P-glycoprotein Expression and Pharmacoresistant Epilepsy: Cause or Consequence?

Transcription

1 Current Literature In Clinical Science P-glycoprotein Expression and Pharmacoresistant Epilepsy: Cause or Consequence? P-glycoprotein Expression and Function in Patients With Temporal Lobe Epilepsy: A Case-control Study. Feldmann M, Asselin MC, Liu J, Wang S, McMahon A, Anton-Rodriguez J, Walker M, Symms M, Brown G, Hinz R, Matthews J, Bauer M, Langer O, Thom M, Jones T, Vollmar C, Duncan JS, Sisodiya SM, Koepp MJ. Lancet Neurol 2013 Aug;12(8): doi: /S (13) Epub 2013 Jun 18. BACKGROUND: Studies in rodent models of epilepsy suggest that multidrug efflux transporters at the blood-brain barrier, such as P-glycoprotein, might contribute to pharmacoresistance by reducing target-site concentrations of antiepileptic drugs. We assessed P-glycoprotein activity in vivo in patients with temporal lobe epilepsy. METHODS: We selected 16 patients with pharmacoresistant temporal lobe epilepsy who had seizures despite treatment with at least two antiepileptic drugs, eight patients who had been seizure-free on antiepileptic drugs for at least a year after 3 or more years of active temporal lobe epilepsy, and 17 healthy controls. All participants had a baseline PET scan with the P-glycoprotein substrate (R)-[(11)C]verapamil. Pharmacoresistant patients and healthy controls then received a 30-min infusion of the P-glycoprotein-inhibitor tariquidar followed by another (R)-[(11)C]verapamil PET scan 60 min later. Seizure-free patients had a second scan on the same day, but without tariquidar infusion. Voxel-by-voxel, we calculated the (R)-[(11)C]verapamil plasma-to-brain transport rate constant, K1 (ml/min/cm(3)). Low baseline K1 and attenuated K1 increases after tariquidar correspond to high P-glycoprotein activity. FINDINGS: Between October, 2008, and November, 2011, we completed (R)-[(11)C]verapamil PET studies in 14 pharmacoresistant patients, eight seizure-free patients, and 13 healthy controls. Voxel-based analysis revealed that pharmacoresistant patients had lower baseline K1, corresponding to higher baseline P-glycoprotein activity, than seizure-free patients in ipsilateral amygdala (0 031 vs ml/min/cm(3); p=0 014), bilateral parahippocampus (0 032 vs 0 037; p<0 0001), fusiform gyrus (0 036 vs 0 041; p<0 0001), inferior temporal gyrus (0 035 vs 0 041; p<0 0001), and middle temporal gyrus (0 038 vs 0 044; p<0 0001). Higher P-glycoprotein activity was associated with higher seizure frequency in whole-brain grey matter (p=0 016) and the hippocampus (p=0 029). In healthy controls, we noted a 56 8% increase of whole-brain K1 after 2 mg/kg tariquidar, and 57 9% for 3 mg/kg; in patients with pharmacoresistant temporal lobe epilepsy, whole-brain K1 increased by only 21 9% for 2 mg/kg and 42 6% after 3 mg/kg. This difference in tariquidar response was most pronounced in the sclerotic hippocampus (mean 24 5% increase in patients vs mean 65% increase in healthy controls, p<0 0001). INTERPRE- TATION: Our results support the hypothesis that there is an association between P-glycoprotein overactivity in some regions of the brain and pharmacoresistance in temporal lobe epilepsy. If this relation is confirmed, and P-glycoprotein can be identified as a contributor to pharmacoresistance, overcoming P-glycoprotein overactivity could be investigated as a potential treatment strategy. Epilepsy Currents, Vol. 14, No. 3 (May/June) 2014 pp American Epilepsy Society Commentary Since the 1990s, more than a dozen new medications have been introduced for the treatment of epilepsy. Many of these newer molecules have unique pharmacological mechanisms, and most have significantly improved pharmacokinetic profiles as compared to their predecessors. Despite these advances, however, a frustrating statistic remains: Approximately one-third of patients with epilepsy will ultimately inadequately respond to antiepileptic drug (AED) treatment (1). Pharmacoresistance is, by international consensus, defined as a failure of two or more AEDs, given in an appropriate manner, to achieve seizure freedom. While the reasons for this drug resistance is no doubt multifactorial, there is a growing body of evidence derived from both animal models, as well as examinations in brain tissue of patients undergoing surgical resection that drug resistance might be explained by a failure of AEDs to reach their molecular targets (1). P-glycoprotein (Pgp) is a member of the ATP-binding cassette proteins. This transmembrane transporter is found in a number of tissues, including endothelial cells of the blood brain barrier (BBB), and functions as an efflux transporter, pumping both xenobiotics and other toxic substrates from the intracellular space back into the capillary lumen. P-glycopro- 136

2 P-glycoprotein Expression and Pharmacoresistant Epilepsy tein (along with other transmembrane efflux pumps) might best be thought of as a defense mechanism, designed to keep potentially harmful molecules from reaching protected areas, such as the brain. Unfortunately, many of our commonly used AEDs are also substrates for this transport protein, at least in some species. Since the mid-1990s, several investigations have suggested Pgp is overexpressed in brain tissue of patients with treatment resistant seizures (2). Early experiments in animal models helped fuel the enthusiasm for the hypothesis that increased expression or upregulation of this (and perhaps other) efflux transporters was responsible for pharmacoresistant epilepsy (3, 4). Confounding this, however, has been the recognition that marked differences in substrate specificity may exist between both species (i.e., rodent vs human) and tissues (5 8), and the relevance in humans has been questioned (9). Nonetheless, if indeed upregulation of this protein is involved in the development and ultimate expression of pharmacoresistance, how and perhaps more importantly, why might this occur in some, but not all, patients? Genetic variability, exposure to certain AEDs, as well as seizures themselves have all been suggested as potential mechanisms. The gene that encodes for this protein (ABCB1) is highly polymorphic, so it is no surprise that there is great variability in the level of Pgp expression and activity among individuals. Having said that, the available data thus far has failed to show a convincing association between genetic polymorphism of ABCB1 and drug resistant epilepsy (10). Exposure to certain AEDs themselves, particularly enzyme inducing agents, has been proposed as a potential mechanism for Pgp upregulation. The data supporting this notion is controversial. While some experiments have shown that exposure to certain AEDs increased expression of mrna or protein content, there are conflicting data regarding increased Pgp functionality in brain endothelial cells (11, 12). Finally, seizure-associated Pgp upregulation has been documented in a number of animal models and likely involves the excitatory transmitter glutamate and NMDA receptors (13). Again, extrapolating experimental data in rodents to the clinic is controversial. What has been lacking in this story is compelling evidence of the in vivo association between Pgp functionality and pharmacoresistance in humans with temporal lobe epilepsy. Recently, Feldmann and colleagues conducted a prospective, case-control study involving patients who were either pharmacoresistant or seizure free for at least 1 year. Control subjects were healthy volunteers. Functionality of Pgp in these groups was assessed using PET and the Pgp substrate (R)-[11C] verapamil along with, in the case of treatment-resistant and healthy subjects, infusion of the Pgp inhibitor tarquidar. Verapamil entry into the brain is limited by Pgp. In this experimental design, lower regional verapamil uptake would suggest increased Pgp activity. Treatment with tarquidar would be expected to inhibit Pgp activity, thus attenuating any observed increase in Pgp activity. The results of this study are intriguing. At baseline, the patients who had been classified as pharmacoresistant had significantly higher (R)-[11C]verapamil in regions of the hippocampus and temporal lobe as compared to patients deemed seizure free. Moreover, in the pharmacoresistant patient, (R)-[11C]verapamil correlated inversely with average monthly seizure frequency. No significant differences in (R)- [11C]verapamil were seen between seizure-free patients and healthy controls. While the response was not consistent across all patients, following administration of tarquidar, significant regional differences in increased (R)-[11C]verapamil were seen in pharmacoresistant patients as compared to controls, which the authors suggest is indicative of increased Pgp activity in the ipsilateral hippocampus. Interestingly, the pharmacoresistant patients who showed a lesser increase in (R)-[11C] verapamil uptake as compared to controls following tarquidar, all were taking carbamazepine. None of the other patients were receiving this medication. As previously mentioned, while evidence of carbamazepine induction of Pgp in the brain is lacking, the drug has been shown to increase expression of Pgp in other tissues (14, 15). Taken together, this novel study would seem to confirm the notion that the efflux transporter Pgp is indeed increased in the drug-resistant patient. The question of causality remains, however. Evidence from Feldman and colleagues could suggest that upregulation of Pgp might be linked to cause, since increases in activity seemed to be localized around the seizure focus. Data from this study also suggested a relationship between seizure frequency and Pgp activity, at least at one point in time. It is tempting, therefore, to conclude that the underlying cause of resistant epilepsy has a simple pharmacokinetic explanation: patients continue to have seizures because the drug could not reach its site of action. The question is, is the simple explanation always the correct answer? It would seem unlikely that these transport proteins evolved at the blood brain barrier for the sole purpose of limiting access to AEDs. Do we find ourselves caught up in our language of multidrug resistance proteins? Stated differently, is increased Pgp activity the cause of, or perhaps just a marker for, bad epilepsy? Are patients who became seizure free able to do so because they did not overexpress Pgp at the outset, or did Pgp expression return to normal levels after they became seizure free? This would seem to be a chicken and egg quandary. Which came first? Given that the biological role of Pgp is to remove potentially toxic substances from the intracellular space, one could speculate that upregulation of this transporter is a compensatory mechanism to a compromised blood brain barrier and reflects the overall burden of chronic seizures (12, 16). In any case, the work by Feldmann et al. represents an important step forward in our understanding of the pharmacoresistant patient. Perhaps future in vivo investigations will shed light on the relationship between Pgp expression and seizure responsiveness over time. by Barry E. Gidal, PharmD References 1. Kwan P, Schachter S, Brodie M. Drug resistant epilepsy. N Engl J Med 2011;365: Tishler DM, Weinberg KI, Hinton DR, Barbaro N, Annett GM, Raffel C. MDR1 gene expression in brain of patients with medically intractable epilepsy. Epilepsia 1995;36: Stępień K, Tomaszewski M, Tomaszewska J, Czuczwar S. The multidrug transporter P-glycoprotein in pharmacoresistance to antiepileptic drugs. Pharmacol Rep 2012;64:

3 P-glycoprotein Expression and Pharmacoresistant Epilepsy 4. Loscher W, Potschaka H. Role of drug efflux transporters in the brain for drug disposition and treatment of brain diseases. Prog Neurobiol 2005;76: Baltes S, Gastens AM, Fedrowitz M, Potschka H, Kaever V, Loscher W. Differences in the transport of the antiepileptic drugs phenytoin, levetiracetam, and carbamazepine by human and mouse P-glycoprotein. Neuropharmacology 2007;52: Sisodiya SM, Lin WR, Harding BN, Squier MV, Thom M. Drug resistance in epilepsy: Expression of drug resistance proteins in common causes of refractory epilepsy. Brain 2002;125: Aronica E, Gorter JA, Jansen GH, van Veelen CW, van Rijen PC, Leenstra S, Ramkema M, Scheffer GL, Scheper RJ, Troost D. Expression and cellular distribution of multidrug transporter proteins in two major causes of medically intractable epilepsy: Focal cortical dysplasia and glioneuronal tumors. Neuroscience 2003;118: Kubota H, Ishihara H, Langmann T, Schmidz G, Stieger B, Wieser HG, Yonekawa Y, Frei K. Distribution and functional activity of P- glycoprotein and multidrug resistance-associated proteins in human microvascular endothelial cells in hippocampal sclerosis. Epilepsy Res 2006; Anderson GD, Shen DD. Where is the evidence that P-glycoprotein limits brain uptake of antiepileptic drug and contributes to drug resistance in epilepsy? Epilepsia 2007;48: Haerian BS, Roslan H, Raymond A, Tan CT, Lim KS, Zulfiki SZ, Mohamed EHM, Tan HJ, Mohamed Z. ABCB1 C3435T polymorphism and the risk of resistance to antiepileptic drugs in epilepsy: A systematic review and meta-analysis. Seizure 2010;19: Ambroziak K, Kuteykin-Tepyakov K, Luna-Tortos C, Al-Falah M, Fedrowitz M, Loscher W. Exposure to antiepileptic drugs does not alter the functionality of P-glycoprotein in brain capillary endothelial and kidney cell lines. Eur J Pharmacol 2010;628: Liu JY, Thom M, Catarino CB, Martinian L, Figarella-Branger D, Bartolomei F, Koepp M, Sisodiya SM. Neuropathology of the bloodbrain barrier and pharmacoresistance in human epilepsy. Brain 2012;135; Potschaka H. Modulating P-glycoprotein regulation: Future perspectives for pharmacoresistant epilepsies? Epilepsia 2010;51: Lombardo L, Pelliteri R, Balazy M, Cardile V. Induction of nuclear receptors and drug resistance in the brain microvascular endothelial cells treated with antiepileptic drugs. Curr Neurovasc Res 2008;5: Owen A, Goldring C, Morgan P, Park BK, Pirohamed M. Induction of P-glycoprotein in lymphocytes by carbamazepine and rifampicin: the role of nuclear hormone response elements. Br J Clin Pharmacol 2008;62: French JA. P-glycoprotein expression and antiepileptic drug resistance. Neurology 2013;

4 American Epilepsy Society Epilepsy Currents Journal Disclosure of Potential Conflicts of Interest Instructions The purpose of this form is to provide readers of your manuscript with information about your other interests that could influence how they receive and understand your work. Each author should submit a separate form and is responsible for the accuracy and completeness of the submitted information. The form is in four parts. 1. Identifying information. Enter your full name. If you are NOT the main contributing author, please check the box no and enter the name of the main contributing author in the space that appears. Provide the requested manuscript information. 2. The work under consideration for publication. This section asks for information about the work that you have submitted for publication. The time frame for this reporting is that of the work itself, from the initial conception and planning to the present. The requested information is about resources that you received, either directly or indirectly (via your institution), to enable you to complete the work. Checking No means that you did the work without receiving any financial support from any third party that is, the work was supported by funds from the same institution that pays your salary and that institution did not receive third-party funds with which to pay you. If you or your institution received funds from a third party to support the work, such as a government granting agency, charitable foundation or commercial sponsor, check Yes. Then complete the appropriate boxes to indicate the type of support and whether the payment went to you, or to your institution, or both. 3. Relevant financial activities outside the submitted work. This section asks about your financial relationships with entities in the bio-medical arena that could be perceived to influence, or that give the appearance of potentially influencing, what you wrote in the submitted work. For example, if your article is about testing an epidermal growth factor receptor (DGFR) antagonist in lung cancer, you should report all associations with entities pursuing diagnostic or therapeutic strategies in cancer in general, not just in the area of EGFR or lung cancer. Report all sources of revenue paid (or promised to be paid) directly to you or your institution on your behalf over the 36 months prior to submission of the work. This should include all monies from sources with relevance to the submitted work, not just monies from the entity that sponsored the research. Please note that your interactions with the work s sponsor that are outside the submitted work should also be listed here. If there is any question, it is usually better to disclose a relationship than not to do so. For grants you have received for work outside the submitted work, you should disclose support ONLY from entities that could be perceived to be affected financially by the published work, such as drug companies, or foundations supported by entities that could be perceived to have a financial stake in the outcome. Public funding sources, such as government agencies, charitable foundations or academic institutions, need not be disclosed. For example, if a government agency sponsored a study in which you have been involved and drugs were provided by a pharmaceutical company, you need only list the pharmaceutical company. 4. Other relationships Use this section to report other relationships or activities that readers could perceive to have influenced, or that give the appearance of potentially influencing, what you wrote in the submitted work.

5 American Epilepsy Society Epilepsy Currents Journal Disclosure of Potential Conflicts of Interest Section #1 Identifying Information 1. Today s Date: 11/22/13 2. First Name Barry Last Name Gidal Degree PharmD 3. Are you the Main Assigned Author? Yes No If no, enter your name as co-author: 4. Manuscript/Article Title: P-glycoprotein Expression and Pharmacoresistant Epilepsy: Cause or Consequence? 5. Journal Issue you are submitting for: 14.3 Section #2 The Work Under Consideration for Publication Did you or your institution at any time receive payment or services from a third party for any aspect of the submitted work (including but not limited to grants, data monitoring board, study design, manuscript preparation, statistical analysis, etc.)? Complete each row by checking No or providing the requested information. If you have more than one relationship just add rows to this table. Type No Money Paid to You Money to Your Institution* Name of Entity Comments** 1. Grant 2. Consulting fee or honorarium 3. Support for travel to meetings for the study or other purposes 4. Fees for participating in review activities such as data monitoring boards, statistical analysis, end point committees, and the like 5. Payment for writing or reviewing the manuscript 6. Provision of writing assistance, medicines, equipment, or administrative support. 7. Other * This means money that your institution received for your efforts on this study. ** Use this section to provide any needed explanation. Page 2 5/29/2014

6 Section #3 Relevant financial activities outside the submitted work. Place a check in the appropriate boxes in the table to indicate whether you have financial relationships (regardless of amount of compensation) with entities as described in the instructions. Use one line for each entity; add as many lines as you need by clicking the Add box. You should report relationships that were present during the 36 months prior to submission. Complete each row by checking No or providing the requested information. If you have more than one relationship just add rows to this table. Type of relationship (in alphabetical order) No Name of Entity Comments** 1. Board membership Money Paid to You Money to Your Institution* 2. Consultancy 6,000 Eisai, Upsher-Smith 3. Employment 4. Expert testimony 5. Grants/grants pending 6. Payment for lectures including service on speakers bureaus 7,500 UCB, GSK 7. Payment for manuscript preparation. 8. Patents (planned, pending or issued) 9. Royalties 10. Payment for development of educational presentations 11. Stock/stock options 12. Travel/accommodations/meeti ng expenses unrelated to activities listed.** 13. Other (err on the side of full disclosure) * This means money that your institution received for your efforts. ** For example, if you report a consultancy above there is no need to report travel related to that consultancy on this line. Section #4 Other relationships Are there other relationships or activities that readers could perceive to have influenced, or that give the appearance of potentially influencing, what you wrote in the submitted work? No other relationships/conditions/circumstances that present a potential conflict of interest. Yes, the following relationships/conditions/circumstances are present: Thank you for your assistance. Epilepsy Currents Editorial Board Page 3 5/29/2014