-3-(3-[18F]fluoropropyl)-1-propylxanthine ([18F]CPFPX, ) undergoes a fast hepatic metabolism. An optimal design of PET quantitation approaches (e.g., bolus/infusion studies) necessitates the knowledge of factors that influence this metabolism. Metabolites of were separated by radio thin-layer chromatography. Metabolism in vivo, in pooled human liver microsomes and in recombinant human cytochrome isoenzyme preparations was studied. Dynamic PET studies using were performed on three controls and two patients, one treated with the antidepressant and inhibitor of cytochrome CYP1A2 fluvoxamine, the other suffering from liver cirrhosis. CPFPX is metabolized by cytochrome CYP1A2 with high selectivity [KM=1.1 microM (95% confidence interval, or CI, 0.6-2.0 microM) and Vmax=243 pmol min(-1) mg(-1) (95% CI, 112-373 pmol min(-1) mg(-1)) corresponding to 2.4 pmol min(-1) pmol(-1) cytochrome P-450]. This metabolism can competitively be inhibited by fluvoxamine with KI=68 nM (95% CI, 34-138 nM). At least eight compounds found in human plasma and in the CYP1A2 in vitro preparations have an identical migration pattern and account together for >90% and >80% of the respective metabolite yield. Metabolism was considerably delayed in the two patients. In conclusion, is metabolized by cytochrome CYP1A2. Its metabolism is therefore subdued to disease-related or xenobiotic-induced changes of CYP1A2 activity. The identification of the metabolic pathway of 1 allows to optimize image quantification in A1 adenosine receptor PET studies.

Effect of aging on cerebral A1 adenosine receptors: A [18F]CPFPX PET study in humans

Cerebral A(1) adenosine receptors (A(1)AR) fulfill important neuromodulatory and homeostatic functions. The present study examines possible age-related A(1)AR changes in living humans by positron emission tomography (PET) and the A(1)AR ligand [(18)F]CPFPX. Thirty-six healthy volunteers aged 22-74 years were included. The apparent binding potential (BP'2) of [(18)F]CPFPX in various cerebral regions was calculated non-invasively using the cerebellum as reference region. In addition, the total distribution volume (DV't) was assessed in 10 subjects undergoing arterial blood sampling. There was no significant association between regional DV't and age, gender, caffeine consumption or sleep duration. BP'2 showed a significant age-dependent decrease in all regions except cingulate gyrus (p=0.062). The BP'2 decline ranged from -17% (striatum) to -34% (postcentral gyrus), the average cortical decline being -23%. There was no significant effect of gender, caffeine consumption and sleep duration on BP'2. In line with in vitro animal studies, the present in vivo PET study detected an age-dependent A(1)AR loss in humans that may be of pathophysiological importance in various neurological diseases associated with aging. Because of the discrepant results of the invasive (DV't) and the non-invasive (BP'2) analyses the present study needs further validation.

A1 adenosine receptor PET using [18F]CPFPX: Displacement studies in humans

BACKGROUND

Imaging of cerebral A(1) adenosine receptors (A(1)AR) with positron emission tomography (PET) has recently become available for neurological research. To date, it has still not been unraveled if there is a valid reference region without specific radioligand binding that may be used to improve image quantification. We conducted in vivo displacement studies in humans to elucidate this important question using the A(1)AR ligand [(18)F]CPFPX.

METHODS

Five healthy male volunteers underwent [(18)F]CPFPX bolus/infusion PET with short infusion of unlabelled CPFPX as competitor (n = 4; 0.9 to 4.0 mg) or vehicle (n = 1; control condition) after equilibrium of [(18)F]CPFPX distribution was attained.

RESULTS

Infusion of CPFPX induced a rapid displacement of [(18)F]CPFPX binding in all regions, including the cerebellum (region with lowest binding). Even at the highest competitor dose, no full displacement was reached. Displacement was dose-dependent in all regions except the cerebellum where floor effects and/or noise might have obscured dose dependency. Specific binding was estimated to account for about one third and two thirds of total equilibrium uptake in cerebellum and cortex, respectively.

CONCLUSIONS

Although the cerebellum is the region with lowest in vivo [(18)F]CPFPX binding, it is not an ideal reference region devoid of specific binding. Nevertheless, as will be discussed, the use of a reference region analysis may be a useful, non-invasive alternative analysis method in carefully selected applications.