The Effect of Age on Pharmacokinetics of Antiepileptic Drugs

Summary: Management of epilepsy in the elderly requires understanding of the unique biochemical and pharmacologic characteristics of this patient population. Accurate assessment of seizures and identification of epilepsy syndromes, thorough neurologic assessment to define etiology, and comprehensive evaluation of the patient's health and living situation are necessary for informed management decisions. Challenges to treatment include concomitant diseases, polypharmacy with accompanying drug interactions, and changes in physiology, such as changes in renal clearance and hepatic function that alter drug absorption, protein binding, metabolism, and elimination. Elderly patients with declining intellectual function, motor impairment, or altered sensory function may be especially susceptible to dose-related CNS side effects of antiepileptic drugs (AEDs). Drugs pre-scribed for concomitant illnesses such as hypertension, cardiovascular disease, infections, behavioral problems, and gastrointestinal disturbances may alter absorption, distribution, and metabolism of AEDs, with an adverse impact on efficacy and increased occurrence of adverse effects. The AEDs may induce metabolism of other drugs, resulting in decline in target response. Addition of an AED to an elderly patient's medical regimen requires careful review of all prescribed drugs. Optimal care of elderly patients with epilepsy includes use of free drug levels to monitor AED concentrations, careful dose selection, and sensitivity to the social problems that may occur in this population.     

Clinically important drug interactions in epilepsy: interactions between antiepileptic drugs and other drugs

Antiepileptic drugs (AEDs) are commonly prescribed for long periods, up to a lifetime, and many patients will require treatment with other agents for the management of concomitant or intercurrent conditions. When two or more drugs are prescribed together, clinically important interactions can occur. Among old-generation AEDs, carbamazepine, phenytoin, phenobarbital, and primidone are potent inducers of hepatic enzymes, and decrease the plasma concentration of many psychotropic, immunosuppressant, antineoplastic, antimicrobial, and cardiovascular drugs, as well as oral contraceptive steroids. Most new generation AEDs do not have clinically important enzyme inducing effects. Other drugs can affect the pharmacokinetics of AEDs; examples include the stimulation of lamotrigine metabolism by oral contraceptive steroids and the inhibition of carbamazepine metabolism by certain macrolide antibiotics, antifungals, verapamil, diltiazem, and isoniazid. Careful monitoring of clinical response is recommended whenever a drug is added or removed from a patient's AED regimen.     

Differential effect of cimetidine on serum concentrations of carbamazepine and phenytoin

Cimetidine is an inhibitor of drug metabolism. We studied the effects of cimetidine (1,200 mg/d) on steady-state serum concentrations of carbamazepine and phenytoin in 11 epileptic volunteers. The mean serum carbamazepine concentration was unchanged after 7 days of cimetidine treatment. Five subjects were taking phenytoin concurrently; their mean serum phenytoin concentration was significantly increased after 7 and 10 days of cimetidine treatment and returned to baseline 2 weeks after cimetidine was discontinued. Cimetidine apparently inhibits the clearance of phenytoin but not of carbamazepine in adults on chronic therapy.     

Antiepileptic Drug Interactions

Summary: This article reviews the potential interactions of antiepileptic drugs (AEDs) and the pharmacokinetic and pharmacodynamic principles involved. It describes the absorptive and distributive properties of AEDs and the effects on protein binding, hepatic metabolism, and elimination resulting from co-administration of AEDs with food or other drugs. Drug behavior is a function of absorption, metabolism, distribution, and elimination. Administration of either multiple AEDs or a combination of AEDs plus drugs for other conditions can modify any of these physiologic processes, possibly resulting in complex interactions. These may include alterations in the bioavailability and absorption of a drug and changes in half-life and serum level through induction or inhibition of hepatic metabolism. In most cases, increases or decreases in serum concentrations will signal a drug interaction. In other cases, clinically significant drug interactions remain undetected owing to apparently stable serum concentrations. Co-administration of drugs may affect the rate of clearance of one or both drugs. The effect on clearance varies, owing to genetic factors, patient characteristics (age and presence of co-morbidities), and individual responses. AEDs that induce hepatic metabolism can also influence the metabolism of concomitantly administered non-epilepsy medications and can interfere with oral contraceptives, as well as vitamins D and K. Patients with renal insufficiency or advanced age may experience incomplete renal excretion and should receive reduced dosages of drug. Understanding the pharmacokinetics and pharmacodynamic properties of AEDs and the route of metabolism of all competing drugs is important for optimal management of patients with epilepsy and for prevention of avoidable drug interactions.     

Water intoxication in epileptic patients receiving carbamazepine.

Plasma sodium and osmolality were determined in 80 adult epileptic patients receiving chronic treatment with carbamazepine and in 50 control patients treated with other anticonvulsant drugs. Mean plasma osmolality was significantly lower in the carbamazepine-treated patients but mean plasma sodium did not differ in the two groups. Hyponatraemia was found in five of the carbamazine-treated patients and hypo-osmolality in six. None of the control patients had hyponatraemia and only one had a borderline low osmolality. Three of the 13 patients receiving carbamazepine alone were hyponatraemic. Plasma sodium concentration correlated negatively with both daily carbamazepine dose and serum carbamazepine level. Free water clearance after an oral water load was determined in six patients on carbamazepine alone and in six normal subjects not receiving drug therapy. The capacity of some of the patients to excrete the water load was found to be grossly impaired.     

Pharmacotherapy of epilepsy

Antiepileptic drug treatment is essential and provides excellent therapeutic effects in more than the two-third of the epileptic patients. The antiepileptic drugs influence the chronic hyperexcitability of the brain developed during the epileptogenesis. As an effect, it decreases the excitability and/or increases the inhibition of the pathological cells, which prevents the precipitation of the epileptic seizure (anticonvulsive effect). The anticonvulsive effect comes into operation by the influence of the transport of one ore more ion-channels. The anticonvulsive effect is only symptomatic and it doesn't cure the disorder. The drug selection is based on the knowledge of the therapeutic markers and the effectiveness of the drug to be used. This can occur on the basis of the action of the drug or in syndrome-specific way. The pharmacokinetic properties of the drugs determine how they can be used in the practice. The drug interactions can take place in several levels. Among them, the change of the metabolism is the most important. Acute dose-dependent side effects, organ-specific chronic interactions and idiosyncratic reactions must be taken into consideration during the use of antiepileptic drugs. The patient's individual aspects must be considerably taken into account during the treatment. There are other medical areas that can benefit from the antiepileptic drugs. Among them, the most important diseases are: restless legs syndrome, neuropathic pain, trigeminal neuralgia, essential tremor, bulimia and bipolar disorders. There are other pharmacological (adrenocorticotropic hormone, immunoglobulins, neurosteroids) and dietary methods, which may be effective at certain epileptic syndromes. The principles of the pharmacotherapy have been changing continuously during the past decades and since. New drugs have been introduced into the marketing and new expectations are coming into the limelight concerning the treatment. As a consequence this will bring on the modification of antiepileptic drug therapeutic habits.     

Pharmacokinetic behaviour of carbamazepine and its main metabolite-10, 11 epoxide of carbamazepine in monotherapy or in combination with other anti-epileptic drugs

A combination of anti-epileptic drugs is used for the necessary control of seizures. This results in a modulation of the metabolism in the epileptic patient and a concentration of the drugs and their metabolites in serum and in the brain, the target organ. Carbamazepine-10,11 epoxide (CBZE) is the main active metabolite of carbamazepine (CBZ). We have studied their interrelationship in concentrations in the plasma of 68 patients receiving CBZ, either as monotherapy or in combination with phenytoin (PHT) and phenobarbital (PB). The rate of CBZ metabolism was modulated in drug co-administration, which, depending on the grade of the induction of cytochrome p-450, decreases or increases the concentration of CBZE. A graph plotting the relationship between CBZ and CBZE concentrations in patients stabilized on a regime of CBZ alone is linear. The ratio of concentrations of CBZE/CBZ in serum is 0.12 when CBZ is administered as monotherapy, rising to 0.14 (CBZ + PB), 0.18 (CBZ + PHT) and 0.25 (CBZ + PHT + PB) when administered with the other drugs mentioned. From this it can be hypothesized that the additive of induction activities of PHT and PB operates on the mixed function oxidase system.     

Antiepileptic drugs and the immune system

Data on the effects of antiepileptic drugs on the immune system are frequently inconsistent and sometimes conflicting because the effects of drugs cannot be separated from those of seizures, first-generation drugs have been most intensively investigated, the patient's genetic background, the mechanism of action and the pharmacokinetic profile of AEDs and the concurrent use of immunosuppressant drugs may act as confounders. Valproate, carbamazepine, phenytoin, vigabatrin, levetiracetam, and diazepam have been found to modulate the immune system activity by affecting humoral and cellular immunity. AEDs are associated with pharmacokinetic interactions (most frequently occurring with carbamazepine, phenytoin, phenobarbital and valproate). Hepatic metabolism is the primary site of interaction for both AEDs and immunotherapies (ACTH, dexamethasone, hydrocortisone, methylprednisolone, cyclophosphamide, methotrexate, rituximab), which entail induction or inhibition of drug effects. However, the clinical importance of these drug interactions is still far from defined. An important adverse effect of the action of AEDs on the immune system is antiepileptic hypersensitivity syndrome (AHS), a life-threatening, idiosyncratic cutaneous reaction to aromatic AEDs resulting in end organ damage. Phenytoin, carbamazepine, phenobarbital, lamotrigine, oxcarbazepine, felbamate, and zonisamide have been implicated. The pathogenic mechanisms of AHS are incompletely understood.     

Interactions between antiepileptic drugs,and between antiepileptic drugs and other drugs

Interactions between antiepileptic drugs, or between antiepileptic drugs and other drugs, can be pharmacokinetic or pharmacodynamic in nature. Pharmacokinetic interactions involve changes in absorption, distribution or elimination, whereas pharmacodynamic interactions involve synergism and antagonism at the site of action. Most clinically important interactions of antiepileptic drugs result from induction or inhibition of drug metabolism. Carbamazepine, phenytoin, phenobarbital and primidone are strong inducers of cytochrome P450 and glucuronizing enzymes (as well as P‐glycoprotein) and can reduce the efficacy of co‐administered medications such as oral anticoagulants, calcium antagonists, steroids, antimicrobial and antineoplastic drugs through this mechanism. Oxcarbazepine, eslicarbazepine acetate, felbamate, rufinamide, topiramate (at doses ≥200 mg/day) and perampanel (at doses ≥8 mg/day) have weaker inducing properties, and a lower propensity to cause interactions mediated by enzyme induction. Unlike enzyme induction, enzyme inhibition results in decreased metabolic clearance of the affected drug, the serum concentration of which may increase leading to toxic effects. Examples of important interactions mediated by enzyme inhibition include the increase in the serum concentration of phenobarbital and lamotrigine caused by valproic acid. There are also interactions whereby other drugs induce or inhibit the metabolism of antiepileptic drugs, examples being the increase in serum carbamazepine concentration by erythromycin, and the decrease in serum lamotrigine concentration by oestrogen‐containing contraceptives. Pharmacodynamic interactions between antiepileptic drugs may also be clinically important. These interactions can have potentially beneficial effects, such as the therapeutic synergism of valproic acid combined with lamotrigine, or adverse effects, such as the reciprocal potentiation of neurotoxicity observed in patients treated with a combination of sodium channel blocking antiepileptic drugs.     

Effects of common anti-epileptic drug monotherapy on serum levels of homocysteine, vitamin B12, folic acid and vitamin B6.

There is emerging evidence to support the unfavorable effects of some anti-epileptic drugs on the plasma homocysteine concentrations. Elevated homocysteine levels induced by anti-epileptic drug administration can theoretically increase not only the risk of vascular occlusive diseases, but also the risk of resistance to anti-epileptics and development of refractory epilepsy. To investigate the effect of common anti-epileptic drugs on the homocysteine metabolism, a total of 75 epileptic patients receiving phenytoin (n=16), carbamazepine (n=19), or valproic acid (n=22) and no anti-epileptic drug (n=18) were enrolled. Eleven age- and sex-matched healthy subjects served as the control group. Blood concentrations of homocysteine, folic acid, Vitamin B12 and pyridoxal 5'-phosphate (active circulating form of Vitamin B6) were measured. Compared to the control group, epileptic patients on anti-epileptic drug had higher blood levels of homocysteine. No difference in homocysteine concentrations was observed among epileptic patients in terms of the anti-epileptic drug used. Patients receiving phenytoin had significantly lower folic acid levels and those receiving carbamazepine had marginally lower pyridoxal 5'-phosphate levels in comparison with those using other anti-epileptic drugs. A negative correlation between homocysteine and folic acid concentrations was detected in epileptic patients on anti-epileptic drug. The duration of anti-epileptic drug use was correlated to the decrease of folic acid levels, but not with changes observed in homocysteine, Vitamin B12 and pyridoxal 5'-phosphate levels. No relationship between seizure frequency and homocysteine levels was observed in epileptic patients. Our results confirm that common anti-epileptic drugs has disadvantageous effects on homocysteine status. Because there was no significant change in homocysteine concentrations in epileptic patients who were not receiving an anti-epileptic drug, and no positive correlation between seizure frequency and homocysteine levels, we suggest that increase of homocysteine levels may be due to anti-epileptic drug use, rather than being epileptic in origin. Additionally, the underlying mechanism for homocysteine increase seems to be a decrease of cofactor molecules in patients using carbamazepine and phenytoin (pyridoxal 5'-phosphate and folic acid, respectively). However, changes observed are not related to the alteration in the levels of cofactors and remain unclear in the patients using valproic acid.     

Does carbamazepine cause disturbances in calcium metabolism in epileptic patients?

ABSTRACT Calcium metabolism was examined in 30 adult epileptic outpatients on carbamazepine monotherapy. The patients had a normal bone mass, evaluated both on the forearm (100 ± 13 of normal) and on the total skeleton (102 ± 15), and normal serum concentrations of 250HD. The serum calcium was decreased (P < 0.001) and the serum alkaline phosphatase increased (P < 0.001). The clinical significance of our study is that monotherapy with carbamazepine does not have the side effects on bone metabolism known as “anticonvulsant osteomalacia”. Our results further question the connection between liver enzyme induction and anticonvulsant osteomalacia, since carbamazepine possesses the same potency of liver enzyme induction as phenytoin. Further studies on epileptic outpatients will be necessary in order to elucidate the connection between treatment with anticonvulsant drugs and anticonvulsant osteomalacia.     

The importance of drug interactions in epilepsy therapy

Long-term antiepileptic drug (AED) therapy is the reality for the majority of patients diagnosed with epilepsy. One AED will usually be sufficient to control seizures effectively, but a significant proportion of patients will need to receive a multiple AED regimen. Furthermore, polytherapy may be necessary for the treatment of concomitant disease. The fact that over-the-counter drugs and nutritional supplements are increasingly being self-administered by patients also must be considered. Therefore the probability of patients with epilepsy experiencing drug interactions is high, particularly with the traditional AEDs, which are highly prone to drug interactions. Physicians prescribing AEDs to patients with epilepsy must, therefore, be aware of the potential for drug interactions and the effects (pharmacokinetic and pharmacodynamic) that can occur both during combination therapy and on drug discontinuation. Although pharmacokinetic interactions are numerous and well described, pharmacodynamic interactions are few and usually concluded by default. Perhaps the most clinically significant pharmacodynamic interaction is that of lamotrigine (LTG) and valproic acid (VPA); these drugs exhibit synergistic efficacy when coadministered in patients with refractory partial and generalised seizures. Hepatic metabolism is often the target for pharmacokinetic drug interactions, and enzyme-inducing drugs such as phenytoin (PHT), phenobarbitone (PB), and carbamazepine (CBZ) will readily enhance the metabolism of other AEDs [e.g., LTG, topiramate (TPM), and tiagabine (TGB)]. The enzyme-inducing AEDs also enhance the metabolism of many other drugs (e.g., oral contraceptives, antidepressants, and warfarin) so that therapeutic efficacy of coadministered drugs is lost unless the dosage is increased. VPA inhibits the metabolism of PB and LTG, resulting in an elevation in the plasma concentrations of the inhibited drugs and consequently an increased risk of toxicity. The inhibition of the metabolism of CBZ by VPA results in an elevation of the metabolite CBZ-epoxide, which also increases the risk of toxicity. Other examples include the inhibition of PHT and CBZ metabolism by cimetidine and CBZ metabolism by erythromycin. In recent years, a more rational approach has been taken with regard to metabolic drug interactions because of our enhanced understanding of the cytochrome P450 system that is responsible for the metabolism of many drugs, including AEDs. The review briefly discusses the mechanisms of drug interactions and then proceeds to highlight some of the more clinically relevant drug interactions between AEDs and between AEDs and non-AEDs. Understanding the fundamental principles that contribute to a drug interaction may help the physician to better anticipate a drug interaction and allow a graded and planned therapeutic response and, therefore, help to enhance the management of patients with epilepsy who may require treatment with polytherapy regimens.     

Serum elevation of high density lipoprotein (HDL) cholesterol in epileptic patients taking carbamazepine or phenytoin

Serum lipids and the cholesterol concentration in the high density lipoprotein (HDL) fraction were measured in epileptic patients taking either carbamazepine or phenytoin as single drug treatment. There was a significant increase in HDL-cholesterol levels in patients taking both phenytoin and carbamazepine. Serum total-cholesterol concentrations in patients taking both drugs did not differ significantly from those of controls. The ratio of HDL-cholesterol/total cholesterol was increased in both drugs and the increase reached significance in patients taking phenytoin. There was a significant increase in serum triglycerides in females taking carbamazepine.     

Seizure exacerbation and status epileptics related to carbamazepine-10,11-epoxide

Over a 3-year period, we encountered 6 adults whose seizure control unexpectedly deteriorated with the occurrence of partial status epilepticus and daily multiple seizures. Analysis of the case histories and subsequent clinical follow-up for 1 1/2 to 3 years disclosed the following evidence that demonstrates the role of carbamazepine-epoxide in the development of the seizure exacerbation: (1) There were high serum carbamazepine-epoxide concentrations while serum carbamazepine concentrations were lower than or the same as baseline levels; (2) all patients were taking drugs that are known to increase serum carbamazepine-epoxide concentrations; (3) status epilepticus failed to respond to intravenous phenytoin loading; (4) seizure exacerbation in all patients was corrected by withholding carbamazepine dose; (5) seizure exacerbation recurred in 1 patient who resumed the same dose of carbamazepine; and (6) there were no prior status epilepticus or daily multiple seizures despite previous toxicities with other antiepileptic drugs in 3 patients. Our experience shows that inconspicuous elevation of carbamazepine-epoxide levels during polytherapy may precipitate a distinct state of drug toxicity characterized by severe exacerbation of seizures. Mental retardation may be a predisposition to this condition.     

Possible roles for frequent salivary antiepileptic drug monitoring in the management of epilepsy

Salivary levels of phenytoin, phenobarbitone, carbamazepine and carbamazepine-epoxide correlate with the simultaneous plasma water levels of these substances, after correcting for the effects of pH differences between saliva and plasma in the case of phenobarbitone. Saliva is easy and painless to collect, and salivary levels of the drugs are conveniently measured. Frequent (often daily) monitoring of pre-dose morning anticonvulsant drug concentrations in saliva over periods of weeks or months in 3 groups of epileptic subjects showed that (i) in some but not all poorly controlled epileptic patients seizures tended to occur on days when salivary anticonvulsant levels were lower than on non-seizure days, (ii) in such subjects it was possible to estimate an anticipated optimal drug concentration and dose to minimize seizure activity from the plot of seizure frequency against drug concentrations, (iii) in women with 'catamenial' epilepsy, salivary anticonvulsant levels were lower on perimenstrual days than at mid-cycle in half of the subjects studied, and (iv) in pregnant epileptic women the time course of the change in drug levels relative to dose could be followed more closely throughout pregnancy and the post-natal period than was practicable when using blood level measurements. Frequent measurement of salivary anticonvulsant concentrations appears a promising and inexpensive adjunct to the investigation and management of certain problem areas in epilepsy.     

Effect of levetiracetam on the pharmacokinetics of adjunctive antiepileptic drugs: a pooled analysis of data from randomized clinical trials

The purpose of this study was to determine the influence of levetiracetam on the steady-state serum concentrations of other commonly used antiepileptic drugs (AEDs). Serum AED concentrations were measured at baseline and after adjunctive therapy with levetiracetam (1000-4000 mg/day) or placebo in four phase III trials in patients with refractory partial epilepsy receiving stable AED dosages. The data were pooled, and repeated measures covariance analysis was used to calculate the ratio (and 90% confidence intervals) of the geometric mean serum drug concentrations during adjunctive levetiracetam therapy relative to baseline. Levetiracetam did not increase or decrease mean steady-state serum concentrations of carbamazepine, phenytoin, valproic acid, lamotrigine, gabapentin, phenobarbital, or primidone. For each of these AEDs, the 90% confidence interval of the geometric mean drug concentrations ratio was included within the 80-125% bioequivalence range. Serum concentrations of these AEDs did not change over time after adjunctive levetiracetam therapy, irrespective of the dosage of levetiracetam used. For vigabatrin, there was no evidence for a significant change in serum drug concentration after the addition of levetiracetam, but the number of observations was too small for the limits of the confidence interval to fall within the 80-125% range. Thus, adjunctive therapy with levetiracetam does not influence the steady-state serum concentrations of concomitantly administered carbamazepine, phenytoin, valproic acid, lamotrigine, gabapentin, phenobarbital, or primidone. Consequently, no need for adjusting the dosages of these AEDs is anticipated when levetiracetam is added on or removed from a patient's therapeutic regimen.     

Clinically important drug interactions in epilepsy: general features and interactions between antiepileptic drugs

There are two types of interactions between drugs, pharmacokinetic and pharmacodynamic. For antiepileptic drugs (AEDs), pharmacokinetic interactions are the most notable type, but pharmacodynamic interactions involving reciprocal potentiation of pharmacological effects at the site of action are also important. By far the most important pharmacokinetic interactions are those involving cytochrome P450 isoenzymes in hepatic metabolism. Among old generation AEDs, carbamazepine, phenytoin, phenobarbital, and primidone induce the activity of several enzymes involved in drug metabolism, leading to decreased plasma concentration and reduced pharmacological effect of drugs, which are substrates of the same enzymes (eg, tiagabine, valproic acid, lamotrigine, and topiramate). In contrast, the new AEDs gabapentin, lamotrigine, levetiracetam, tiagabine, topiramate, vigabatrin, and zonisamide do not induce the metabolism of other AEDs. Interactions involving enzyme inhibition include the increase in plasma concentrations of lamotrigine and phenobarbital caused by valproic acid. Among AEDs, the least potential interaction is associated with gabapentin and levetiracetam.     

Hair analysis differentiates chronic from acute carbamazepine intoxication

This is a report of a 12-year-old epileptic child undergoing chronic treatment with carbamazepine who was found comatose. He was considered to have acute severe drug toxicity. Measurement of carbamazepine concentration in the patient's hair segments together with the carbamazepine blood levels were both important in determining the chronic nature of the patient's intoxication.     

Anticonvulsant peripheral neuropathy: a clinical and electrophysiological study of patients on single drug treatment with phenytoin, carbamazepine or barbiturates.

Previous studies of phenytoin neuropathy in selected groups of chronic epileptic patients on polytherapy have indicated a widely varying incidence of clinical or electrophysiological abnormalities. In 51 previously untreated epileptic patients followed prospectively on phenytoin or carbamazepine monotherapy, assisted by blood level monitoring, for 1-5 years we found no clinical evidence of neuropathy. Eighteen per cent of the phenytoin group and none of the carbamazepine group had mild electrophysiological changes (abnormalities of sensory action potentials or sensory conduction). In the former group the occurrence of the electrophysiological abnormalities was possibly related to previous exposure to high phenytoin or low folate levels or both. In 10 chronic epileptic patients we demonstrated reversible slowing of sensory nerve conduction during phenytoin intoxication. In six selected epileptic patients on chronic barbiturate monotherapy we found clinical evidence of neuropathy in two and electrophysiological abnormalities in five, including reversible slowing of sensory conduction during intoxication in one. This suggests that barbiturate drugs may, like phenytoin, also contribute to anticonvulsant neuropathy. Careful monitoring of single drug therapy with avoidance of acute toxicity may reduce the risk of chronic anticonvulsant neuropathy.