Plasma protein binding of phenytoin in the aged: in vivo studies

Phenytoin versus fosphenytoin for second-line treatment of status epilepticus: propensity score matching analysis using a nationwide inpatient database

PURPOSE

For status epilepticus, the choice of antiepileptic drugs for second-line treatment after benzodiazepine remains controversial: phenytoin or fosphenytoin are recommended, however, it has been unknown which is better. Using a nationwide database, we compared the efficacy and safety of them.

METHOD

An observational study conducted with the Japanese Diagnosis Procedure Combination inpatient database identified adult patients who had been admitted for status epilepticus and who had received intravenous diazepam on the day of admission from January 1, 2011 through December 31, 2015. Propensity score matching was applied to compare outcomes of the phenytoin and fosphenytoin groups.

RESULTS

The analysis examined data of 5265 patients: 2969 patients received phenytoin; 2296 received fosphenytoin, on the day of admission. One-to-one propensity score matching created 1871 matched pairs. No significant difference was found for vasopressor use on the day of admission (4.2 % vs. 4.4 %; odds ratio 1.07; 95 % confidence intervals 0.77-1.48; p = 0.69), or for mechanical ventilation on the day of admission, in-hospital mortality, length of hospital stay, or total hospitalization cost. Higher age, comorbidity of cardiac diseases and lower body mass index were associated significantly with increased vasopressor use, whereas the dose of phenytoin equivalents and the choice of fosphenytoin were not.

CONCLUSIONS

This nationwide observational study found no evidence that fosphenytoin provides higher efficacy or safety than phenytoin for treatment of status epilepticus in adults after diazepam. Age, cardiac disease and low body mass index were identified as independent risk factors for vasopressor use in both phenytoin and fosphenytoin.

Challenges and innovations of drug delivery in older age

Both drug delivery performance and various age-related physical, mental and physiological changes can affect drug effectiveness and safety in elderly patients. The many drug delivery systems developed over the years include recent novel transdermal, nasal, pulmonary and orally disintegrating tablets that provide consistent, precise, timely and more targeted drug delivery. Certain drug delivery systems may be associated with suboptimal outcomes in the elderly because of the nature of drug present, a lack of appreciation of the impact of age-related changes in drug absorption, distribution and clearance, the limited availability of pharmacokinetic, safety and clinical data. Polypharmacy, patient morbidity and poor adherence can also contribute to sub-optimal drug delivery systems outcomes in the elderly. The development of drug delivery systems for the elderly is a poorly realised opportunity, with each system having specific advantages and limitations. A key challenge is to provide the innovation that best meets the specific physiological, psychological and multiple drug requirements of individual elderly patients.

The quantitative effect of serum albumin, serum urea, and valproic acid on unbound phenytoin concentrations in children

Dosing of phenytoin is difficult in children because of its variable pharmacokinetics and protein binding. Possible covariates for this protein binding have mostly been univariately investigated in small, and often adult, adult populations. We conducted a study to identify and quantify these covariates in children. We extracted data on serum phenytoin concentrations, albumin, triglycerides, urea, total bilirubin and creatinine concentrations and data on coadministration of valproic acid or carbamazepine in 186 children. Using nonlinear mixed effects modeling the effects of covariates on the unbound phenytoin fraction were investigated. Serum albumin, serum urea concentrations, and concomitant valproic acid use significantly influenced the unbound phenytoin fraction. For clinical practice, we recommend that unbound phenytoin concentrations are measured routinely. However, if this is impossible, we suggest to use our model to calculate the unbound concentration. In selected children, close treatment monitoring and dose reductions should be considered to prevent toxicity.

Pharmacokinetics of Antiepileptic Drugs in Elderly Nursing Home Residents

With approximately 10% of elderly nursing home residents taking antiepileptic drugs (AEDs), it is critical to understand the pharmacokinetics, dosing, and possible adverse reactions of these AEDs. In this chapter, five AEDs commonly prescribed to nursing home residents will be discussed. Phenytoin (PHT), the most commonly used AED in this population, is extensively metabolized by the cytochrome P450 enzyme system, is highly protein bound, and interacts with many concomitant medications. Up to 45% of nursing home residents who receive PHT have concentrations below the range (subtherapeutic) used in adults (<65 years), while approximately 10% of residents have concentrations that are potentially toxic (>20 microg/ml). In addition, serum PHT concentrations can vary greatly within an individual resident and may be subtherapeutic one day and potentially toxic the next. Valproic acid is taken by approximately 9-17% of nursing home residents who are administered AEDs, with over half using it for nonseizure indications. Doses are approximately 16 mg/kg/day in elderly nursing home residents, but doses and serum concentrations are lower in the oldest age group (> or =85 years). A majority of residents are maintained at serum concentrations considered subtherapeutic for epilepsy, whereas relatively few (approximately 3%) are maintained at toxic levels. The average (+/-SD) carbamazepine (CBZ) dose is 8.8 +/- 4.7 mg/kg/day, yielding a mean serum concentration of 6.3 +/- 2.2 mg/liter. Subtherapeutic concentrations are found in up to 20% of serum measurements, while 2.5% of serum measurements are in the toxic range. CBZ is highly bound to serum albumin and alpha1-acid glycoprotein and is metabolized to carbamazepine-10,11-epoxide, an active metabolite thought to be responsible for some side effects. Phenobarbital (PB) is frequently combined with PHT. This combination can cause devastating side effects because both PB and PHT can produce cognitive side effects. Gabapentin is one of the newer AEDs frequently administered to nursing home residents. Its lack of both hepatic metabolism and protein binding potentially makes it a safer drug in this population.

Clinical evaluation of plasma free phenytoin measurement and factors influencing its protein binding

The relationship between free phenytoin concentrations and clinical responses, and the factors influencing protein binding of phenytoin were investigated. A total of 119 plasma samples from 70 patients treated orally with phenytoin were analysed. The mean free phenytoin concentration was significantly higher in the patients who received phenytoin monotherapy and were classified as having a complete response (1.25 +/- 1.09 microg/ml) than that in the partial response group (0.59 +/- 0.07 microg/ml), whereas the mean total concentrations were not significantly different between the two groups. Samples were divided into three groups based on the free fraction of phenytoin, i.e. low, <5%; medium, 5%-10%; high, > 10%. The mean age (55.3 +/- 10.9 years) was significantly higher in the high group than in the low (42.7 +/- 21.2 years) and medium (42.8 +/- 16.0 years) groups. The mean creatinine clearance (CLcr) (55.3 +/- 10.9 ml/min) and serum albumin concentration (3.30 +/- 1.25 g/dl) were significantly lower in the high group than the low (88.3 +/- 29.0 ml/min and 4.08 +/- 0.50 g/dl, respectively) and medium (95.0 +/- 32.8 ml/min and 3.95 +/- 0.92 g/dl, respectively) groups. These results suggest that the free phenytoin concentration, rather than the total concentration, is more useful for monitoring antiepileptic effects in patients receiving phenytoin monotherapy. It was also found that the free phenytoin fraction was significantly influenced by aging, CLcr and serum albumin levels.

Aging biology and geriatric clinical pharmacology

Population aging evokes doomsday economic and sociological prognostication, despite a minority of older people suffering significant dependency and the potential for advances in therapeutics of age-related disease and primary aging. Biological aging processes are linked mechanistically to altered drug handling, altered physiological reserve, and pharmacodynamic responses. Parenteral loading doses need only be adjusted for body weight as volumes of distribution are little changed, whereas oral loading doses in some cases may require reduction to account for age-related increases in bioavailability. Age-related reduction of hepatic blood flow and hepatocyte mass and primary aging changes in hepatic sinusoidal endothelium with effects on drug transfer and oxygen delivery reduce hepatic drug clearance. Primary renal aging is evident, although renal clearance reduction in older people is predominantly disease-related and is poorly estimated by standard methods. The geriatric dosing axiom, "start low and go slow" is based on pharmacokinetic considerations and concern for adverse drug reactions, not from clinical trial data. In the absence of generalizable dosage guidelines, individualization via effect titration is required. Altered pharmacodynamics are well documented in the cardiovascular system, with changes in the autonomic system, autacoid receptors, drug receptors, and endothelial function to modify baseline cardiovascular tone and responses to stimuli such as postural change and feeding. Adverse drug reactions and polypharmacy represent major linkages to avoidable morbidity and mortality. This, combined with a deficient therapeutic evidence base, suggests that extrapolation of risk-benefit ratios from younger adults to geriatric populations is not necessarily valid. Even so, therapeutic advances generally may convert healthy longevity from an asset of fortunate individuals into a general social benefit.

Influence of aging on serum phenytoin concentrations: a pharmacokinetic analysis based on therapeutic drug monitoring data

The influence of aging on the pharmacokinetics of phenytoin at steady-state was evaluated retrospectically by comparing apparent oral clearance values (CL/F) in 75 patients aged 65-90 years (mean, 71.7 +/- 5.3 years) receiving phenytoin alone (n = 58) or in combination with phenobarbital (n = 17) and in an equal number of control patients aged 20-50 years (mean, 36.7 +/- 8.5 years) matched for gender, body weight, and comedication. All data were derived from the database of the therapeutic drug monitoring service (TDMS) of an academic neurological hospital. On average, elderly patients were found to exhibit slightly higher CL/F values compared with controls (14.6 +/- 4.7 ml h(-1) kg(-1) versus 13.1 +/- 4.2 ml h(-1) kg(-1), P < 0.05), the difference being probably related to the dose-dependent nature of phenytoin metabolism and the fact that elderly patients received lower dosages (4.4 +/- 1.1 mg kg(-1)day(-1) versus 5.3 +/- 1.1 mg kg(-1) day(-1), P < 0.001) and had lower serum phenytoin concentrations (14.1 +/- 5.7 microg ml(-1) versus 18.6 +/- 6.8 microg ml(-1), P < 0.0001). Gender and phenobarbital comedication were not found to exert any statistically significant influence on phenytoin CL/F. By contrast, in the elderly group, CL/F values were negatively correlated with age. On average, CL/F values decreased by about one-third between 65 and 85 years of age, but interindividual variability was considerable and age explained only 7.8% of the variation in CL/F in the elderly group. Overall, these findings indicate that aging is associated with a progressive decline in phenytoin clearance, presumably as a result of decreased drug metabolizing capacity. Because assessment was based on total serum phenytoin concentrations and the unbound fraction of phenytoin is known to decrease in old age, the influence of aging as quantified in this study may underestimate the magnitude of changes in the clearance of unbound, pharmacologically active drug. Based on these data, it is prudent to utilize initially smaller phenytoin dosages in old patients, and to make subsequent dose adjustments based on clinical response and serum drug level measurements. Interpretation of the latter, however, should take into account the possibility of an increase in the fraction of unbound drug.

Fosphenytoin: clinical pharmacokinetics and comparative advantages in the acute treatment of seizures

Fosphenytoin is a phosphate ester prodrug developed as an alternative to intravenous phenytoin for acute treatment of seizures. Advantages include more convenient and rapid intravenous administration, availability for intramuscular injection, and low potential for adverse local reactions at injection sites. Drawbacks include the occurrence of transient paraesthesias and pruritus at rapid infusion rates, and cost. Fosphenytoin is highly bound (93-98%) to plasma proteins. Saturable binding at higher plasma concentrations accounts for an increase in its distribution volume and clearance with increasing dose and infusion rate. Fosphenytoin is entirely eliminated through metabolism to phenytoin by blood and tissue phosphatases. The bioavailability of the derived phenytoin relative to intravenous phenytoin is approximately 100% following intravenous or intramuscular administration. The half-life for conversion of fosphenytoin to phenytoin ranges from 7-15 minutes. Faster intravenous infusion rates and competitive displacement of derived phenytoin from plasma protein binding sites by fosphenytoin compensate for the expected conversion-related delay in appearance of phenytoin in the plasma. Unbound phenytoin plasma concentrations achieved with intravenous fosphenytoin loading doses of 100-150 or 50-100mg phenytoin sodium equivalents/min are comparable, and achieved at similar times, to those with equimolar doses of intravenous phenytoin at 50 (maximum recommended rate) or 20-40 mg/min, respectively. The rapid achievement of effective concentrations permits the use of fosphenytoin in emergency situations, such as status epilepticus. Following intramuscular administration, therapeutic phenytoin plasma concentrations are observed within 30 minutes and maximum plasma concentrations occur at approximately 30 minutes for fosphenytoin and at 2-4 hours for derived phenytoin. Plasma concentration profiles for fosphenytoin and total and unbound phenytoin in infants and children closely approximate those in adults following intravenous or intramuscular fosphenytoin at comparable doses and infusion rates. Earlier and higher unbound phenytoin plasma concentrations, and thus an increase in systemic adverse effects, may occur following intravenous fosphenytoin loading doses in patients with a decreased ability to bind fosphenytoin and phenytoin (renal or hepatic disease, hypoalbuminaemia, the elderly). Close monitoring and reduction in the infusion rate by 25-50% are recommended when intravenous loading doses of fosphenytoin are administered in these patients. The potential exists for clinically significant interactions when fosphenytoin is coadministered with other highly protein bound drugs. The pharmacokinetic properties of fosphenytoin permit the drug to serve as a well tolerated and effective alternative to parenteral phenytoin in the emergency and non-emergency management of acute seizures in children and adults.

Safety and tolerability of oxcarbazepine in elderly patients with epilepsy

Despite the high incidence of seizures and epilepsy in the elderly, the tolerability and safety of anticonvulsants are rarely evaluated in this patient population. We compared the safety and tolerability of oxcarbazepine in a cohort of 52 patients aged 65 years and older and a group of 1574 adult patients ranging in age between 18 and 64 years. There was no significant difference between the two groups with respect to premature discontinuation due to adverse events. The four most common adverse events experienced by patients in the elderly group, irrespective of their causal relationship to oxcarbazepine, were vomiting (19%), dizziness (17%), nausea (17%), and somnolence (15%). Three patients developed an asymptomatic hyponatremia, with at least one serum sodium level below 125mEq/L. Elderly patients on concomitant natriuretic drugs were significantly more likely to develop serum sodium levels below 135mEq/L. The results indicate that oxcarbazepine is safe to use in elderly patients and that its tolerability in this age group is similar to that of younger adult patients.

Variability of total phenytoin serum concentrations within elderly nursing home residents

BACKGROUND

Approximately 6% of all elderly nursing home residents receive phenytoin. Phenytoin concentrations are often measured to guide therapy.

OBJECTIVE

To evaluate the intraresident variability among multiple measurements of total phenytoin serum concentrations in nursing home residents.

METHODS

This was an observational study of 56 elderly (>or=65 years) nursing home residents from 32 nursing homes who had at least 3 phenytoin concentrations measured while on the same dose of phenytoin for at least 4 weeks and who were not taking any interfering concomitant medications. These were a subset of 387 elderly nursing home residents from 112 nursing homes across the United States who had total phenytoin concentration measurements between June 1998 and December 2000.

RESULTS

The mean age was 80.1 years (range, 65 to 100 years) and 58.9% were women. The mean daily dose of phenytoin per resident was 4.9 +/- 1.5 mg/kg. Total phenytoin concentrations within an elderly nursing home resident varied as much as two- to threefold, even though there was no change in dose. The person with the smallest variability had a minimum concentration of 10.0 micro g/mL and a maximum of 10.4 micro g/mL. The person with the largest variability had a minimum concentration of 9.7 micro g/mL and a maximum of 28.8 micro g/mL.

CONCLUSIONS

There is considerable variability in the total phenytoin concentrations in the elderly nursing home resident and measurement of a single total phenytoin concentration should not be used to guide treatment.

Differential kinetics of phenytoin in elderly patients

The elderly have a relatively high risk of developing adverse drug reactions. Phenytoin continues to be a preferred drug for treating generalised tonic-clonic seizures in the elderly and simple partial seizures that generalise. Phenytoin is eliminated almost entirely by hepatic oxidation. The principle enzymes responsible are cytochrome P450 (CYP)2C9 and CYP2C19. CYP2C9 is saturated by therapeutic doses of phenytoin, and at steady state both enzymes are probably operant in most people. The nonlinear pharmacokinetics of phenytoin make it a difficult drug for which to establish safe and effective administration regimens. An important area of inquiry is whether the differential disposition kinetics of phenytoin in the elderly render its administration an even more difficult challenge. Moreover, since the elderly are generally subject to more polypharmacy than younger adults, are they, as a result, subject to either more frequent or more severe drug interactions with phenytoin than younger adults? In order to examine these issues we were interested in learning the extent to which old age might affect the plasma protein binding of phenytoin, its hepatic metabolism and, ultimately, its pharmacokinetic profile. With regard to the latter we looked carefully at the methods that have been used to characterise the disposition kinetics of phenytoin in general, and in the elderly, in particular. There are many conflicting findings with regard to the effect of age on the disposition kinetics of phenytoin. However, the strategies used for estimating kinetic parameters for phenytoin [viz the maximum rate of metabolism/elimination (Vmax) and the Michaelis-Menton constant (Km)] exhibit deficiencies that could account for some of the disparate findings. Certainly, more careful prospective studies focusing on the effects of age on phenytoin disposition kinetics are warranted. However, in light of the information currently available, no special attention need be paid to the initiation of phenytoin administration in elderly patients who are taking multiple anticonvulsants. On the other hand, for the elderly receiving phenytoin monotherapy, the initiation of phenytoin administration should occur at lower doses than would be customary for younger adults, and phenytoin blood concentrations should be appropriately monitored in order to evaluate individual Vmax and Km values for informed dosage adjustments.

The aging liver. Drug clearance and an oxygen diffusion barrier hypothesis

A change in drug clearance with age is considered an important factor in determining the high prevalence of adverse drug reactions associated with prescribing medications for the elderly. Despite this, no general principles have been available to guide drug administration in the elderly, although a substantial body of clearance and metabolism data has been generated in humans and experimental animals. A review of age-related change in drug clearances established that patterns of change are not simply explained in terms of hepatic blood flow, hepatic mass and protein binding changes. In particular, the maintained clearance of drugs subject to conjugation processes while oxygen-dependent metabolism declines, and all in vitro tests of enzyme function have been normal, requires new explanations. Reduction in hepatic oxygen diffusion as part of a general change in hepatocyte surface membrane permeability and conformation does provide one explanation for the paradoxical patterns of drug metabolism, and increased hepatocyte volume would also modify oxygen diffusion path lengths (the 'oxygen diffusion barrier' hypothesis). The reduction in clearances of high extraction drugs does correlate with observed reduction in hepatic perfusion. Dosage guidelines emerge from these considerations. The dosage of high clearance drugs should be reduced by approximately 40% in the elderly while the dosage of low clearance drugs should be reduced by approximately 30%, unless the compound is principally subject to conjugation mechanisms. If the hepatocyte diffusion barrier hypothesis is substantiated, this concept may lead to therapeutic (preventative and/or restorative) approaches to increased hepatocyte oxygenation in the elderly. This may lead to approaches for modification of the aging process in the liver.

Anticonvulsant therapy in aged patients. Clinical pharmacokinetic considerations

Alterations in drug disposition that occur with aging are now becoming widely recognised, and there is an increasing number of drugs for which the approach to therapy in elderly patients can be based on pharmacokinetic data. Both healthy aging and comorbid disease can alter the responsiveness of the body to drugs and to their absorption, distribution and elimination. Altered absorption in the elderly has not been documented after oral ingestion of any anticonvulsant drug. Increased adipose tissue in the elderly may raise the apparent volume of distribution (Vd) of lipid-soluble drugs. An increased Vd in the elderly has been shown for diazepam and clobazam, but not midazolam. The data are inconclusive for phenytoin and valproic acid (sodium valproate). The decreased plasma protein binding that often occurs in the elderly has few clinical consequences. The reduced liver function that to occur with aging seems to affect the elimination of drugs that are mainly cleared by oxidative metabolism [e.g. carbamazepine, phenytoin and phenobarbital (phenobarbitone)]. Reduced clearances for methylphenobarbital (methylphenobarbitone), diazepam, midazolam and clobazam occur in elderly men, but not in women. The reduced renal function that is seen in old age affects the disposition of drugs that are eliminated mainly by direct renal excretion. Thus. the clearances of vigabatrin and gabapentin correlate with creatinine clearance. Such considerations may help guide anticonvulsant dosage in the elderly.

Management of Seizures in the Elderly: A Survey of UK Geriatricians

The incidence and prevalence of epilepsy increase substantially with old age. Despite this, the investigation and management of this patient population remains a grey area. Four hundred and eleven (53%) consultant geriatricians responded to a questionnaire exploring their approach to seizures in the elderly in order to establish an overview of current clinical practice. Between one and five patients presenting with seizures, predominantly aged between 75–85 years, were reviewed monthly. Seventy per cent of geriatricians undertook to investigate the patients themselves with biochemical and haematological profiles performed by most. Electroencephalography and computerized tomographic scanning were routinely requested by a quarter of responders. Only 58% would themselves initiate therapy with antiepileptic drugs, with 16% of consultants starting treatment following the first seizure, 59% after a second and 5% after a third. Phenytoin was first choice for generalized tonic-clonic seizures with carbamazepine preferred for partial seizures. If good control was not obtained, 67% would substitute another first line drug, while 27% would add in a second. Less than 3% would use the new anticonvulsants lamotrigine or vigabatrin. Sixty per cent monitored anticonvulsant concentrations in patients with poor control or suspected toxicity. A wide variability was seen in the current approach to seizures in the elderly, which reflects a lack of established practice. Epilepsy clinics for the elderly would encourage structured research into the many unanswered questions affecting the care of older people with seizures.

Comparison of linear regression methods when both variables contain error: relation to clinical studies

Five common linear regression methods were evaluated for their ability to determine the correct values of slope and intercept of a known function after random errors were added to x and y. The error variances were controlled to simulate research problems commonly studied by linear regression. The total error of each method was assessed by the absolute value of the bias in the estimate of slope. Whenever differences among methods were observed, the mean of the slope determined by two reciprocal techniques performed as well as or better than orthogonal regression, regression of y upon x, or x upon y. All the methods studied appeared to perform equally well when x and y errors were heteroscedastic or when the data set was small (n = 7). Regression of y upon x was equal or superior to other methods when n = 7 or n = 20 and y and x errors were homoscedastic. When the data set was large (n = 50) and the error in x greater than that in y, the standard method (regression of y upon x) was inferior to all other methods. It is suggested that linear regression by the traditional method of y upon x (a method present in many hand-held calculators) is appropriate in the majority of clinical situations, but when n is large and errors in x are much larger than those in y, orthogonal regression or the averaging method may be preferable.

Plasma protein binding of drugs in the elderly

Binding to plasma proteins can affect the pharmacokinetics and pharmacodynamics of drugs. Age is one of many factors which can affect plasma protein binding of drugs. Unfortunately, very few generalities can be drawn from the studies of the effect of age on protein binding. Whether age has an effect on protein binding is dependent not only on the drug, but also on the manner in which the study is conducted. Several studies involve patients with various disease states making assessment of the effect of age alone on protein binding difficult. Results of different studies on the same drug do not always agree--in one case finding no change in protein binding with age and in another, a significant increase or decrease in protein binding. Most drugs which exhibit increased binding (decreased free fraction) in elderly subjects are basic and tend to have a greater affinity for alpha 1-acid glycoprotein than for albumin. The list of drugs exhibiting decreased binding (increased free fraction) in the elderly is longer and includes both acidic and basic drugs. The impact of changes in protein binding with age is dependent on the magnitude of the change, on the pharmacokinetic characteristics of the drug and on its therapeutic index. Some changes, although statistically significant, are not likely to be of importance clinically. From the studies reviewed, the free fraction is changed by greater than 50% in the elderly for only a few drugs, e.g. acetazolamide, diflunisal, etomidate, naproxen, salicylate, valproate and zimeldine.

Drug monitoring in the elderly

Although the gut changes markedly with age no important differences in drug absorption have yet been demonstrated for any of a wide variety of drugs. Alterations in body structure and function can have important consequences on drug distribution in the elderly. There is a considerable reduction in total body mass with age and the proportion of lean body tissue decreases while there is a relative increase in the amount of adipose tissue. Protein binding of drugs can also be affected by age. Acidic drugs may be bound less in many older patients, mainly because of a general decrease in plasma albumin concentrations. Alpha-1-acid glycoprotein is of importance in the binding of basic drugs and its elevation in the elderly suggests that it may have an important effect on free drug concentration. Drug metabolism generally involves oxidative and conjugative reactions by the hepatic microsomal enzymes. There is little change in conjugative capacity with age although some oxidative pathways have been shown to be impaired. The steady decline in glomerular and tubular function with age is reflected in corresponding decreases in drug elimination for those drugs removed by this route.

Anticonvulsant drugs. An update

A considerable amount of information is now available concerning the clinical pharmacology of the anticonvulsant drugs. Some of the more important data are reviewed in this article. In recent years, valproic acid (or sodium valproate) has found a place as a major anticonvulsant agent, while older drugs such as troxidone and sulthiame seem to be disappearing from use. Although much information is available, the essential mechanisms of action of the anticonvulsant drugs are still not understood, either at a molecular or at an electrophysiological level. The pharmacokinetics of the anticonvulsants in common use are now reasonably well documented, though some minor questions are still to be answered. Numerous interactions between anticonvulsants and endogenous substances or other drugs administered concurrently (including other anticonvulsants) have been recorded, but much work still needs to be done to elucidate the frequency and mechanisms of the various interactions. Many adverse effects of the anticonvulsants are known, but further unwanted effects of long-established drugs continue to emerge from time to time, including the still somewhat controversial matter of anticonvulsant-related dysmorphogenesis. The use of valproic acid and its sodium salt has been associated with a worrying incidence of serious liver and pancreatic toxicity. Adequate basic data are now available to put the clinical use of anticonvulsants on a rational basis, but much work remains to be done in this area. In particular, the question of 'therapeutic ranges' of plasma concentrations of the various drugs needs to be reinvestigated in a rigorous statistical fashion, and in relation to different clinical types of epilepsy. The usefulness of monitoring free rather than total drug concentrations also needs further investigation. The ultimate test of the validity of all background scientific pharmacological information about anticonvulsants is its usefulness in the treatment of patients with epilepsy.

Do saliva concentrations predict plasma unbound theophylline concentrations? A problem re-examined

On the assumption that plasma unbound drug concentrations are therapeutically active, the value of saliva concentrations in predicting plasma unbound theophylline concentrations was investigated in 25 ambulatory adults (aged 27 to 84 years) receiving theophylline (225-1350 mg aminophylline daily) for asthma or chronic bronchitis. Plasma samples from all patients were ultrafiltered, and the plasma unbound theophylline (F) concentrations were compared with the corresponding total plasma (P), citric acid stimulated saliva (S) and non-stimulated saliva (Ns) theophylline concentrations. Plasma unbound theophylline concentrations correlated significantly with P (r = 0.97) and S (r = 0.973), but less well with Ns (r = 0.883), emphasising the benefit of saliva stimulation. The ability of S to predict F theophylline concentrations was assessed using the mean ratio of 0.7297. In 92% of the patients, predicted F concentrations were within +/- 1 microgram/ml of the measured concentrations. Similarly, using the mean F/P ratio of 0.418, predicted P were within +/- 1 microgram/ml of obtained P in 84% patients, and using the mean S/P ratio of 0.568, predicted P were within +/- 1 microgram/ml of obtained P in 81%. An accuracy of +/- 1 microgram/ml in estimating F from S concentrations would be sufficient to indicate appropriate dose adjustments, and we therefore advocate the use of stimulated saliva samples for routine monitoring of theophylline therapy.