Cyclosporine, a P-glycoprotein modulator, increases [18F]MPPF uptake in rat brain and peripheral tissues: microPET and ex vivo studies

Cardiac PET Imaging of ATP Binding Cassette (ABC) Transporters: Opportunities and Challenges

Adenosine triphosphate binding cassette (ABC) transporters are a broad family of membrane protein complexes that use energy to transport molecules across cells and/or intracellular organelle lipid membranes. Many drugs used to treat cardiac diseases have an affinity for these transporters. Among others, P-glycoprotein (P-gp) plays an essential role in regulating drug concentrations that reach cardiac tissue and therefore contribute to cardiotoxicity. As a molecular imaging modality, positron emission tomography (PET) has emerged as a viable technique to investigate the function of P-gp in organs and tissues. Using PET imaging to evaluate cardiac P-gp function provides new insights for drug development and improves the precise use of medications. Nevertheless, information in this field is limited. In this review, we aim to examine the current applications of ABC transporter PET imaging and its tracers in the heart, with a specific emphasis on P-gp. Furthermore, the opportunities and challenges in this novel field will be discussed.

Positron Emission Tomography (PET) Imaging Tracers for Serotonin Receptors

Serotonin (5-hydroxytryptamine, 5-HT) and 5-HT receptors (5-HTRs) have crucial roles in various neuropsychiatric disorders and neurodegenerative diseases, making them attractive diagnostic and therapeutic targets. Positron emission tomography (PET) is a noninvasive nuclear molecular imaging technique and is an essential tool in clinical diagnosis and drug discovery. In this context, numerous PET ligands have been developed for "visualizing" 5-HTRs in the brain and translated into human use to study disease mechanisms and/or support drug development. Herein, we present a comprehensive repertoire of 5-HTR PET ligands by focusing on their chemotypes and performance in PET imaging studies. Furthermore, this Perspective summarizes recent 5-HTR-focused drug discovery, including biased agonists and allosteric modulators, which would stimulate the development of more potent and subtype-selective 5-HTR PET ligands and thus further our understanding of 5-HTR biology.

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.

Preclinical validation of [18F]2FNQ1P as a specific PET radiotracer of 5-HT6 receptors in rat, pig, non-human primate and human brain tissue

INTRODUCTION

The aim of this study was to perform in-vitro and in-vivo radiopharmacological characterizations of [¹⁸F]2FNQ1P, a new PET radiotracer of 5-HT₆ receptors, in rat, pig, non-human primate and human tissues. The 5-HT₆ receptor is one of the more recently identified serotonin receptors in central nervous system and, because of its role in memory and cognitive processes, is considered as a promising therapeutic target.

METHODS

In-vitro autoradiography and saturation binding assays were performed in postmortem brain tissues from rat, pig, non-human primate and human caudate nucleus, completed by serum stability assessment in all species and cerebral radiometabolite and biodistribution studies in rat.

RESULTS

In all species, autoradiography data revealed high binding levels of [¹⁸F]2FNQ1P in cerebral regions with high 5-HT₆ receptor density. Binding was blocked by addition of SB258585 as a specific antagonist. Binding assays provided K_D and B_max values of respectively 1.34 nM and 0.03 pmol·mg^-1 in rat, 0.60 nM and 0.04 pmol·mg^-1 in pig, 1.38 nM and 0.07 pmol·mg^-1 in non-human primate, and 1.39 nM and 0.15 pmol·mg^-1 in human caudate nucleus. In rat brain, the proportion of unmetabolized [¹⁸F]2FNQ1P was >99% 5 min after iv injection and 89% at 40 min. The biodistribution studies found maximal radioactivity in lungs and kidneys (3.5 ± 1.2% ID/g and 2.0 ± 0.7% ID/g, respectively, 15 min post-injection).

CONCLUSION

These radiopharmacological data confirm that [¹⁸F]2FNQ1P is a specific radiotracer for molecular imaging of 5-HT₆ receptors and suggest that it could be used as a radiopharmaceutical in humans.

Towards selective CNS PET imaging of the 5-HT7 receptor system: Past, present and future

Since its discovery in 1993, the serotonin receptor subtype 7 (5-HT₇) has attracted significant attention as a potential drug target; due to its elucidated roles in conditions such as insomnia, schizophrenia, and more. Therefore, it is unsurprising that there has been relatively early efforts undertaken to develop a positron emission tomography (PET) imaging agent for said receptor system. PET can be clinically used to probe receptor systems in vivo, permitting information such as a drug's occupancy against this system to be investigated. This review focuses on the efforts towards the development of a 5-HT₇R selective PET CNS tracer over the last 20 years, critically reflecting on applied strategies and commonly employed chemical frameworks and suggests future considerations that are needed to successfully develop a PET tracer for this clinically relevant target. This article is part of the special issue entitled 'Serotonin Research: Crossing Scales and Boundaries'.

Noninvasive Evaluation of Cellular Proliferative Activity in Brain Neurogenic Regions in Rats under Depression and Treatment by Enhanced [18F]FLT-PET Imaging

Neural stem cells in two neurogenic regions, the subventricular zone and the subgranular zone (SGZ) of the hippocampal dentate gyrus, can divide and produce new neurons throughout life. Hippocampal neurogenesis is related to emotions, including depression/anxiety, and the therapeutic effects of antidepressants, as well as learning and memory. The establishment of in vivo imaging for proliferative activity of neural stem cells in the SGZ might be used to diagnose depression and to monitor the therapeutic efficacy of antidepressants. Positron emission tomography (PET) imaging with 3'-deoxy-3'-[(18)F]fluoro-l-thymidine ([(18)F]FLT) has been studied to allow visualization of proliferative activity in two neurogenic regions of adult mammals; however, the PET imaging has not been widely used because of lower accumulation of [(18)F]FLT, which does not allow quantitative assessment of the decline in cellular proliferative activity in the SGZ under the condition of depression. We report the establishment of an enhanced PET imaging method with [(18)F]FLT combined with probenecid, an inhibitor of drug transporters at the blood-brain barrier, which can allow the quantitative visualization of neurogenic activity in rats. Enhanced PET imaging allowed us to evaluate reduced cell proliferation in the SGZ of rats with corticosterone-induced depression, and further the recovery of proliferative activity in rats under treatment with antidepressants. This enhanced [(18)F]FLT-PET imaging technique with probenecid can be used to assess the dynamic alteration of neurogenic activity in the adult mammalian brain and may also provide a means for objective diagnosis of depression and monitoring of the therapeutic effect of antidepressant treatment.

SIGNIFICANCE STATEMENT

Adult hippocampal neurogenesis may play a role in major depression and antidepressant therapy. Establishment of in vivo imaging for hippocampal neurogenic activity may be useful to diagnose depression and monitor the therapeutic efficacy of antidepressants. Positron emission tomography (PET) imaging has been studied to allow visualization of neurogenic activity; however, PET imaging has not been widely used due to the lower accumulation of the PET tracer in the neurogenic regions. Here, we succeeded in establishing highly quantitative PET imaging for neurogenic activity in adult brain with an inhibitor for drug transporter. This enhanced PET imaging allowed evaluation of the decline of neurogenic activity in the hippocampus of rats with depression and the recovery of neurogenic activity by antidepressant treatment.

P-Glycoprotein, not BCRP, Limits the Brain Uptake of [(18)F]Mefway in Rodent Brain

PURPOSE

The aim of this study was to determine whether the brain uptake of [(18)F]Mefway is influenced by the action of P-glycoprotein (P-gp) and breast cancer resistance protein (Bcrp) in rodents.

PROCEDURES

[(18)F]Mefway was applied to rats pharmacologically inhibited with tariquidar (TQD) and to genetically disrupted mice.

RESULTS

Pretreatment of TQD results in 160% higher hippocampal uptake compared with control rats. In genetically disrupted mice, a maximal brain uptake value of 3.2 SUV in the triple knockout mice (tKO, Mdr1a/b((-/-))Bcrp1((-/-))) was comparable to that of the double knockout mice (dKO, Mdr1a/b((-/-))) and 2-fold those of the wild-type and Bcrp1((-/-)) knockout mice. The differences of binding values were statistically insignificant between control and experimental groups. The brain-to-plasma ratios for tKO mice were also two to five times higher than those for other groups.

CONCLUSIONS

[(18)F]Mefway is modulated by P-gp, and not by Bcrp in rodents.

Biodistribution and Radiation Dosimetry of [18F]Mefway in Humans

PURPOSE

[¹⁸F]Mefway is a positron emission tomography (PET) radioligand for quantification of the brain serotonin 1A (5-HT_1A) receptor density. The purpose of this study was to evaluate the radiation safety of [¹⁸F]Mefway in humans.

PROCEDURES

Six healthy volunteers (three males and three females) were whole-body PET scanned for 114 min after injection of [¹⁸F]Mefway (226 ± 35 MBq). Estimated radiation doses were determined by the OLINDA/EXM software.

RESULTS

[¹⁸F]Mefway was safe and well tolerated by all subjects. Residence time ranges from 0 (gallbladder) to 0.822 h (urinary bladder wall). While the estimated radiation doses in the reproductive and blood-forming organs were below 13.35-22.87 μSv/MBq, radiation dose in the urinary bladder wall was 471 μSv/MBq. The mean effective dose was 40.23 ± 6.63 μSv/MBq.

CONCLUSION

For a typical single injection of 185 MBq (5 mCi), the dose will result in 87.1 mSv for the urinary bladder wall. To reduce radiation burden, the bladder voiding can be used before [¹⁸F]Mefway PET scan.

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.

Imaging the Impact of the P-Glycoprotein (ABCB1) Function on the Brain Kinetics of Metoclopramide

The effects of metoclopramide on the central nervous system (CNS) in patients suggest substantial brain distribution. Previous data suggest that metoclopramide brain kinetics may nonetheless be controlled by ATP-binding cassette (ABC) transporters expressed at the blood-brain barrier. We used (11)C-metoclopramide PET imaging to elucidate the kinetic impact of transporter function on metoclopramide exposure to the brain.

METHODS

(11)C-metoclopramide transport by P-glycoprotein (P-gp; ABCB1) and the breast cancer resistance protein (BCRP; ABCG2) was tested using uptake assays in cells overexpressing P-gp and BCRP. (11)C-metoclopramide brain kinetics were compared using PET in rats (n = 4-5) in the absence and presence of a pharmacologic dose of metoclopramide (3 mg/kg), with or without P-gp inhibition using intravenous tariquidar (8 mg/kg). The (11)C-metoclopramide brain distribution (VT based on Logan plot analysis) and brain kinetics (2-tissue-compartment model) were characterized with either a measured or an imaged-derived input function. Plasma and brain radiometabolites were studied using radio-high-performance liquid chromatography analysis.

RESULTS

(11)C-metoclopramide transport was selective for P-gp over BCRP. Pharmacologic dose did not affect baseline (11)C-metoclopramide brain kinetics (VT = 2.28 ± 0.32 and 2.04 ± 0.19 mL⋅cm(-3) using microdose and pharmacologic dose, respectively). Tariquidar significantly enhanced microdose (11)C-metoclopramide VT (7.80 ± 1.43 mL⋅cm(-3)) with a 4.4-fold increase in K1 (influx rate constant) and a 2.3-fold increase in binding potential (k3/k4) in the 2-tissue-compartment model. In the pharmacologic situation, P-gp inhibition significantly increased metoclopramide brain distribution (VT = 6.28 ± 0.48 mL⋅cm(-3)) with a 2.0-fold increase in K1 and a 2.2-fold decrease in k2 (efflux rate), with no significant impact on binding potential. In this situation, only parent (11)C-metoclopramide could be detected in the brains of P-gp-inhibited rats.

CONCLUSION

(11)C-metoclopramide benefits from favorable pharmacokinetic properties that offer reliable quantification of P-gp function at the blood-brain barrier in a pharmacologic situation. Using metoclopramide as a model of CNS drug, we demonstrated that P-gp function not only reduces influx but also mediates the efflux from the brain back to the blood compartment, with additional impact on brain distribution. This PET-based strategy of P-gp function investigation may provide new insight on the contribution of P-gp to the variability of response to CNS drugs between patients.

5-HT2A receptor SPECT imaging with [¹²³I]R91150 under P-gp inhibition with tariquidar: More is better?

INTRODUCTION

Pharmacological P-glycoprotein (P-gp) inhibition with tariquidar (TQD) is considered a promising strategy for the augmentation of radiotracer brain uptake. However, a region-dependent effect may compromise the robustness of quantitative studies. For this reason, we studied the effect of a TQD pretreatment on 5-HT2A imaging with [(123)I]R91150 and compared results with those obtained in Mdr1a knock-out (KO) rats.

METHODS

Ex vivo autoradiography was performed in TQD (15 mg/kg) pretreated wild-type (WT-TQD), Mdr1a knock-out (KO) and untreated WT rats for Specific Binding Ratio (SBR) estimation. In vivo dynamic SPECT imaging with serial arterial blood sampling was performed in the former two groups of rats and kinetic analysis was performed with a one tissue-compartment (1TC) model and the Specific Uptake Ratio (SUR). Results were analyzed statistically using repeated measures ANOVA.

RESULTS

SBR values differed between WT-TQD, Mdr1a KO and WT rats in a region-dependent manner (p<0.0001). In vivo brain uptake of radiotracer did not differ between groups. Similarly, kinetic analysis provided distribution volume (V(T)) values that did not differ significantly between groups. SUR binding potential (BPND) values from both groups highly correlated with corresponding V(T) (r=0.970, p<0.0001 and r=0.962, p<0.0001, respectively). However, SUR measured over averaged images between 100 and 120 min, using cerebellum as reference region, demonstrated values that were, by average, 2.99±0.53 times higher in the WT-TQD group, with the difference between groups being region-dependent (p<0.001). In addition, coefficient of variation of the SUR BPND values across brain regions was significantly higher in the WT-TQD rats (41.25%±9.63% versus 11.13%±5.59%, p<0.0001).

CONCLUSION

P-gp inhibition with TQD leads to region-dependent effect in the rat brain, with probably sub-optimal effect in cerebellum. This warrants attention when it is used as a reference region for quantitative studies.

18F-Mefway PET imaging of serotonin 1A receptors in humans: a comparison with 18F-FCWAY

Introduction

The purpose of this research is to evaluate the prospects for the use of 4-(trans-¹⁸F-fluoranylmethyl)-N-[2-[4-(2-methoxyphenyl)piperazin-1-yl]ethyl]-N-pyridin-2-ylcyclohexane-1-carboxamide (¹⁸F-Mefway) in comparison to ¹⁸F-trans-4-fluoro-N-2-[4-(2-methoxyphenyl)piperazin-1-yl]ethyl]-N-(2-pyridyl)cyclohexanecarboxamide (¹⁸F-FCWAY) for the quantification of 5-HT_1A receptors in human subjects.

Method

Five healthy male controls were included for two positron emission tomography (PET) studies: ¹⁸F-FCWAY PET after the pretreatment with 500 mg of disulfiram and two months later, ¹⁸F-Mefway PET without disulfiram. Regional time-activity curves (TACs) were extracted from nine cortical and subcortical regions in dynamic PET images. Using cerebellar cortex without vermis as reference tissue, in vivo kinetics for both radioligands were compared based on the distribution volume ratio (DVR) calculated by non-invasive Logan graphical analysis and area under the curve ratio of the TACs (AUC ratio).

Result

Although the pattern of regional uptakes in the ¹⁸F-Mefway PET was similar to that of the ¹⁸F-FCWAY PET (highest in the hippocampus and lowest in the cerebellar cortex), the amount of regional uptake in ¹⁸F-Mefway PET was almost half of that in ¹⁸F-FCWAY PET. The skull uptake in ¹⁸F-Mefway PET was only 25% of that in ¹⁸F-FCWAY PET with disulfiram pretreatment. The regional DVR values and AUC ratio values for ¹⁸F-Mefway were 17—40% lower than those of ¹⁸F-FCWAY. In contrast to a small overestimation of DVR values by AUC ratio values (< 10%) in ¹⁸F-FCWAY PET, the overestimation bias of AUC ratio values was much higher (up to 21%) in ¹⁸F-Mefway PET.

Conclusion

As ¹⁸F-Mefway showed lower DVR values and greater overestimation bias of AUC ratio values, ¹⁸F-Mefway may appear less favorable than ¹⁸F-FCWAY. However, in contrast to ¹⁸F-FCWAY, the resistance to in vivo defluorination of ¹⁸F-Mefway obviates the need for the use of a defluorination inhibitor. Thus, ¹⁸F-Mefway may be a good candidate PET radioligand for 5-HT_1A receptor imaging in human.

[(18)F]FDG is not transported by P-glycoprotein and breast cancer resistance protein at the rodent blood-brain barrier

INTRODUCTION

Transport of 2-[(18)F]fluoro-2-deoxy-d-glucose ([(18)F]FDG) by the multidrug efflux transporters P-glycoprotein (ABCB1) and breast cancer resistance protein (ABCG2) at the blood-brain barrier (BBB) may confound the interpretation of [(18)F]FDG brain PET data. Aim of this study was to assess the influence of ABCB1 and ABCG2 at the BBB on brain distribution of [(18)F]FDG in vivo by performing [(18)F]FDG PET scans in wild-type and transporter knockout mice and by evaluating changes in [(18)F]FDG brain distribution after transporter inhibition.

METHODS

Dynamic small-animal PET experiments (60min) were performed with [(18)F]FDG in groups of wild-type and transporter knockout mice (Abcb1a/b((-/-)), Abcg2((-/-)) and Abcb1a/b((-/-))Abcg2((-/-))) and in wild-type rats without and with i.v. pretreatment with the known ABCB1 inhibitor tariquidar (15mg/kg, given at 2h before PET). Blood was sampled from animals from the orbital sinus vein at the end of the PET scans and measured in a gamma counter. Brain uptake of [(18)F]FDG was expressed as the brain-to-blood radioactivity concentration ratio in the last PET time frame (Kb,brain).

RESULTS

Kb,brain values of [(18)F]FDG were not significantly different between different mouse types both without and with tariquidar pretreatment. The blood-to-brain transfer rate constant of [(18)F]FDG was significantly lower in tariquidar-treated as compared with vehicle-treated rats (0.350±0.025mL/min/g versus 0.416±0.024mL/min/g, p=0.026, paired t-test) but Kb,brain values were not significantly different between both rat groups.

CONCLUSION

Our results show that [(18)F]FDG is not transported by Abcb1 at the mouse and rat BBB in vivo. In addition we found no evidence for Abcg2 transport of [(18)F]FDG at the mouse BBB.

ADVANCES IN KNOWLEDGE AND IMPLICATIONS FOR PATIENT CARE

Our findings imply that functional activity of ABCB1 and ABCG2 at the BBB does not need to be taken into account when interpreting brain [(18)F]FDG PET data.

Determination of optimal acquisition time of [(18)F]FCWAY PET for imaging serotonin 1A receptors in the healthy male subjects

The purpose of this research is to find optimal acquisition time point of [(18)F]FCWAY PET for the assessment of serotonin 1A receptor (5-HT1A) density. To achieve this goal, we examined the specific-to-nonspecific ratios in various brain regions. The cerebellum has very few 5-HT1A receptors in the brain, so we set this region as the reference tissue. As a result, specific-to-nonspecific binding ratios in the frontal, temporal cortex and the hippocampus were steadily increased at 90 min after injection and remained stable at 120 min. In addition, the binding ratio of the late time was significantly higher than that of the previous time points. From these results, we recommend that 90 min p.i. is a better single time point for the analysis rather than previous time points for assessing [(18)F]FCWAY binding to 5-HT1A receptors.

[¹⁸F]Altanserin and small animal PET: impact of multidrug efflux transporters on ligand brain uptake and subsequent quantification of 5-HT₂A receptor densities in the rat brain

INTRODUCTION

The selective 5-hydroxytryptamine type 2a receptor (5-HT(2A)R) radiotracer [(18)F]altanserin is a promising ligand for in vivo brain imaging in rodents. However, [(18)F]altanserin is a substrate of P-glycoprotein (P-gp) in rats. Its applicability might therefore be constrained by both a differential expression of P-gp under pathological conditions, e.g. epilepsy, and its relatively low cerebral uptake. The aim of the present study was therefore twofold: (i) to investigate whether inhibition of multidrug transporters (MDT) is suitable to enhance the cerebral uptake of [(18)F]altanserin in vivo and (ii) to test different pharmacokinetic, particularly reference tissue-based models for exact quantification of 5-HT(2A)R densities in the rat brain.

METHODS

Eighteen Sprague-Dawley rats, either treated with the MDT inhibitor cyclosporine A (CsA, 50 mg/kg, n=8) or vehicle (n=10) underwent 180-min PET scans with arterial blood sampling. Kinetic analyses of tissue time-activity curves (TACs) were performed to validate invasive and non-invasive pharmacokinetic models.

RESULTS

CsA application lead to a two- to threefold increase of [(18)F]altanserin uptake in different brain regions and showed a trend toward higher binding potentials (BP(ND)) of the radioligand.

CONCLUSIONS

MDT inhibition led to an increased cerebral uptake of [(18)F]altanserin but did not improve the reliability of BP(ND) as a non-invasive estimate of 5-HT(2A)R. This finding is most probable caused by the heterogeneous distribution of P-gp in the rat brain and its incomplete blockade in the reference region (cerebellum). Differential MDT expressions in experimental animal models or pathological conditions are therefore likely to influence the applicability of imaging protocols and have to be carefully evaluated.

Assessment of cerebral P-glycoprotein expression and function with PET by combined [11C]inhibitor and [11C]substrate scans in rats

INTRODUCTION

The adenosine triphosphate-binding cassette (ABC) transporter P-glycoprotein (Pgp) protects the brain from accumulation of lipophilic compounds by active efflux transport across the blood-brain barrier. Changes in Pgp function/expression may occur in neurological disorders, such as epilepsy, Alzheimer's or Parkinson's disease. In this work we investigated the suitability of the radiolabeled Pgp inhibitors [(11)C]elacridar and [(11)C]tariquidar to visualize Pgp density in rat brain with PET.

METHODS

Rats underwent a first PET scan with [(11)C]elacridar (n = 5) or [(11)C]tariquidar (n = 6) followed by a second scan with the Pgp substrate (R)-[(11)C]verapamil after administration of unlabeled tariquidar at a dose which half-maximally inhibits cerebral Pgp (3 mg/kg). Compartmental modeling using an arterial input function and Logan graphical analysis were used to estimate rate constants and volumes of distribution (VT) of radiotracers in different brain regions.

RESULTS

Brain PET signals of [(11)C]elacridar and [(11)C]tariquidar were very low (~0.5 standardized uptake value, SUV). There was a significant negative correlation between VT and K1 (i.e. influx rate constant from plasma into brain) values of [(11)C]elacridar or [(11)C]tariquidar and VT and K1 values of (R)-[(11)C]verapamil in different brain regions which was consistent with binding of [(11)C]inhibitors to Pgp and efflux of (R)-[(11)C]verapamil by Pgp.

CONCLUSION

The small Pgp binding signals obtained with [(11)C]elacridar and [(11)C]tariquidar limit the applicability of these tracers to measure cerebral Pgp density. PET tracers with higher (i.e. subnanomolar) binding affinities will be needed to visualize the low density of Pgp in brain.

Radiosynthesis and in vivo evaluation of fluorinated huprine derivates as PET radiotracers of acetylcholinesterase

INTRODUCTION

Developing positron emission tomography (PET) radiotracers for non-invasive study of the cholinergic system is crucial to the understanding of neurodegenerative diseases. Although several acetylcholinesterase (AChE) PET tracers radiolabeled with carbon-11 exist, no fluorinated radiotracer is currently used in clinical imaging studies. The purpose of the present study is to describe the first fluorinated PET radiotracer for this brain enzyme.

METHODS

Three structural analogs of huprine, a specific AChE inhibitor presenting high affinity towards AChE in vitro, were synthesized and labeled with fluorine-18 via a mesylate/fluoro-nucleophilic aliphatic substitution: ([(18)F]-FHUa, [(18)F]-FHUb and [(18)F]-FHUc). Initial biological evaluation included in vitro autoradiography in rat with competition with an AChE inhibitor at different concentrations, and microPET-scan on anesthetized rats. In vivo PET studies in anesthetized cat focused on [(18)F]-FHUa.

RESULTS AND CONCLUSIONS

Although radiosynthesis of these huprine analogs was straightforward, they showed poor brain penetration potential, partially reversed after pharmacological inhibition of P-glycoprotein. These results indicated that current huprine analogs are not suitable for PET mapping of brain AChE receptors, but require physicochemical modulation in order to increase brain penetration.

Discrepancies in the P-glycoprotein-mediated transport of (18)F-MPPF: a pharmacokinetic study in mice and non-human primates

PURPOSE

Several in vivo studies have found that the 5-HT(1A) PET radioligand (18)F-MPPF is a substrate of rodent P-glycoprotein (P-gp). However, in vitro assays suggest that MPPF is not a substrate of human P-gp. We have now tested the influence of inhibiting P-gp on the brain kinetics of (18)F-MPPF in mice and non-human primates.

METHODS

We measured the peripheral kinetics (arterial input function, metabolism, free fraction in plasma (f(P))) during (18)F-MPPF brain PET scanning in baboons with or without cyclosporine A (CsA) infusion. We measured (3)H-MPPF transport at the mouse BBB using in situ brain perfusion in P-gp/Bcrp deficient mice and after inhibiting P-gp with PSC833.

RESULTS

There was an unexpected 1.9-fold increase in brain area under the curve in CsA-treated baboons (n = 4), with no change in radiometabolite-corrected arterial input. However, total volume of distribution corrected for f(P) (V(T)/f(P)) remained unchanged. In situ brain perfusion showed that P-gp restricted the permeability of the mouse BBB to (3)H-MPPF while Bcrp did not.

CONCLUSION

These and previous in vitro results suggest that P-gp may not influence the permeability of human BBB to (18)F-MPPF. However, CsA treatment increased (18)F-MPPF free fraction, which is responsible for a misleading, P-gp unrelated enhanced brain uptake.

PET and SPECT radiotracers to assess function and expression of ABC transporters in vivo

Adenosine triphosphate-binding cassette (ABC) transporters, such as P-glycoprotein (Pgp, ABCB1), breast cancer resistance protein (BCRP, ABCG2) and multidrug resistance-associated proteins (MRPs) are expressed in high concentrations at various physiological barriers (e.g. blood-brain barrier, blood-testis barrier, blood-tumor barrier), where they impede the tissue accumulation of various drugs by active efflux transport. Changes in ABC transporter expression and function are thought to be implicated in various diseases, such as cancer, epilepsy, Alzheimer's and Parkinson's disease. The availability of a non-invasive imaging method which allows for measuring ABC transporter function or expression in vivo would be of great clinical use in that it could facilitate the identification of those patients that would benefit from treatment with ABC transporter modulating drugs. To date three different kinds of imaging probes have been described to measure ABC transporters in vivo: i) radiolabelled transporter substrates ii) radiolabelled transporter inhibitors and iii) radiolabelled prodrugs which are enzymatically converted into transporter substrates in the organ of interest (e.g. brain). The design of new imaging probes to visualize efflux transporters is inter alia complicated by the overlapping substrate recognition pattern of different ABC transporter types. The present article will describe currently available ABC transporter radiotracers for positron emission tomography (PET) and single-photon emission computed tomography (SPECT) and critically discuss strengths and limitations of individual probes and their potential clinical applications.

Serotonin-1A receptors in major depression quantified using PET: controversies, confounds, and recommendations

The serotonin-1A (5-HT(1A)) receptor is of particular interest in human positron emission tomography (PET) studies of major depressive disorder (MDD). Of the eight studies investigating this issue in the brains of patients with MDD, four reported decreased 5-HT(1A) receptor density, two reported no change, and two reported increased 5-HT(1A) receptor density. While clinical heterogeneity may have contributed to these differing results, methodological factors by themselves could also explain the discrepancies. This review highlights several of these factors, including the use of the cerebellum as a reference region and the imprecision of measuring the concentration of parent radioligand in arterial plasma, the method otherwise considered to be the 'gold standard'. Other potential confounds also exist that could restrict or unexpectedly affect the interpretation of results. For example, the radioligand may be a substrate for an efflux transporter - like P-gp - at the blood-brain barrier; furthermore, the binding of the radioligand to the receptor in various stages of cellular trafficking is unknown. Efflux transport and cellular trafficking may also be differentially expressed in patients compared to healthy subjects. We believe that, taken together, the existing disparate findings do not reliably answer the question of whether 5-HT(1A) receptors are altered in MDD or in subgroups of patients with MDD. In addition, useful meta-analysis is precluded because only one of the imaging centers acquired all the data necessary to address these methodological concerns. We recommend that in the future, individual centers acquire more thorough data capable of addressing methodological concerns, and that multiple centers collaborate to meaningfully pool their data for meta-analysis.

P-glycoprotein influence on the brain uptake of a 5-HT(2A) ligand: [(18)F]MH.MZ

BACKGROUND/AIMS

The serotonergic system, especially the 5-HT(2A) receptor, is involved in various diseases and conditions. We have recently developed a new [(18)F]-5-HT(2A) receptor ligand using an analogue, MDL 100907, as a basis for molecular imaging with positron emission tomography. This tracer, [(18)F]MH.MZ, has been shown to be an adequate tool to visualize the 5-HT(2A) receptors in vivo. However, [(18)F]altanserin, similar in chemical structure, is a substrate of efflux transporters, such as P-glycoprotein (P-gp), of the blood-brain barrier, thus limiting its availability in the central nervous system. The aim of this study was to determine whether transport by P-gp influences the distribution ratio of [(18)F]MH.MZ in the frontal cortex.

METHODS

The approach was based on P-gp knockout mice which were compared with wild-type mice under several conditions. In vivo pharmacokinetic and microPET investigations were carried out.

RESULTS

All analyses showed that [(18)F]MH.MZ entered the brain and was sensitive to P-gp transport. In P-gp knockout mice, brain concentrations of MH.MZ were about 5-fold higher than in wild-type animals which is reflected by a 2-fold increase in standardized uptake values of [(18)F]MH.MZ in the frontal cortex of P-gp knockout mice.

CONCLUSION

Our results give evidence for a functional role of transport mechanisms at the blood-brain barrier, specifically of P-gp, and its subregional distribution. Investigation of these mechanisms will benefit the development of more efficient radioligands and drugs for molecular imaging and pharmacotherapy of the mentally ill.

5-HT radioligands for human brain imaging with PET and SPECT

The serotonergic system plays a key modulatory role in the brain and is the target for many drug treatments for brain disorders either through reuptake blockade or via interactions at the 14 subtypes of 5-HT receptors. This review provides the history and current status of radioligands used for positron emission tomography (PET) and single photon emission computerized tomography (SPECT) imaging of human brain serotonin (5-HT) receptors, the 5-HT transporter (SERT), and 5-HT synthesis rate. Currently available radioligands for in vivo brain imaging of the 5-HT system in humans include antagonists for the 5-HT(1A), 5-HT(1B), 5-HT(2A), and 5-HT(4) receptors, and for SERT. Here we describe the evolution of these radioligands, along with the attempts made to develop radioligands for additional serotonergic targets. We describe the properties needed for a radioligand to become successful and the main caveats. The success of a PET or SPECT radioligand can ultimately be assessed by its frequency of use, its utility in humans, and the number of research sites using it relative to its invention date, and so these aspects are also covered. In conclusion, the development of PET and SPECT radioligands to image serotonergic targets is of high interest, and successful evaluation in humans is leading to invaluable insight into normal and abnormal brain function, emphasizing the need for continued development of both SPECT and PET radioligands for human brain imaging.

[F-18]FDDNP microPET imaging correlates with brain Aβ burden in a transgenic rat model of Alzheimer disease: effects of aging, in vivo blockade, and anti-Aβ antibody treatment

In vivo detection of Alzheimer's disease (AD) neuropathology in living patients using positron emission tomography (PET) in conjunction with high affinity molecular imaging probes for β-amyloid (Aβ) and tau has the potential to assist with early diagnosis, evaluation of disease progression, and assessment of therapeutic interventions. Animal models of AD are valuable for exploring the in vivo binding of these probes, particularly their selectivity for specific neuropathologies, but prior PET experiments in transgenic mice have yielded conflicting results. In this work, we utilized microPET imaging in a transgenic rat model of brain Aβ deposition to assess [F-18]FDDNP binding profiles in relation to age-associated accumulation of neuropathology. Cross-sectional and longitudinal imaging demonstrated that [F-18]FDDNP binding in the hippocampus and frontal cortex progressively increases from 9 to 18months of age and parallels age-associated Aβ accumulation. Specificity of in vivo [F-18]FDDNP binding was assessed by naproxen pretreatment, which reversibly blocked [F-18]FDDNP binding to Aβ aggregrates. Both [F-18]FDDNP microPET imaging and neuropathological analyses revealed decreased Aβ burden after intracranial anti-Aβ antibody administration. The combination of this non-invasive imaging method and robust animal model of brain Aβ accumulation allows for future longitudinal in vivo assessments of potential therapeutics for AD that target Aβ production, aggregation, and/or clearance. These results corroborate previous analyses of [F-18]FDDNP PET imaging in clinical populations.

Effect of cyclosporin A on the uptake of D3-selective PET radiotracers in rat brain

INTRODUCTION

Four benzamide analogs having a high affinity and selectivity for D(3) versus D(2) receptors were radiolabeled with (11)C or (18)F for in vivo evaluation.

METHODS

Precursors were synthesized, and the four D(3) selective benzamide analogs were radiolabeled. The tissue distribution and brain uptake of the four compounds were evaluated in control rats and rats pretreated with cyclosporin A, a modulator of P-glycoprotein and an inhibitor of other ABC efflux transporters that contribute to the blood brain barrier. Micro-positron emission tomographic (PET) imaging was carried out for [(11)C]6 in a control and a cyclosporin A pretreated rat.

RESULTS

All four compounds showed low brain uptake in control rats at 5 and 30 min post-injection; despite recently reported rat behavioral studies conducted on analogs 6 (WC-10) and 7 (WC-44). Following administration of cyclosporin A, increased brain uptake was observed with all four PET radiotracers at both 5 and 30 min post-intravenous injection. An increase in brain uptake following modulation/inhibition of the ABC transporters was also observed in the microPET study.

CONCLUSIONS

These data suggest that D3 selective conformationally-flexible benzamide analogs which contain a N-2-methoxyphenylpiperazine moiety are substrates for P-glycoprotein or other adenosine triphosphate (ATP)-binding cassette transporters expressed at the blood-brain barrier, and that PET radiotracers containing this pharmacophore may display low brain uptake in rodents due to the action of these efflux transporters.

Dose-response assessment of tariquidar and elacridar and regional quantification of P-glycoprotein inhibition at the rat blood-brain barrier using (R)-[(11)C]verapamil PET

PURPOSE

Overactivity of the multidrug efflux transporter P-glycoprotein (P-gp) at the blood-brain barrier (BBB) is believed to play an important role in resistance to central nervous system drug treatment. (R)-[(11)C]verapamil (VPM) PET can be used to measure the function of P-gp at the BBB, but low brain uptake of VPM hampers the mapping of regional differences in cerebral P-gp function and expression. The aim of this study was to evaluate the dose-response relationship of two potent P-gp inhibitors and to investigate if increased brain uptake of VPM mediated by P-gp inhibition can be used to assess regional differences in P-gp activity.

METHODS

Two groups of Sprague-Dawley rats (n = 12) underwent single VPM PET scans at 120 min after administration of different doses of the P-gp inhibitors tariquidar and elacridar. In an additional six rats, paired VPM PET scans were performed before and after administration of 3 mg/kg tariquidar.

RESULTS

Inhibitor administration resulted in an up to 11-fold increase in VPM brain distribution volumes (DV) with half-maximum effective dose (ED(50)) values of 3.0 +/- 0.2 and 1.2 +/- 0.1 mg/kg for tariquidar and elacridar, respectively. In paired PET scans, 3 mg/kg tariquidar resulted in regionally different enhancement of brain activity distribution, with lowest DV in cerebellum and highest DV in thalamus.

CONCLUSION

Our data show that tariquidar and elacridar are able to increase VPM brain distribution in rat brain up to 11-fold over baseline at maximum effective doses, with elacridar being about three times more potent than tariquidar. Regional differences in tariquidar-induced modulation of VPM brain uptake point to regional differences in cerebral P-gp function and expression in rat brain.

PET radiotracers: crossing the blood-brain barrier and surviving metabolism

Radiotracers for imaging protein targets in the living human brain with positron emission tomography (PET) are increasingly useful in clinical research and in drug development. Such radiotracers must fulfill many criteria, among which an ability to enter brain adequately and reversibly without contamination by troublesome radiometabolites is desirable for accurate measurement of the density of a target protein (e.g. neuroreceptor, transporter, enzyme or plaque). Candidate radiotracers can fail as a result of poor passive brain entry, rejection from brain by efflux transporters or undesirable metabolism. These issues are reviewed. Emerging PET radiotracers for measuring efflux transporter function and new strategies for ameliorating radiotracer metabolism are discussed. A growing understanding of the molecular features affecting the brain penetration, metabolism and efflux transporter sensitivity of prospective radiotracers should ultimately lead to their more rational and efficient design, and also to their greater efficacy.

Regional differences in capillary density, perfusion rate, and P-glycoprotein activity: a quantitative analysis of regional drug exposure in the brain

The in situ brain perfusion technique was used to assess the impact of local capillary density, blood flow rate and P-gp-mediated efflux activity on regional drug exposure for the P-gp substrates colchicine, quinidine, verapamil, and loperamide, the perfusion flow rate marker diazepam, and the vascular volume marker inulin, in mdr1a(+/+) and mdr1a(-/-) mice. Regional perfusion flow rate varied 7.5-fold, and capillary density (based on vascular volume) varied 3.7-fold, across the 13 brain regions examined. The rate of regional flow, as well as P-gp-mediated colchicine efflux activity, was directly proportional to local capillary density. A decrease in perfusion rate attenuated verapamil brain uptake and had significant effect on P-gp-mediated efflux activity for this substrate in brain regions with lower capillary density. Regional brain uptake and calculated logD at pH 7.4 (clogD(7.4)) were well-related in P-gp-deficient mice, indicating that in the absence of P-gp-mediated efflux, physicochemical properties of the compound (i.e., lipophilicity) serve as the primary determinant of regional brain uptake. Loperamide regional brain uptake and P-gp effect during a 60-s brain perfusion or at 30min after subcutaneous administration were significantly correlated with local capillary density. The highest P-gp-mediated efflux activity was consistently observed in cerebral cortex and midbrain regions for loperamide following short-term brain perfusion and at all time points following subcutaneous administration. These results in intact animal emphasize that the regionality of substrate exposure in brain as measured by the in situ brain perfusion technique is actually biologically relevant.