Pharmacoresistance in epilepsy: a pilot PET study with the P-glycoprotein substrate R-[(11)C]verapamil

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.

Reproducibility of quantitative (R)-[11C]verapamil studies

Background

P-glycoprotein [Pgp] dysfunction may be involved in neurodegenerative diseases, such as Alzheimer's disease, and in drug resistant epilepsy. Positron emission tomography using the Pgp substrate tracer (R)-[¹¹C]verapamil enables in vivo quantification of Pgp function at the human blood-brain barrier. Knowledge of test-retest variability is important for assessing changes over time or after treatment with disease-modifying drugs. The purpose of this study was to assess reproducibility of several tracer kinetic models used for analysis of (R)-[¹¹C]verapamil data.

Methods

Dynamic (R)-[¹¹C]verapamil scans with arterial sampling were performed twice on the same day in 13 healthy controls. Data were reconstructed using both filtered back projection [FBP] and partial volume corrected ordered subset expectation maximization [PVC OSEM]. All data were analysed using single-tissue and two-tissue compartment models. Global and regional test-retest variability was determined for various outcome measures.

Results

Analysis using the Akaike information criterion showed that a constrained two-tissue compartment model provided the best fits to the data. Global test-retest variability of the volume of distribution was comparable for single-tissue (6%) and constrained two-tissue (9%) compartment models. Using a single-tissue compartment model covering the first 10 min of data yielded acceptable global test-retest variability (9%) for the outcome measure K₁. Test-retest variability of binding potential derived from the constrained two-tissue compartment model was less robust, but still acceptable (22%). Test-retest variability was comparable for PVC OSEM and FBP reconstructed data.

Conclusion

The model of choice for analysing (R)-[¹¹C]verapamil data is a constrained two-tissue compartment model.

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.

Transporter hypothesis of drug-resistant epilepsy: challenges for pharmacogenetic approaches

Drug resistance in epilepsy is considered a complex and multifactorial problem. Overexpression of efflux transporters at the blood-brain barrier is discussed as one factor that might limit brain penetration and efficacy of antiepileptic drugs. Whereas experimental data render support for this hypothesis, there is still a lack of sufficient clinical evidence indicating a functional role of efflux transporters. Pharmacogenetic analysis has been considered as one approach in the evaluation of a putative link between transporters and drug-resistant epilepsy. However, the likelihood of a multifactorial nature of drug resistance and the complexity of the events regulating transporters pose a major challenge to any attempt at linking selected genetic polymorphisms to the outcome of drug therapy. In this article, the evidence for an impact of efflux transporters on the response to antiepileptic drugs is discussed, focusing in particular on the different issues presenting a challenge for pharmacogenetic approaches in this field.

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.

PET study on mice bearing human colon adenocarcinoma cells using [11C]GF120918, a dual radioligand for P-glycoprotein and breast cancer resistance protein

OBJECTIVE

To evaluate the functions of P-glycoprotein (Pgp) and breast cancer resistance protein (BCRP) in human colon adenocarcinoma (Caco-2), we carried out an in-vitro study and a small animal positron emission tomography (PET) study using [C]GF120918 (elacridar).

METHODS

[C]GF120918 was synthesized by reacting the desmethyl precursor with [C]CH3I. An in-vitro study using [C]GF120918 was carried out in Caco-2 and Madin-Darby canine kidney cells in the presence or absence of a transporter inhibitor (cyclosporine A and unlabeled GF120918). The biodistribution of radioactivity after the injection of [C]GF120918 was determined in Caco-2-bearing mice using a small animal PET scanner.

RESULTS

In Caco-2 cells expressing Pgp and BCRP, coincubation with unlabeled GF120918 caused an approximately two-fold increase in [C]GF120918 uptake compared with that of the control ([C]GF120918 only). In Caco-2-bearing mice, PET results indicated that [C]GF120918 uptake in the tumor was low, but was significantly increased by treatment with unlabeled GF120918. In metabolite analysis, the radioactive component in the tumor almost corresponded to intact [C]GF120918.

CONCLUSION

A PET study combining the administration of [C]GF120918 with unlabeled GF120918 may be a useful tool for evaluating the functions of Pgp and BCRP in tumors.

(R)-[11C]verapamil PET studies to assess changes in P-glycoprotein expression and functionality in rat blood-brain barrier after exposure to kainate-induced status epilepticus

BACKGROUND

Increased functionality of efflux transporters at the blood-brain barrier may contribute to decreased drug concentrations at the target site in CNS diseases like epilepsy. In the rat, pharmacoresistant epilepsy can be mimicked by inducing status epilepticus by intraperitoneal injection of kainate, which leads to development of spontaneous seizures after 3 weeks to 3 months. The aim of this study was to investigate potential changes in P-glycoprotein (P-gp) expression and functionality at an early stage after induction of status epilepticus by kainate.

METHODS

(R)-[11C]verapamil, which is currently the most frequently used positron emission tomography (PET) ligand for determining P-gp functionality at the blood-brain barrier, was used in kainate and saline (control) treated rats, at 7 days after treatment. To investigate the effect of P-gp on (R)-[11C]verapamil brain distribution, both groups were studied without or with co-administration of the P-gp inhibitor tariquidar. P-gp expression was determined using immunohistochemistry in post mortem brains. (R)-[11C]verapamil kinetics were analyzed with approaches common in PET research (Logan analysis, and compartmental modelling of individual profiles) as well as by population mixed effects modelling (NONMEM).

RESULTS

All data analysis approaches indicated only modest differences in brain distribution of (R)-[11C]verapamil between saline and kainate treated rats, while tariquidar treatment in both groups resulted in a more than 10-fold increase. NONMEM provided most precise parameter estimates. P-gp expression was found to be similar for kainate and saline treated rats.

CONCLUSIONS

P-gp expression and functionality does not seem to change at early stage after induction of anticipated pharmacoresistant epilepsy by kainate.

Celecoxib treatment restores pharmacosensitivity in a rat model of pharmacoresistant epilepsy

BACKGROUND AND PURPOSE

A functional link between seizure-induced P-glycoprotein overexpression at the blood-brain barrier and therapeutic failure has been suggested by several studies using rodent epilepsy models and human epileptic tissue. Recently, we reported that interference with the mechanisms that up-regulate P-glycoprotein in response to seizure activity might provide a novel approach to control its expression in the epileptic brain. Based on these data, we hypothesized that blocking the appropriate signalling cascade by cyclooxygenase-2 inhibition should improve brain penetration of antiepileptic drugs and help to overcome drug resistance.

EXPERIMENTAL APPROACH

Effects of the selective cyclooxygenase-2 inhibitor celecoxib on the response to the P-glycoprotein substrate, phenobarbital, was evaluated in a chronic model of drug-resistant temporal lobe epilepsy in rats. Drug-resistant rats selected from this model exhibit a marked overexpression of P-glycoprotein in the hippocampus and other limbic brain regions.

KEY RESULTS

Responders and non-responders were selected from a group of rats with spontaneous recurrent seizures after prolonged treatment with phenobarbital at maximum tolerated doses. The efficacy of phenobarbital was re-evaluated following a 6 day treatment with celecoxib and the frequency of spontaneous recurrent seizures was significantly reduced in both groups of rats, phenobarbital responders or non-responders selected from the previous drug trial.

CONCLUSIONS AND IMPLICATIONS

Pretreatment with the cyclooxygenase-2 inhibitor restored the anticonvulsant activity of phenobarbital in rats that failed to exhibit a relevant response before celecoxib treatment. Our data provide further support for a novel therapeutic approach to overcome transporter-mediated drug resistance in epilepsies.

New developments in antiepileptic drug resistance: an integrative view

Current theories on drug resistance in epilepsy include the drug transporter hypothesis, the drug target hypothesis, and a novel approach called the inherent severity model of epilepsy, which posits that the severity of the disease determines its relative response to medication. Valuable as each of these hypotheses is, none is currently a stand-alone theory that is able to convincingly explain drug resistance in human epilepsy. As a consequence, it may be of interest to update and integrate the various hypotheses of drug resistance and to explore possible links to the severity of epilepsy. The observation that a high frequency of seizures prior to onset of treatment is a prognostic signal of increased severity and future drug failure suggests that common neurobiological factors may underlie both disease severity and pharmacoresistance. Such a link has been proposed for depression; however, the evidence for a direct mechanistic link, genetic or otherwise, between drug response and disease severity of human epilepsy is still elusive. Although emerging data from experimental studies suggest that alterations in GABA(A) receptors may present one example of a mechanistic link, clearly more work is needed to explore whether common neurobiological factors may underlie both epilepsy severity and drug failure.

Modulating P-glycoprotein regulation: future perspectives for pharmacoresistant epilepsies?

Enhanced brain efflux of antiepileptic drugs by the blood-brain barrier transporter P-glycoprotein is discussed as one mechanism contributing to pharmacoresistance of epilepsies. P-glycoprotein overexpression has been proven to occur as a consequence of seizure activity. Therefore, blocking respective signaling events should help to improve brain penetration and efficacy of P-glycoprotein substrates. A series of recent studies revealed key signaling factors involved in seizure-associated transcriptional activation of P-glycoprotein. These data suggested several interesting targets, including the N-methyl-d-aspartate (NMDA) receptor, the inflammatory enzyme cyclooxygenase-2, and the prostaglandin E2 EP1 receptor. These targets have been further evaluated in rodent models, demonstrating that targeting these factors can control P-glycoprotein expression, improve antiepileptic drug brain penetration, and help to overcome pharmacoresistance. In general, the approach offers particular advantages over transporter inhibition as it preserves basal transporter function. In this review the different strategies for blocking P-glycoprotein upregulation, including their therapeutic promise and drawbacks are discussed. Moreover, pros and cons of the approach are compared to those of alternative strategies to overcome transporter-associated resistance. Regarding future perspectives of the novel approach, there is an obvious need to more clearly define the clinical relevance of transporter overexpression. In this context current efforts are discussed, including the development of imaging tools that allow an evaluation of P-glycoprotein function in individual patients.

N-desmethyl-loperamide is selective for P-glycoprotein among three ATP-binding cassette transporters at the blood-brain barrier

[(11)C]N-desmethyl-Loperamide ([(11)C]dLop) is used in positron emission tomography (PET) to measure the in vivo activity of efflux transporters that block the passage of drugs across the blood-brain barrier. The three most prevalent ATP-binding cassette efflux transporters at the blood-brain barrier are P-glycoprotein (P-gp), multidrug resistance protein 1 (Mrp1), and breast cancer resistance protein (BCRP). We sought to measure the selectivity of dLop among these three transporters. The selectivity of dLop at low concentrations (< or =1 nM) was measured both as the accumulation of [(3)H]dLop in human cells that overexpress each transporter and as the uptake of [(11)C]dLop in brains of mice that lack genes encoding P-gp, Mrp1, or BCRP. The selectivity of dLop at high concentrations (> or =20 microM) was measured as the inhibition of uptake of a fluorescent substrate and the change in cytotoxicity of drugs effluxed at each transporter. Accumulation of [(3)H]dLop was lowest in cells overexpressing P-gp, and the uptake of [(11)C]dLop was highest in brains of mice lacking P-gp. At high concentrations, dLop selectively inhibited P-gp function and also decreased the resistance of only the P-gp-expressing cells to cytotoxic agents. dLop is selective for P-gp among these three transporters, but its activity is dependent on concentration. At low concentrations (< or =1 nM), dLop acts only as a substrate; at high concentrations (> or =20 microM), it acts as both a substrate and an inhibitor (i.e., a competitive substrate). Because low concentrations of radiotracer are used for PET imaging, [(11)C]dLop acts selectively and only as a substrate for P-gp.

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.

Evaluation of transport of common antiepileptic drugs by human multidrug resistance-associated proteins (MRP1, 2 and 5) that are overexpressed in pharmacoresistant epilepsy

Resistance to antiepileptic drugs (AEDs) is one of the most serious problems in the treatment of epilepsy. Accumulating experimental evidence suggests that increased expression of the drug efflux transporter P-glycoprotein (Pgp) at the blood-brain barrier may be involved in the mechanisms leading to AED resistance. In addition to Pgp, increased expression of several multidrug resistance-associated proteins (MRPs) has been determined in epileptogenic brain regions of patients with pharmacoresistant epilepsy. However, it is not known whether AEDs are substrates for MRPs. In the present experiments, we evaluated whether common AEDs are transported by human MRPs (MRP1, 2 and 5) that are overexpressed in AED resistant epilepsy. For this purpose, we used a highly sensitive assay (concentration equilibrium transport assay; CETA) in polarized kidney cell lines (LLC, MDCKII) transfected with human MRPs. The assay was validated by known MRP substrates, including calcein-AM (MRP1), vinblastine (MRP2) and chloromethylfluorescein diacetate (CMFDA; MRP5). The directional transport determined with these drugs in MRP-transfected cell lines could be blocked with the MRP inhibitor MK571. However, in contrast to transport of known MRP substrates, none of the common AEDs (carbamazepine, valproate, levetiracetam, phenytoin, lamotrigine and phenobarbital) used in this study was transported by MRP1, MRP2 or MRP5. A basolateral-to-apical transport of valproate, which could be inhibited by MK571 and probenecid, was determined in LLC cells (both wildtype and transfected), but the specific transporter involved was not identified. The data indicate that common AEDs are not substrates for human MRP1, MRP2 or MRP5, at least in the in vitro models used in this study.

Targeting the brain--surmounting or bypassing the blood-brain barrier

The constituents of the blood-brain barrier, including its efflux transporter system, can efficiently limit brain penetration of potential CNS therapeutics. Effective extrusion from the brain by transporters is a frequent reason for the pharmaceutical industry to exclude novel compounds from further development for CNS therapeutics. Moreover, high transporter expression levels that are present in individual patients or may be generally associated with the pathophysiology seem to be a major cause of therapeutic failure in a variety of CNS diseases including brain tumors, epilepsy, brain HIV infection, and psychiatric disorders. Increasing knowledge of the structure and function of the blood-brain barrier creates a basis for the development of strategies which aim to enhance brain uptake of beneficial pharmaceutical compounds. The different strategies discussed in this review aim to modulate blood-brain barrier function or to bypass constituents of the blood-brain barrier.

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.

Evaluation of [11C]laniquidar as a tracer of P-glycoprotein: radiosynthesis and biodistribution in rats

At present, P-glycoprotein (P-gp) function can be studied using positron emission tomography (PET) together with a labelled P-gp substrate such as R-[11C]verapamil. Such a tracer is, however, less suitable for investigating P-gp (over)expression. Laniquidar is a third-generation P-gp inhibitor, which has been used in clinic trials for modulating multidrug resistance transporters. The purpose of the present study was to develop the radiosynthesis of [11C]laniquidar and to assess its suitability as a tracer of P-gp expression. The radiosynthesis of [11C]laniquidar was performed by methylation of the carboxylic acid precursor with [11C]CH3I. The product was purified by HPLC and reformulated over a tC18 Seppak, yielding a sterile solution of [11C]laniquidar in saline. For evaluating [11C]laniquidar, rats were injected with 20 MBq [11C]laniquidar via a tail vein and sacrificed at 5, 15, 30 and 60 min after injection. Several tissues and distinct brain regions were dissected and counted for radioactivity. In addition, uptake of [11C]laniquidar in rats pretreated with cyclosporine A and valspodar (PSC 833) was determined at 30 min after injection. Finally, the metabolic profile of [11C]laniquidar in plasma was determined. [11C]Laniquidar could be synthesized in moderate yields with high specific activity. Uptake in brain was low, but significantly increased after administration of cyclosporine A. Valspodar did not have any effect on cerebral uptake of [11C]laniquidar. In vivo rate of metabolism was relatively low. Further kinetic studies are needed to investigate the antagonistic behaviour of [11C]laniquidar at tracer level.

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.

COX-2 inhibition controls P-glycoprotein expression and promotes brain delivery of phenytoin in chronic epileptic rats

Epileptic seizures drive expression of the blood-brain barrier efflux transporter P-glycoprotein via a glutamate/cyclooxygenase-2 mediated signalling pathway. Targeting this pathway may represent an innovative approach to control P-glycoprotein expression in the epileptic brain and to enhance brain delivery of antiepileptic drugs. Therefore, we tested the effect of specific cyclooxygenase-2 inhibition on P-glycoprotein expression in two different status epilepticus models. Moreover, the impact of a cyclooxygenase-2 inhibitor on expression of the efflux transporter and on brain delivery of an antiepileptic drug was evaluated in rats with recurrent spontaneous seizures. The highly selective cyclooxygenase-2 inhibitors SC-58236 and NS-398 both counteracted the status epilepticus-associated increase in P-glycoprotein expression in the parahippocampal cortex and the ventral hippocampus. In line with our working hypothesis, a sub-chronic 2-week treatment with SC-58236 in the chronic epileptic state kept P-glycoprotein expression at control levels. As described previously, enhanced P-glycoprotein expression in chronic epileptic rats was associated with a significant reduction in the brain penetration of the antiepileptic drug phenytoin. Importantly, the brain delivery of phenytoin was significantly enhanced by sub-chronic cyclooxygenase-2 inhibition in rats with recurrent seizures. In conclusion, the data substantiate targeting of cyclooxygenase-2 in the chronic epileptic brain as a promising strategy to control the expression levels of P-glycoprotein despite recurrent seizure activity. Cyclooxygenase-2 inhibition may therefore help to increase concentrations of antiepileptic drugs at the target sites in the epileptic brain. It needs to be further evaluated whether the approach also enhances efficacy.

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 the function of P-glycoprotein with radiotracers: pharmacokinetics and in vivo applications

P-glycoprotein (P-gp), an efflux transporter, controls the pharmacokinetics of various compounds under physiological conditions. P-gp-mediated drug efflux has been suggested as playing a role in various disorders, including multidrug-resistant cancer and medication-refractory epilepsy. However, P-gp inhibition has had, to date, little or no clinically significant effect in multidrug-resistant cancer. To enhance our understanding of its in vivo function under pathophysiological conditions, substrates of P-gp have been radiolabeled and imaged using single-photon emission computed tomography (SPECT) and positron emission tomography (PET). To accurately quantify P-gp function, a radiolabeled P-gp substrate should be selective for P-gp, produce a large signal after P-gp blockade, and generate few radiometabolites that enter the target tissue. Furthermore, quantification of P-gp function via imaging requires pharmacological inhibition of P-gp, which requires knowledge of P-gp density at the target site. By meeting these criteria, imaging can elucidate the function of P-gp in various disorders and improve the efficacy of treatments.

Prevention of seizure-induced up-regulation of endothelial P-glycoprotein by COX-2 inhibition

In the epileptic brain, seizure activity induces expression of the blood-brain barrier efflux transporter, P-glycoprotein, thereby limiting brain penetration and therapeutic efficacy of antiepileptic drugs. We recently provided the first evidence that seizures drive P-glycoprotein induction through a pathway that involves glutamate-signaling through the NMDA receptor and cyclooxygenase-2 (COX-2). Based on these data, we hypothesized that selective inhibition of COX-2 could prevent seizure-induced P-glycoprotein up-regulation. In the present study, we found that the highly selective COX-2 inhibitors, NS-398 and indomethacin heptyl ester, blocked the glutamate-induced increase in P-glycoprotein expression and transport function in isolated rat brain capillaries. Importantly, consistent with this, the COX-2 inhibitor, celecoxib, blocked seizure-induced up-regulation of P-glycoprotein expression in brain capillaries of rats in vivo. To explore further the role of COX-2 in signaling P-glycoprotein induction, we analyzed COX-2 protein expression in capillary endothelial cells in brain sections from rats that had undergone pilocarpine-induced seizures and in isolated capillaries exposed to glutamate and found no change from control levels. However, in isolated rat brain capillaries, the COX-2 substrate, arachidonic acid, significantly increased P-glycoprotein transport activity and expression indicating that enhanced substrate flux to COX-2 rather than increased COX-2 expression drives P-glycoprotein up-regulation. Together, these results provide the first in vivo proof-of-principle that specific COX-2 inhibition may be used as a new therapeutic strategy to prevent seizure-induced P-glycoprotein up-regulation at the blood-brain barrier for improving pharmacotherapy of drug-resistant epilepsy.

Several major antiepileptic drugs are substrates for human P-glycoprotein

One of the current hypotheses of pharmacoresistant epilepsy proposes that transport of antiepileptic drugs (AEDs) by drug efflux transporters such as P-glycoprotein (Pgp) at the blood-brain barrier may play a significant role in pharmacoresistance in epilepsy by extruding AEDs from their intended site of action. However, several recent in vitro studies using cell lines that overexpress efflux transporters indicate that human Pgp may not transport AEDs to any relevant extent. In this respect it has to be considered that most AEDs are highly permeable, so that conventional bi-directional transport assays as used in these previous studies may fail to identify AEDs as Pgp substrates, particularly if these drugs are not high-affinity substrates for Pgp. In the present study, we used a modified transport assay that allows evaluating active transport independently of the passive permeability component. In this concentration equilibrium transport assay (CETA), the drug is initially added at identical concentration to both sides of a polarized, Pgp-overexpressing cell monolayer instead of applying the drug to either the apical or basolateral side for studying bi-directional transport. Direct comparison of the conventional bi-directional (concentration gradient) assay with the CETA, using MDR1-transfected LLC cells, demonstrated that CETA, but not the conventional assay, identified phenytoin and phenobarbital as substrates of human Pgp. Furthermore, directional transport was determined for lamotrigine and levetiracetam, but not carbamazepine. Transport of AEDs could be completely or partially (>50%) inhibited by the selective Pgp inhibitor, tariquidar. However, transport of phenobarbital and levetiracetam was also inhibited by MK571, which preferentially blocks transport by multidrug resistance transporters (MRPs), indicating that, in addition to Pgp, these AEDs are substrates of MRPs. The present study provides the first direct evidence that several AEDS are substrates of human Pgp, thus further substantiating the transporter hypothesis of pharmacoresistant epilepsy.

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.

Nordihydroguaiaretic acid increases glutamate uptake in vitro and in vivo: therapeutic implications for amyotrophic lateral sclerosis

Synaptic accumulation of glutamate causes neuronal death in many neurodegenerative pathologies including amyotrophic lateral sclerosis. Drugs capable of increasing glutamate uptake could therefore be therapeutically effective. We screened in a cell-based assay a library of 1040 FDA-approved drugs and nutrients for compounds that could enhance glutamate uptake. Nordihydroguaiaretic acid (NDGA), an anti-inflammatory drug that inhibits lipoxygensases, potently enhanced glutamate uptake in MN-1 cells. Given subcutaneously at 1 mg/day for 30 days in mice, NDGA increased glutamate uptake in spinal cord synaptosomes persistently throughout the treatment. However, when administered following the same regimen to the SOD1-G93A transgenic mouse model of ALS at disease onset, NDGA did not extend survival of these mice. We found that NDGA failed to sustain increased glutamate uptake in the SOD1-G93A mice despite an initial upregulation measured during the first 10 days of treatment. SOD1-G93A mice displayed a progressive increase in spinal cord expression levels of the efflux transporter P-glycoprotein beginning at disease onset. This increase was not mediated by the NDGA treatment because it was measured in untreated SOD1-G93A mice. Since P-glycoproteins control the extrusion of a broad range of toxins and xenobiotics and are responsible for drug resistance in many diseases including cancer and brain diseases such as epilepsy, we propose that the failure of NDGA in maintaining glutamate uptake upregulated in SOD1-G93A mice and its therapeutic inefficacy are due to acquired pharmacoresistance mediated by the increased expression of P-glycoprotein.

Association of ABCB1 genetic variants 3435C>T and 2677G>T to ABCB1 mRNA and protein expression in brain tissue from refractory epilepsy patients

PURPOSE

There is evidence from studies in rodents that P-glycoprotein (P-gp) overexpression is implicated in the causation of refractory epilepsy. Genetic variants in the human ABCB1 (MDR1) gene were shown to affect the expression levels of the transporter in various tissues and to be associated with refractory epilepsy. However, the effect of the genetic variants on the P-gp level in epileptogenic brain tissue is poorly investigated. In the present study, we examined the impact of putatively functional polymorphisms 3435C>T and 2677G>T in the ABCB1 gene on the ABCB1 mRNA expression and P-gp content in human brain tissue from epileptogenic foci of the patients with refractory epilepsy.

METHODS

Fresh brain tissue specimens were obtained from therapy-refractory epilepsy patients during neurosurgery of the epileptogenic focus. We determined the ABCB1 mRNA expression in 23 samples using 5' exonuclease-based real-time polymerase chain reaction (PCR) as well as the P-gp content in 32 samples determined by immunohistochemistry, genotyping was performed by PCR/restriction fragment length polymorphism (RFLP).

RESULTS

There was lack of association of 3435C>T and 2677G>T as well as diplotype configurations on ABCB1 mRNA expression and P-gp content in epileptogenic brain tissues.

CONCLUSIONS

We cannot exclude an association of ABCB1 variants on P-gp function, but our results suggest that brain ABCB1 mRNA and protein expression is not substantially influenced by major ABCB1 genetic variants thus explaining in part results from case-control studies obtaining lack of association of ABCB1 polymorphisms to the risk of refractory epilepsy.

P-glycoprotein--a clinical target in drug-refractory epilepsy?

ATP-binding cassette transporters such as P-glycoprotein (Pgp), multidrug resistance-associated protein, and breast cancer resistance protein are known to transport a wide range of substrates and are highly expressed in the capillary endothelial cells that form part of the blood-brain barrier. It is noteworthy that P-glycoprotein has been shown to be up-regulated in animal models of refractory epilepsy, and adding a Pgp inhibitor to treatment regimens has been shown to reverse the drug-resistant phenotype. Limited data have suggested a role for Pgp in epilepsy in humans as well. However, few epilepsy drugs have been shown to be transported by Pgp, leading to controversy over whether Pgp actually plays a role in drug-resistant epilepsy. In this issue of Molecular Pharmacology, Bauer et al. (p. 1444) demonstrate that glutamate can cause localized up-regulation of Pgp via cyclooxygenase-2 (COX-2) and that this phenomenon can be prevented with COX-2 inhibitors. Localized rather than global up-regulation of Pgp may explain some of the difficulty investigators have had in proving a role for Pgp in epilepsy. The results add new support for future clinical trials targeting Pgp expression in drug-refractory epilepsy.

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

PURPOSE

(R)-[(11)C]verapamil is a new PET tracer for P-glycoprotein-mediated transport at the blood-brain barrier. For kinetic analysis of (R)-[(11)C]verapamil PET data the measurement of a metabolite-corrected arterial input function is required. The aim of this study was to assess peripheral (R)-[(11)C]verapamil metabolism in patients with temporal lobe epilepsy and compare these data with previously reported data from healthy volunteers.

METHODS

Arterial blood samples were collected from eight patients undergoing (R)-[(11)C]verapamil PET and selected samples were analysed for radiolabelled metabolites of (R)-[(11)C]verapamil by using an assay that measures polar N-demethylation metabolites by solid-phase extraction and lipophilic N-dealkylation metabolites by HPLC.

RESULTS

Peripheral metabolism of (R)-[(11)C]verapamil was significantly faster in patients compared to healthy volunteers (AUC of (R)-[(11)C]verapamil fraction in plasma: 29.4 +/- 3.9 min for patients versus 40.8 +/- 5.0 min for healthy volunteers; p < 0.0005, Student's t-test), which resulted in lower (R)-[(11)C]verapamil plasma concentrations (AUC of (R)-[(11)C]verapamil concentration, normalised to injected dose per body weight: 25.5 +/- 2.1 min for patients and 30.5 +/- 5.9 min for healthy volunteers; p = 0.038). Faster metabolism appeared to be mainly due to increased N-demethylation as the polar [(11)C]metabolite fraction was up to two-fold greater in patients.

CONCLUSIONS

Faster metabolism of (R)-[(11)C]verapamil in epilepsy patients may be caused by hepatic cytochrome P450 enzyme induction by antiepileptic drugs. Based on these data caution is warranted when using an averaged arterial input function derived from healthy volunteers for the analysis of patient data. Moreover, our data illustrate how antiepileptic drugs may decrease serum levels of concomitant medication, which may eventually lead to a loss of therapeutic efficacy.