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`Antiepileptic Drugs
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`Therapeutic Drug Monitoring of the Newer Generation Drugs
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`Author: Matthew D. Krasowski, MD, PhD // Date: JUN.1.2013 // Source: Clinical Laboratory News
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`Topics: Specializations, Toxicology, TDM, Forensics
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`Antiepileptic drugs (AEDs) used to treat seizure disorders are today among the most common medications for which clinical laboratories
`perform therapeutic drug monitoring (TDM) (1, 2). The first-generation of AEDs—carbamazepine, ethosuximide, phenobarbital, phenytoin,
`primidone, and valproic acid—were introduced by U.S. and European drug manufacturers several decades ago, and TDM quickly became
`part of using them in clinical practice. Generally speaking, these AEDs have complicated pharmacokinetics, including absorption,
`distribution, metabolism, and excretion, as well as narrow therapeutic ranges that cause significant differences in individuals' therapeutic
`dosages.
`
`In the last 20 years, 14 more so-called newer generation AEDs entered the market: eslicarbazepine acetate, felbamate, gabapentin,
`lacosamide, lamotrigine, levetiracetam, oxcarbazepine, pregabalin, rufinamide, stiripentol, tiagabine, topiramate, vigabatrin, and
`zonisamide (3). Only eslicarbazepine acetate and stiripentol are not approved in the U.S., while all of the drugs are available in Europe.
`Compared to first-generation AEDs, the newer agents generally have wider therapeutic ranges and fewer serious adverse effects.
`
`Despite the fact that evidence for the clinical benefit of monitoring blood levels of AEDs in patients is mostly anecdotal and retrospective,
`TDM today continues to be widely used in clinical practice. Only two randomized, controlled studies have been published, and neither
`showed clear clinical benefits (4, 5). Moreover, these two studies and others indicate that pre- and post-analytical errors occur frequently,
`particularly in timing blood draws and interpreting drug levels (1, 2).
`
`This article will present an overview of the newer AEDs and why improved education on the proper use of TDM is an important goal for
`maximizing the safety and benefits of these drugs for patients.
`
`Current Status of TDM for AEDs
`
`TDM of AEDs is challenging because no simple diagnostic tests can assess the clinical efficacy of any of the drugs. Careful clinical
`observation and labor-intensive electroencephalograms (EEG) remain the mainstays of clinical assessment. Furthermore, seizures by
`nature occur irregularly and unpredictably, making diagnosis difficult.
`
`The basic assumption of TDM is that the measured drug concentration correlates with the concentration at the target site of its action,
`usually an ion channel or neurotransmitter transporter in the brain, and therefore with the therapeutic effect. However, the correlation of
`drug concentration with clinical effect is reduced by factors such as irreversibility of drug action or an individual's tolerance of the drug. For
`example, TDM has limited utility for vigabatrin because it irreversibly binds to its molecular target.
`
`Laboratories usually perform TDM on serum or plasma samples, and less commonly on cerebrospinal fluid or saliva. However, the
`popularity of saliva as a specimen for TDM of AEDs is growing. It is easy to collect and transport saliva (6), although it is not a viable
`specimen type for some AEDs (7). For drugs with active metabolites, TDM may also include measuring the concentration of metabolite
`alone or together with the parent drug. For example, TDM of oxcarbazepine often focuses on its active metabolite, 10-hydroxycarbazepine.
`
`Commercially Available Immunoassays for Newer AEDs
`Anti-Epileptic Drug
`Immunoassay
`Eslicarbazepine
`Felbamate
`Gabapentin
`Lacosamide
`Lamotrigine
`Levetiracetam
`Oxcarbazepine
`Pregabalin
`Rufinamide
`Stiripentol
`Tiagabine
`Topiramate
`Vigabatrin
`
`Not available
`Not available
`ARK Diagnostics
`Not available
`ARK Diagnostics; Thermo Scientific
`ARK Diagnostics
`Not available
`Not available
`Not available
`Not available
`Not available
`ARK Diagnostics; Thermo Scientific
`Not available
`
`https://www.aacc.org/publications/cln/articles/2013/june/antiepileptic-drugs
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`5/20/2015
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`ARGENTUM Exhibit 1162
`Argentum Pharmaceuticals LLC v. Research Corporation Technologies, Inc.
`IPR2016-00204
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`00001
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`Zonisamide
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`ARK Diagnostics; Thermo Scientific
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`Clinical Need for TDM
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`Clinicians rely upon TDM for managing patients' AED therapy for multiple reasons. Perhaps the most common is that the patients exhibit
`significant inter-individual variability in their pharmacokinetic responses to most AEDs (1, 2). However, a few AEDs have predictable and
`consistent pharmacokinetics and generally require little or no TDM.
`
`The major pharmacokinetic factor affecting a patient's therapeutic drug levels is how quickly the liver metabolizes the AED. It is well known
`that the cytochrome P450 (CYP) enzyme system plays a major role. Some drugs actually increase or induce liver metabolism by CYP
`enzymes, leading to quicker metabolism. Classic inducers include carbamazepine, phenobarbital, phenytoin, rifampin (a tuberculosis drug),
`and St. John's wort (an herbal antidepressant). These drugs may lower patients' blood levels below what is optimal unless the AED dose is
`increased.
`
`Other drugs inhibit CYP enzyme metabolism of some AEDs. This inhibition can lead to excessively high drug concentrations unless the
`clinician reduces the patient's dose. Inter-individual differences may also result from: impaired organ function, typically kidney or liver; drug-
`drug or drug-food interactions; or genetic or pharmacogenetics factors. In renal failure patients, AEDs may be removed during dialysis
`procedures, especially those that only bind plasma proteins weakly. A number of factors also may alter serum protein concentrations
`including liver disease, advanced age, pregnancy, uremia, and other drugs (e.g., valproic acid) that also bind serum proteins.
`
`Clinicians also rely upon TDM to assess patient compliance. Typically, they prescribe the drug for the patient for months or even years,
`despite the absence of seizures. Patients may skip doses or stop taking the medication altogether because they haven't had a seizure, or
`they may quit taking it due to adverse effects or the cost of the medication. Lastly, AED levels may also be useful in managing suspected
`toxicity due to inadvertent or intentional overdose.
`
`The Reference Range Dilemma
`
`Although the newer AEDS offer many benefits for patients, laboratories have struggled to establish reference ranges for TDM, primarily
`because many of the drugs are effective over a wide range of serum/plasma concentrations (1, 2, 8). Furthermore, some individuals show
`good clinical response at levels above or below the standard reference range. Reference ranges also vary with different types of seizures,
`as well as with whether the AED is taken as monotherapy or in combination with other AEDs.
`
`A noted clinical pharmacologist, Emilio Perucca, MD, PhD, has promoted the concept of "individual therapeutic concentrations" for AEDs,
`wherein a patient is treated until good seizure control is achieved (8). In this model, the clinician assesses the AED's serum/plasma
`concentration at a clinical endpoint and uses it as the patient's individual therapeutic concentration. The frequency of TDM can be adjusted
`as needed when any changes occur that might alter the AED's pharmacokinetics.
`
`Table 1 summarizes some of the pharmacokinetic properties of the newer AEDs, and Table 2 presents a summary of the factors that
`influence their clinical use and interpretation.
`
`Analytical Methods
`
`Click here for Tables 1 and 2, and Figure 1
`
`Another reason that TDM of the newer AEDs has been challenging is that homogeneous immunoassays have only recently become
`available on standard chemistry analyzers. Early on, some clinical laboratories developed analytical methods using chromatography
`techniques, with or without mass spectrometry (MS) (9). Many laboratories, however, send out samples to reference laboratories.
`
`Today, most reference laboratories employ high-performance liquid chromatography (HPLC) and liquid chromatography-tandem mass
`spectrometry (LC-MS/MS) for measuring levels of newer-generation AEDs. Immunoassays are also commercially available for gabapentin,
`lamotrigine, levetiracetam, topiramate, and zonisamide, with assays for other AEDs in development. Table 3 summarizes the analytical
`methodologies used for measuring AED serum/plasma concentrations (9), as well as the viability of saliva as a specimen type (7).
`
`Table 3
`
`Analytical Methods for Therapeutic Drug Monitoring of Newer
`Antiepileptic Drugs
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`Generic
`
`Primary Analytical
`Methodology(ies)*
`
`Viability of Saliva as a
`Specimen Type
`
`Drug Name
`Eslicarbazepine
`Felbamate
`
`Gabapentin
`
`Lacosamide
`
`Lamotrigine
`
`Levetiracetam
`
`Oxcarbazepine
`Pregabalin
`
`HPLC
`GC, HPLC, LC-MS/MS
`HPLC, LC-MS/MS,
`immunoassay
`HPLC, LC-MS/MS
`HPLC, LC-MS/MS,
`immunoassay
`GC, HPLC, LC-MS/MS,
`immunoassay
`HPLC, LC-MS/MS
`HPLC, LC-MS/MS
`
`Unknown
`Unknown
`Limited, low concentrations
`in saliva
`
`Yes
`
`Yes
`
`Yes
`
`Yes
`Unknown
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`Rufinamide
`Stiripentol
`Tiagabine
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`Topiramate
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`Vigabatrin
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`Zonisamide
`
`HPLC, LC-MS/MS
`HPLC
`LC-MS/MS
`GC, HPLC, LC-MS/MS,
`immunoassay
`HPLC
`HPLC, LC-MS/MS,
`immunoassay
`* Abbreviations: GC, gas chromatography; HPLC, high-performance liquid
`chromatography; LC-MS/MS, liquid chromatography-tandem mass
`spectrometry.
`
`Unknown
`Unknown
`Unknown
`
`Yes
`
`No
`
`Yes
`
`A Closer Look
`
`The newer AEDs are a welcome addition to therapeutic options for treating epilepsy; however, the large number of drugs along with the
`variability in patients' responses presents a wide array of challenges for assessing their levels in patients. Laboratorians need to be aware
`of the key clinical and pharmacokinetic properties of these AEDs in order to optimize their TDM. Below is a brief review of each drug that
`summarizes key parameters.
`
`Eslicarbazepine Acetate. Approved in Europe, but not in the U.S., eslicarbazepine acetate is a pro-drug that is rapidly metabolized by liver
`esterases to form eslicarbazepine, the active metabolite that is the target of TDM. Overall, the drug has relatively predictable
`pharmacokinetics; therefore, TDM has a minimal role in eslicarbazepine therapy, except when it is taken by renal insufficiency patients who
`may have impaired drug clearance.
`
`Felbamate. The Food and Drug Administration (FDA) approved felbamate for treating adults with partial seizures and children with Lennox-
`Gastaut Syndrome, a type of childhood epilepsy that is often refractory to standard AED therapy. By 1994, however, clinicians identified
`cases of aplastic anemia, some of which progressed to severe liver failure, that were associated with felbamate therapy. While the drug
`has remained on the market, FDA sought revised labeling and restricted its use.
`
`Patients taking a typical dose of felbamate have serum/plasma concentrations of 30–60 mg/L, but children clear the drug 20–65% faster
`than adults. Overall, felbamate TDM has relatively modest utility, and unfortunately, toxicity cannot be easily predicted from laboratory
`studies. Given felbamate's potential adverse effects, laboratorians should advise clinicians to closely monitor blood counts and liver
`function of patients receiving this therapy.
`
`Gabapentin. FDA originally approved gabapentin for treating epilepsy, but the drug has achieved far greater popularity as an adjunctive
`therapy for chronic pain. Gabapentin is not metabolized, shows little binding to serum proteins, and is cleared almost entirely by the
`kidneys. A wide range of serum/plasma concentrations, 2–20 mg/L, are associated with effective seizure control.
`
`Other than optimizing dosing in renal insufficiency patients, TDM has overall low utility in gabapentin therapy. Saliva concentrations can be
`monitored, but they are only 5–10% of those in serum/plasma.
`
`Lacosamide. This drug has predictable pharmacokinetics across all ages and is cleared almost equally by the liver and kidney. Clinically
`significant drug-drug interactions involving lacosamide are also uncommon. Lacosamide TDM has relatively low utility except in patients
`with severe liver and/or kidney failure.
`
`Lamotrigine. Approved in the U.S., lamotrigine is widely used to treat partial seizures, as well as bipolar disorder. Several first-generation
`AEDs have been associated with severe birth defects, but lamotrigine has a solid safety record in pregnancy, making it the AED of choice
`to treat pregnant women experiencing seizures. Dermatologic reactions occur frequently, however, and patients should seek medical
`attention promptly if any skin reactions occur.
`
`Lamotrigine's pharmacokinetics are well understood, but fairly complex. The drug exhibits: increased metabolism over time (auto-
`induction); drug-drug interactions with CYP enzyme inducers and inhibitors; and impaired clearance in renal failure. But lamotrigine's
`clearance is higher in children and markedly higher (~300%) in pregnancy.
`
`Researchers have proposed a reference range of 3–14 mg/L for refractory epilepsy therapy; however, the incidence of toxic effects is
`significantly increased when serum/plasma concentrations are above 15 mg/L. Given the complicated pharmacokinetics and well-defined
`toxicity level, TDM plays a major role in lamotrigine therapy. In addition to serum/plasma, saliva is a viable specimen type for lamotrigine
`TDM.
`
`Levetiracetam. A widely used newer AED, levetiracetam is available in both oral and intravenous formulations, with the intravenous form
`used for acute management in the hospital setting. The drug does not bind serum proteins, has predictable pharmacokinetics, and limited
`drug-drug interactions because it is not metabolized by the liver.
`
`Laboratories that perform TDM of levetiracetam should separate serum or plasma from whole blood rapidly, as hydrolysis of levetiracetam
`can occur in the blood tube. Saliva is also a viable specimen type for levetiracetam. Researchers have proposed a therapeutic reference
`range of 12–46 mg/L, but laboratorians should be aware that samples drawn shortly after a dose of intravenous levetiracetam can appear
`to have very high drug levels. The main value of TDM for levetiracetam is adjusting dosage for pregnant patients and those with renal
`insufficiency.
`
`Oxcarbazepine. Oxcarbazepine is structurally related to carbamazepine, but it has a lower incidence of adverse effects, such as
`agranulocytosis and drug-drug interactions. The drug is metabolized primarily to 10-hydroxycarbazepine, which accounts for much of the
`anti-seizure activity. For TDM, oxcarbazepine is treated like a pro-drug, with monitoring focused on 10-hydroxycarbazepine. Clearance is
`reduced in the elderly and in individuals with renal insufficiency, but increased in pregnant women and patients taking liver enzyme-
`inducing drugs.
`
`Laboratories have observed that a wide range of serum concentrations, 3–35 mg/L, are clinically effective in seizure treatment, with toxic
`side effects more common at >35 mg/L. Overall, TDM is useful especially in renal insufficiency, pregnancy, and cases with suspected
`drug-drug interactions. Saliva is also a possible specimen type, although 10-hydroxycarbazepine has a shorter half-life in saliva compared
`to serum.
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`Pregabalin. Pharmaceutical researchers designed pregabalin to be a more potent analog of gabapentin. Similar to its predecessor,
`pregabalin is effective in treating chronic pain, and since its introduction, FDA approved a separate indication for treating fibromyalgia.
`Pregabalin has predictable pharmacokinetics with no reported drug-drug interactions and minimal binding to serum proteins; however,
`renal failure patients generally take lower dosages.
`
`Within the reference range of 2.8–8.3 mg/L, patients experience beneficial anti-seizure effects. However, other than adjusting dosage for
`renal failure patients or assessing adherence to therapy, TDM has minimal benefit in pregabalin therapy.
`
`Rufinamide. Approved in the U.S. for Lennox-Gastaut syndrome, rufinamide has very complicated metabolism pathways and a high
`potential for drug-drug interactions.
`
`Serum/plasma levels within a broad reference range of 3–30 mg/L correlate well with seizure control. Monitoring serum levels can be
`especially helpful in patients taking concomitant liver enzyme inducers or who are receiving hemodialysis. Overall, TDM is quite useful for
`this drug.
`
`Stiripentol. Approved in Europe in 2001, stiripentol has yet to be approved in the U.S. It has very complex pharmacokinetics, including non-
`linear elimination kinetics, high serum protein binding, and extensive metabolism by the liver, which resembles the classic AED phenytoin
`more than newer AEDs. The stiripentol reference range is not well-defined, but serum concentrations of 4–22 mg/L correlate with good
`management of seizures in children.
`
`Monitoring the free-drug fraction of stiripentol would appear to be beneficial, but no methods have been reported to date. Overall, clinical
`use of the drug is limited, and therefore so is experience with TDM.
`
`Tiagabine. The AED drug tiagabine is not widely prescribed in the U.S. or Europe. Its limited use has been attributed to a propensity to
`cause non-convulsive status epilepticus, a serious adverse effect (10). It shows significant binding to proteins (>96%), as well as variability
`in inter-individual metabolism by the liver. Although therapeutic levels of the drug are substantially lower than those for other newer
`generation AEDs, tiagabine has a broad reference range of 0.02–0.2 mg/L for its anti-seizure effect. Overall, TDM is very useful for
`tiagabine due to its complex and variable pharmacokinetics.
`
`Topiramate. Approved for treating epilepsy in children and migraine headaches in adults, topiramate is metabolized in the liver and has the
`potential for drug-drug interactions. Researchers have proposed a reference range of 5–20 mg/L for epilepsy therapy. TDM of topiramate is
`valuable because individuals' metabolism is quite variable. Saliva is also a viable specimen type.
`
`Vigabatrin. An irreversible inhibitor of GABA transaminase, vigabatrin breaks one of the principle assumptions of TDM, namely that the
`concentration in serum/plasma correlates with the concentration at the target site. This may be one reason why laboratories have observed
`a wide range, 0.8–36 mg/L, of trough serum/plasma concentrations found in patients successfully treated with the drug. Consequently,
`other than to assess patient compliance or to evaluate possible drug overdose, there is little benefit in monitoring vigabatrin.
`
`Zonisamide. Metabolism of zonisamide is affected by drugs that induce or inhibit CYP enzyme activity. There is significant inter-individual
`variability in metabolism of the drug, especially in patients who are on concomitant therapy with other drugs that also affect expression of
`liver enzymes. Toxic side effects are uncommon at serum concentrations <30 mg/L, and researchers have proposed a reference range of
`10–40 mg/L in serum/plasma for managing seizures. Saliva is also a viable specimen type for zonisamide TDM. Overall, TDM is useful for
`zonisamide.
`
`The Take-Home Message
`
`The past 20 years have seen the introduction of 14 newer AEDs for treatment of seizure disorders. Clinicians also prescribe the newer
`agents for "off-label" conditions, including bipolar disorder, chronic pain syndromes such as fibromyalgia and trigeminal neuralgia, or
`migraine headaches.
`
`As use of these drugs increases, clinical laboratories will likely see more requests from clinicians to monitor patients' drug levels. Although
`physicians often order the tests to assess adherence to therapy, laboratorians should approach TDM for conditions other than seizure
`disorders cautiously, as it is not well-defined at present.
`
`The availability of more automated immunoassays for measuring these drugs, as well as the expected development and introduction of
`more assays by manufacturers, will allow larger numbers of laboratories to perform in-house TDM of the newer AEDs. Laboratorians would
`do well to recognize this trend and evaluate new assays carefully so that they can best help clinicians understand the clinical utility of the
`results.
`
`REFERENCES
`
`1. Neels HM, Sierens AC, Naelerts K, et al. Therapeutic drug monitoring of old and newer anti-epileptic drugs. Clin Chem Lab Med
`2004;42:1228–55.
`2. Patsalos PN, Berry DJ, Bourgeois BFD, et al. Antiepileptic drugs—best practice guidelines for therapeutic drug monitoring: A
`position paper by the subcommission on therapeutic drug monitoring, ILAE Commission on Therapeutic Strategies. Epilepsia
`2008;49:1239–76.
`3. LaRoche SM and Helmers SL. The new antiepileptic drugs: Clinical applications. JAMA 2004;291:615–20.
`4. Fröscher W, Eichelbaum M, Gugler R, et al. A prospective randomized trial on the effect of monitoring plasma anticonvulsant levels
`in epilepsy. J Neurol 1981;224:193–201.
`5. Januzzi G, Cian P, Fattore C, et al. A multicenter randomized controlled trial on the clinical impact of therapeutic drug monitoring in
`patients with newly diagnosed epilepsy. Epilepsia 2000;41:222–30.
`6. Jones MD, Ryan M, Miles MV, et al. Stability of salivary concentrations of the newer antiepileptic drugs in the postal system. Ther
`Drug Monit 2005;27:576–9.
`7. Patsalos PN and Berry DJ. Therapeutic drug monitoring of antiepileptic drugs by use of saliva. Ther Drug Monit 2013;35:4–29.
`8. Perucca E. Clinical pharmacology and therapeutic use of the new antiepileptic drugs. Fundam Clin Pharmacol 2001;15:405–17.
`9. Krasowski MD. Therapeutic drug monitoring of the newer anti-epilepsy medications. Pharmaceuticals (Basel) 2010;3:1909–35.
`10. Schapel G and Chadwick D. Tiagabine and non-convulsive status epilepticus. Seizure 1996; 5:153–6.
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`Matthew D. Krasowski, MD, PhD, is a clinical associate professor and medical director of the Clinical Chemistry and Point of Service Laboratories, and
`director of clinical laboratories in the Department of Pathology at the University of Iowa Hospitals and Clinics, Iowa City, Iowa.
`
`Email: matthew-krasowski@uiowa.edu.
`Disclosure: The author has nothing to disclose.
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