Single-dose pharmacokinetics of felbamate in patients with renal dysfunction

Effect of compromised liver function and acute kidney injury on the pharmacokinetics of thymoquinone in a rat model

The current research explored the effect of hepatic and renal dysfunctions on the pharmacokinetics of thymoquinone (TQ) in a rat model.An acute kidney injury was induced using gentamicin and a liver damage was elicited using a single dose of d-galactosamine. For the pharmacokinetic studies, TQ was administered as IV injection or and PO route to rats.The concentrations of TQ and pharmacokinetic parameters were calculated using a non-compartmental analysis. The systemic clearance (Cl) of TQ after IV dosing was slightly reduced in the liver dysfunction group compared to healthy controls (p = 0.0013). Similarly, the estimated volume of distribution at steady state (Vss) was marginally decreased (p = 0.001). However, in rats with acute kidney injury exhibited a larger Vss as opposed to normal renal function (511.28 ± 21.03 ml/kg vs. 442.25 ± 31.43 ml/kg; p = 0.0001). Whereas oral Cl and terminal volume of distribution (Vz) of TQ were reduced by ∼50% in the liver dysfunction group (p = 0.0001). These changes were associated with more systemic exposure as measured by AUC_0-∞ in rats with compromised liver functions. The estimated plasma protein binding TQ was 99.84 ± 0.03% in healthy controls, 97.05 ± 0.57% with kidney injury rats, and 95.75 ± 0.64% in liver dysfunctionThe findings of the present study suggest that liver dysfunction could potentially modify the disposition of TQ administered orally, and therefore, a smaller maintenance dose is probably required to avoid accumulation.

An Updated Overview on Therapeutic Drug Monitoring of Recent Antiepileptic Drugs

Given the distinctive characteristics of both epilepsy and antiepileptic drugs (AEDs), therapeutic drug monitoring (TDM) can make a significant contribution to the field of epilepsy. The measurement and interpretation of serum drug concentrations can be of benefit in the treatment of uncontrollable seizures and in cases of clinical toxicity; it can aid in the individualization of therapy and in adjusting for variable or nonlinear pharmacokinetics; and can be useful in special populations such as pregnancy. This review examines the potential for TDM of newer AEDs such as eslicarbazepine acetate, felbamate, gabapentin, lacosamide, lamotrigine, levetiracetam, perampanel, pregabalin, rufinamide, retigabine, stiripentol, tiagabine, topiramate, vigabatrin, and zonisamide. We describe the relationships between serum drug concentration, clinical effect, and adverse drug reactions for each AED as well as the different analytical methods used for serum drug quantification. We discuss retrospective studies and prospective data on the serum drug concentration-efficacy of these drugs and present the pharmacokinetic parameters, oral bioavailability, reference concentration range, and active metabolites of newer AEDs. Limited data are available for recent AEDs, and we discuss the connection between drug concentrations in terms of clinical efficacy and nonresponse. Although we do not propose routine TDM, serum drug measurement can play a beneficial role in patient management and treatment individualization. Standardized studies designed to assess, in particular, concentration-efficacy-toxicity relationships for recent AEDs are urgently required.

Evaluation of Brain Pharmacokinetic and Neuropharmacodynamic Attributes of an Antiepileptic Drug, Lacosamide, in Hepatic and Renal Impairment: Preclinical Evidence

The knowledge of pharmacokinetic and pharmacodynamic properties of antiepileptic drugs is helpful in optimizing drug therapy for epilepsy. This study was designed to evaluate the pharmacokinetic and pharmacodynamic properties of lacosamide in experimentally induced hepatic and renal impairment in seizure animals. Hepatic or renal impairment was induced by injection of carbon tetrachloride or diclofenac sodium, respectively. After induction, the animals were administered a single dose of lacosamide. At different time points, maximal electroshock (MES) seizure recordings were made followed by isolation of plasma and brain samples for drug quantification and pharmacodynamic measurements. Our results showed a significant increase in the area under the curve of lacosamide in hepatic and renal impairment groups. Reduced clearance of lacosamide was observed in animals with renal impairment. Along with pharmacokinetic alterations, the changes in pharmacodynamic effects of lacosamide were also observed in all the groups. Lacosamide showed a significant protection against MES-induced seizures, oxidative stress, and neuroinflammatory cytokines. These findings revealed that experimentally induced hepatic or renal impairment could alter the pharmacokinetic as well as pharmacodynamic properties of lacosamide. Hence, these conditions may affect the safety and efficacy of lacosamide.

Pharmacokinetic Properties of New Antiepileptic Drugs

Eight new antiepileptic drugs (AEDs) have been approved for use within the United States within the past decade. They are felbamate, gabapentin, lamotrigine, levetiracetam, oxcarbazepine, tiagabine, topiramate, and zonisamide. These afford clinicians with more options to increase efficacy and tolerability in the treatment of patients with epilepsy. Pharmacokinetic properties and drug interactions with other AEDs and other medications taken for comorbidities are individually discussed for each of these new agents. Drug concentrations are not routinely monitored for these newer agents, and there have been few studies designed to investigate their concentration-effect relationships. For most of these medications, the concentrations observed in responders and nonresponders overlap considerably and levels associated with efficacy are often associated with adverse events, complicating the definition of target ranges. Also, epilepsy manifests itself sporadically causing difficulty in clinically monitoring efficacy of medications. Therapeutic drug monitoring provides for the individualization of treatment for these agents, which is important because they demonstrate significant variability in inter- and intraindividual pharmaco-kinetic properties. Therapeutic drug monitoring also allows for identification of noncompliance, drug interactions, and toxicity. Current knowledge of the relationships between efficacy, toxicity, and drug concentrations is discussed.

Pharmacokinetic of antiepileptic drugs in patients with hepatic or renal impairment

Many factors influence choice of antiepileptic drugs (AEDs), including efficacy of the drug for the indication (epilepsy, neuropathic pain, affective disorder, migraine), tolerability, and toxicity. The first-generation AEDs and some newer AEDs are predominately eliminated by hepatic metabolism. Other recent AEDs are eliminated by renal excretion of unchanged drug or a combination of hepatic metabolism and renal excretion. The effect of renal and hepatic disease on the dosing will depend on the fraction of the AED eliminated by hepatic and/or renal excretion, the metabolic isozymes involved, as well as the extent of protein binding, if therapeutic drug monitoring is used. For drugs that are eliminated by renal excretion, methods of estimating creatinine clearance can be used to determine dose adjustments. For drugs eliminated by hepatic metabolism, there are no specific markers of liver function that can be used to provide guidance in dosage adjustments. Based on studies with probe drugs, the hepatic metabolic enzymes are differentially affected depending on the cause and severity of hepatic disease, which can aid in predicting dose adjustment when clinical data are not available. Several AEDs are also associated with laboratory markers of mild hepatic dysfunction and, rarely, more severe hepatic injury. In contrast, the risk of renal injury from AEDs is generally low. In general, co-morbid hepatic or renal diseases influence the decision for the selection of an AED. For some patients dosing changes to their existing AEDs may be appropriate. For others, a change to another AED may be a better option.

Effect of experimentally induced hepatic and renal failure on the pharmacokinetics of topiramate in rats

We aimed to investigate the effect of induced hepatic and renal failure on the pharmacokinetics of topiramate (TPM) in rats. Twenty-four Sprague-Dawley rats were used in this study. Renal or hepatic failure was induced by a single i.p. dose of 7.5 mg/kg cisplatin (n = 8) or 0.5 mL/kg carbon tetrachloride (CCl4) (n = 8), respectively. Three days after cisplatin dose or 24 h after CCl4 dose, the rats were administered a single oral dose of 20 mg/kg TPM. The plasma samples were quantified by LC-MS/MS method. Compared to control, plasma concentration-time profile in CCl4-treated and, to a lesser extent, in cisplatin-treated rats decreased more slowly particularly in the elimination phase. TPM oral clearance (CL/F) in CCl4-treated group was significantly lower than that in control (P < 0.001), whereas AUC0-∞, T1/2, and Vd/F were significantly higher in CCl4-treated rats compared to the control (P < 0.01). The CL/F was not significantly different between cisplatin-treated rats and control (P > 0.05). However, in cisplatin-treated rats, the T1/2 and Vd/F were significantly higher than that in the control group (P < 0.01). Both conditions failed to cause a significant effect on Cmax or Tmax. The present findings suggest that induced hepatic or renal failure could modify the pharmacokinetic profile of TPM in the rat.

Use of antiepileptic drugs in hepatic and renal disease

The use of antiepileptic drugs in patients with renal or hepatic disease is common in clinical practice. Since the liver and kidney are the main organs involved in the elimination of most drugs, their dysfunction can have important effects on the disposition of antiepileptic drugs. Renal or hepatic disease can prolong the elimination of the parent drug or an active metabolite leading to accumulation and clinical toxicity. It can also affect the protein binding, distribution, and metabolism of a drug. The protein binding of anionic acidic drugs, such as phenytoin and valproate, can be reduced significantly by renal failure, causing difficulties in the interpretation of total serum concentrations commonly used in clinical practice. Dialysis can further modify the pharmacokinetic parameters or result in significant removal of the antiepileptic drugs. Antiepileptic drugs that are eliminated unchanged by the kidneys or undergo minimal metabolism include gabapentin, pregabalin, vigabatrin, and topiramate when used as monotherapy. Drugs eliminated predominantly by biotransformation include phenytoin, valproate, carbamazepine, tiagabine, and rufinamide. Drugs eliminated by a combination of renal excretion and biotransformation include levetiracetam, lacosamide, zonisamide, primidone, phenobarbital, ezogabine/retigabine, oxcarbazepine, eslicarbazepine, ethosuximide, and felbamate. Drugs in the latter group can be used cautiously in patients with either renal or liver failure. Antiepileptic drugs that are at high risk of being extracted by hemodialysis include ethosuximide, gabapentin, lacosamide, levetiracetam, pregabalin and topiramate. The use of antiepileptic drugs in the presence of hepatic or renal disease is complex and requires great familiarity with the pharmacokinetics of these agents. Closer follow-up of the patients and more frequent monitoring of serum concentrations are required to optimize clinical outcomes.

[Therapeutic drug monitoring of felbamate]

Felbamate is a derivative of meprobamate used in second-line partial epilepsy and in the Lennox-Gastaut syndrome. Felbamate is well absorbed and has linear kinetics: C(max) and AUC increasing linearly with dose. The metabolism takes place in the liver. Metabolites represent 40 to 60% of excretion and are eliminated via the urine. The half-life is between 15 and 23 hours. Clearance is dependent on renal function. There is a concentration - efficacy and concentration - toxicity relationship. These arguments are in favour of a TDM but the therapeutic range is not clearly established. Potentially fatal side effects can be caused by felbamate (aplastic anemia, acute liver failure), which limits its use because they are dose-independant.

Emerging evidence of the impact of kidney disease on drug metabolism and transport

Several lines of emerging evidence indicate that kidney disease differentially affects uptake and efflux transporters and metabolic enzymes in the liver and gastrointestinal (GI) tract, and uremic toxins have been implicated as the cause. In patients with kidney disease, even drugs that are eliminated by nonrenal transport and metabolism could lead to important unintended consequences if they are administered without dose adjustment for reduced renal function. This is particularly so in the case of drugs with narrow therapeutic windows and may translate into clinically significant variations in exposure and response.

Use of Antiepileptic Drugs in Patients with Kidney Disease

The number of medications used to treat different types of seizures has increased over the last 10-15 years. Most of the newer antiepileptic drugs (AEDs) are likely to be unfamiliar to many nephrologists. For both the older and newer AEDs, basic pharmacokinetic information, recommendations for drug dosing in patients with reduced kidney function or who are on dialysis, and adverse renal and fluid-electrolyte effects are reviewed. Newer AEDs are less likely to have significant drug-drug interactions than older agents, but are more likely to need dosage adjustment in patients with reduced kidney function. The most common renal toxicities of these drugs include metabolic acidosis, hyponatremia, and nephrolithiasis; interstitial nephritis and other adverse effects are less common. Little is known about the clearance of most of the newer AEDs with high-efficiency hemodialyzers or with peritoneal dialysis. Monitoring of drug levels when available, careful clinical assessment of patients taking AEDs, and close collaboration with neurologists is essential to the management of patients taking AEDs.

Pharmacokinetic Variability of Newer Antiepileptic Drugs

A new generation of antiepileptic drugs (AEDs) has reached the market in recent years with ten new compounds: felbamate, gabapentin, lamotrigine, levetiracetam, oxcarbazepine, pregabalin, tiagabine, topiramate, vigabatrin and zonisamide. The newer AEDs in general have more predictable pharmacokinetics than older AEDs such as phenytoin, carbamazepine and valproic acid (valproate sodium), which have a pronounced inter-individual variability in their pharmacokinetics and a narrow therapeutic range. For these older drugs it has been common practice to adjust the dosage to achieve a serum drug concentration within a predefined 'therapeutic range', representing an interval where most patients are expected to show an optimal response. However, such ranges must be interpreted with caution, since many patients are optimally treated when they have serum concentrations below or above the suggested range. It is often said that there is less need for therapeutic drug monitoring (TDM) with the newer AEDs, although this is partially based on the lack of documented correlation between serum concentration and drug effects. Nevertheless, TDM may be useful despite the shortcomings of existing therapeutic ranges, by utilisation of the concept of 'individual reference concentrations' based on intra-individual comparisons of drug serum concentrations. With this concept, TDM may be indicated regardless of the existence or lack of a well-defined therapeutic range. The ten newer AEDs all have different pharmacological properties, and therefore, the usefulness of TDM for these drugs has to be assessed individually. For vigabatrin, a clear relationship between drug concentration and clinical effect cannot be expected because of its unique mode of action. Therefore, TDM of vigabatrin is mainly to check compliance. The mode of action of the other new AEDs would not preclude the applicability of TDM. For the prodrug oxcarbazepine, TDM is also useful, since the active metabolite licarbazepine is measured. For drugs that are eliminated renally completely unchanged (gabapentin, pregabalin and vigabatrin) or mainly unchanged (levetiracetam and topiramate), the pharmacokinetic variability is less pronounced and more predictable. However, the dose-dependent absorption of gabapentin increases its pharmacokinetic variability. Drug interactions can affect topiramate concentrations markedly, and individual factors such as age, pregnancy and renal function will contribute to the pharmacokinetic variability of all renally eliminated AEDs. For those of the newer AEDs that are metabolised (felbamate, lamotrigine, oxcarbazepine, tiagabine and zonisamide), pharmacokinetic variability is just as relevant as for many of the older AEDs. Therefore, TDM is likely to be useful in many clinical settings for the newer AEDs. The purpose of the present review is to discuss individually the potential value of TDM of these newer AEDs, with emphasis on pharmacokinetic variability.

Drug dosing in chronic kidney disease

Patients with chronic kidney disease (CKD) are at high risk for adverse drug reactions and drug-drug interactions. Drug dosing in these patients often proves to be a difficult task. Renal dysfunction-induced changes in human pathophysiology regularly results may alter medication pharmacodynamics and handling. Several pharmacokinetic parameters are adversely affected by CKD, secondary to a reduced oral absorption and glomerular filtration; altered tubular secretion; and reabsorption and changes in intestinal, hepatic, and renal metabolism. In general, drug dosing can be accomplished by multiple methods; however, the most common recommendations are often to reduce the dose or expand the dosing interval, or use both methods simultaneously. Some medications need to be avoided all together in CKD either because of lack of efficacy or increased risk of toxicity. Nevertheless, specific recommendations are available for dosing of certain medications and are an important resource, because most are based on clinical or pharmacokinetic trials.

Hepatic drug metabolism and transport in patients with kidney disease

The disposition of many drugs is altered in patients with acute (AKD) and chronic kidney disease (CKD). A decline in renal clearance of several drugs has been correlated significantly with residual renal function (ie, creatinine clearance) of subjects. Reductions in nonrenal clearance of some compounds also have been reported and associated with clearance of markers of oxidative and/or conjugative metabolism or P-glycoprotein-mediated transport. Although initial accounts of reduced hepatic microsomal cytochrome P-450 (CYP) content and activity in animal models of AKD and CKD were published almost 25 years ago, it is only in the last decade that technical advances in molecular biology and clinical pharmacology have enabled researchers to begin to characterize the phenotypic expression of individual enzymes and, importantly, distinguish the molecular and/or genetic basis for these changes. The selective modulation of hepatic CYP enzyme activity observed in kidney disease is caused, at least in part, by differentially altered expression of several CYP isoforms. This review summarizes data available through June 2003 regarding the effect of AKD and CKD on drug metabolism. Knowledge of the impact and nature of these alterations associated with kidney disease may facilitate the individualization of medication management in this patient population.

Therapeutic drug monitoring of the newer antiepileptic drugs

The aim of the present review is to discuss the potential value of therapeutic drug monitoring (TDM) of the newer antiepileptic drugs (AEDs) felbamate, gabapentin, lamotrigine, levetiracetam, oxcarbazepine, tiagabine, topiramate, vigabatrin, and zonisamide. Studies of the relationship between serum concentrations and clinical efficacy of these drugs are reviewed, and the potential value of TDM of the drugs is discussed based on their pharmacokinetic properties and mode of action. Analytical methods for the determination of the serum concentrations of these drugs are also briefly described. There are only some prospective data on the serum concentration-effect relationships, and few studies have been designed primarily to study these relationships. As TDM is not widely practiced for the newer AEDs, there are no generally accepted target ranges for any of these drugs, and for most a wide range in serum concentration is associated with clinical efficacy. Furthermore, a considerable overlap in drug concentrations related to toxicity and nonresponse is reported. Nevertheless, the current tentative target ranges for felbamate, gabapentin, lamotrigine, levetiracetam, oxcarbazepine (10-hydroxy-carbazepine metabolite), tiagabine, topiramate, vigabatrin, and zonisamide are 125 to 250 micromol/L, 70 to 120 micromol/L, 10 to 60 micromol/L, 35 to 120 micromol/L, 50 to 140 micomol/L, 50 to 250 nmol/L, 15 to 60 micromol/L, 6 to 278 micromol/L, and 45 to 180 micromol/L, respectively. Further systematic studies designed specifically to evaluate concentration-effect relationships of the new AEDs are urgently needed. Although routine monitoring in general cannot be recommended at present, measurements of some of the drugs is undoubtedly of help with individualization of treatment in selected cases in a particular clinical setting.

Coadministration of phenytoin and felbamate: evidence of additional phenytoin dose-reduction requirements based on pharmacokinetics and tolerability with increasing doses of felbamate

PURPOSE

This open-label study investigated the pharmacokinetic interaction of phenytoin (PHT) and felbamate (FBM).

METHODS

Ten subjects with epilepsy receiving PHT monotherapy were administered increasing doses of FBM (1,200, 1,800, 2,400-3,600 mg/day) at 2-week intervals. PHT doses were reduced by 20% on an individual basis when evidence of clinically significant intolerance was present. With intolerance, the PHT dose was reduced before the next incremental FBM dose. Blood samples were analyzed for FBM, PHT, and PHT metabolite 5-(4-hydroxyphenyl)-5-phenylhydantoin (HPPH).

RESULTS

Total PHT plasma concentrations increased with coadministered FBM. PHT Cmax increased from 15.9 microg/ml at baseline to 20.9 microg/ml after 1,200 mg/day FBM and to 26.8 microg/ml after 1,800 mg/day FBM. Four subjects required a 20% PHT dose reduction after 1,800 mg/day FBM and six after the administration of 2,400 mg/day FBM. All subjects required further 20% PHT reductions before 3,600 mg/day FBM. FBM Cmax and AUCT were reduced, and apparent clearance increased compared with data from FBM monotherapy.

CONCLUSIONS

With the initiation of FBM therapy in subjects receiving PHT, the PHT dosage should be reduced by 20%. Further PHT dose reductions are likely to be necessary if the FBM dose is increased. The requirements for reductions in dose might be predicted by clinical signs of PHT intolerance.