Peripheral metabolism of (R)-[11C]verapamil in epilepsy patients

Performance and Sensitivity of [99mTc]Tc-sestamibi Compared with Positron Emission Tomography Radiotracers to Measure P-glycoprotein Function in the Kidneys and Liver

P-glycoprotein (P-gp, encoded in humans by the ABCB1 gene and in rodents by the Abcb1a/b genes) is a membrane transporter that can restrict the intestinal absorption and tissue distribution of many drugs and may also contribute to renal and hepatobiliary drug excretion. The aim of this study was to compare the performance and sensitivity of currently available radiolabeled P-gp substrates for positron emission tomography (PET) with the single-photon emission computed tomography (SPECT) radiotracer [99mTc]Tc-sestamibi for measuring the P-gp function in the kidneys and liver. Wild-type, heterozygous (Abcb1a/b(+/-)), and homozygous (Abcb1a/b(-/-)) Abcb1a/b knockout mice were used as models of different P-gp abundance in excretory organs. Animals underwent either dynamic PET scans after intravenous injection of [11C]N-desmethyl-loperamide, (R)-[11C]verapamil, or [11C]metoclopramide or consecutive static SPECT scans after intravenous injection of [99mTc]Tc-sestamibi. P-gp in the kidneys and liver of the mouse models was analyzed with immunofluorescence labeling and Western blotting. In the kidneys, Abcb1a/b() mice had intermediate P-gp abundance compared with wild-type and Abcb1a/b(-/-) mice. Among the four tested radiotracers, renal clearance of radioactivity (CLurine,kidney) was significantly reduced (-83%) in Abcb1a/b(-/-) mice only for [99mTc]Tc-sestamibi. Biliary clearance of radioactivity (CLbile,liver) was significantly reduced in Abcb1a/b(-/-) mice for [11C]N-desmethyl-loperamide (-47%), [11C]metoclopramide (-25%), and [99mTc]Tc-sestamibi (-79%). However, in Abcb1a/b(+/-) mice, CLbile,liver was significantly reduced (-47%) only for [99mTc]Tc-sestamibi. Among the tested radiotracers, [99mTc]Tc-sestamibi performed best in measuring the P-gp function in the kidneys and liver. Owing to its widespread clinical availability, [99mTc]Tc-sestamibi represents a promising probe substrate to assess systemic P-gp-mediated drug-drug interactions and to measure renal and hepatic P-gp function under different (patho-)physiological conditions.

Impact of Cytochrome Induction or Inhibition on the Plasma and Brain Kinetics of [11C]metoclopramide, a PET Probe for P-Glycoprotein Function at the Blood-Brain Barrier

[11C]metoclopramide PET imaging provides a sensitive and translational tool to explore P-glycoprotein (P-gp) function at the blood-brain barrier (BBB). Patients with neurological diseases are often treated with cytochrome (CYP) modulators which may impact the plasma and brain kinetics of [11C]metoclopramide. The impact of the CYP inducer carbamazepine or the CYP inhibitor ritonavir on the brain and plasma kinetics of [11C]metoclopramide was investigated in rats. Data obtained in a control group were compared with groups that were either orally pretreated with carbamazepine (45 mg/kg twice a day for 7 days before PET) or ritonavir (20 mg/kg, 3 h before PET) (n = 4 per condition). Kinetic modelling was performed to estimate the brain penetration (VT) of [11C]metoclopramide. CYP induction or inhibition had negligible impact on the plasma kinetics and metabolism of [11C]metoclopramide. Moreover, carbamazepine neither impacted the brain kinetics nor VT of [11C]metoclopramide (p > 0.05). However, ritonavir significantly increased VT (p < 0.001), apparently behaving as an inhibitor of P-gp at the BBB. Our data suggest that treatment with potent CYP inducers such as carbamazepine does not bias the estimation of P-gp function at the BBB with [11C]metoclopramide PET. This supports further use of [11C]metoclopramide for studies in animals and patients treated with CYP inducers.

A companion to the preclinical common data elements and case report forms for in vivo rodent neuroimaging: A report of the TASK3-WG3 Neuroimaging Working Group of the ILAE/AES Joint Translational Task Force

The International League Against Epilepsy/American Epilepsy Society (ILAE/AES) Joint Translational Task Force established the TASK3 working groups to create common data elements (CDEs) for various aspects of preclinical epilepsy research studies, which could help improve the standardization of experimental designs. In this article, we discuss CDEs for neuroimaging data that are collected in rodent models of epilepsy, with a focus on adult rats and mice. We provide detailed CDE tables and case report forms (CRFs), and with this companion manuscript, we discuss the methodologies for several imaging modalities and the parameters that can be collected.

Use of PET Imaging to Evaluate Transporter-Mediated Drug-Drug Interactions

Several membrane transporters belonging to the adenosine triphosphate-binding cassette (ABC) and solute carrier (SLC) families can transport drugs and drug metabolites and thereby exert an effect on drug absorption, distribution, and excretion, which may potentially lead to transporter-mediated drug-drug interactions (DDIs). Some transporter-mediated DDIs may lead to changes in organ distribution of drugs (eg, brain, liver, kidneys) without affecting plasma concentrations. Positron emission tomography (PET) is a noninvasive imaging method that allows studying of the distribution of radiolabeled drugs to different organs and tissues and is therefore the method of choice to quantitatively assess transporter-mediated DDIs on a tissue level. There are 2 approaches to how PET can be used in transporter-mediated DDI studies. When the drug of interest is a potential perpetrator of DDIs, it may be administered in unlabeled form to assess its influence on tissue distribution of a generic transporter-specific PET tracer (probe substrate). When the drug of interest is a potential victim of DDIs, it may be radiolabeled with carbon-11 or fluorine-18 and used in combination with a prototypical transporter inhibitor (eg, rifampicin). PET has already been used both in preclinical species and in humans to assess the effects of transporter-mediated DDIs on drug disposition in different organ systems, such as brain, liver, and kidneys, for which examples are given in the present review article. Given the growing importance of membrane transporters with respect to drug safety and efficacy, PET is expected to play an increasingly important role in future drug development.

Effect of P-glycoprotein inhibition at the blood-brain barrier on brain distribution of (R)-[11 C]verapamil in elderly vs. young subjects

AIMS

The efflux transporter P-glycoprotein (ABCB1) acts at the blood-brain barrier (BBB) to restrict the distribution of many different drugs from blood to the brain. Previous data suggest an age-associated decrease in the expression and function of ABCB1 at the BBB. In the present study, we investigated the influence of age on the magnitude of an ABCB1-mediated drug-drug interaction (DDI) at the BBB.

METHODS

We performed positron emission tomography scans using the model ABCB1 substrate (R)-[¹¹ C]verapamil in five young [26 ± 1 years, (mean ± standard deviation)] and five elderly (68 ± 6 years) healthy male volunteers before and after intravenous administration of a low dose of the ABCB1 inhibitor tariquidar (3 mg kg^-1 ).

RESULTS

In baseline scans, the total distribution volume (V_T ) of (R)-[¹¹ C]verapamil in whole-brain grey matter was not significantly different between the elderly (V_T  = 0.78 ± 0.15) and young (V_T  = 0.79 ± 0.10) group. After partial (incomplete) ABCB1 inhibition, V_T values were significantly higher (P = 0.040) in the elderly (V_T  = 1.08 ± 0.15) than in the young (V_T  = 0.80 ± 0.18) group. The percentage increase in (R)-[¹¹ C]verapamil V_T following partial ABCB1 inhibition was significantly greater (P = 0.032) in elderly (+40 ± 17%) than in young (+2 ± 17%) volunteers. Tariquidar plasma concentrations were not significantly different between the young (786 ± 178 nmol l^-1 ) and elderly (1116 ± 347 nmol l^-1 ) group.

CONCLUSIONS

Our results provide the first direct evidence of an increased risk for ABCB1-mediated DDIs at the BBB in elderly persons, which may have important consequences for pharmacotherapy of the elderly.

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.

Using positron emission tomography to study transporter-mediated drug-drug interactions in tissues

Drug disposition is highly regulated by membrane transporters. Some transporter-mediated drug–drug interactions (DDIs) may not manifest themselves in changes in systemic exposure but rather in changes in tissue exposure of drugs. To better assess the impact of transporter-mediated DDIs in tissues, positron emission tomography (PET)—a noninvasive imaging method—plays an increasingly important role. In this article, we provide examples of how PET can be used to assess transporter-mediated DDIs in different organs.

P-glycoprotein expression and function in patients with temporal lobe epilepsy: a case-control study

BACKGROUND

Studies in rodent models of epilepsy suggest that multidrug efflux transporters at the blood-brain barrier, such as P-glycoprotein, might contribute to pharmacoresistance by reducing target-site concentrations of antiepileptic drugs. We assessed P-glycoprotein activity in vivo in patients with temporal lobe epilepsy.

METHODS

We selected 16 patients with pharmacoresistant temporal lobe epilepsy who had seizures despite treatment with at least two antiepileptic drugs, eight patients who had been seizure-free on antiepileptic drugs for at least a year after 3 or more years of active temporal lobe epilepsy, and 17 healthy controls. All participants had a baseline PET scan with the P-glycoprotein substrate (R)-[(11)C]verapamil. Pharmacoresistant patients and healthy controls then received a 30-min infusion of the P-glycoprotein-inhibitor tariquidar followed by another (R)-[(11)C]verapamil PET scan 60 min later. Seizure-free patients had a second scan on the same day, but without tariquidar infusion. Voxel-by-voxel, we calculated the (R)-[(11)C]verapamil plasma-to-brain transport rate constant, K1 (mL/min/cm(3)). Low baseline K1 and attenuated K1 increases after tariquidar correspond to high P-glycoprotein activity.

FINDINGS

Between October, 2008, and November, 2011, we completed (R)-[(11)C]verapamil PET studies in 14 pharmacoresistant patients, eight seizure-free patients, and 13 healthy controls. Voxel-based analysis revealed that pharmacoresistant patients had lower baseline K1, corresponding to higher baseline P-glycoprotein activity, than seizure-free patients in ipsilateral amygdala (0·031 vs 0·036 mL/min/cm(3); p=0·014), bilateral parahippocampus (0·032 vs 0·037; p<0·0001), fusiform gyrus (0·036 vs 0·041; p<0·0001), inferior temporal gyrus (0·035 vs 0·041; p<0·0001), and middle temporal gyrus (0·038 vs 0·044; p<0·0001). Higher P-glycoprotein activity was associated with higher seizure frequency in whole-brain grey matter (p=0·016) and the hippocampus (p=0·029). In healthy controls, we noted a 56·8% increase of whole-brain K1 after 2 mg/kg tariquidar, and 57·9% for 3 mg/kg; in patients with pharmacoresistant temporal lobe epilepsy, whole-brain K1 increased by only 21·9% for 2 mg/kg and 42·6% after 3 mg/kg. This difference in tariquidar response was most pronounced in the sclerotic hippocampus (mean 24·5% increase in patients vs mean 65% increase in healthy controls, p<0·0001).

INTERPRETATION

Our results support the hypothesis that there is an association between P-glycoprotein overactivity in some regions of the brain and pharmacoresistance in temporal lobe epilepsy. If this relation is confirmed, and P-glycoprotein can be identified as a contributor to pharmacoresistance, overcoming P-glycoprotein overactivity could be investigated as a potential treatment strategy.

FUNDING

EU-FP7 programme (EURIPIDES number 201380).

P-glycoprotein imaging in temporal lobe epilepsy: in vivo PET experiments with the Pgp substrate [11C]-verapamil

Overexpression of the multidrug efflux transporter P-glycoprotein (Pgp) at the blood-brain barrier (BBB) is thought to be involved in pharmacoresistance in epilepsy by extruding antiepileptic drugs (AEDs) from their target site. To explore this hypothesis, positron emission tomography (PET) scans were performed with the Pgp substrate-verapamil (VPM) in animal models before and after status epilepticus (SE) and in patients with temporal lobe epilepsy (TLE) and healthy controls. In addition to baseline scans, a second VPM-PET scan was performed after administration of the Pgp inhibitor tariquidar (TQD), showing that VPM uptake at baseline and its increase after Pgp inhibition are reduced in animals following SE compared to baseline, and in refractory TLE relative to healthy controls. In animal models, brain regions with increased Pgp expression (cerebellum, thalamus, and hippocampus) showed reduced influx rate constants from blood to brain, K(1), of the radiolabeled Pgp substrate relative to control animals. In human studies, preliminary findings are lower K(1) values in refractory compared to seizure-free patients and attenuated increase of K(1) for temporal lobe regions in patients with TLE compared to healthy controls. In summary, there is lower brain uptake of the Pgp substrate VPM in Pgp-rich areas of animals 2 days following SE, as well as lower increase in VPM brain uptake after TQD in patients with refractory TLE compared to healthy controls, supporting the hypothesis of increased cerebral Pgp function following prolonged seizures and as a mechanism contributing to drug resistance in refractory epilepsy. The observation of reduced VPM uptake in refractory compared to seizure-free patients with TLE is consistent with multiple mechanisms affecting Pgp function, including uncontrolled seizures.

Interaction of HM30181 with P-glycoprotein at the murine blood-brain barrier assessed with positron emission tomography

HM30181, a potent and selective inhibitor of the adenosine triphosphate-binding cassette transporter P-glycoprotein (Pgp), was shown to enhance oral bioavailability and improve antitumour efficacy of paclitaxel in mouse tumour models. In search for a positron emission tomography (PET) radiotracer to visualise Pgp expression levels at the blood-brain barrier (BBB), we examined the ability of HM30181 to inhibit Pgp at the murine BBB. HM30181 was shown to be approximately equipotent with the reference Pgp inhibitor tariquidar in inhibiting rhodamine 123 efflux from CCRF-CEM T cells (IC(50), tariquidar: 8.2 ± 2.0 nM, HM30181: 13.1 ± 2.3 nM). PET scans with the Pgp substrate (R)-[(11)C]verapamil in FVB wild-type mice pretreated i.v. with HM30181 (10 or 21 mg/kg) failed to show significant increases in (R)-[(11)C]verapamil brain uptake compared with vehicle treated animals. PET scans with [(11)C]HM30181 showed low and not significantly different brain uptake of [(11)C]HM30181 in wild-type, Mdr1a/b((-/-)) and Bcrp1((-/-)) mice and significantly, i.e. 4.7-fold (P<0.01), higher brain uptake, relative to wild-type animals, in Mdr1a/b((-/-))Bcrp1((-/-)) mice. This was consistent with HM30181 being at microdoses a dual substrate of Pgp and breast cancer resistance protein (Bcrp). In vitro autoradiography on low (EMT6) and high (EMT6Ar1.0) Pgp expressing murine breast tumour sections showed 1.9 times higher binding of [(11)C]HM30181 in EMT6Ar1.0 tumours (P<0.001) which was displaceable with unlabelled tariquidar, elacridar or HM30181 (1 μM). Our data suggest that HM30181 is not able to inhibit Pgp at the murine BBB at clinically feasible doses and that [(11)C]HM30181 is not suitable as a PET tracer to visualise cerebral Pgp expression levels.

Synthesis and preclinical evaluation of the radiolabeled P-glycoprotein inhibitor [(11)C]MC113

OBJECTIVES

With the aim to develop a PET tracer to visualize P-glycoprotein (Pgp) expression levels in different organs, the Pgp inhibitor MC113 was labeled with (11)C and evaluated using small-animal PET.

METHODS

[(11)C]MC113 was synthesized by reaction of O-desmethyl MC113 with [(11)C]methyl triflate. Small-animal PET was performed with [(11)C]MC113 in FVB wild-type and Mdr1a/b((-/-)) mice (n=3 per group) and in a mouse model of high (EMT6Ar1.0) and low (EMT6) Pgp expressing tumor grafts (n=5). In the tumor model, PET scans were performed before and after administration of the reference Pgp inhibitor tariquidar (15mg/kg).

RESULTS

Brain uptake of [(11)C]MC113, expressed as area under the time-activity curve from time 0 to 60min (AUC(0-60)), was moderately but not significantly increased in Mdr1a/b((-/-)) compared with wild-type mice (mean±SD AUC(0-60), Mdr1a/b((-/-)): 88±7min, wild-type: 62±6min, P=0.100, Mann Whitney test). In the tumor model, AUC(0-60) values were not significantly different between EMT6Ar1.0 and EMT6 tumors. Neither in brain nor in tumors was activity concentration significantly changed in response to tariquidar administration. Half-maximum effect concentrations (IC(50)) for inhibition of Pgp-mediated rhodamine 123 efflux from CCRFvcr1000 cells were 375±60nM for MC113 versus 8.5±2.5nM for tariquidar.

CONCLUSION

[(11)C]MC113 showed higher brain uptake in mice than previously described Pgp PET tracers, suggesting that [(11)C]MC113 was only to a low extent effluxed by Pgp. However, [(11)C]MC113 was found unsuitable to visualize Pgp expression levels presumably due to insufficiently high Pgp binding affinity of MC113 in relation to Pgp densities in brain and tumors.

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.

The antiepileptic drug mephobarbital is not transported by P-glycoprotein or multidrug resistance protein 1 at the blood-brain barrier: a positron emission tomography study

Aim of this study was to determine whether the carbon-11-labeled antiepileptic drug [(11)C]mephobarbital is a substrate of P-glycoprotein (Pgp) and can be used to assess Pgp function at the blood-brain barrier (BBB) with positron emission tomography (PET). We performed paired PET scans in rats, wild-type (FVB) and Mdr1a/b((-/-)) mice, before and after intravenous administration of the Pgp inhibitor tariquidar (15mg/kg). Brain-to-blood AUC(0-60) ratios in rats and brain AUC(0-60) values of [(11)C]mephobarbital in wild-type and Mdr1a/b((-/-)) mice were similar in scans 1 and 2, respectively, suggesting that in vivo brain distribution of [(11)C]mephobarbital is not influenced by Pgp efflux. Absence of Pgp transport was confirmed in vitro by performing concentration equilibrium transport assay in cell lines transfected with MDR1 or Mdr1a. PET experiments in wild-type mice, with and without pretreatment with the multidrug resistance protein (MRP) inhibitor MK571 (20mg/kg), and in Mrp1((-/-)) mice suggested that [(11)C]mephobarbital is also not transported by MRPs at the murine BBB, which was also supported by in vitro transport experiments using human MRP1-transfected cells. Our results are surprising, as phenobarbital, the N-desmethyl derivative of mephobarbital, has been shown to be a substrate of Pgp, which suggests that N-methylation abolishes Pgp affinity of barbiturates.

Pgp-mediated interaction between (R)-[11C]verapamil and tariquidar at the human blood-brain barrier: a comparison with rat data

Using positron emission tomography (PET) imaging we assessed, in vivo, the interaction between a microdose of (R)-[(11)C]verapamil (a P-glycoprotein (Pgp) substrate) and escalating doses of the Pgp inhibitor tariquidar (3, 4, 6, and 8 mg/kg) at the blood-brain barrier (BBB) in healthy human subjects. We compared the dose-response relationship of tariquidar in humans with data obtained in rats using a similar methodology. Tariquidar was equipotent in humans and rats in its effect of increasing (R)-[(11)C]verapamil brain uptake (expressed as whole-brain volume of distribution (V(T))), with very similar half-maximum-effect concentrations. Both in humans and in rats, brain V(T) approached plateau levels at plasma tariquidar concentrations >1,000 ng/ml. However, Pgp inhibition in humans led to only a 2.7-fold increase in brain V(T) relative to baseline scans (before administration of tariquidar) as compared with 11.0-fold in rats. The results of this translational study add to the accumulating evidence that there are marked species-dependent differences in Pgp expression and functionality at the BBB.

A novel positron emission tomography imaging protocol identifies seizure-induced regional overactivity of P-glycoprotein at the blood-brain barrier

Approximately one-third of epilepsy patients are pharmacoresistant. Overexpression of P-glycoprotein and other multidrug transporters at the blood-brain barrier is thought to play an important role in drug-refractory epilepsy. Thus, quantification of regionally different P-glycoprotein activity in the brain in vivo is essential to identify P-glycoprotein overactivity as the relevant mechanism for drug resistance in an individual patient. Using the radiolabeled P-glycoprotein substrate (R)-[(11)C]verapamil and different doses of coadministered tariquidar, which is an inhibitor of P-glycoprotein, we evaluated whether small-animal positron emission tomography can quantify regional changes in transporter function in the rat brain at baseline and 48 h after a pilocarpine-induced status epilepticus. P-glycoprotein expression was additionally quantified by immunohistochemistry. To reveal putative seizure-induced changes in blood-brain barrier integrity, we performed gadolinium-enhanced magnetic resonance scans on a 7.0 tesla small-animal scanner. Before P-glycoprotein modulation, brain uptake of (R)-[(11)C]verapamil was low in all regions investigated in control and post-status epilepticus rats. After administration of 3 mg/kg tariquidar, which inhibits P-glycoprotein only partially, we observed increased regional differentiation in brain activity uptake in post-status epilepticus versus control rats, which diminished after maximal P-glycoprotein inhibition. Regional increases in the efflux rate constant k(2), but not in distribution volume V(T) or influx rate constant K(1), correlated significantly with increases in P-glycoprotein expression measured by immunohistochemistry. This imaging protocol proves to be suitable to detect seizure-induced regional changes in P-glycoprotein activity and is readily applicable to humans, with the aim to detect relevant mechanisms of pharmacoresistance in epilepsy in vivo.

A combined accelerator mass spectrometry-positron emission tomography human microdose study with 14C- and 11C-labelled verapamil

BACKGROUND AND OBJECTIVE

In microdose studies, the pharmacokinetic profile of a drug in blood after administration of a dose up to 100 μg is measured with sensitive analytical techniques, such as accelerator mass spectrometry (AMS). As most drugs exert their effect in tissue rather than blood, methodology is needed for extending pharmacokinetic analysis to different tissue compartments. In the present study, we combined, for the first time, AMS analysis with positron emission tomography (PET) in order to determine the pharmacokinetic profile of the model drug verapamil in plasma and brain of humans. In order to assess pharmacokinetic dose linearity of verapamil, data were acquired and compared after administration of an intravenous microdose and after an intravenous microdose administered concomitantly with an oral therapeutic dose.

METHODS

Six healthy male subjects received an intravenous microdose [0.05 mg] (period 1) and an intravenous microdose administered concomitantly with an oral therapeutic dose [80 mg] of verapamil (period 2) in a randomized, crossover, two-period study design. The intravenous dose was a mixture of (R/S)-[14C]verapamil and (R)-[11C]verapamil and the oral dose was unlabelled racaemic verapamil. Brain distribution of radioactivity was measured with PET whereas plasma pharmacokinetics of (R)- and (S)-verapamil were determined with AMS. PET data were analysed by pharmacokinetic modelling to estimate the rate constants for transfer (k) of radioactivity across the blood-brain barrier.

RESULTS

Most pharmacokinetic parameters of (R)- and (S)-verapamil as well as parameters describing exchange of radioactivity between plasma and brain (influx rate constant [K(1)] = 0.030 ± 0.003 and 0.031 ± 0.005 mL/mL/min and efflux rate constant [k(2)] = 0.099 ± 0.006 and 0.095 ± 0.008 min-1 for period 1 and 2, respectively) were not statistically different between the two periods although there was a trend for nonlinear pharmacokinetics for the (R)-enantiomer. On the other hand, all pharmacokinetic parameters (except for the terminal elimination half-life [t1/2;)]) differed significantly between the (R)- and (S)-enantiomers for both periods. The maximum plasma concentration (C(max)), area under the plasma concentration-time curve (AUC) from 0 to 24 hours (AUC(24)) and AUC from time zero to infinity (AUC(∞)) were higher and the total clearance (CL), volume of distribution (V(d)) and volume of distribution at steady state (V(ss)) were lower for the (R)- than for the (S)-enantiomer.

CONCLUSION

Combining AMS and PET microdosing allows long-term pharmacokinetic data along with information on drug tissue distribution to be acquired in the same subjects thus making it a promising approach to maximize data output from a single clinical study.

Regional increase in P-glycoprotein function in the blood-brain barrier of patients with chronic schizophrenia: a PET study with [(11)C]verapamil as a probe for P-glycoprotein function

P-glycoprotein (P-gp), a major efflux pump in the blood-brain barrier (BBB) has a profound effect on entry of drugs, peptides and other substances into the central nervous system (CNS). The brain's permeability can be negatively influenced by modulation of the transport function of P-gp. Inflammatory mediators play a role in schizophrenia, and may be able to influence the integrity of the BBB, via P-gp modulation. We hypothesized that P-gp function in the BBB is changed in patients with schizophrenia. Positron-emission tomography was used to measure brain uptake of [(11)C]verapamil, which is normally extruded from the brain by P-gp. We found that patients with chronic schizophrenia under treatment with antipsychotic drugs compared with healthy controls showed a significant decrease in [(11)C]verapamil uptake in the temporal cortex, the basal ganglia, and the amygdala, and amygdalae, and a trend towards a significant decrease was seen throughout the brain. The decrease of [(11)C]verapamil uptake correlates with an increased activity of the P-gp pump. Increased P-gp activity may be a factor in drug resistance in schizophrenia, induced by the use of antipsychotic agents.

In vivo evaluation of [123I]-4-(2-(bis(4-fluorophenyl)methoxy)ethyl)-1-(4-iodobenzyl)piperidine, an iodinated SPECT tracer for imaging the P-gp transporter

INTRODUCTION

P-glycoprotein (P-gp) is an energy-dependent transporter that contributes to the efflux of a wide range of xenobiotics at the blood-brain barrier playing a role in drug-resistance or therapy failure. In this study, we evaluated [(123)I]-4-(2-(bis(4-fluorophenyl)methoxy)ethyl)-1-(4-iodobenzyl)piperidine ([(123)I]-FMIP) as a novel single photon emission computed tomography (SPECT) tracer for imaging P-gp at the brain in vivo.

METHODS

The tissue distribution and brain uptake as well as the metabolic profile of [(123)I]-FMIP in wild-type and mdr1a (-/-) mice after pretreatment with physiological saline or cyclosporin A (CsA) (50 mg/kg) was investigated. The influence of increasing doses CsA on brain uptake of [(123)I]-FMIP was explored. microSPECT images of mice brain after injection of 11.1 MBq [(123)I]-FMIP were obtained for different treatment strategies thereby using the Milabs U-SPECT-II.

RESULTS

Modulation of P-gp with CsA (50 mg/kg) as well as mdr1a gene depletion resulted in significant increase in cerebral uptake of [(123)I]-FMIP with only minor effect on blood activity. [(123)I]-FMIP is relative stable in vivo with >80% intact [(123)I]-FMIP in brain at 60 min p.i. in the different treatment regiments. A dose-dependent sigmoidal increase in brain uptake of [(123)I]-FMIP with increasing doses of CsA was observed. In vivo region of interest-based SPECT measurements correlated well with the observations of the biodistribution studies.

CONCLUSIONS

These findings indicate that [(123)I]-FMIP can be applied to assess the efficacy of newly developed P-gp modulators. It is also suggested that [(123)I]-FMIP is a promising SPECT tracer for imaging P-gp at the blood-brain barrier.

Development of 2D chiral chromatography with accelerator mass spectrometry for quantification of 14C-labeled R- and S-verapamil in plasma

Background: A microdose study was performed where 50 µg R/S-14C-verapamil was dosed intravenously to human volunteers. In order to quantify the individual R- and S-enantiomers in human plasma a 2D chiral HPLC method with subsequent analysis by accelerator mass spectrometry was verified. Results: R/S-verapamil was separated on a C18 column and the isolated fraction was applied to a chiral column where the verapamil enantiomers were separated. Experimental recovery (∼73% [coefficient of variation {CV} = 16%] and 66% [CV = 21%] for R- and S-verapamil, respectively) was accounted for by the use of internal standardization from the fluorescence response of nonlabeled R- and S-verapamil. The precision of the assay ranged from 4.1 to 15.9% CV and the limit of quantitation was 1.95–4.81 pg/ml for R-verapamil and 1.76–3.34 pg/ml for S-verapamil. Conclusion: This method was successfully applied to the analysis of R- and S-verapamil in human plasma.

Assessment of regional differences in tariquidar-induced P-glycoprotein modulation at the human blood-brain barrier

We attempted to assess regional differences in cerebral P-glycoprotein (P-gp) function by performing paired positron emission tomography (PET) scans with the P-gp substrate (R)-[(11)C]verapamil in five healthy subjects before and after i.v. infusion of tariquidar (2 mg/kg). Comparison of tariquidar-induced changes in distribution volumes (DVs) in 42 brain regions of interest (ROIs) failed to detect significant differences among brain ROIs. Statistical parametric mapping analysis of parametric DV images visualized symmetrical bilateral clusters with moderately higher DV increases in response to tariquidar administration in cerebellum, parahippocampal gyrus, olfactory gyrus, and middle temporal lobe and cortex, which might reflect moderately decreased P-gp function and expression.

Imaging of P-glycoprotein function and expression to elucidate mechanisms of pharmacoresistance in epilepsy

The issue of pharmacoresistance in epilepsy has received considerable attention in recent years, and a number of plausible hypotheses have been proposed. Of these, the so-called transporter hypothesis is the most extensively researched and documented. This hypothesis assumes that refractory epilepsy is associated with a localised over-expression of drug transporter proteins such as P-glycoprotein (Pgp) in the region of the epileptic focus, which actively extrudes antiepileptic drugs (AEDs) from their intended site of action. However, although this hypothesis has biological plausibility, there is no clinical evidence to support the assertion that AEDs are sufficiently strong substrates for transporter-mediated extrusion from the brain. The use of modern brain imaging techniques to determine Pgp function in patients with refractory epilepsy has started only recently, and may ultimately determine whether increased expression and function of Pgp or other efflux transporters are involved in AED resistance.

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.

Synthesis and small-animal positron emission tomography evaluation of [11C]-elacridar as a radiotracer to assess the distribution of P-glycoprotein at the blood-brain barrier

With the aim to develop a positron emission tomography (PET) tracer to assess the distribution of P-glycoprotein (P-gp) at the blood-brain barrier (BBB) in vivo, the potent third-generation P-gp inhibitor elacridar (1) was labeled with (11)C by reaction of O-desmethyl 1 with [(11)C]-methyl triflate. In vitro autoradiography and small-animal PET imaging of [(11)C]-1 was performed in rats (n = 3), before and after administration of unlabeled 1, as well as in wild-type, Mdr1a/b((-/-)) and Bcrp1((-/-)) mice (n = 3). In PET experiments in rats, administration of unlabeled 1 increased brain activity uptake 5.4-fold, whereas blood activity levels remained unchanged. In Mdr1a/b((-/-)) mice, brain activity uptake was 2.5-fold higher compared to wild-type animals, whereas in Bcrp1((-/-)) mice, brain activity uptake was only 1.3-fold higher. In vitro autoradiography showed that 63% of [(11)C]-1 binding was displaceable by an excess of unlabeled 1. As the signal obtained with [(11)C]-1 appeared to be specific for P-gp at the BBB, its utility for the visualization of cerebral P-gp merits further investigation.

A pilot study to assess the efficacy of tariquidar to inhibit P-glycoprotein at the human blood-brain barrier with (R)-11C-verapamil and PET

Tariquidar, a potent, nontoxic, third-generation P-glycoprotein (P-gp) inhibitor, is a possible reversal agent for central nervous system drug resistance. In animal studies, tariquidar has been shown to increase the delivery of P-gp substrates into the brain by severalfold. The aim of this study was to measure P-gp function at the human blood-brain barrier (BBB) after tariquidar administration using PET and the model P-gp substrate (R)-(11)C-verapamil.

METHODS

Five healthy volunteers underwent paired (R)-(11)C-verapamil PET scans and arterial blood sampling before and at 2 h 50 min after intravenous administration of tariquidar (2 mg/kg of body weight). The inhibition of P-gp on CD56-positive peripheral lymphocytes of each volunteer was determined by means of the (123)Rh efflux assay. Tariquidar concentrations in venous plasma were quantified using liquid chromatography/mass spectrometry.

RESULTS

Tariquidar administration resulted in significant increases (Wilcoxon test for paired samples) in the distribution volume (DV, +24% +/- 15%) and influx rate constant (K(1), +49% +/- 36%) of (R)-(11)C-verapamil across the BBB (DV, 0.65 +/- 0.13 and 0.80 +/- 0.07, P = 0.043; K(1), 0.034 +/- 0.009 and 0.049 +/- 0.009, P = 0.043, before and after tariquidar administration, respectively). A strong correlation was observed between the change in brain DV after administration of tariquidar and tariquidar exposure in plasma (r = 0.90, P = 0.037). The mean plasma concentration of tariquidar achieved during the second PET scan (490 +/- 166 ng/mL) corresponded to 100% inhibition of P-gp function in peripheral lymphocytes.

CONCLUSION

Tariquidar significantly increased brain penetration of (R)-(11)C-verapamil-derived activity due to increased influx. As opposed to peripheral P-gp function, central P-gp inhibition appeared to be far from complete after the administered tariquidar dose.

Age dependency of cerebral P-gp function measured with (R)-[11C]verapamil and PET

PURPOSE

The aim of this study was to assess the influence of age on the functional activity of the multidrug efflux transporter P-glycoprotein (P-gp) at the human blood-brain barrier.

METHODS

Seven young (mean age: 27 +/- 4 years) and six elderly (mean age: 69 +/- 9 years) healthy volunteers underwent dynamic (R)-[(11)C]verapamil (VPM) positron emission tomography (PET) scans and arterial blood sampling. Parametric distribution volume (DV) images were generated using Logan linearisation, and age groups were compared with statistical parametric mapping (SPM). Brain regions that SPM analysis had shown to be most affected by age were analysed by a region of interest (ROI)-based approach using a maximum probability brain atlas, before and after partial volume correction (PVC).

RESULTS

SPM analysis revealed significant clusters of DV increases in cerebellum, temporal and frontal lobe of elderly compared to younger subjects. In the ROI-based analysis, elderly subjects showed significant DV increases in amygdala (+30%), insula (+26%) and cerebellum (+25%) before PVC, and in insula (+33%) after PVC.

CONCLUSIONS

Increased VPM DV values in the brains of elderly subjects suggest a decrease in cerebral P-gp function with increasing age.

Locally increased P-glycoprotein function in major depression: a PET study with [11C]verapamil as a probe for P-glycoprotein function in the blood-brain barrier

The aetiology of depressive disorder remains unknown, although genetic susceptibility and exposure to neurotoxins are currently being discussed as possible contributors to this disorder. In normal circumstances, the brain is protected against bloodborne toxic influences by the blood-brain barrier, which includes the molecular efflux pump P-glycoprotein (P-gp) in the vessel wall of brain capillaries. We hypothesized that P-gp function in the blood-brain barrier is changed in patients with major depression. Positron emission tomography was used to measure brain uptake of [11C]verapamil, which is normally expelled from the brain by P-gp. Cerebral volume of distribution (V(T)) of [11C]verapamil was used as a measure of P-gp function. Both region-of-interest (ROI) analysis and voxel analysis using statistical parametric mapping (SPM2) were performed to assess regional brain P-gp function. We found that patients with a major depressive episode, using antidepressants, compared to healthy controls showed a significant decrease of [11C]verapamil uptake in different areas throughout the brain, in particular in frontal and temporal regions. The decreased [11C]verapamil uptake correlates with an increased function of the P-gp protein and may be related to chronic use of psychotropic drugs. Our results may explain why treatment-resistant depression can develop.

Imaging of cyclosporine inhibition of P-glycoprotein activity using 11C-verapamil in the brain: studies of healthy humans

The multiple-drug resistance (MDR) transporter P-glycoprotein (P-gp) is highly expressed at the human blood-brain barrier (BBB). P-gp actively effluxes a wide variety of drugs from the central nervous system, including anticancer drugs. We have previously demonstrated P-gp activity at the human BBB using PET of (11)C-verapamil distribution into the brain in the absence and presence of the P-gp inhibitor cyclosporine-A (CsA). Here we extend the initial noncompartmental analysis of these data and apply compartmental modeling to these human verapamil imaging studies.

METHODS

Healthy volunteers were injected with (15)O-water to assess blood flow, followed by (11)C-verapamil to assess BBB P-gp activity. Arterial blood samples and PET images were obtained at frequent intervals for 5 and 45 min, respectively, after injection. After a 60-min infusion of CsA (intravenously, 2.5 mg/kg/h) to inhibit P-gp, a second set of water and verapamil PET studies was conducted, followed by (11)C-CO imaging to measure regional blood volume. Blood flow was estimated using dynamic (15)O-water data and a flow-dispersion model. Dynamic (11)C-verapamil data were assessed by a 2-tissue-compartment (2C) model of delivery and retention and a 1-tissue-compartment model using the first 10 min of data (1C(10)).

RESULTS

The 2C model was able to fit the full dataset both before and during P-pg inhibition. CsA modulation of P-gp increased blood-brain transfer (K(1)) of verapamil into the brain by 73% (range, 30%-118%; n = 12). This increase was significantly greater than changes in blood flow (13%; range, 12%-49%; n = 12, P < 0.001). Estimates of K(1) from the 1C(10) model correlated to estimates from the 2C model (r = 0.99, n = 12), indicating that a short study could effectively estimate P-gp activity.

CONCLUSION

(11)C-verapamil and compartmental analysis can estimate P-gp activity at the BBB by imaging before and during P-gp inhibition by CsA, indicated by a change in verapamil transport (K(1)). Inhibition of P-gp unmasks verapamil trapping in brain tissue that requires a 2C model for long imaging times; however, transport can be effectively measured using a short scan time with a 1C(10) model, avoiding complications with labeled metabolites and tracer retention.

Rapid solid-phase extraction method to quantify [(11)C]-verapamil, and its [(11)C]-metabolites, in human and macaque plasma

INTRODUCTION

P-glycoprotein (P-gp), an efflux transporter, is a significant barrier to drug entry into the brain and the fetus. The positron emission tomography (PET) ligand, [(11)C]-verapamil, has been used to measure in vivo P-gp activity at various tissue-blood barriers of humans and animals. Since verapamil is extensively metabolized in vivo, it is important to quantify the extent of verapamil metabolism in order to interpret such P-gp activity. Therefore, we developed a rapid solid-phase extraction (SPE) method to separate, and then quantify, verapamil and its radiolabeled metabolites in plasma.

METHODS

Using high-performance liquid chromatography (HPLC), we established that the major identifiable circulating radioactive metabolite of [(11)C]-verapamil in plasma of humans and the nonhuman primate, Macaca nemestrina, was [(11)C]-D-617/717. Using sequential and differential pH elution on C(8) SPE cartridges, we developed a rapid method to separate [(11)C]-verapamil and [(11)C]-D-617/717. Recovery was measured by spiking the samples with the corresponding nonradioactive compounds and assaying these compounds by HPLC.

RESULTS

Verapamil and D-617/717 recovery with the SPE method was >85%. When the method was applied to PET studies in humans and nonhuman primates, significant plasma concentration of D-617/717 and unknown polar metabolite(s) were observed. The SPE and the HPLC methods were not significantly different in the quantification of verapamil and D-617/717.

CONCLUSIONS

The SPE method simultaneously processes multiple samples in less than 5 min. Given the short half-life of [(11)C], this method provides a valuable tool to rapidly determine the concentration of [(11)C]-verapamil and its [(11)C]-metabolites in human and nonhuman primate plasma.

Tariquidar-induced P-glycoprotein inhibition at the rat blood-brain barrier studied with (R)-11C-verapamil and PET

The multidrug efflux transporter P-glycoprotein (P-gp) is expressed in high concentrations at the blood-brain barrier (BBB) and is believed to be implicated in resistance to central nervous system drugs. We used small-animal PET and (R)-11C-verapamil together with tariquidar, a new-generation P-gp modulator, to study the functional activity of P-gp at the BBB of rats. To enable a comparison with human PET data, we performed kinetic modeling to estimate the rate constants of radiotracer transport across the rat BBB.

METHODS

A group of 7 Wistar Unilever rats underwent paired (R)-11C-verapamil PET scans at an interval of 3 h: 1 baseline scan and 1 scan after intravenous injection of tariquidar (15 mg/kg, n = 5) or vehicle (n = 2).

RESULTS

After tariquidar administration, the distribution volume (DV) of (R)-11C-verapamil was 12-fold higher than baseline (3.68 +/- 0.81 vs. 0.30 +/- 0.08; P = 0.0007, paired t test), whereas the DVs were essentially the same when only vehicle was administered. The increase in DV could be attributed mainly to an increased influx rate constant (K1) of (R)-11C-verapamil into the brain, which was about 8-fold higher after tariquidar. A dose-response assessment with tariquidar provided an estimated half-maximum effect dose of 8.4 +/- 9.5 mg/kg.

CONCLUSION

Our data demonstrate that (R)-11C-verapamil PET combined with tariquidar administration is a promising approach to measure P-gp function at the BBB.