Intraindividual variability in theophylline pharmacokinetics: statistical verification in 39 of 60 healthy young adults

Adjustment of the area under the concentration curve by terminal rate constant for bioequivalence assessment in a parallel-group study of lamotrigine

AIM

A new strength of lamotrigine extended-release formulation unexpectedly failed to show bioequivalence with the existing strengths at the same dose in a parallel-group study. We report the post-hoc analyses conducted to identify the cause and propose an approach for future evaluations in similar situations.

METHODS

A seemingly bimodal distribution of the half-life among the study participants prompted the use of terminal-phase-rate-constant-adjusted area under the concentration curve as the endpoint for bioequivalence assessment. Population pharmacokinetic modelling was also performed to assess the bimodal distribution of apparent clearance and the potential treatment effects on bioavailability.

RESULTS

The cause for failing to achieve bioequivalence appeared to be a biased representation of a bimodal clearance distribution between the groups. The pharmacokinetic modelling with a mixture routine identified two subpopulations: 88% had a mean clearance of 1.99 l h-1 ; 12% had a mean clearance of 0.64 l h-1 . The low-clearance population was unequally represented by 13% and 4% of subjects in the reference and test groups, respectively, and treatment appeared to have no significant effect on oral bioavailability. The bioequivalence comparison using the adjusted area concluded with a 90% confidence interval of 0.91-1.06, suggesting that treatment had no significant effect on bioavailability and the formulations would meet regulatory criteria for bioequivalence.

CONCLUSIONS

The adjustment of the area under the concentration curve adjusted by terminal-phase rate constant should be considered for situational application in bioequivalence assessment when there are multiple clearance subpopulations in a parallel-group study.

Application of a Physiologically Based Pharmacokinetic Model to Study Theophylline Metabolism and Its Interactions With Ciprofloxacin and Caffeine

Theophylline is a commonly used bronchodilator. However, due to its narrow therapeutic range, moderate elevation of serum concentration can result in adverse drug reactions (ADRs). ADRs occur because of interhuman pharmacokinetic variability and interactions with coprescribed medicines. We developed a physiologically based pharmacokinetic (PBPK) model of theophylline, caffeine, and ciprofloxacin metabolisms to: examine theophylline pharmacokinetic variability, and predict population‐level outcomes of drug–drug interactions (DDIs). A simulation‐based equation for personalized dosing of theophylline was derived. Simulations of DDI show that calculated personalized doses are safe even after cotreatment with large doses of strong inhibitors. Simulations of adult populations indicate that the elderly are most susceptible to ADRs stemming from theophylline–ciprofloxacin and theophylline–caffeine interactions. Females, especially Asians, due to their smaller average size, are more susceptible to DDI‐induced ADRs following typical dosing practices. Our simulations also show that the higher adipose and lower muscle fractions in females significantly alter the pharmacokinetics of theophylline or ciprofloxacin.

Bayesian inference for generalized linear mixed model based on the multivariate t distribution in population pharmacokinetic study

This article provides a fully bayesian approach for modeling of single-dose and complete pharmacokinetic data in a population pharmacokinetic (PK) model. To overcome the impact of outliers and the difficulty of computation, a generalized linear model is chosen with the hypothesis that the errors follow a multivariate Student t distribution which is a heavy-tailed distribution. The aim of this study is to investigate and implement the performance of the multivariate t distribution to analyze population pharmacokinetic data. Bayesian predictive inferences and the Metropolis-Hastings algorithm schemes are used to process the intractable posterior integration. The precision and accuracy of the proposed model are illustrated by the simulating data and a real example of theophylline data.

Gamma generalized linear models for pharmacokinetic data

This article considers the modeling of single-dose pharmacokinetic data. Traditionally, so-called compartmental models have been used to analyze such data. Unfortunately, the mean function of such models are sums of exponentials for which inference and computation may not be straightforward. We present an alternative to these models based on generalized linear models, for which desirable statistical properties exist, with a logarithmic link and gamma distribution. The latter has a constant coefficient of variation, which is often appropriate for pharmacokinetic data. Inference is convenient from either a likelihood or a Bayesian perspective. We consider models for both single and multiple individuals, the latter via generalized linear mixed models. For single individuals, Bayesian computation may be carried out with recourse to simulation. We describe a rejection algorithm that, unlike Markov chain Monte Carlo, produces independent samples from the posterior and allows straightforward calculation of Bayes factors for model comparison. We also illustrate how prior distributions may be specified in terms of model-free pharmacokinetic parameters of interest. The methods are applied to data from 12 individuals following administration of the antiasthmatic agent theophylline.

Differential pharmacokinetics of theophylline in elderly patients

The clinical use of theophylline as a first-line bronchodilator has declined during the last two decades. However, in many clinical settings, such as an emergency bronchial asthma attack, theophylline may have a first-line role, in combination with beta(2)-adrenoreceptor agonists and corticosteroids, for improving the asthmatic status. Furthermore, many therapeutic mechanisms of theophylline for bronchial asthma have been reported, and recent studies have suggested that theophylline therapy may have a beneficial role in the management of chronic stable asthma as well as exacerbated disease. However, theophylline has a low therapeutic index because the bronchodilation it produces has a linear relationship with logarithmic increases in serum concentration for the therapeutic range of 5-20 mg/L. Thus, the knowledge of its basic pharmacokinetics and the factors that can alter its clearance is clinically relevant for physicians. Especially when used in elderly asthmatic patients, dosage adjustment of theophylline is a requisite since the elderly have several risk factors that may increase the plasma theophylline level, such as reduced clearance, various underlying diseases and multiple coadministered drugs. After theophylline treatment has been initiated, therapeutic drug monitoring is required.

New mathematical implementation of generalized pharmacodynamic models: method and clinical evaluation

A new method and experimental design are presented to unambiguously estimate the transduction function (phi) and the conduction function (psi) of the generalized pharmacodynamic model: E = phi (psi * r), when measured pharmacokinetic response r is (i) drug plasma concentration and (ii) drug input rate into the systemic circulation. phi relates the observed pharmacologic effect E to the concentration at the effect site: ce = (psi * r), psi defines transfer of drug from plasma site to effect site or from input site to effect site, and * represents the convolution integral. The model functions psi and phi were expressed as cubic splines giving a very flexible description of those processes which is not biased by the structured assumptions of more conventional models, e.g., effect compartment models. The experimental design proposed addresses the problem of ambiguous identification of the model functions typical of these models; that is, there is more than one pair of very different functions describing the effect data collected after a single drug administration. We tested the hypothesis that the simultaneous fitting of at least two administrations allows the unambiguous identification of the model functions without the need for unlikely or cumbersome constraints. The performance of the mathematical implementation and the robustness of the methods with respect to measurement noise and possible failure of some assumptions, such as intraindividual variability, were tested by computer simulations. The method was then applied to the results of a clinical study of verapamil pharmacodynamics in 6 healthy subjects. Results of these studies demonstrated that the mathematical implementation does not introduce bias or artifact into the estimated functions and that the models and the proposed methods are suitable for application to clinical research. Two drug administrations were sufficient to unambiguously describe verapamil pharmacodynamics in the 6 human subjects studied.

The efficacy and tolerability of inhaled salmeterol and individually dose-titrated, sustained-release theophylline in patients with reversible airways disease

In a double-blind, double-dummy, cross-over, multicentre study, 141 patients with moderate reversible airways disease were randomized to receive either 50 micrograms salmeterol via a metered dose inhaler or individually dose-titrated oral theophylline, both twice daily for a 2-week period. Just over half (n = 77) the patients had received theophylline before, but 64 patients were new to theophylline therapy. Efficacy was based on lung function data and symptom scores. Salmeterol produced significantly higher increase in mean morning peak expiratory flow (PEF) of 161 min-1 (P < 0.001; 95% Confidence Interval (CI); 8-241 min-1) and mean evening PEF of 151 min-1 (p < 0.001; 95% CI; 7-221 min-1) compared with theophylline therapy. Further analysis of the data revealed that the increase in PEF with salmeterol compared with theophylline was highest in a sub-group of patients new to theophylline therapy. Patients on salmeterol had significantly less night-time awakenings than those on theophylline (P = 0.011) and significantly less daytime symptoms (P = 0.017). There was also a significant increase in the number of nights (P = 0.013) and days (P < 0.001) on salmeterol when no additional salbutamol was required compared with theophylline. Theophylline produced a higher incidence of adverse events compared with salmeterol. The results of this study show that inhaled salmeterol is more effective and better tolerated than individually dose-titrated oral theophylline over a 2-week study period in patients with moderate asthma.

Pharmacokinetic optimisation of asthma treatment

Asthma is generally managed with bronchodilator therapy and/or anti-inflammatory drugs. Guidelines now advocate selection of drugs and pharmaceutical formulations (long-acting vs short-acting, inhaled vs systemic) on the basis of disease severity. Theophylline has a narrow therapeutic margin. Clearance is highly variable and plasma concentrations should be monitored to avoid the occurrence of plasma concentration-related adverse effects. The rate of absorption of theophylline differs depending on the sustained release formulation administered. Some products do not provide sufficient plasma drug concentrations for therapeutic efficacy over a 12-hour period, particularly in patients with high clearance rates (e.g. children and patients who smoke). Administration of drugs via inhalation offers several advantages over systemic routes of administration (e.g. adverse effects are decreased). Inhalation is now advocated as first-line therapy. Aerosol medications available for the treatment of asthma are beta 2-agonist (including the newer long-acting agents such as salmeterol), corticosteroids, anticholinergic drugs, sodium cromoglycate (cromolyn sodium) and nedocromil. To reach the airways, aerosolised particles should be 1 to 5 microns in diameter. Particles of this size can be produced by nebuliser for continuous administration or by metered-dose inhaler and drug powder inhaler for unit dose medication. For efficient use of the metered-dose inhaler, slow inhalation and actuation must be coordinated. However, efficacy and convenience can be improved when spacer devices are used. Furthermore, spacer devices lessen the oropharyngeal adverse effects of inhaled corticosteroids. Dry powder inhalers are more easily used by children and elderly patients than metered-dose inhalers. Regardless of the device used, a maximum of 10% of the inhaled dose reaches the airways. The rest of the dose is swallowed and absorbed through the gastrointestinal tract. Most inhaled drugs have low oral bioavailability, either because of a high first-pass metabolism (beta 2-agonists and glucocorticoids) or because of lack of absorption (sodium cromoglycate). Sulphation of beta 2-agonists occurs in the wall of the gastrointestinal tract and extensive metabolism of inhaled corticosteroids occurs in the liver. Low bioavailability of the swallowed fraction contributes to reduced adverse effects. The pharmacokinetic properties of an inhaled drug are of interest. The fraction of the dose absorbed through the lung has the same disposition characteristics as an intravenous dose, and the swallowed fraction has the same disposition as an orally administered dose. However, for many drugs, pharmacokinetic data after inhalation are limited and cannot be used as a criteria for selection of therapy.(ABSTRACT TRUNCATED AT 400 WORDS)

Theophylline. Current thoughts on the risks and benefits of its use in asthma

Theophylline and its derivatives have been used in the treatment of asthma for over 50 years, but since the advent of more potent bronchodilators their use has become cloaked in controversy. Their continued existence results from their undoubted usefulness in severe acute asthma, nocturnal asthma, childhood asthma and moderate to severe chronic airflow limitation, and because of habitual use by physicians in other situations. The precise mechanism of action of theophylline remains uncertain. The role of phosphodiesterase inhibition and adenosine antagonism has been reviewed and the clinical significance of the anti-inflammatory action of theophylline discussed. Theophylline has unpredictable metabolism when first administered, and continued monitoring of drug concentrations is essential. Commonly encountered adverse effects may occur at therapeutic serum concentrations, frequently necessitating drug withdrawal. The overlapping therapeutic and toxic theophylline serum ranges can lead to life-threatening adverse effects at the upper end of the therapeutic range, especially in the elderly in whom special precaution is required.

Placental transfer of theophylline during in situ perfusion in the rabbit

Many physiological changes take place during pregnancy, and the disposition profile of endogenous and exogenous compounds may change, too. Thus knowledge of the disposition pattern of a compound may be useful in relation to its therapeutic effect(s) and its potential toxicity on the fetus and the newborn. Because the amount of a compound received by the fetus is a product of placental transfer rate, and available maternal amount, and because it is difficult to control and evaluate the factors that may affect such a transfer in women, we set up an in situ perfused placental model in the rabbit. The reliability of the model was borne out by comparing the placental transfer of theophylline with antipyrine, a commonly used marker of placental exchange, at steady state after a two-step infusion at mean arterial plasma concentrations of 8 and 5 mg/L, respectively for theophylline and antipyrine. The rabbit placenta was perfused in situ with a modified Earle's buffer at a 1-mL/min flow rate. During perfusion, maternal plasma, placental perfusate, biochemical parameters, gas exchange, body temperature, and electrocardiogram were carefully monitored. The maternal plasma and perfusate drug concentrations over time were fitted by appropriate models and kinetic parameters were calculated. Umbilical vein/maternal artery concentration ratios reached equilibrium soon after the loading infusion was stopped for both drugs. Placental clearance averaged 0.62 and 0.77 mL/min for theophylline and antipyrine, respectively, and the clearance index of theophylline was 0.81 +/- 0.07. Although human and rabbit placentas are structurally dissimilar, the rabbit placenta perfused in situ appears to be a useful preparation for measuring the transfer processes and the related and governing factors, of different compounds.

Lack of effect of a high-fat meal on the volume of distribution of theophylline in humans

The pharmacokinetics of intravenously administered theophylline were studied in five healthy nonsmokers. Each subject received 5 mg/kg of theophylline as aminophylline after an overnight fast and again after a standard high-fat meal. Although there was wide between-day variation in the elimination rate constant in three of the five subjects, no statistically significant differences were observed in area under the time-versus-concentration curve, maximum serum theophylline concentration, elimination rate constant, or apparent volume of distribution between the two treatments. A statistical power analysis indicated that if differences in volume of distribution and maximum serum theophylline concentration occur in the general population, the mean differences are less less than 15% and 20%, respectively. This suggests that alterations in intravascular drug distribution resulting from eating a high-fat meal do not contribute importantly to previously reported effects of food on serum theophylline concentrations after oral dosing.

Pharmacokinetic interactions between theophylline and other medication (Part II)

Part I of this article, which appeared in the previous issue of the Journal, covered the effects or lack of effects on theophylline clearance of sympathomimetics, corticosteroids, antihistamines and other antiallergy drugs, antimicrobial agents, phenytoin, carbamazepine, barbiturates, antacids and activated charcoal. In Part II, this discussion is extended to the effects of other agents. Overall summaries, both textual and tabular, appear in Part I.

The effect of respiratory disorders on clinical pharmacokinetic variables

Respiratory disorders induce several pathophysiological changes involving gas exchange and acid-base balance, regional haemodynamics, and alterations of the alveolocapillary membrane. The consequences for the absorption, distribution and elimination of drugs are evaluated. Drug absorption after inhalation is not significantly impaired in patients. With drugs administered by this route, an average of 10% of the dose reaches the lungs. It is not completely clear whether changes in pulmonary endothelium in respiratory failure enhance lung absorption. The effects of changes in blood pH on plasma protein binding and volume of distribution are discussed, but relevant data are not available to explain the distribution changes observed in acutely ill patients. Lung diffusion of some antimicrobial agents is enhanced in patients with pulmonary infections. Decreased cardiac output and hepatic blood flow in patients under mechanical ventilation cause an increase in the plasma concentration of drugs with a high hepatic extraction ratio, such as lidocaine (lignocaine). On a theoretical basis, hypoxia should lead to decreased biotransformation of drugs with a low hepatic extraction ratio, but in vivo data with phenazone (antipyrine) or theophylline are conflicting. The effects of disease on the lung clearance of drugs are discussed but clinically relevant data are lacking. The pharmacokinetics of drugs in patients with asthma or chronic obstructive pulmonary disease are reviewed. Stable asthma and chronic obstructive pulmonary disease do not appear to affect the disposition of theophylline or beta 2-agonists such as salbutamol (albuterol) or terbutaline. Important variations in theophylline pharmacokinetics have been reported in critically ill patients, the causes of which are more likely to be linked to the poor condition of the patients than to a direct effect of hypoxia or hypercapnia. Little is known regarding the pharmacokinetics of cromoglycate, ipratropium, corticoids or antimicrobial agents in pulmonary disease. In patients under mechanical ventilation, the half-life of midazolam, a new benzodiazepine used as a sedative, has been found to be lengthened but the underlying mechanism is not well understood. Pulmonary absorption of pentamidine was found to be increased in patients under mechanical ventilation. Pharmacokinetic impairment does occur in patients with severe pulmonary disease but more work is needed to understand the exact mechanisms and to propose proper dosage regimens.

Capillary gas chromatographic-mass spectrometric determination of stable isotopically labelled and unlabelled theophylline in serum and urine and of 1,3-dimethyluric acid in urine

A capillary gas chromatographic-mass spectrometric method for the determination of [13C,15N2]theophylline and unlabelled theophylline (TP) and of 1,3-dimethyluric acid (1,3-DMU), a major metabolite of TP, is described. TP and the metabolite were extracted separately from serum or urine, purified by high-performance liquid chromatography and converted into alkyl derivatives (monoethyl-TP and dibutyl-1,3-DMU). The internal standards used for the respective determinations were [2H6]TP and [2H6]1,3-DMU. Detection was performed by monitoring the molecular ions of the alkyl derivatives. The method needed no complex corrections for contributions and provides good accuracy and precision.

Effects of equimolar doses of cimetidine and ranitidine on theophylline elimination

The disposition of theophylline was studied on four occasions in eight healthy adult males. The control mean theophylline half-life and clearance were 7.32 h and 0.86 ml min-1 kg-1, respectively. After 5 days pretreatment with placebo the corresponding values of 7.01 and 0.88 were not significantly different, as were those of 7.43 and 0.85 after 5 days pretreatment with ranitidine (1.2 g daily). Five days pretreatment with cimetidine (1.0 g daily) resulted in a significant 44.4 per cent rise in the mean theophylline half-life and a 36.1 per cent fall in clearance. The fall in clearance correlated positively (r = 0.9407) with the initial value. The volume of distribution did not change significantly throughout the study period. The fact that, at as large a dose as 1.2 g daily, ranitidine did not impair theophylline metabolism suggests that similar results reported earlier with therapeutic doses of 300 mg daily cannot be ascribed to the lower dose of ranitidine employed. It is also suggested that the risk of theophylline toxicity consequent on cimetidine coadministration will be more likely in individuals with initial high theophylline clearance.

The consistency of theophylline absorption from a sustained-release formulation in asthmatic children

Theophylline absorption was studied in asthmatic children ages 2-7 years. A bead-filled capsule of sustained-release theophylline (Slo-Bid, William H. Rorer, Inc., Fort Washington, PA) was administered in equal doses twice daily (8 AM and 8 PM) to produce serum theophylline concentrations (STCs) between 10 and 20 micrograms ml. After 7 days, blood samples were obtained every 2 hours for 24 hours. This sequence was repeated after another seven days of treatment. Elimination values were calculated after an intravenous reference dose. Slo-Bid was completely absorbed (mean +/- SD percentage of dose absorbed of 117.1 +/- 26.1 on day 1, and 104.8 +/- 16.1% on day 2), and the pattern of absorption was consistent from day to day, with maximum STCs occurring 2-6 hours after the dose, and minimum STCs at or near the time of PM dose. The percentage of fluctuation of 149.7 +/- 49.7% and 163.0 +/- 114.0% (mean +/- SD) on days 1 and 2, respectively, was slightly higher than desired, but probably was a result of rapid theophylline clearance in this group of children (119.9 +/- 25.8 ml/hr/kg). Mean residence time for absorption after AM and PM doses indicated significantly longer time for absorption (ta) after the PM doses: mean +/- SEM ta for AM doses on days 1 and 2 was 2.8 +/- 0.3 and 2.7 +/- 0.2 hours, respectively, and increased to 3.9 +/- 0.3 and 3.7 +/- 0.5 hours after PM doses on days 1 and 2, respectively (p less than 0.0001).(ABSTRACT TRUNCATED AT 250 WORDS)

Inter- and intrasubject variation of digoxin renal clearance in normal adult males

Three methods of estimating inter- and intrasubject variances and coefficients of variation of digoxin renal clearance are discussed and illustrated. All three methods gave essentially the same values for intrasubject variation; two of the methods (II and III), based on two-factor and one-factor analyses of variance, respectively, gave the same estimates of intersubject variation. The preferred method I involves directly calculating variances from the rows and columns of the tabular data, where the rows are repetitive treatments in the same subject and the mean variance represents intrasubject variance; a column represents different subjects' results on a given treatment and the mean of the column variances represents intersubject variance. Problems in attempting to use components of variance are discussed. Intrasubject coefficients of variation of digoxin renal clearance averaged 23.8 percent by method I and ranged from 15.2 to 28.6 percent. Intersubject coefficients of variation averaged 30.5 percent with a range of 17.5-41.8 percent by method I and averaged 42.1 percent with a range of 19.2-49.7 percent by methods II and III.

Methylxanthines: toxicity to humans. 1. Theophylline

While there are several comprehensive reviews on the toxic effects of theophylline, caffeine and theobromine in animals, data on the toxicity of these methylxanthines in humans have not been extensively reviewed in one document. This question will be addressed in a series of three papers. This paper provides an overview of the human toxicity of theophylline. Only pertinent and recent information on theophylline toxicity is summarized. In addition, some information regarding the use and benefits of theophylline, the mechanism of its effects and factors that affect variability in its clearance and half-life is also provided. Some problems in the analytical methodology of theophylline, problems that may be responsible for the controversy in the reported dose-response effects, are critically reviewed.

A comparison of the accuracy of a least squares regression, a Bayesian, Chiou's and the steady-state clearance method of individualising theophylline dosage

We compared a least squares regression method, used prospectively to individualise the intravenous aminophylline and oral theophylline dosage of 48 patients, with 3 other pharmacokinetic methods - Chiou's, the steady-state clearance and the Bayesian - used retrospectively to analyse the same patient data. Methods were compared on the basis of the similarity of their parameter estimates and the accuracy with which serum concentrations during subsequent intravenous and oral therapy could be forecast, assuming each method's parameter estimates. The least squares and Bayesian programs were able to fit data from all but 4 and 2 patients, respectively. Mean absolute prediction errors were of the order of 20% for serum concentrations during intravenous therapy, and of the order of 40% for serum concentrations during oral therapy. The accuracy of the least squares, Bayesian and steady-state clearance methods were similar, but the accuracy of Chiou's method was comparable only when the 2 serum concentrations needed for the method were measured between 11 and 17 hours apart; an interval which corresponds to the 1.0 to 1.5 half-lives previously suggested as desirable for implementation of the Chiou method.

Analytical methodology to determine stable isotopically labelled and unlabelled theophylline in human plasma using capillary gas chromatography-mass spectrometry

A method is described for the simultaneous determination of [1,3-15N] theophylline and unlabelled theophylline in human plasma using gas chromatography-mass spectrometry. Plasma samples were subjected to extractive alkylation and the stable isotopically labelled and unlabelled forms of the drug were analysed as their N-pentafluorobenzyl derivatives on an SE-52 fused-silica capillary column. Quantitation was made by selected-ion monitoring employing as the internal standard 3-isobutyl-1-methylxanthine. The method has been used to study the absorption kinetics and bioavailability of a sustained release formulation of the drug when co-administered to human volunteers with a conventional formulation of the drug labelled with the stable isotope.

Intrasubject variability of theophylline clearance in a morbidly obese patient

It is well appreciated that theophylline pharmacokinetics exhibits wide intersubject variation. Within-subject changes in clearance have been generally reported in patients with acute exacerbations of disease states such as cor pulmonale or heart failure. Apparent random changes in theophylline clearance within the same patient have recently been reported. This report describes a case of suspected changes in the clearance of theophylline in a morbidly obese patient (250 kg) over a two-month period of time. Frequent dosage adjustments were required to maintain theophylline concentrations within the range of 8.2 to 27.3 micrograms/ml. Possible explanations for these changes included hypoxia and medically induced starvation. Daily theophylline doses ranged from 600 to 1600 mg/d. It is concluded that in some individuals, within-subject changes in theophylline clearance can be substantial, thus requiring frequent monitoring of theophylline concentration.

Theophylline pharmacokinetics after intravenous infusion with ethylenediamine or sodium glycinate

Six healthy volunteers received two theophylline preparations by short intravenous infusion, at weekly intervals in a randomized cross-over design. The solubilizer was ethylenediamine or sodium glycinate. Plasma concentrations of theophylline and ethylenediamine were measured by h.p.l.c. for up to 24 h after dosing. The pharmacokinetics of theophylline did not show any significant intra-subject variation associated with the solubilizing agent used.

Use of simultaneous computer fitting to estimate the apparent absorption rate constant

In this study we explored an alternative approach to specifically determine the apparent absorption rate constant (ka) and the initial disposition macro-rate constant (alpha) without intravenous dose data. Data sets were simulated with ka values in a range of +/- 70% of alpha. A 10%, uniformly distributed, random error was added to all the simulation parameters and to the concentration values. The data sets were fit individually (one data set) and simultaneously (all possible combinations of two, four, or six data sets) using NONLIN. The parameters alpha, beta (the terminal disposition rate constant), and k21 (the exit rate constant from the peripheral compartment) were assumed to be common parameters between the data sets. If a single data set was used in fitting the equation, large deviations in the estimates for ka and alpha occurred as ka became less than alpha. However, when multiple data sets were fit simultaneously, correct values for ka and alpha were consistently predicted regardless of the relative magnitude of the initial estimates.

Theophylline poisoning. Pharmacological considerations and clinical management

The recent marketing of slow release preparations of theophylline and new indications for the use of the drug have resulted in a marked increase in the sale of theophylline products. This phenomenon combined with the drug's highly variable pharmacokinetics has led to an increase in the number of theophylline intoxications. The morbidity and mortality rates associated with theophylline intoxication are significant. Therefore it is essential that clinicians are aware of the pathophysiology, clinical presentation and treatment of this poisoning. Theophylline intoxication mainly affects the gastrointestinal, cardiovascular and central nervous systems. Signs and symptoms range from mild gastrointestinal upset to serious central nervous system manifestations such as seizures, a symptom often associated with a bad prognosis. Theophylline serum concentrations are very useful for making decisions regarding treatment. However, their interpretation should take into account several factors such as the age of the patient and the type of intoxication (acute versus chronic). Prevention of gastrointestinal absorption should be the principal objective of treatment of an oral theophylline poisoning. The repetitive administration of activated charcoal not only prevents theophylline absorption but also increases its rate of Once absorbed, external methods such as haemodialysis and haemoperfusion can significantly accelerate the elimination of the drug from the body. Finally, the rapid suppression of seizures and cardiac arrhythmias are essential to prevent severe neurological sequelae and death. Since theophylline intoxication can be potentially life-threatening, its administration should be monitored with regular measurements of the serum theophylline concentration, especially in the very young and the very old.

Comparison of single and multiple dose pharmacokinetics of theophylline using stable isotopes

Theophylline, enriched with the stable isotopes 13C and 15N, was administered intravenously in a dose of 10 mg to 8 healthy men following single (200 mg) and multiple (200 mg 8-hourly for 5 days) oral dose administration of aminophylline. Total plasma clearance, volume of distribution, and half-time determined from the intravenous data were similar, demonstrating that the pharmacokinetics of theophylline after chronic dosing can be predicted from the pharmacokinetics of a single dose. With chronic oral dosing, however, the mean trough concentration was 12% higher at 9 a.m. than at 5 p.m., the end of the dose interval (3.94 +/- 0.55 vs. 3.50 +/- 0.45 micrograms X ml-1). The AUC following oral dosing was 25% higher in the multiple dose study than in the single dose study. Simulation analysis suggested that these results could be explained by diurnal variation in the clearance or absorption rate or a combination of both. Thus, the systemic availability of theophylline measured during a single dosage interval after chronic oral dosing to steady state would be overestimated in comparison with that measured after a single oral dose.

Methylxanthine therapy and reversible airway obstruction

Theophylline is a medium-potency bronchodilator that is useful in the treatment of reversible airway obstruction from any cause. It also improves contraction of the diaphragm, accelerates mucociliary transport, lowers pulmonary artery pressures, and limits the release of the mediators of immediate hypersensitivity from mast cells. The pharmacologic effects correlate closely with the plasma concentration of this drug, and within the defined therapeutic limits, only minor adverse effects occur. The dose of theophylline required to achieve therapeutic plasma levels varies considerably between subjects, mainly because of differences in clearance. The latter varies with age, smoking, alterations in hepatic function, cardiac failure, viral infections, and concurrent administration of other drugs. For the treatment of acute symptoms, either intravenous administration or a rapidly absorbed compound such as uncoated tablets or liquids is required. For the long-term management of reversible airway obstruction, sustained-release theophyllines are available. These formulations maintain therapeutic serum theophylline concentrations for prolonged periods, permitting longer dosing intervals and improved patient compliance.

New method for bioavailability assessment of slow-release preparations of theophylline

Variability in an individual's clearance of theophylline is an important consideration when estimating bioavailability. A method is described for compensating for this problem, using the serum concentration of theophylline and urinary excretion data on its major metabolites to make an estimation of the clearance after oral administration using the intravenous dose as reference. The method is particularly useful for assessing the bioavailability of slow-release theophylline preparations.

Theophylline absorption in young asthmatic children receiving sustained-release formulations

Theophylline absorption from sustained-release formulations intended for administration every 8 hours and every 12 hours was examined in children ages 2 to 6 years during multiple dosing intervals. By generally applied measurements, including mean serum theophylline concentration, bioavailability over a single daytime dosing interval, and percent change in serum theophylline concentration over a single dosing interval, the preparations did not differ. However, over multiple dosing intervals, the 8-hour preparation varied in rate and extent of absorption, with subsequent large variations in serum theophylline concentrations. The 12-hour preparation, on the other hand, was completely bioavailable during each dosing interval, although the rate of absorption did differ from day to night, and was associated with generally acceptable changes in serum concentrations. Thus, analysis of dose-to-dose absorption was required to reveal the differences between the two study preparations. This indicates that traditional analysis of a single daytime dosing interval may be inadequate in the evaluation of preparations of sustained-release theophylline.

Lack of influence of an intensive antacid regimen on theophylline bioavailability

We examined the influence of a large-volume, therapeutic antacid regimen, administered for three full days, on the steady-state bioavailability of a conventional-release and sustained-release theophylline product, Aminophyllin and Theodur, respectively. Nine stable asthmatics voluntarily completed a four-phase investigation requiring a total stay of 12 days in the Clinical Research Unit. The treatments consisted of administration of the formulations mentioned with and without antacids to each patient in a randomized sequence. Four patients participated in an additional phase where antacids were administered q2h around the clock for three days. After coadministration of theophylline plus antacids for two days, theophylline therapy was discontinued while numerous blood samples were obtained over 22 hr and analyzed for theophylline content via radioimmunoassay. Antacids had no predictable, consistent influence on theophylline absorption rate as determined by the absorption rate constant, the time to maximal theophylline concentration, or the lag time for theophylline absorption. Antacids had no detectable influence on theophylline elimination half-life and had no consistent, statistically significant effect on the extent of theophylline bioavailability, according to measurements of maximal concentration, AUC measured over the appropriate steady-state dosing interval, or elimination-rate adjusted AUC. The substantial intraindividual changes for all parameters of theophylline bioavailability that occurred for control and treatment phases likely represent spontaneous, random between-day variability in theophylline disposition independent of antacid administration, as evidenced by the comparability of the percent coefficient of variation for parameters of bioavailability across all phases. Our data demonstrate that therapeutic antacid administration has no effect on steady-state theophylline bioavailability and does not alter the intrinsic variability in theophylline absorption. Based on the results of our data, it is unlikely that a clinically significant (greater than 20%) decrease in theophylline absorption would occur in any patient treated intensively with antacids concurrently.

Effect of using an incorrect elimination rate constant in application of the Wagner-Nelson method to theophylline data in cases of zero order absorption

Some authors have used an elimination rate constant derived from one theophylline treatment (e.g. the elixir) to apply the Wagner--Nelson method to concentration--time data from another treatment (e.g. a sustained-release form). Since there is considerable intrasubject variation in the elimination rate constant of theophylline such a practice usually involves use of an incorrect rate constant. By means of theoretical treatment and simulations the effects of such a practice are shown and illustrated.

Lack of clinically important interaction between erythromycin and theophylline

In 11 healthy volunteers the kinetics of theophylline and the plasma levels and the urinary excretion of its metabolites were studied before and after treatment with erythromycin for 10 days. Theophylline was administered as an intravenous bolus injection (280 mg) followed by a constant intravenous infusion (23.8 +/- 4.1 mg/h) for 6 hours. The total clearance of theophylline at steady-state (63.4 +/- 9.9 vs 63.8 +/- 14.4 ml/min, before vs after erythromycin treatment) and the elimination half-life after cessation of the infusion (6.7 +/- 2.6 vs 7.5 +/- 1.8 h, before vs after treatment) did not change during the treatment with erythromycin. No difference in the formation of metabolites before and after treatment with erythromycin was detected; the findings in urine were 40.4 +/- 5.0 vs 42.1 +/- 5.4% 1,3-dimethyluric acid, 29.6 +/- 4.6 vs 30.1 +/- 5.9% 1-methyluric acid and 13.4 +/- 3.5 vs 12.5 +/- 2.2% 3-methylxanthine before and after erythromycin treatment, respectively. It is concluded that a clinically relevant interaction between erythromycin and theophylline does not occur.

Patient compliance with oral theophylline therapy

Day-to-day variations in plasma theophylline concentrations at steady-state have been assessed in 29 hospital inpatients who required theophylline for obstructive airways disease. Plasma concentrations were measured at 09.00h and 14.00h for four consecutive days in the equilibrium state in 13 patients taking 350 mg/day and 16 patients taking 700 mg/day. Analysis of variance gave 95% confidence limits for day-to-day variation of +/- 2.9 micrograms/ml at 350 mg/day and +/- 4.8 micrograms/ml at 700 mg/day. In a separate study, compliance with sustained-release theophylline therapy has been assessed in a group of 63 patients receiving the drug in general practice but not attending hospital. Compliance was estimated by comparing plasma theophylline concentrations before and after a 7-day period of measured theophylline consumption and by tablet counting. Of the 63 patients, 16 had discontinued their theophylline prior to being contacted and two did so during the first week: these were considered non-compliant. Three patients had plasma concentrations which increased by more than the day-to-day variation for their dose level during monitored intake and one other took less than 80% of his tablets. These patients were also considered non-compliant. A further four patients in whom plasma theophylline levels were zero on at least one occasion during the study were also adjudged non-compliant. Thus non-compliance with prescribed theophylline dosage occurred in 26 (41.3%) of the patients studied. In the majority of these, treatment had been discontinued and the non-compliance was gross.(ABSTRACT TRUNCATED AT 250 WORDS)

The preparation and characterization of ethylcellulose-walled theophylline microcapsules

The use of sustained-release preparations of theophylline has become routine in the treatment of bronchial asthma. Microencapsulation provides a possible means of producing such a product. The present paper examines, by means of triangular diagrams, the phases present in the system ethylcellulose-petroleum ether-toluene. The separated cellulose was used to coat the drug theophylline. The effects of temperature, petroleum ether fraction, solvent:non-solvent ratio and rate of non-solvent addition on the size and size distribution of the microcapsules was studied.

Prediction of maintenance oral theophylline dosage using single oral doses in patients with obstructive airways disease

This study was designed to test whether single oral doses of theophylline, rather than the specified single intravenous infusions, could be used with a nomogram designed to predict the maintenance oral dose necessary to establish plasma theophylline concentrations of 10 micrograms ml (Koup et al., 1979) in patients with obstructive airways disease. A test dose of theophylline (5 mg kg) was administered on two separate occasions as an oral elixir and as an intravenous infusion. Exactly 6 h after each test dose, plasma theophylline concentration was measured, and the result used to predict daily maintenance dose requirements. During subsequent oral administration, a sustained-release preparation was given in equally divided 12-hourly doses, and 'steady-state' concentrations were measured at trough. Nineteen patients completed the study, and on doses ranging from 4.2 to 21.1 mg kg-1 day-1, trough concentrations of 5.4 to 15.8 micrograms ml (mean 10.0 +/- 0.7 micrograms ml) were achieved. No significant differences were noted between the dose predictions made following either the oral or the intravenous test doses, and the predictive accuracy using the oral route was not compromised. This would suggest that the test dose of theophylline may be administered orally, instead of intravenously, with this nomogram without reducing its efficiency.

Prediction of optimum oral theophylline dose in patients with obstructive airways disease

The hypothesis that a logarithmic correlation exists between the plasma theophylline concentrations 6 h after a test dose C(6) and the maintenance dose calculated to achieve a desired drug concentration during chronic oral administration (DM,CALC) was tested. A nomogram based on this relationship was evaluated as a means of predicting the optimum dose of theophylline in 14 patients with obstructive airways disease (DM,PRED). Each patient was given 5 mg/kg theophylline intravenously (i.v.) and plasma theophylline concentrations were measured for 12 h thereafter including one exactly 6 h after commencing the i.v. infusion C(6). Pharmacokinetic parameters derived from the plasma concentration-time curve were used to establish DM,CALC for a concentration of 10 micrograms/ml. DM,PRED was obtained from the nomogram using both the optimum and the actual values for C(6). Subsequently oral sustained-release theophylline was administered and the dose adjusted to establish a trough concentration of approximately 10 micrograms/ml. This dose was then corrected arithmetically, assuming a linear relationship between dose and plasma level, to represent that required to achieve a plasma concentration of exactly 10 micrograms/ml (DM,ACT). The correlation between C(6) and log DM,CALC was confirmed (r = 0.97 P less than 0.001), validating the hypothesis. DM,ACT was found to correlate significantly with DM,PRED (r = 0.90, P less than 0.01) substantiating the value of the nomogram. In nine of the 14 patients, DM,ACT corresponded satisfactorily to DM,PRED. In the remaining five, for whom DM,ACT lay outside the 95% confidence limits for the predicted dose, DM,PRED in general underestimated DM,ACT, an advantage in a drug with a low therapeutic index. The predictive error for DM,PRED was lower than that for DM,CALC, and the bias using either method was not significant. The results suggest that a single plasma theophylline assay following a test dose, and the nomogram may be useful in simplifying optimal theophylline administration.

Estimation of absolute bioavailability assuming steady state apparent volume of distribution remains constant

The limitations of using estimates of extent of bioavailability (F) based on the assumption that either clearance (CL) or Varea remain, constant are discussed in relation to the situation where CL changes between doses. When estimates of F assume CL to remain constant, the extent of the error is the same for all drugs where the percentage change in CL is the same. Assuming Varea to remain constant, the error in F will vary between drugs for similar percentage changes in CL and is related to the extent to which the kinetics of the disposition process deviate from a one compartment body model. A noncompartmental method is described where, provided the reference dose is given intravenously, F can be estimated based on the assumption that Vss remains constant between doses. This method is more accurate than those based on the assumption that either CL or Varea remain, constant when CL changes between doses, but is subject to error when the terminal log-linear slope of Cp vs. time better reflects the process of absorption rather than elimination.

A method for estimating within-individual variability in clearance and in volume of distribution from standard bioavailability studies

Bioavailability studies are commonly undertaken, and most, because they involve subjects taking repeated doses of a drug, contain information on intraindividual variability in pharmacokinetics. However, because in such studies bioavailability itself is unknown, it is difficult to resolve which pharmacokinetic parameters vary within individuals. A mathematical model is presented which permits estimation of variability in clearance and in volume of distribution. When applied to pooled data arising from five theophylline bioavailability studies, this model has given statistical evidence that clearance of theophylline is inherently more variable within individuals (coefficient of variation, 13%) than volume of distribution (8%). As a result, use of the measurement AUC-beta rather than AUC as a more precise index of bioavailability is justified in studies where beta is measured with reasonable precision. The model could be applied to estimation of within-batch within-person variability in bioavailability.

Evaluation of the absorption from 15 commercial theophylline products indicating deficiencies in currently applied bioavailability criteria

The bioavailability of theophylline from alcoholic and aqueous oral solutions was compared to that from an intravenous dose in 12 normal adults. The alcoholic elixir surprisingly gave rise to a significantly greater (114 +/- 14%, mean +/- SD) amount absorbed than did the intravenous dose. The aqueous solution (99 +/- 8%) and intravenous dose were statisticlly indistinguishable in this respect, and, furthermore, the extent of absorption from a 300-mg dose of the aqueous solution was 99 +/- 10% of that from a 500-mg dose, and not statistically different. The aqueous solution was thus employed in three subsequent studies as a standard with which to compare 13 different types of theophylline tablets, all marketed in the United States. Of the 13 tablets, eight showed bioavailability statistically distinguishable from that of the standard. Nevertheless, for only two tablets could it be claimed with 95% confidence level that the bioavailability was less than 95%. For none can it be stated at this confidence level that the bioavailability is less than 90%. Bioavailability studies should include criteria of clinical significance in addition to criteria of statistical significance. Contrary to the usual rationale behind choice of a bioavailability standard, nine of the 12 uncoated tablets appeared to allow more rapid absorption of theophylline than did the standard oral solution, an aqueous syrup. Increasing the dose of syrup decreased the rate of theophylline absorption. Orally administered drug solutions may have properties more absorption rate limiting than the disintegration of many brands of tablet.

Evaluation of the absorption from some commercial enteric-release theophylline products

In a single-dose bioavailability study, Wales, Robinson, Columbia, and Choledyl (Warner/Chilcott) enteric-coated tablets all allowed a bioavailability of theophylline (99% +/- 25%, 102% +/- 23%, 103% +/- 18%, and 98% +/- 15%; mean +/- SD, n = 12) statistically indistinguishable from that of the standard uncoated tablet (Searle 200 mg aminophylline). Only the Wales and Choledyl tablets (7.6, 4.2 hr) could be shown (p < 0.5) to generate a peak plasma theophylline concentration later than the standard (1.4 hr). All tablet brands demonstrated a significant lag time before appearance of theophylline in the plasma, and both Wales and Choledyl tablets also had a (tpeak-tlag) statistically different from that of the standard. Despite misleading indications from the mean plasma profile (plasma concentrations at each sampling time averaged over all subjects), plasma data from the individual participants and in vitro dissolution data show that, while release of theophylline from the Wales tablet might be inordinately slow, this is not a sustained-release preparation. Of the enteric-coated tablets only the Columbia product allowed significant levels in the first sample after dosage. Five of the 18 Columbia doses gave rise to 40-99% of the peak concentration in the 1-hr sample. In vitro, it takes 39 +/- 14 min for 40% of the theophylline content of Columbia tablets to dissolve in simulated intestinal fluid. Suprisingly rapid delivery of an enteric-coated tablet to the duodenum would appear to be required to allow a significant percentage of theophylline to be dissolved and absorbed before 1 hr. None of 12 Columbia tablets tested in vitro, however, allowed dissolution of more than 1.2% of their theophylline content during 1 hr immersion in simulated gastric fluid. Since once in intestinal fluid Columbia tablets appear to dissolve more rapidly than the other enteric products, it is not clear whether the five Columbia tablets in question had imperfections or wheter, in fact, this tablet brand more closely than the others represents the ideal of immediate release once in the duodenum. Plasma samples should be taken as early as 15 min after dosage when evaluating the bioavailability of enteric release products.