Theophylline-terbutaline, a steady state study on possible pharmacokinetic interactions with special reference to chronopharmacokinetic aspects

Clinical pharmacokinetics of terbutaline in humans: a systematic review

Terbutaline is used for the management of bronchospasm associated with asthma, bronchitis, emphysema, and chronic obstructive pulmonary disease. A systematic review would be beneficial to assess the impact of routes of administration, stereoisomerism, disease states, smoking, age, exercise, and chronobiology on pharmacokinetics (PK) of terbutaline in humans. PubMed and Google Scholar databases were searched to screen all the relevant articles consisting of at least one of the PK parameters after administration of oral, inhaled, and intravenous (IV) terbutaline in humans. Oral studies of terbutaline depicted a linear relationship between plasma concentration (C_p) and the administered dose. The IV studies demonstrated multi-exponential behavior for disposition and renal clearance. Higher systemic availability was observed with inhaled as compared to oral route, and chrono-pharmacokinetic behavior was notable. Time to reach maximum plasma concentration (T_max) was prolonged, and maximum plasma concentration (C_max) was lowered after exercise. The primary route of excretion in chronic kidney disease (CKD) patients is reported to be nonrenal. In pregnant women, the C_p of terbutaline is lowered and clearance is increased. The addition of theophylline to terbutaline did not affect the PK of terbutaline; hence, both can be used without dose adjustment. This review summarizes all the available PK parameters of terbutaline, and it may be helpful for researchers in the development and evaluation of PK models as well as in designing optimal dosage regimens in different clinical conditions.

The novel combination of theophylline and bambuterol as a potential treatment of hypoxemia in humans

Hypoxemia can be life-threatening, both acutely and chronically. Because hypoxemia causes vascular dysregulation that further restricts oxygen availability to tissue, it can be pharmacologically addressed. We hypothesized that theophylline can be safely combined with the β2-adrenergic vasodilator bambuterol to improve oxygen availability in hypoxemic patients. Ergogenicity and hemodynamic effects of bambuterol and theophylline were measured in rats under hypobaric and normobaric hypoxia (12% O₂). Feasibility in humans was assessed using randomized, double-blind testing of the influence of combined slow-release theophylline (300 mg) and bambuterol (20 mg) on adverse events (AEs), plasma K⁺, pulse, blood pressure, and drug interaction. Both drugs and their combination significantly improved hypoxic endurance in rats. In humans, common AEs were low K⁺ (<3.5 mmol/L; bambuterol: 12, theophylline: 4, combination: 13 episodes) and tremors (10, 0, 14 episodes). No exacerbation or serious AE occurred when drugs were combined. A drop in plasma K⁺ coincided with peak bambuterol plasma concentrations. Bambuterol increased heart rate by approximately 13 bpm. Drug interaction was present but small. We report promise, feasibility, and relative safety of combined theophylline and bambuterol as a treatment of hypoxemia in humans. Cardiac safety and blood K⁺ will be important safety endpoints when testing these drugs in hypoxemic subjects.

Effect of gender on theophylline clearance in the asthmatic acute phase in Japanese pediatric patients

The effect of gender on theophylline clearance was investigated retrospectively in 96 Japanese pediatric patients (63 males and 33 females) ranging in age from 0.5 to 8 years and in weight from 6.3 to 36.8 kg. All patients received intravenous constant-rate infusion of aminophylline in the asthmatic acute phase. The theophylline clearances in males and females were 56.2+/-15.4 and 50.1+/-14.2 ml/h/kg for ages 0.5-<2 years, 58.7+/-18.8 and 48.3+/-6.5 ml/h/kg for ages 2-<4 years, and 65.7+/-12.0 and 52.1+/-16.8 ml/h/kg for ages 4-<9 years, respectively. At ages from 2 to 8 years, the theophylline clearance was 20% higher in males than in females (p<0.05). Our findings suggested that the initial dosage of theophylline should be adjusted according to the gender of pediatric patients and particularly in the case of infants.

The influence of circadian rhythms on the kinetics of drugs in humans

In clinical practice, it is important to consider circadian rhythms in pharmacokinetics and cell responses to therapy in order to design proper protocols for drug administration. Scientists have arrived at this conclusion after several experiments in animals and in humans have clearly demonstrated that all organisms are highly organised according to circadian rhythms. These temporal cycles influence different physiological functions and, consequently, can influence the pharmacokinetic phases of drugs. A drug's pharmacokinetics can be modified according to the time of drug administration. In fact, the circadian changes of > 100 different compounds have been documented. The results obtained have led several scientific societies to provide guidelines concerning the timing of drug dosing for anticancer, cardiovascular, respiratory, anti-ulcer, anti-inflammatory, immunosuppressive and antiepileptic drugs. Absorption may be influenced by circadian rhythms and most lipophilic drugs seem to be absorbed faster when the drug is taken in the morning compared with the evening; for water-soluble compounds, no circadian variation in the absorption of drugs has been found. Concerning drug distribution, the higher the blood flow fraction an organ receives, the higher the rate constant for transferring drugs out of the capillaries. This drug pharmacokinetic phase may be influenced by circadian variations in the protein binding of acidic and basic drugs. Drug metabolism may be influenced by daily modifications of blood flow. For drugs with a high extraction ratio, metabolism depends on hepatic blood flow, while that of drugs with a low extraction ratio depends on liver enzyme activity. Hepatic blood flow has been shown to be greatest at 8 am and metabolism seems to be reduced during the night. Finally, concerning drug elimination, the clearance of 'flow-limited' drugs that present a high extraction rate is affected by the blood flow delivered to the organ, independent of the cardiac output fraction supplied. Chronopharmacokinetics can explain individual differences in drug levels revealed by therapeutic drug monitoring and can be used to optimise the management of patients receiving drug therapy.

Clinical chronopharmacology: the importance of time in drug treatment

Nearly all functions of the body, including those influencing pharmacokinetic parameters such as drug absorption and distribution, drug metabolism and renal elimination, show significant daily variations: these include liver metabolism, hepatic blood flow and the first-pass effect; glomerular filtration, renal plasma flow and urine volume and pH; blood pressure, heart rate and organ perfusion rates; acid secretion in the gastro-intestinal tract and gastric emptying time. The onset and symptoms of diseases such as asthma attacks, coronary infarction, angina pectoris, stroke and ventricular tachycardia are circadian phase dependent. In humans, variations during the 24 h day in pharmacokinetics (chrono-pharmacokinetics) have been shown for cardiovascularly active drugs (propranolol, nifedipine, verapamil, enalapril, isosorbide 5-mononitrate and digoxin), anti-asthmatics (theophylline and terbutaline), anticancer drugs, psychotropics, analgesics, local anaesthetics and antibiotics, to mention but a few. Even more drugs have been shown to display significant variations in their effects throughout the day (chronopharmacodynamics and chronotoxicology) even after chronic application or constant infusion. Moreover, there is clear evidence that even dose/concentration-response relationships can be significantly modified by the time of day. Thus, circadian time has to be taken into account as an important variable influencing a drug's pharmacokinetics and its effects or side-effects.

Chronopharmacology and controlled drug release

Circadian rhythms in the body are well established and are an important factor to consider when administering drugs. Many diseases display symptoms and onset characteristics that are not randomly distributed within 24 h (e.g., coronary infarction, angina pectoris, asthmatic attacks and peptic ulcer perforations); therefore, it is not surprising that the effects and/or pharmacokinetics of drugs can display significant daily variations. Recent data, primarily concerned with the chronopharmacokinetics of antiasthmatics, histamine H2-blockers and cardiovascular active drugs (e.g., propanolol, organic nitrate and nifedipine) are described as representative examples in this review. The data demonstrate that biological rhythms should have been taken into account when evaluating drug delivery systems, galenic formulations and pharmacokinetics as a basis for drug treatment.

[Circadian rhythms and clinical pharmacology]

Almost all physiological functions in animal and man including vital signs display significant daily variations. The existence of internal clock(s) triggering circadian rhythms is now well established. In man, also the onset of certain diseases such as asthma attacks, coronary infarction, angina pectoris and peptic ulcers is not randomly distributed over 24 hours of a day. These rhythmic changes may have implications for drug therapy: In man more than 60 different drugs were shown to exhibit pronounced daily variations in their pharmacokinetics and/or in their effects or side effects. This data demonstrate that the time of day has to be taken into account as an additional parameter influencing the pharmacokinetics, the efficacy and the therapeutic range of drug therapy.

Relevance for chronopharmacology in practical medicine

Nearly all functions of the body, including those influencing pharmacokinetic parameters, such as drug absorption and distribution, drug metabolism, and renal elimination display significant daily variations. Also, the onset and symptoms of diseases such as asthma attacks, coronary infarction, angina pectoris, stroke, and ventricular tachycardia are circadian-phase dependent. Asthma attacks predominantly occur around 4 o'clock at night. Blood pressure and heart rate in normotensives and essential (primary) hypertensive patients display highest values during daytime followed by a nightly drop and an early morning rise. In about 70% of forms of secondary hypertension, however, this rhythmic pattern is abolished or even reversed exhibiting nightly peaks in blood pressure. Similar findings were obtained in children. This form of hypertension is accompanied by increased end organ damages. These observations call for a circadian time-specified drug treatment. In nocturnal asthma unequal dosing of antiasthmatic drugs with a higher/single evening dose is recommended. In secondary hypertension not only the elevated blood pressure must be reduced but the disturbed blood pressure profile should be normalized, too, possibly best achieved by evening dosing. Pharmacokinetics may also not be constant within 24 hours of a day as shown for cardiovascular active drugs, antiasthmatics, anticancer drugs, psychotropics, analgesics and local anesthetics, antibiotics to mention but a few. Far more drugs were shown to display significant daily variations in their effects even after chronic application or constant infusion. Because circadian rhythms undergo maturation with development, drug therapy in children can/may also be modified by circadian time of drug dosing as shown for anticancer drugs. In conclusion, there is clear evidence that the dose/concentration-response relationship of drugs can be significantly dependent on the time of day. Thus, circadian time has to be taken into account as an important variable influencing a drug's pharmacokinetics and/or its effects or side effects.

A double blind cross over trial of theophylline prophylaxis for sleep hypoxaemia in Duchenne muscular dystrophy

Twelve non-ambulant patients with Duchenne muscular dystrophy underwent a double-blind cross-over clinical trial of slow-release theophylline for the treatment of asymptomatic sleep hypoxaemia. Eight channel polysomnography was carried out at home to identify patients with sleep hypoxaemia and this was repeated whilst on the theophylline and placebo limbs of the trial. The trial design was effective in maintaining blinding but theophylline adversely affected sleep quality and sleep hypoxaemia. The overnight oxygen saturation was further reduced whilst on theophylline with reduced sleep efficiency and increased sleep fragmentation. This trial lends further evidence towards the theory that the underlying aetiology of sleep hypoxaemia is upper airway obstruction.

Influence of endogenous and exogenous effectors on the pharmacokinetics of theophylline. Focus on biotransformation

Theophylline has been widely used as a bronchodilatory drug for the treatment of neonatal apnoea in premature newborns and patients with obstructive airways disease. The development of analytical equipment and procedures to determine the systemic concentration of theophylline renders it possible to improve the effectiveness of theophylline therapy and reduce the incidence of toxic and adverse effects. Since the beginning of the 1970s, endogenous and exogenous factors (e.g. age, blood pH, concomitant diseases and drug therapy, meal preparation procedure, nutritional habits, pregnancy, gender, smoking and, to a lesser extent, biorhythms), influencing nearly all parameters of theophylline pharmacokinetics have been described. Drug absorption depends on galenic formulation, drug delivery, nutritional habits and the chemical derivatives used. The mean plasma protein binding rates depend on the method of plasma protein determination: acidic blood pH values and advanced age may result in reduced plasma proteins. The volume of distribution depends primarily on age; it is 2-fold greater in newborns than in adults. Furthermore, changes in blood pH values, the plasma protein content and the administration of concomitant drugs may vary this parameter. Biotransformation is the most clinically important pharmacokinetic parameter. Hepatic metabolism accounts for 90% of the metabolism of theophylline. Essentially, 2 microsomal isoenzymes of the cytochrome P450 system appear to be responsible for the N-methylation and 8-hydroxylation of the drug. Age and concomitant disease are the major endogenous effectors influencing biotransformation of theophylline, whereas biorhythms, gender and pregnancy are of lesser importance. Exogenous factors, such as concomitantly administered drugs, smoking and nutritional factors, affect biotransformation by inducing or inhibiting the metabolising enzymes. Because of intra- and interindividual variability in the pharmacokinetics of theophylline, which may be increased by the presence of endogenous and/or exogenous effectors, it is necessary to supervise theophylline therapy by therapeutic drug monitoring if target concentrations are to be achieved.

Steady-state population pharmacokinetics of sustained release theophylline in adult asthmatic patients

The steady-state population pharmacokinetics of theophylline were studied in 52 asthmatic adult patients who received sustained-release theophylline as armophylline or euphylline. A total of 92 steady-state plasma theophylline concentration-dosage pairs were analyzed using a nonlinear mixed effects model. The pharmacokinetic model used was a one-compartment open model with single path Michaelis-Menten elimination. Dosage was adjusted to body weight. The effects of age, gender, alcohol consumption, cigarette smoking, dosage form, concurrent treatment with beta-agonists or steroids, outpatient dosing, and plasma caffeine concentration on maximum elimination rate (Vm) and Michaelis constant for theophylline metabolism (Km) were investigated. Hypothesis testing produced a final model in which Km = 0.42 (mg/l), and Vm (mg/kg per day) was based on cigarette smoking and dosage form, with Vm = 7.54 + 2.01 (smoking) + 1.08 (euphylline). Estimated coefficients of variation for interindividual variability in Km and Vm were 162.6% and 48.1%, respectively. Residual variability in dosage rates was estimated as 0.90 mg/kg per day. The identification of factors influencing theophylline disposition should prove useful for the a priori design of theophylline dosage regimens and monitoring of drug levels during therapy.

Circadian rhythms in theophylline disposition: simulations and observations in the dog

A constant-rate iv infusion of aminophylline approaching steady state in four dogs was used to investigate a circadian rhythm in theophylline disposition. A significant circadian rhythm (p less than 0.05, group cosinor analysis) was observed for serum theophylline concentrations and for the urinary excretion rates of theophylline and 3-methylxanthine. The phase relationship of the observed circadian rhythms in urinary pH to circadian changes in serum theophylline concentrations were supportive of a circadian rhythm in renal clearance of theophylline. Computer simulations indicated that circadian changes in serum theophylline concentration and urinary excretion of theophylline and its metabolites could be accounted for by a circadian rhythm in theophylline renal clearance and metabolism to 3-methylxanthine. A circadian rhythm in theophylline volume of distribution could not be totally dismissed as potentially having some effect on disposition. The timing of food intake is hypothesized to play a role in synchronizing the observed rhythm in serum theophylline concentrations.

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.

Clinical pharmacokinetics of beta-agonists

The beta-agonists have found wide clinical use as racemic mixtures for 20 years, but information on their pharmacokinetics is not comprehensive. They are well absorbed orally, but have low systemic availability due to extensive first-pass sulphation. When administered by inhalation, very little of the administered dose reaches the lungs, but the small amount that does produces effective bronchodilatation. Plasma protein binding of most beta-agonists is negligible, and there is substantial extravascular distribution of the administered dose. Elimination of intravenous drug is predominantly renal, whereas oral doses are mostly eliminated by biotransformation. Renal clearance correlates with creatinine clearance; therefore, dose reduction should be considered if renal function is impaired, such as in the elderly or in cardiac failure. The elimination half-life of most beta-agonists is relatively short, and pharmacokinetics are independent of dose and duration of treatment. Differences in the pharmacokinetics of the enantiomers are evident. There is very large variation in pharmacodynamic response for a given plasma beta 2-agonist concentration among different subjects, and as treatment proceeds in an individual subject. Therefore, in most cases therapeutic response and side effects are more useful for the monitoring of beta 2-agonist treatment than measurement of plasma drug concentrations. The pharmacokinetics of beta 2-agonists are not greatly altered in pregnancy although these agents cause a marked reduction in maternal renal function. Placental transfer is relatively rapid, and side effects are observed in fetus and neonate. Elimination may be somewhat faster in children (8 to 15 years) than in young adults. Asthma does not appear to influence the pharmacokinetics of beta 2-agonists; the only recorded drug interaction of clinical significance is an increase in theophylline clearance by intravenous isoprenaline (isoproterenol). Controlled release oral preparations do not reduce side effects, but may improve compliance due to less frequent dosing. The application of pharmacokinetic principles may improve the clinical usage of beta-agonists, at least when they are used in premature labour and in cardiac failure.