Duration and degree of cyclosporin induced P-glycoprotein inhibition in the rat blood-brain barrier can be studied with PET

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.

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

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

SIGNIFICANCE STATEMENT

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

Increasing nerve agent treatment efficacy by P-glycoprotein inhibition

One of the shortcomings of current treatment of nerve agent poisoning is that not all drugs effectively penetrate the blood-brain barrier (BBB), whereas most nerve agents easily do. P-glycoprotein (Pgp) efflux transporters at the BBB may contribute to this aspect. It was previously shown that Pgp inhibition by tariquidar enhanced the efficacy of nerve agent treatment when administered as a pretreatment. In the present study soman-induced seizures were also substantially prevented when the animals were intravenously treated with tariquidar post-poisoning, in addition to HI-6 and atropine. In these animals, approximately twice as much AChE activity was present in their brain as compared to control rats. The finding that tariquidar did not affect distribution of soman to the brain indicates that the potentiating effects were a result of interactions of Pgp inhibition with drug distribution. In line with this, atropine appeared to be a substrate for Pgp in in vitro studies in a MDR1/MDCK cell model. This indicates that tariquidar might induce brain region specific effects on atropine distribution, which could contribute to the therapeutic efficacy increase found. Furthermore, the therapeutic enhancement by tariquidar was compared to that of the less specific and less potent Pgp inhibitor cyclosporine A. This compound appeared to induce a protective effect similar to tariquidar. In conclusion, treatment with a Pgp inhibitor resulted in enhanced therapeutic efficacy of HI-6 and atropine in a soman-induced seizure model in the rat. The mechanism underlying these effects should be further investigated. To that end, the potentiating effect of nerve agent treatment should be addressed against a broader range of nerve agents, for oximes and atropine separately, and for those at lower doses. In particular when efficacy against more nerve agents is shown, a Pgp inhibitor such as tariquidar might be a valid addition to nerve agent antidotes.

Clinical Applications of Simultaneous PET/MR Imaging Using (R)-[11C]-Verapamil with Cyclosporin A: Preliminary Results on a Surrogate Marker of Drug-Resistant Epilepsy

BACKGROUND AND PURPOSE

The development of resistance to antiepileptic drugs is explained well by the transporter hypothesis, which suggests that drug resistance is caused by inadequate penetration of drugs into the brain barrier as a result of increased levels of efflux transporter such as p-glycoprotein. To evaluate the brain expression of p-glycoprotein in patients with drug-resistant epilepsy, including neocortical epilepsy, we developed a noninvsive quantitative analysis including asymmetry indices based on (R)-[(11)C]-verapamil PET/MR imaging with cyclosporin A, a p-glycoprotein inhibitor.

MATERIALS AND METHODS

Six patients with drug-resistant epilepsy, 5 patients with drug-sensitive epilepsy, and 8 healthy controls underwent dynamic (R)-[(11)C]-verapamil PET/MR imaging with an intravenous infusion of cyclosporin A. Asymmetry indices [(Right Region - Left Region)/(Right Region + Left Region) × 200%] of the standard uptake values in each of the paired lobes were calculated.

RESULTS

All patients with drug-resistant epilepsy had significantly different asymmetry from the healthy controls, whereas all patients with drug-sensitive epilepsy had asymmetry similar to that in healthy controls. In the temporal lobe, the asymmetry indices of patients with left temporal lobe drug-resistant epilepsy were more positive than those of healthy controls (healthy controls: 4.0413 ± 1.7452; patients: 7.2184 ± 1.8237; P = .048), and those of patients with right temporal drug-resistant epilepsy were more negative (patients: -1.6496 ± 3.4136; P = .044). In addition, specific regions that had significant asymmetry were different between the lateral and medial temporal lobe epilepsy groups. In the frontal lobe, the asymmetry index of patients with right frontal lobe drug-resistant epilepsy was more negative than that in healthy controls.

CONCLUSIONS

We confirmed that statistical parametric mapping analysis by using asymmetry indices of (R)-[(11)C]-verapamil PET/MR imaging with cyclosporin A could be used as a surrogate marker for drug-resistant epilepsy, and this approach might be helpful for localizing or lateralizing the epileptic zone.

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.

Small and Innovative Molecules as New Strategy to Revert MDR

Multidrug resistance (MDR) is a complex phenomenon principally due to the overexpression of some transmembrane proteins belonging to the ATP binding cassette (ABC) transporter family. Among these transporters, P-glycoprotein (P-gp) is mostly involved in MDR and its overexpression is the major cause of cancer therapy failure. The classical approach used to overcome MDR is the co-administration of a P-gp inhibitor and the classic antineoplastic drugs, although the results were often unsatisfactory. Different classes of P-gp ligands have been developed and, among them, Tariquidar has been extensively studied both in vitro and in vivo. Although Tariquidar has been considered for several years as the lead compound for the development of P-gp inhibitors, recent studies demonstrated it to be a substrate and inhibitor, in a dose-dependent manner. Moreover, Tariquidar structure-activity relationship studies were difficult to carry out because of the complexity of the structure that does not allow establishing the role of each moiety for P-gp activity. For this purpose, SMALL molecules bearing different scaffolds such as tetralin, biphenyl, arylthiazole, furoxane, furazan have been developed. Many of these ligands have been tested both in in vitro assays and in in vivo PET studies. These preliminary evaluations lead to obtain a library of P-gp interacting agents useful to conjugate chemotherapeutic agents displaying reduced pharmacological activity and appropriate small molecules. These molecules could get over the limits due to the antineoplastic-P-gp inhibitor co-administration since pharmacokinetic and pharmacodynamic profiles are related to a dual innovative drug.

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

INTRODUCTION

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

(R)-[(11)C]verapamil is selectively transported by murine and human P-glycoprotein at the blood-brain barrier, and not by MRP1 and BCRP

INTRODUCTION

Positron emission tomography (PET) with [(11)C]verapamil, either in racemic form or in form of the (R)-enantiomer, has been used to measure the functional activity of the adenosine triphosphate-binding cassette (ABC) transporter P-glycoprotein (Pgp) at the blood-brain barrier (BBB). There is some evidence in literature that verapamil inhibits two other ABC transporters expressed at the BBB, i.e. multidrug resistance protein 1 (MRP1) and breast cancer resistance protein (BCRP). However, previous data were obtained with micromolar concentrations of verapamil and do not necessarily reflect the transporter selectivity of verapamil at nanomolar concentrations, which are relevant for PET experiments. The aim of this study was to assess the selectivity of verapamil, in nanomolar concentrations, for Pgp over MRP1 and BCRP.

METHODS

Concentration equilibrium transport assays were performed with [(3)H]verapamil (5 nM) in cell lines expressing murine or human Pgp, human MRP1, and murine Bcrp1 or human BCRP. Paired PET scans were performed with (R)-[(11)C]verapamil in female FVB/N (wild-type), Mrp1((-/-)), Mdr1a/b((-/-)), Bcrp1((-/-)) and Mdr1a/b((-/-))Bcrp1((-/-)) mice, before and after Pgp inhibition with 15 mg/kg tariquidar.

RESULTS

In vitro transport experiments exclusively showed directed transport of [(3)H]verapamil in Mdr1a- and MDR1-overexpressing cells which could be inhibited by tariquidar (0.5μM). In PET scans acquired before tariquidar administration, brain-to-blood ratio (Kb,brain) of (R)-[(11)C]verapamil was low in wild-type (1.3 ± 0.1), Mrp1((-/-)) (1.4 ± 0.1) and Bcrp1((-/-)) mice (1.8 ± 0.1) and high in Mdr1a/b((-/-)) (6.9 ± 0.8) and Mdr1a/b((-/-))Bcrp1((-/-)) mice (7.9 ± 0.5). In PET scans after tariquidar administration, Kb,brain was significantly increased in Pgp-expressing mice (wild-type: 5.0 ± 0.3-fold, Mrp1((-/-)): 3.2 ± 0.6-fold, Bcrp1((-/-)): 4.3 ± 0.1-fold) but not in Pgp knockout mice (Mdr1a/b((-/-)) and Mdr1a/b((-/-))Bcrp1((-/-))).

CONCLUSION

Our combined in vitro and in vivo data demonstrate that verapamil, in nanomolar concentrations, is selectively transported by Pgp and not by MRP1 and BCRP at the BBB, which supports the use of (R)-[(11)C]verapamil or racemic [(11)C]verapamil as PET tracers of cerebral Pgp function.

Interaction of 11C-tariquidar and 11C-elacridar with P-glycoprotein and breast cancer resistance protein at the human blood-brain barrier

The adenosine triphosphate-binding cassette transporters P-glycoprotein (Pgp) and breast cancer resistance protein (BCRP) are 2 major gatekeepers at the blood-brain barrier (BBB) that restrict brain distribution of several clinically used drugs. In this study, we investigated the suitability of the radiolabeled Pgp/BCRP inhibitors (11)C-tariquidar and (11)C-elacridar to assess Pgp density in the human brain with PET.

METHODS

Healthy subjects underwent a first PET scan of 120-min duration with either (11)C-tariquidar (n = 6) or (11)C-elacridar (n = 5) followed by a second PET scan of 60-min duration with (R)-(11)C-verapamil. During scan 1 (at 60 min after radiotracer injection), unlabeled tariquidar (3 mg/kg) was intravenously administered. Data were analyzed using 1-tissue 2-rate-constant (1T2K) and 2-tissue 4-rate-constant (2T4K) compartment models and either metabolite-corrected or uncorrected arterial input functions.

RESULTS

After injection of (11)C-tariquidar or (11)C-elacridar, the brain PET signal corrected for radioactivity in the vasculature was low (~0.1 standardized uptake value), with slow washout. In response to tariquidar injection, a moderate but statistically significant rise in brain PET signal was observed for (11)C-tariquidar (+27% ± 15%, P = 0.014, paired t test) and (11)C-elacridar (+21% ± 15%, P = 0.014) without changes in plasma activity concentrations. Low levels of radiolabeled metabolites (<25%) were detected in plasma up to 60 min after injection of (11)C-tariquidar or (11)C-elacridar. The 2T4K model provided better data fits than the 1T2K model. Model outcome parameters were similar when metabolite-corrected or uncorrected input functions were used. There was no significant correlation between distribution volumes of (11)C-tariquidar or (11)C-elacridar and distribution volumes of (R)-(11)C-verapamil in different brain regions.

CONCLUSION

The in vivo behavior of (11)C-tariquidar and (11)C-elacridar was consistent with that of dual Pgp/BCRP substrates. Both tracers were unable to visualize cerebral Pgp density, most likely because of insufficiently high binding affinities in relation to the low density of Pgp in human brain (∼1.3 nM). Despite their inability to visualize Pgp density, (11)C-tariquidar and (11)C-elacridar may find use as a new class of radiotracers to study the interplay of Pgp and BCRP at the human BBB in limiting brain uptake of dual substrates.

[11C]quinidine and [11C]laniquidar PET imaging in a chronic rodent epilepsy model: impact of epilepsy and drug-responsiveness

INTRODUCTION

To analyse the impact of both epilepsy and pharmacological modulation of P-glycoprotein on brain uptake and kinetics of positron emission tomography (PET) radiotracers [(11)C]quinidine and [(11)C]laniquidar.

METHODS

Metabolism and brain kinetics of both [(11)C]quinidine and [(11)C]laniquidar were assessed in naive rats, electrode-implanted control rats, and rats with spontaneous recurrent seizures. The latter group was further classified according to their response to the antiepileptic drug phenobarbital into "responders" and "non-responders". Additional experiments were performed following pre-treatment with the P-glycoprotein modulator tariquidar.

RESULTS

[(11)C]quinidine was metabolized rapidly, whereas [(11)C]laniquidar was more stable. Brain concentrations of both radiotracers remained at relatively low levels at baseline conditions. Tariquidar pre-treatment resulted in significant increases of [(11)C]quinidine and [(11)C]laniquidar brain concentrations. In the epileptic subgroup "non-responders", brain uptake of [(11)C]quinidine in selected brain regions reached higher levels than in electrode-implanted control rats. However, the relative response to tariquidar did not differ between groups with full blockade of P-glycoprotein by 15 mg/kg of tariquidar. For [(11)C]laniquidar differences between epileptic and control animals were only evident at baseline conditions but not after tariquidar pretreatment.

CONCLUSIONS

We confirmed that both [(11)C]quinidine and [(11)C]laniquidar are P-glycoprotein substrates. At full P-gp blockade, tariquidar pre-treatment only demonstrated slight differences for [(11)C]quinidine between drug-resistant and drug-sensitive animals.

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

INTRODUCTION

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

METHODS

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

RESULTS

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

CONCLUSION

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

Advances in PET imaging of P-glycoprotein function at the blood-brain barrier

Efflux transporter P-glycoprotein (P-gp) at the blood-brain barrier (BBB) restricts substrate compounds from entering the brain and may thus contribute to pharmacoresistance observed in patient groups with refractory epilepsy and HIV. Altered P-gp function has also been implicated in neurodegenerative diseases such as Alzheimer's and Parkinson's disease. Positron emission tomography (PET), a molecular imaging modality, has become a promising method to study the role of P-gp at the BBB. The first PET study of P-gp function was conducted in 1998, and during the past 15 years two main categories of P-gp PET tracers have been investigated: tracers that are substrates of P-gp efflux and tracers that are inhibitors of P-gp function. PET, as a noninvasive imaging technique, allows translational research. Examples of this are preclinical investigations of P-gp function before and after administering P-gp modulating drugs, investigations in various animal and disease models, and clinical investigations regarding disease and aging. The objective of the present review is to give an overview of available PET radiotracers for studies of P-gp and to discuss how such studies can be designed. Further, the review summarizes results from PET studies of P-gp function in different central nervous system disorders.

Pharmacokinetic modeling of P-glycoprotein function at the rat and human blood-brain barriers studied with (R)-[11C]verapamil positron emission tomography

Background

This study investigated the influence of P-glycoprotein (P-gp) inhibitor tariquidar on the pharmacokinetics of P-gp substrate radiotracer (R)-[¹¹C]verapamil in plasma and brain of rats and humans by means of positron emission tomography (PET).

Methods

Data obtained from a preclinical and clinical study, in which paired (R)-[¹¹C]verapamil PET scans were performed before, during, and after tariquidar administration, were analyzed using nonlinear mixed effects (NLME) modeling. Administration of tariquidar was included as a covariate on the influx and efflux parameters (Q_in and Q_out) in order to investigate if tariquidar increased influx or decreased outflux of radiotracer across the blood–brain barrier (BBB). Additionally, the influence of pilocarpine-induced status epilepticus (SE) was tested on all model parameters, and the brain-to-plasma partition coefficient (V_T-NLME) was calculated.

Results

Our model indicated that tariquidar enhances brain uptake of (R)-[¹¹C]verapamil by decreasing Q_out. The reduction in Q_out in rats during and immediately after tariquidar administration (sevenfold) was more pronounced than in the second PET scan acquired 2 h after tariquidar administration (fivefold). The effect of tariquidar on Q_out in humans was apparent during and immediately after tariquidar administration (twofold reduction in Q_out) but was negligible in the second PET scan. SE was found to influence the pharmacological volume of distribution of the central brain compartment V_br1. Tariquidar treatment lead to an increase in V_T-NLME, and pilocarpine-induced SE lead to increased (R)-[¹¹C]verapamil distribution to the peripheral brain compartment.

Conclusions

Using NLME modeling, we were able to provide mechanistic insight into the effects of tariquidar and SE on (R)-[¹¹C]verapamil transport across the BBB in control and 48 h post SE rats as well as in humans.

Inhibition of P-glycoprotein enhances transport of imipramine across the blood-brain barrier: microdialysis studies in conscious freely moving rats

BACKGROUND AND PURPOSE

Recent studies indicate that efflux of antidepressants by the multidrug resistance transporter P-glycoprotein (P-gp) at the blood-brain barrier (BBB) may contribute to treatment-resistant depression (TRD) by limiting intracerebral antidepressant concentrations. In addition, clinical experience shows that adjunctive treatment with the P-gp inhibitor verapamil may improve the clinical outcome in TRD. Therefore, the present study aimed to investigate the effect of P-gp inhibition on the transport of the tricyclic antidepressant imipramine and its active metabolite desipramine across the BBB.

EXPERIMENTAL APPROACH

Intracerebral microdialysis in rats was used to monitor brain levels of imipramine and desipramine following i.v. imipramine administration, with or without pretreatment with one of the P-gp inhibitors verapamil or cyclosporin A (CsA). Plasma drug levels were also determined at regular intervals.

KEY RESULTS

Pretreatment with either verapamil or CsA resulted in significant increases in imipramine concentrations in the microdialysis samples, without altering imipramine plasma pharmacokinetics. Furthermore, pretreatment with verapamil, but not CsA, led to a significant elevation in plasma and brain levels of desipramine.

CONCLUSIONS AND IMPLICATIONS

The present study demonstrated that P-gp inhibition enhanced the intracerebral concentration of imipramine , thus supporting the hypothesis that P-gp activity restricts brain levels of certain antidepressants, including imipramine. These findings may help to explain reports of a beneficial response to adjunctive therapy with verapamil in TRD.

[11C]phenytoin revisited: synthesis by [11C]CO carbonylation and first evaluation as a P-gp tracer in rats

BACKGROUND

At present, several positron emission tomography (PET) tracers are in use for imaging P-glycoprotein (P-gp) function in man. At baseline, substrate tracers such as R-[11C]verapamil display low brain concentrations with a distribution volume of around 1. [11C]phenytoin is supposed to be a weaker P-gp substrate, which may lead to higher brain concentrations at baseline. This could facilitate assessment of P-gp function when P-gp is upregulated. The purpose of this study was to synthesize [11C]phenytoin and to characterize its properties as a P-gp tracer.

METHODS

[11C]CO was used to synthesize [11C]phenytoin by rhodium-mediated carbonylation. Metabolism and, using PET, brain pharmacokinetics of [11C]phenytoin were studied in rats. Effects of P-gp function on [11C]phenytoin uptake were assessed using predosing with tariquidar.

RESULTS

[11C]phenytoin was synthesized via [11C]CO in an overall decay-corrected yield of 22 ± 4%. At 45 min after administration, 19% and 83% of radioactivity represented intact [11C]phenytoin in the plasma and brain, respectively. Compared with baseline, tariquidar predosing resulted in a 45% increase in the cerebral distribution volume of [11C]phenytoin.

CONCLUSIONS

Using [11C]CO, the radiosynthesis of [11C]phenytoin could be improved. [11C]phenytoin appeared to be a rather weak P-gp substrate.

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.

Assessment of receptor occupancy-over-time of two dopamine transporter inhibitors by [(11)C]CIT and target controlled infusion

INTRODUCTION

Occupancy-over-time was determined for two dopamine transporter (DAT) inhibitors through modeling of their ability to displace the PET ligand [(11)C]CIT. The tracer was held at a pseudo steady state in a reference tissue by target controlled infusion.

METHODS

Rhesus monkeys (n = 5) were given [(11)C]CIT and studied with a PET scanner. Tracer uptake in the reference tissue cerebellum was held at a pseudo steady state by use of target controlled infusion. The pharmacokinetics/pharmacodynamics(PK/PD) of [(11)C]CIT was assessed through the simplified reference tissue model (SRTM). Bupropion (n = 2) and GBR-12909 (n = 2) receptor occupancies were estimated through modeling of their effects on [(11)C]CIT displacement.

RESULTS

There was a high uptake of [(11)C]CIT in striatum, which contains a high DAT density. The reference tissue cerebellum had a comparatively low uptake. The modeling of [(11)C]CIT PK/PD properties in striatum showed high binding potential (BP = 5.34 ± 0.78). Both DAT inhibitors caused immediate displacement of [(11)C]CIT after administration. The occupancy-over-time was modeled as a mono-exponential function, describing initial maximal occupancy (Occ(0)) and rate of ligand-receptor dissociation (k(off)). GBR-12909 showed irreversible binding (k(off) = 0) after an initial occupancy of 76.1%. Bupropion had a higher initial occupancy (84.5%) followed by a release half-life of 33 minutes (k(off) = 0.021).

CONCLUSIONS

The proposed model can be used for assessment of in-vivo occupancy-over-time of DAT ligands by use of target controlled infusion of [(11)C]CIT. The concept of assessing drug-receptor interactions by studying perturbations of a PET tracer from a pseudo steady state can be transferred to other CNS systems.

(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.

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.

Multidrug resistance in oncology and beyond: from imaging of drug efflux pumps to cellular drug targets

Resistance of tumor cells to several structurally unrelated classes of natural products, including anthracyclines, taxanes, and epipodophyllotoxines, is often referred as multidrug resistance (MDR). This is associated with ATP-binding cassette transporters, which function as drug efflux pumps such as P-glycoprotein (Pgp) and multidrug resistance-associated protein 1 (MRP1). Because of the hypothesis in the early eighties that blockade of these efflux pumps by modulators would improve the effect of chemotherapy, extensive effort has been put to visualize these pumps using nuclear imaging with several specific tracers, using both SPECT and PET techniques. The methods and possibilities to visualize these pumps in both the tumor and the blood-brain barrier will be discussed. Because of the fact that the addition of Pgp or MRP modulators has not shown any clinical benefit in patient outcome, these specific MDR tracers are not routinely used in clinical practice. Evidence emerges that combination of chemotherapeutic drugs involved in MDR with the so-called targeted agents can improve patient outcome. The concept of molecular imaging can also be used to visualize the targets for these agents, such as HER2/neu and angiogenic factors such as vascular endothelial growth factor (VEGF). Potentially visualizing molecular drug targets in the tumor can function as biomarkers to support treatment decision for the individual patient.

Evaluation of [C]S14506 and [F]S14506 in rat and monkey as agonist PET radioligands for brain 5-HT(1A) receptors

In vitro and ex vivo measurements have shown that the binding of the selective high-affinity agonist, S14506 (1-[2-(4-fluorobenzoylamino)ethyl]-4-(7-methoxy-naphthyl)piperazine), to 5-HT(1A) receptors, is similar in affinity (K(d) = 0.79 nM) and extent (B(max)) to that of the antagonist, WAY 100635. We aimed to test whether S14506, labeled with a positron-emitter, might serve as a radioligand for imaging brain 5-HT(1A) receptors in vivo with positron emission tomography (PET). Here we evaluated [(11)C]S14506 and [(18)F]S14506 in rat and rhesus monkey in vivo. After intravenous administration of [(11)C]S14506 into rat, radioactivity entered brain, reaching 210% SUV at 2 min. Radioactivity uptake into brain was higher (~ 350% SUV) in rats pre-treated with the P-glycoprotein (P-gp) inhibitor, cyclosporin A. In rhesus monkey, peak brain uptake of radioactivity after administration of [(11)C]S14506 or [(18)F]S14506 was also moderate and for [(11)C]S14506 increased from ~ 170% SUV after 7 min, to 240% SUV in a monkey pre-treated with the P-gp inhibitor, tariquidar. The ratios of radioactivity in 5-HT(1A) receptor-rich regions, such as cingulate or hippocampus to that in receptor-poor cerebellum reached between 1.35 and 1.5 at 60 min for both [(11)C]S14506 and [(18)F]S14506. [(11)C]S14506 gave one major polar radiometabolite in monkey plasma, and [(18)F]S14506 gave three and two more polar radiometabolites in rat and monkey plasma, respectively. The rat radiometabolites of [(18)F]S14506 did not accumulate in brain. [(18)F]S14506 was not radiodefluorinated in monkey. Thus, despite high-affinity and lack of troublesome brain radiometabolites, both [(11)C]S14506 and [(18)F]S14506 were ineffective for imaging rat or monkey brain 5-HT(1A) receptors in vivo, even under P-gp inhibited conditions. Explanations for the failure of these radioligands are offered.

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.

Drug interactions at the blood-brain barrier: fact or fantasy?

There is considerable interest in the therapeutic and adverse outcomes of drug interactions at the blood-brain barrier (BBB) and the blood-cerebrospinal fluid barrier (BCSFB). These include altered efficacy of drugs used in the treatment of CNS disorders, such as AIDS dementia and malignant tumors, and enhanced neurotoxicity of drugs that normally penetrate poorly into the brain. BBB- and BCSFB-mediated interactions are possible because these interfaces are not only passive anatomical barriers, but are also dynamic in that they express a variety of influx and efflux transporters and drug metabolizing enzymes. Based on studies in rodents, it has been widely postulated that efflux transporters play an important role at the human BBB in terms of drug delivery. Furthermore, it is assumed that chemical inhibition of transporters or their genetic ablation in rodents is predictive of the magnitude of interaction to be expected at the human BBB. However, studies in humans challenge this well-established paradigm and claim that such drug interactions will be lesser in magnitude but yet may be clinically significant. This review focuses on current known mechanisms of drug interactions at the blood-brain and blood-CSF barriers and the potential impact of such interactions in humans. We also explore whether such drug interactions can be predicted from preclinical studies. Defining the mechanisms and the impact of drug-drug interactions at the BBB is important for improving efficacy of drugs used in the treatment of CNS disorders while minimizing their toxicity as well as minimizing neurotoxicity of non-CNS drugs.

Pharmacokinetics of P-glycoprotein inhibition in the rat blood-brain barrier

This article describes the experimental set-up and pharmacokinetic modeling of P-glycoprotein function in the rat blood-brain barrier using [(11)C]verapamil as the substrate and cyclosporin A as an inhibitor of P-gp. [(11)C]verapamil was administered to rats as an i.v. bolus dose followed by graded infusions to obtain steady-state concentrations in the brain during 70 min. CsA was administered as a bolus followed by a constant infusion 20 min after the start of the [(11)C]verapamil infusion. The brain uptake of [(11)C]verapamil over 2 h was portrayed in a sequence of PET scans in parallel with measurement of [(11)C]verapamil concentrations in blood and plasma and CsA concentrations in blood. Mixed effects modeling in NONMEM was used to build a pharmacokinetic model of CsA-induced P-gp inhibition. The brain pharmacokinetics of [(11)C]verapamil was well described by a two-compartment model. The effect of CsA on the uptake of [(11)C]verapamil in the brain was best described by an inhibitory indirect effect model with an effect on the transport of [(11)C]verapamil out of the brain. The CsA concentration required to obtain 50% of the maximal inhibition was 4.9 microg/mL (4.1 microM). The model parameters indicated that 93% of the outward transport of [(11)C]verapamil was P-gp mediated.

Species differences in blood-brain barrier transport of three positron emission tomography radioligands with emphasis on P-glycoprotein transport

Species differences occur in the brain concentrations of drugs, but the reasons for these differences are not yet apparent. This study was designed to compare brain uptake of three radiolabeled P-glycoprotein (P-gp) substrates across species using positron emission tomography. Brain concentrations and brain-to-plasma ratios were compared; [(11)C]verapamil in rats, guinea pigs, and monkeys; [(11)C](S)-(2-methoxy-5-(5-trifluoromethyltetrazol-1-yl)-phenylmethylamino)-2(S)-phenylpiperidine (GR205171) in rats, guinea pigs, monkeys, and humans; and [(18)F]altanserin in rats, minipigs, and humans. The fraction of the unbound radioligand in plasma was studied along with its metabolism. The effect of P-gp inhibition was investigated by administering cyclosporin A (CsA). Pronounced species differences were found in the brain and brain-to-plasma concentrations of [(11)C]verapamil, [(11)C]GR205171, and [(18)F]altanserin with higher brain distribution in humans, monkeys, and minipigs than in rats and guinea pigs. For example, the brain-to-plasma ratio of [(11)C]GR205171 was almost 9-fold higher in humans compared with rats. The species differences were still present after P-gp inhibition, although the increase in brain concentrations after P-gp inhibition was somewhat greater in rats than in the other species. Differences in plasma protein binding and metabolism did not explain the species-related differences. The findings are important for interpretation of brain drug delivery when extrapolating preclinical data to humans. Compounds found to be P-gp substrates in rodents are likely to also be substrates in higher species, but sufficient blood-brain barrier permeability may be retained in humans to allow the compound to act at intracerebral targets.

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.

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

PURPOSE

Pretreatment with cyclosporine, a P-glycoprotein (P-gp) modulator increases brain uptake of 4-(2'-methoxyphenyl)-1-[2'-(N-2"-pyridinyl)-p-[(18)F]fluorobenzamido]ethylpiperazine ([(18)F]MPPF) for binding to hydroxytryptamine(1A) (5-HT(1A)) receptors. Those increases were quantified in rat brain with in vivo microPET and ex vivo tissue studies.

MATERIALS AND METHODS

Each Sprague-Dawley rat (n = 4) received a baseline [(18)F]MPPF microPET scan followed by second scan 2-3 weeks later that included cyclosporine pretreatment (50 mg/kg, i.p.). Maximum a posteriori reconstructed images and volumetric ROIs were used to generate dynamic radioactivity concentration measurements for hippocampus, striatum, and cerebellum, with simplified reference tissue method (SRTM) analysis. Western blots were used to semiquantify P-gp regional distribution in brain.

RESULTS

MicroPET studies showed that hippocampus uptake of [(18)F]MPPF was increased after cyclosporine; ex vivo studies showed similar increases in hippocampus and frontal cortex at 30 min, and for heart and kidney at 2.5 and 5 min, without concomitant increases in [(18)F]MPPF plasma concentration. P-gp content in cerebellum was twofold higher than in hippocampus or frontal cortex.

CONCLUSIONS

These studies confirm and extend prior ex vivo results (J. Passchier, et al., Eur J Pharmacol, 2000) that showed [(18)F]MPPF as a substrate for P-gp. Our microPET results showed that P-gp modulation of [(18)F]MPPF binding to 5-HT(1A) receptors can be imaged in rat hippocampus. The heterogeneous brain distribution of P-gp appeared to invalidate the use of cerebellum as a nonspecific reference region for SRTM modeling. Regional quantitation of P-gp may be necessary for accurate PET assessment of 5-HT(1A) receptor density when based on tracer uptake sensitive to P-gp modulation.

A computerized infusion pump for control of tissue tracer concentration during positron emission tomography in vivo pharmacokinetic/pharmacodynamic measurements

BACKGROUND

A computer controlled infusion pump (UIPump) for regulation of target tissue concentration of radioactive compounds was developed for use in biological research and tracer development for PET.

METHODS

Based on observed tissue or plasma kinetics after a bolus injection of the tracer an algorithm calculates the infusion needed to obtain a specified target kinetic curve. A computer feeds this infusion scheme into an infusion pump connected to an animal via a venous catheter. The concept was validated using [11C]Flumazenil administrated to Sprague-Dawley rats where the whole brain distribution and kinetic of the tracer was measured over time using a microPET-scanner. The accuracy and precision of the system was assessed by producing steady-state levels of the tracer and by mimicking kinetics after oral administration.

RESULTS

Various kinetic profiles could be generated, including rapid achievement of constant levels, or step-wise increased levels. The resulting tissue curves had low deviation from the target curves according to the specified criteria: AUC (%): 4.2 +/- 2.8, Maximal deviation (%): 13.6 +/- 5.0 and R2: 0.95 +/- 0.02.

CONCLUSION

The UIPump-system is suitable for use in PET-research for assessment of PK/PD properties by simulation of different tracer tissue kinetics in vivo.

Positron emission tomography and target-controlled infusion for precise modulation of brain drug concentration

INTRODUCTION

There are several instances when it is desirable to control brain concentration of pharmaceuticals, e.g., to modulate the concentration of anesthetic agents to different desired levels fitting to different needs during the course of surgery. This has so far only been possible using indirect estimates of drug concentration such as assuming constant relation between tissue and blood including extrapolations from animals.

METHODS

A system for controlling target tissue concentration (UIPump) was used to regulate whole-brain concentrations of a central benzodiazepine receptor antagonist at therapeutic levels with input from brain kinetics as determined with PET. The system was tested by using pharmacological doses of flumazenil mixed with tracer amounts of [11C]flumazenil. Flumazenil was used as a model compound for anesthesia. An infusion scheme to produce three different steady-state levels in sequence was designed based on kinetic curves obtained after bolus injection. The subjects (Sprague-Dawley rats, n=6) were monitored in a microPET scanner during the whole experiment to verify resulting brain kinetic curves.

RESULTS

A steady-state brain concentration was rapidly achieved corresponding to a whole-brain concentration of 118+/-6 ng/ml. As the infusion rate decreased to lower the exposure by a factor of 2, the brain concentration decreased to 56+/-4 ng/ml. A third increased steady-state level of anesthesia corresponding to a whole-brain concentration of 107+/-7 ng/ml was rapidly achieved.

CONCLUSION

The experimental setup with computerized pump infusion and PET supervision enables accurate setting of target tissue drug concentration.

On the rate and extent of drug delivery to the brain

To define and differentiate relevant aspects of blood–brain barrier transport and distribution in order to aid research methodology in brain drug delivery. Pharmacokinetic parameters relative to the rate and extent of brain drug delivery are described and illustrated with relevant data, with special emphasis on the unbound, pharmacologically active drug molecule. Drug delivery to the brain can be comprehensively described using three parameters: K_p,uu (concentration ratio of unbound drug in brain to blood), CL_in (permeability clearance into the brain), and V_u,brain (intra-brain distribution). The permeability of the blood–brain barrier is less relevant to drug action within the CNS than the extent of drug delivery, as most drugs are administered on a continuous (repeated) basis. K_p,uu can differ between CNS-active drugs by a factor of up to 150-fold. This range is much smaller than that for log BB ratios (K_p), which can differ by up to at least 2,000-fold, or for BBB permeabilities, which span an even larger range (up to at least 20,000-fold difference). Methods that measure the three parameters K_p,uu, CL_in, and V_u,brain can give clinically valuable estimates of brain drug delivery in early drug discovery programmes.

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.

The role of permeability in drug ADME/PK, interactions and toxicity--presentation of a permeability-based classification system (PCS) for prediction of ADME/PK in humans

PURPOSE

The objective was to establish in vitro passive permeability (Pe) vs in vivo fraction absorbed (fa)-relationships for each passage through the human intestine, liver, renal tubuli and brain, and develop a Pe-based ADME/PK classification system (PCS).

MATERIALS AND METHODS

Pe- and intestinal fa-data were taken from an available data set. Hepatic fa was calculated based on extraction ratios of the unbound fraction of drugs (with support from animal in vivo uptake data). Renal fa (reabsorption) was estimated using renal pharmacokinetic data, and brain fa was predicted using animal in vitro and in vivo brain Pe-data. Hepatic and intestinal fa-data were used to predict bile excretion potential.

RESULTS

Relationships were established, including predicted curves for bile excretion potential and minimum oral bioavailability, and a 4-Class PCS was developed: I (very high Pe; elimination mainly by metabolism); II (high Pe) and III (intermediate Pe and incomplete fa); IV (low Pe and fa). The system enables assessment of potential drug-drug transport interactions, and drug and metabolite organ trapping.

CONCLUSIONS

The PCS and high quality Pe-data (with and without active transport) are believed to be useful for predictions and understanding of ADME/PK, elimination routes, and potential interactions and organ trapping/toxicity in humans.

Irradiation of rat brain reduces P-glycoprotein expression and function

The blood–brain barrier (BBB) hampers delivery of several drugs including chemotherapeutics to the brain. The drug efflux pump P-glycoprotein (P-gp), expressed on brain capillary endothelial cells, is part of the BBB. P-gp expression on capillary endothelium decreases 5 days after brain irradiation, which may reduce P-gp function and increase brain levels of P-gp substrates. To elucidate whether radiation therapy reduces P-gp expression and function in the brain, right hemispheres of rats were irradiated with single doses of 2–25 Gy followed by 10 mg kg⁻¹ of the P-gp substrate cyclosporine A (CsA) intravenously (i.v.), with once 15 Gy followed by CsA (10, 15 or 20 mg kg⁻¹), or with fractionated irradiation (4 × 5 Gy) followed by CsA (10 mg kg⁻¹) 5 days later. Additionally, four groups of three rats received 25 Gy once and were killed 10, 15, 20 or 25 days later. The brains were removed and P-gp detected immunohistochemically. P-gp function was assessed by [¹¹C]carvedilol uptake using quantitative autoradiography. Irradiation increased [¹¹C]carvedilol uptake dose-dependently, to a maximum of 20% above non irradiated hemisphere. CsA increased [¹¹C]carvedilol uptake dose-dependently in both hemispheres, but more (P<0.001) in the irradiated hemisphere. Fractionated irradiation resulted in a lost P-gp expression 10 days after start irradiation, which coincided with increased [¹¹C]carvedilol uptake. P-gp expression decreased between day 15 and 20 after single dose irradiation, and increased again thereafter. Rat brain irradiation results in a temporary decreased P-gp function.

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

PURPOSE AND METHODS

Regional overexpression of the multidrug transporter P-glycoprotein (P-gp) in epileptic brain tissue may lower target site concentrations of antiepileptic drugs and thus contribute to pharmacoresistance in epilepsy. We used the P-gp substrate R-[(11)C]verapamil and positron emission tomography (PET) to test for differences in P-gp activity between epileptogenic and nonepileptogenic brain regions of patients with drug-resistant unilateral temporal lobe epilepsy (n = 7). We compared R-[(11)C]verapamil kinetics in homologous brain volumes of interest (VOIs) located ipsilateral and contralateral to the seizure focus.

RESULTS

Among different VOIs, radioactivity was highest in the choroid plexus. The hippocampal VOI could not be used for data analysis because it was contaminated by spill-in of radioactivity from the adjacent choroid plexus. In several other temporal lobe regions that are known to be involved in seizure generation and propagation ipsilateral influx rate constants K(1) and efflux rate constants k(2) of R-[(11)C]verapamil were descriptively increased as compared to the contralateral side. Parameter asymmetries were most prominent in parahippocampal and ambient gyrus (K(1), range: -3.8% to +22.3%; k(2), range: -2.3% to +43.9%), amygdala (K(1), range: -20.6% to +31.3%; k(2), range: -18.0% to +38.9%), medial anterior temporal lobe (K(1), range: -8.3% to +14.5%; k(2), range: -14.5% to +31.0%) and lateral anterior temporal lobe (K(1), range: -20.7% to +16.8%; k(2), range: -24.4% to +22.6%). In contrast to temporal lobe VOIs, asymmetries were minimal in a region presumably not involved in epileptogenesis located outside the temporal lobe (superior parietal gyrus, K(1), range: -3.7% to +4.5%; k(2), range: -4.2% to +5.8%). In 5 of 7 patients, ipsilateral efflux (k(2)) increases were more pronounced than ipsilateral influx (K(1)) increases, which resulted in ipsilateral reductions (10%-26%) of R-[(11)C]verapamil distribution volumes (DV). However, for none of the examined brain regions, any of the differences in K(1), k(2) and DV between the epileptogenic and the nonepileptogenic hemisphere reached statistical significance (p > 0.05, Wilcoxon matched pairs test).

CONCLUSIONS

Even though we failed to detect statistically significant differences in R-[(11)C]verapamil model parameters between epileptogenic and nonepileptogenic brain regions, it cannot be excluded from our pilot data in a small sample size of patients that regionally enhanced P-gp activity might contribute to drug resistance in some patients with temporal lobe epilepsy.