Effects of tobacco smoking and oral contraceptive use on theophylline disposition

Clinical Relevance of Drug Interactions with Cannabis: A Systematic Review

Concomitant use of cannabis with other drugs may lead to cannabis–drug interactions, mainly due to the pharmacokinetic mechanism involving the family of CYP450 isoenzymes. This narrative systematic review aimed to systematize the available information regarding clinical relevance of cannabis–drug interactions. We utilized the PubMed/Medline database for this systematic review, using the terms drug interactions and cannabis, between June 2011 and June 2021. Articles with cannabis–drug interactions in humans, in English or Spanish, with full-text access were selected. Two researchers evaluated the article’s inclusion. The level of clinical relevance was determined according to the severity and probability of the interaction. Ninety-five articles were identified and twenty-six were included. Overall, 19 pairs of drug interactions with medicinal or recreational cannabis were identified in humans. According to severity and probability, 1, 2, 12, and 4 pairs of cannabis–drug interactions were classified at levels 1 (very high risk), 2 (high risk), 3 (medium risk), and 5 (without risk), respectively. Cannabis–warfarin was classified at level 1, and cannabis–buprenorphine and tacrolimus at level 2. This review provides evidence for both the low probability of the occurrence of clinically relevant drug interactions and the lack of evidence regarding cannabis–drug interactions.

A Review of Cannabis in Chronic Kidney Disease Symptom Management

Purpose of Review:

Physical and psychological symptom burden in patients with advanced chronic kidney disease (CKD) is significantly debilitating; yet, it is often inadequately treated. Legalization of cannabis in Canada may attract increasing interest from patients for its medical use in refractory symptom management, but its indications and long-term adverse health impacts are poorly established, creating a challenge for clinicians to support its use. In this review, we summarize key clinical studies and the level of evidence for nonsynthetic cannabinoids in the treatment of common symptoms encountered in advanced stages of CKD, including chronic pain, nausea and vomiting, anorexia, pruritus, and insomnia.

Sources of Information:

Medline and Embase

Methods:

A search was conducted in MEDLINE and EMBASE (inception to March 1, 2018) on cannabis and CKD symptoms of interest, complemented with a manual review of bibliographies. Studies that examined synthetic cannabinoids that are manufactured to mimic the effects of ∆9-tetrahydrocannabinol such as dronabinol, levonantradol, nabilone, and ajulemic acid were excluded. We focused on studies with higher level of evidence where available, and quality of studies was graded based on the Oxford Centre for Evidence-based Medicine Levels of Evidence (1a to 5).

Findings:

Based on studies conducted in patients without renal impairment, those treated with nonsynthetic cannabinoids were 43% to 300% more likely to report a ≥30% reduction in chronic neuropathic pain compared with placebo. However, there is currently insufficient evidence to recommend nonsynthetic cannabinoids for other medical indications, although preliminary investigation into topical endocannabinoids for uremia-induced pruritus in end-stage renal disease is promising. Finally, any benefits of cannabis may be offset by potential harms in the form of cognitive impairment, increased risk of mortality post-myocardial infarction, orthostatic hypotension, respiratory irritation, and malignancies (with smoked cannabis).

Limitations:

Nonsynthetic cannabinoid preparations were highly variable between studies, sample sizes were small, and study durations were short. Due to an absence of studies conducted in CKD, recommendations were primarily extrapolated from the general population.

Implications:

Until further studies are conducted, the role of nonsynthetic cannabinoids for symptom management in patients with CKD should be limited to the treatment of chronic neuropathic pain. Clinicians need to be cognizant that nonsynthetic cannabinoid preparations, particularly smoked cannabis, can pose significant health risks and these must be cautiously weighed against the limited substantiated therapeutic benefits of cannabis in patients with CKD.

Pharmacokinetic Drug Interactions with Tobacco, Cannabinoids and Smoking Cessation Products

Tobacco smoke contains a large number of compounds in the form of metals, volatile gases and insoluble particles, as well as nicotine, a highly addictive alkaloid. Marijuana is the most widely used illicit drug of abuse in the world, with a significant increase in the USA due to the increasing number of states that allow medical and recreational use. Of the over 70 phytocannabinoids in marijuana, Δ9-tetrahydrocannabinol (Δ9THC), cannabidiol (CBD) and cannibinol are the three main constituents. Both marijuana and tobacco smoking induce cytochrome P450 (CYP) 1A2 through activation of the aromatic hydrocarbon receptor, and the induction effect between the two products is additive. Smoking cessation is associated with rapid downregulation of CYP1A enzymes. On the basis of the estimated half-life of CYP1A2, dose reduction of CYP1A drugs may be necessary as early as the first few days after smoking cessation to prevent toxicity, especially for drugs with a narrow therapeutic index. Nicotine is a substrate of CYP2A6, which is induced by oestrogen, resulting in lower concentrations of nicotine in females than in males, especially in females taking oral contraceptives. The significant effects of CYP3A4 inducers and inhibitors on the pharmacokinetics of Δ9THC/CBD oromucosal spray suggest that CYP3A4 is the primary enzyme responsible for the metabolism of Δ9THC and CBD. Limited data also suggest that CBD may significantly inhibit CYP2C19. With the increasing use of marijuana and cannabis products, clinical studies are needed in order to determine the effects of other drugs on pharmacokinetics and pharmacodynamics.

Inhibition of cytochrome P450 by ethambutol in human liver microsomes

Although cytochrome P450 inhibition is the major drug-drug interaction (DDI) mechanism in clinical pharmacotherapy, DDI of a number of well-established drugs have not been investigated. Rifampicin, isoniazid, pyrazinamide and ethambutol combination therapy inhibits clearance of theophylline in patients with tuberculosis. We determined the inhibitory effects of ethambutol on the activities of nine CYP isoforms including CYP1A2, 2A6, 2B6, 2C8, 2C9, 2C19, 2D6, 2E1 and 3A4 in pooled human liver microsomes (HLM). As measured by liquid chromatography-electrospray ionization tandem mass spectrometry, ethambutol exhibited strong inhibitory potential against CYP1A2 and CYP2E1, moderate against CYP2C19 and CYP2D6 and weak against CYP2A6, CYP2C9 and CYP3A4, based on the IC50 values. The K(i) value of ethambutol for CYP1A2 was 1.4 μM and for CYP2E1 was 2.9 μM. Inhibition of CYP1A2 and CYP2E1 was not increased by preincubation with ethambutol and β-nicotinamideadenine dinucleotide phosphate (NADPH), suggesting that the ethambutol-induced CYP inhibition may not be metabolism-dependent. Kinetic analysis showed that the inhibition of CYP1A2 and CYP2E1 by ethambutol was best fit to a competitive inhibition model. Formation of 1-methylxanthene and 1,3-dimethyluric acid from theophylline in HLM was decreased to 47% and 36%, respectively, by 3.0 μM ethambutol, which is comparable to its IC50 value against CYP1A2. Considering its maximal plasma concentrations of ~10 μM and long half-life of ~22 h, our findings raise the possibility that ethambutol causes significant DDIs in clinical situations with drugs with narrow therapeutic index, such as theophylline, in clinical situations.

Effect of a Triphasic Oral Contraceptive on Drug-Metabolizing Enzyme Activity as Measured by the Validated Cooperstown 5+1 Cocktail

The effects of a common oral contraceptive preparation on the activity of 7 drug-metabolizing enzymes were investigated using the validated Cooperstown 5+1 Cocktail. In a randomized crossover fashion, 10 premenopausal women received caffeine, dextromethorphan, omeprazole, intravenous midazolam, and warfarin + vitamin K with and without a triphasic oral contraceptive (ethinyl estradiol 35 microg) and varying doses of daily norgestimate (0.18, 0.215, and 0.25 mg). Bioequivalence testing showed nonequivalence in drug versus no-drug treatment on the activity of drug-metabolizing enzymes (as reflected by metabolite ratios following probe drug administration); the activity of CYP1A2, CYP2C19, and NAT-2 decreased following the oral contraceptive, whereas the activity of CYP2C9 and CYP2D6 increased. No effects on xanthine oxidase or hepatic CYP3A were seen. Application of a non-parametric statistical testing approach revealed a significant difference only for CYP1A2 and CYP2C19. This triphasic oral contraceptive may have a clinically significant effect on the activity of some drug-metabolizing enzymes.

Prediction of drug response and safety in clinical practice

Many clinicians hoped that the completion of the Human Genome Project would result in "individualized drug therapy," i.e., determining the right medication at the right dose 100% of the time based upon the individual's genetics. The pharmacogenomic prediction of drug efficacy and safety has not become a reality due to continuing realization of the complexity dictating the human-drug interaction. New methods of metabolomics, proteomics, and transcriptomics that account for this complexity hold promise for translational researchers hoping to increase drug efficacy and decrease drug toxicity.

Prediction of drug clearance in a smoking population: modeling the impact of variable cigarette consumption on the induction of CYP1A2

PURPOSE

To derive estimates of CYP1A2 abundance as a function of daily cigarette consumption and use these values to predict the clearances of CYP1A2 substrates in smokers.

METHODS

Smoking-induced changes in hepatic CYP1A2 abundance were extrapolated from reported in vivo caffeine clearance data for sub-groups of a smoking population that were categorized according to their daily cigarette consumption. These abundance values together with in vitro-in vivo extrapolation (IVIVE) within the Simcyp population-based Simulator were used to predict the clearances of caffeine, theophylline, and clozapine in smokers. The model was used subsequently to predict differences in oral clearance between smoker and non-smoker cohorts in a Phase 1 clinical trial involving PF-2400013, a drug metabolized by CYP1A2.

RESULTS

Estimated hepatic CYP1A2 abundance values were 52, 64, 79, 90, and 94 pmol/mg microsomal protein for subjects smoking 0, 1-5, 6-10, 11-20, and >20 cigarettes/day respectively. Predicted -fold increases in oral clearance of caffeine, theophylline and clozapine in smokers relative to non-smokers were consistent with observed data. The validated model was able to recover the smoking-induced increase in oral clearance of PF-2400013; predicted and observed mean (CV%) values in male nonsmokers and smokers were 90 L/h (40%) and 141 L/h (34%) respectively, and 100 L/h (58%) and 131 L/h (33%) respectively.

CONCLUSIONS

This study demonstrates that it may be possible to predict the clearance of CYP1A2 substrates in smoking populations using quantitative estimates of CYP1A2 abundance based on daily cigarette consumption in conjunction with an IVIVE approach.

Pharmacokinetics and metabolism of natural methylxanthines in animal and man

Caffeine, theophylline, theobromine, and paraxanthine administered to animals and humans distribute in all body fluids and cross all biological membranes. They do not accumulate in organs or tissues and are extensively metabolized by the liver, with less than 2% of caffeine administered excreted unchanged in human urine. Dose-independent and dose-dependent pharmacokinetics of caffeine and other dimethylxanthines may be observed and explained by saturation of metabolic pathways and impaired elimination due to the immaturity of hepatic enzyme and liver diseases. While gender and menstrual cycle have little effect on their elimination, decreased clearance is seen in women using oral contraceptives and during pregnancy. Obesity, physical exercise, diseases, and particularly smoking and the interactions of drugs affect their elimination owing to either stimulation or inhibition of CYP1A2. Their metabolic pathways exhibit important quantitative and qualitative differences in animal species and man. Chronic ingestion or restriction of caffeine intake in man has a small effect on their disposition, but dietary constituents, including broccoli and herbal tea, as well as alcohol were shown to modify their plasma pharmacokinetics. Using molar ratios of metabolites in plasma and/or urine, phenotyping of various enzyme activities, such as cytochrome monooxygenases, N-acetylation, 8-hydroxylation, and xanthine oxidase, has become a valuable tool to identify polymorphisms and to understand individual variations and potential associations with health risks in epidemiological surveys.

Environmental tobacco smoke and its effect on the symptoms and medication in children with asthma

Environmental tobacco smoke (ETS) worsens asthmatic symptoms. We analyzed the relationship between levels of ETS and asthmatic symptoms and medication. We asked parents of 282 asthmatic children about the general condition, smoke exposure and medication. Patients were classified into three groups: no-ETS (no smoking), mild-ETS (smoking in the house but not in the same room as patient), and heavy-ETS (smoking in the same room as patient). We classified 116 children in no-ETS group, 124 children in mild-ETS group and 42 children in heavy-ETS group. The symptoms were worst and prevalence of leukotriene receptor antagonist and long-acting beta(2)-agonist use were highest in heavy-ETS group. However, there was no statistical difference between no-ETS and mild-ETS groups in prevalence of anti-asthmatic drug use and symptoms. We conclude that a smoking ban in rooms used by asthmatic children is an easy way to reduce ETS, asthmatic symptoms and the use of anti-asthmatic drugs.

Effects of gender and moderate smoking on the pharmacokinetics and effects of the CYP1A2 substrate tizanidine

OBJECTIVE

We studied the effects of gender and smoking on the pharmacokinetics and effects of the cytochrome P450 (CYP) 1A2 substrate tizanidine.

METHODS

Seventy-one healthy young volunteers (male and female nonsmokers, male smokers) ingested 4 mg tizanidine. Plasma concentrations and pharmacodynamics of tizanidine were measured, and a caffeine test was performed.

RESULTS

Among nonsmokers, the peak concentration (C(max)) and area under concentration-time curve from 0 to infinity [AUC(0-infinity)] of tizanidine did not differ significantly between females and males. However, the half-life (t(1/2)) was 9% shorter in female nonsmokers than in male nonsmokers (P < 0.05). In male smokers, the t(1/2) was 10% shorter and the weight-adjusted AUC(0-infinity) 33% smaller than in male nonsmokers (P < 0.05). The caffeine/paraxanthine ratio was 35-40% smaller (P = 0.001) in male smokers than in nonsmoking males or females, but did not differ between males and females. Tizanidine lowered blood pressure and caused drowsiness significantly (P < 0.05) more in females than in either male groups. The effects on blood pressure were smallest in male smokers (P < 0.05).

CONCLUSIONS

Gender by itself seems to have no clinically significant effect on the pharmacokinetics of tizanidine, whereas smoking reduces plasma concentrations and effects of tizanidine. Any possible effect of gender and smoking is largely outweighed by individual variability in CYP1A2 activity due to genetic and environmental factors and in body weight. Careful dosing of tizanidine is warranted in small females, whereas male smokers can require higher than average doses.

Assessment of CYP1A2 Activity in Clinical Practice: Why, How, and When?

The cytochrome P450 enzyme CYP1A2 mediates the rate-limiting step in the metabolism of many drugs including theophylline, clozapine, and tacrine as well as in the bioactivation of procarcinogens. CYP1A2 activity shows both pronounced intra- and interindividual variability, which is, among other factors, related to smoking causing enzyme induction, to drug intake and to dietary factors which may result in induction or inhibition. In contrast to these exogenous factors, genetic influences on enzyme activity seem to be less pronounced. Therefore, phenotyping of CYP1A2, i.e. the determination of the actual activity of the enzyme in vivo, represents a useful approach both for scientific and clinical applications. CYP1A2 is almost exclusively expressed in the liver. Since liver tissue cannot be obtained for direct phenotyping, a probe drug which is metabolized by CYP1A2 has to be given. Proposed probe drugs include caffeine, theophylline, and melatonin. Caffeine is most often used because of the predominating role of CYP1A2 in its overall metabolism and the excellent tolerability. Various urinary, plasma, saliva, and breath based CYP1A2 caffeine metrics have been applied. While caffeine clearance is considered as the gold standard, the salivary or plasma ratio of paraxanthine to caffeine in a sample taken approximately 6 hr after a defined dose of caffeine is a more convenient, less expensive but also fully validated CYP1A2 phenotyping metric. CYP1A2 phenotyping is applied frequently in epidemiologic and drug-drug interaction studies, but its clinical use and usefulness remains to be established.

Uncertainty factors for chemical risk assessment. human variability in the pharmacokinetics of CYP1A2 probe substrates

A 100-fold uncertainty factor is used to derive acceptable daily intakes for compounds causing thresholded toxicity. The 10-fold factor for human variability can be further subdivided into two factors of 10(0.5) (3.16) to allow for toxicokinetics and toxicodynamics. The validity of the human kinetic subfactor has been analysed in relation to CYP1A2 metabolism using published in vivo pharmacokinetic parameters selected to reflect chronic exposure (metabolic and total clearances and area under the plasma concentration-time curve: CLm, CL and AUC) and acute exposure (the peak plasma concentration, C(max)). The variability in CYP1A2 activity in healthy adults, based on data after oral and intravenous dosage (CLm, CL and AUC), ranged from 34 to 42%. The variability in C(max) was 21%. The default kinetic factor of 3.16 would cover at least 99% of the healthy adult population, assuming that the data were log-normally distributed, but would give lower protection for some subgroups (pregnant women at term, healthy elderly, patients with liver disease), and was inadequate for neonates. This analysis of in vivo kinetic data for CYP1A2 substrates illustrates the importance of quantifying human variability in specific metabolic pathways, and of identifying potentially susceptible subgroups of the human population, in order to determine the scientific validity of uncertainty factors.

Pharmacokinetic changes of theophylline and amikacin through the menstrual cycle in healthy women

The objective of this open-label, single-dose study was to clarify the influence of the menstrual cycle on the pharmacokinetics of theophylline (n = 10) and amikacin (n = 8) in young healthy Japanese women with regular menstrual cycles. Each subject received an intravenous infusion of theophylline or amikacin sulfate at four different phases--mid-follicular (phase I), peri-ovulatory (phase II), mid-luteal (phase III), and premenstrual days (phase IV). In the theophylline study, there were no significant differences in the pharmacokinetic parameters among the four phases studied. In the amikacin study, CLtot was 15% higher in phase III than in phase I (p < 0.01). Vd beta was 35% higher in phase III than in phase I (p < 0.05). The other pharmacokinetic parameters of amikacin were not significantly altered during the menstrual cycle. Evidence suggests that the phase of the menstrual cycle may be a factor in determining the pharmacokinetics of amikacin.

Influence of oral contraceptive use and cigarette smoking, alone and together, on antipyrine pharmacokinetics

The pharmacokinetics of antipyrine following a single 1-g intravenous dose was determined in 63 healthy women. Subjects were divided into 4 groups as follows: 1) cigarette smokers using low-dose oral contraceptives (n = 15); 2) nonsmokers using low-dose oral contraceptives (n = 12); 3) cigarette smokers not using oral contraceptives (n = 10); and 4) controls, neither cigarette smokers nor oral contraceptive users. Plasma antipyrine concentrations during 24 to 48 hours after dosage were measured by high-performance liquid chromatography. Mean kinetic variables in the nonsmoking, non-oral contraceptive using control group were: volume of distribution, 37.7 L; elimination half-life, 13.2 hours; and clearance, 34.4 mL/min. In cigarette smoking, non-oral contraceptive users versus controls, elimination half-life was reduced (8.0 vs. 13.2 hours, P < 0.05) and clearance increased (56.0 vs. 34.4 mL/min, P < 0.05). In nonsmoker oral contraceptive users, the reverse was true (elimination half-life was significantly increased: 16.6 vs. 13.2 hours, P < 0.05; and clearance was significantly decreased: 24.8 vs. 34.4 mL/min, P < 0.05). In smokers who were using oral contraceptives, values were not significantly different from controls (elimination half-life, 11.2 hours; clearance, 39.5 mL/min). Volume of distribution did not differ among the four groups. Thus the opposing effects on antipyrine clearance of the induction of metabolism by cigarette smoking and the inhibition due to low dose oral contraceptive use in effect negate each other when combined in humans.

The influence of endogenous and exogenous sex hormones in adolescents with attention to oral contraceptives and anabolic steroids

The endogenous sex hormones produce dispositional changes in the developing child as well as imparting unique male and female dispositional patterns. Age-related changes have been observed for digoxin disposition and caffeine and theophylline metabolism. These age-related dispositional changes have led to age-dependent dosing recommendations. Studies with caffeine and antipyrine indicate that this change in drug disposition occurs over a short period of time, is seen earlier in girls than in boys, and is related to pubertal (Tanner) stage and the "growth spurt". Significant changes in endogenous sex hormone concentrations occur during the menstrual cycle and during pregnancy, leading to alterations in drug binding, distribution, and clearance. Oral contraceptives (OCs) inhibit the metabolism of certain drugs resulting in toxicity or lack of efficacy. Rifampin induces the OC metabolism, resulting in decreased clinical effectiveness. Most studies did not examine these kinetic and dynamic interactions between adult and adolescent users. It is estimated that there are over 45 anabolic steroid compounds available for abuse by athletes, and their use is increasing among male and female adolescents. Although the adolescent is at increased risk of developing adverse effects from these agents, a systematic evaluation of the long-term effects of anabolic steroid abuse has not been undertaken in this population. Further research is needed regarding the influence of endogenous and exogenous hormones on adolescent drug kinetics and dynamics. Because of their frequency of use among adolescents, OCs and anabolic steroids require particular emphasis.

Effect of cigarette smoke on pharmacokinetics of oral, intrarectal, or intravenous indomethacin in rats

The effect of cigarette smoke exposure on the pharmacokinetics of indomethacin administered orally, intravenously or intrarectally was investigated in rats. When cigarette smoke exposure was performed for 10 min using a Hamburg II smoking machine immediately after the oral administration of indomethacin (5 mg/kg), the plasma indomethacin concentration was significantly lowered during the first 2 h after administration. However, there was no significant difference in plasma indomethacin concentration between the cigarette smoke-exposed and nonexposed control rats thereafter. Cigarette smoke exposure caused a significant decrease in the area under the concentration-time curve from 0 to 4 h (AUC0-4) and a prolongation of the time to reach the maximum concentration (tmax). The plasma level of O-desmethyl-indomethacin (a major metabolite) was not significantly changed by cigarette smoke. When indomethacin (5 mg/kg) was administered to rats intravenously or intrarectally, cigarette smoke exposure did not have any influence on the pharmacokinetics of indomethacin or 0-desmethyl-indomethacin. The pharmacokinetic effect of cigarette smoke on orally administered indomethacin was mimicked by the subcutaneous injection of nicotine at 0.3 mg/kg but not at 0.1 mg/kg. These results suggest that acute exposure to cigarette smoke decreases the plasma concentration of indomethacin when it is administered orally but not intrarectally or intravenously.

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.

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.

Influence of oral contraceptives on drug therapy

Interferences between drugs and oral contraceptives are considered to alter pharmacokinetics and thus the efficacy of steroidal hormones. It should be noted, however, that steroids can also modify the metabolism and pharmacodynamic effects of various substances. To the present knowledge, phase I (i.e., oxidation, demethylation) and phase II reactions (conjugation) are concerned. Drugs sharing those enzymatic systems with oral contraceptives experience either an increase in bioavailability by inhibition of oxidative metabolism or undergo accelerated elimination by induced conjugation. Such interaction may be of practical interest in subjects who take oral contraceptives and are simultaneously treated with antidepressants, antihypertensives, insulin, synthetic glucocorticoids, theophylline, and caffeine.

Influence of the menstrual cycle on theophylline pharmacokinetics in asthmatics

The effect of menstrual cycle on the pharmacokinetics of theophylline has been studied in 9 female asthmatic patients. At three different times during the cycle (first, tenth and twentieth days) each subject received an i.v. infusion of 240 mg theophylline. Significant changes were found in the kinetics of theophylline according to the menstrual cycle. The maximum plasma drug concentration, minimum mean residence time, and minimum elimination half-life were observed at mid-cycle. The lowest clearance was observed at D 20. The AUC and volume of distribution did not vary significantly during the cycle.

Effect of oral contraceptive steroids on the pharmacokinetics of phenprocoumon

1. The effect of chronic administration of oral contraceptive steroids (OCS) on the pharmacokinetics of the oral anticoagulant phenprocoumon was investigated in seven healthy females. 2. Plasma concentrations of phenprocoumon and the urinary recovery of unchanged as well as conjugated drug were measured following a single oral dose of 0.22 mg kg-1. A group of seven non-smoking, drug-free women matched for age and body weight served as controls. 3. Administration of OCS was associated with a significant increase in the clearance of phenprocoumon from 1.6 +/- 0.7 to 2.0 +/- 0.7 ml min-1 kg-1 (P less than 0.05). The urinary recovery of phenprocoumon glucuronide was significantly higher in OCS users (21.0 +/- 16 vs 14.0 +/- 10 (% of dose); P less than 0.05). No difference in plasma protein binding of phenprocoumon was observed, being 99.2 +/- 0.07 in both groups. 4. The accelerated glucuronidation of phenprocoumon in OCS users suggests the need for careful monitoring of the anticoagulatory response in these subjects, especially when the OCS are withdrawn.

Sustained release theophylline preparations. Practical recommendations for prescribing and therapeutic drug monitoring

Theophylline is an important antiasthmatic medication which has bronchodilator properties. With increased understanding of the relationships of serum theophylline concentration and effect, both adverse and beneficial, oral dosage forms were developed to provide consistent serum theophylline concentrations with the benefit of convenient dosage intervals for long term use. Since factors such as concurrent disease states, drug interactions and age have a profound effect on theophylline disposition, relatively sophisticated dosage guidelines have evolved. Theophylline is in fact a model drug for the application of pharmacokinetic principles to the individualization of a treatment regimen. The purpose of this discussion is to review the relationship of serum theophylline concentration and pharmacodynamic effect and the special properties of oral sustained release theophylline formulations, and to provide a practical approach to prescribing theophylline. Guidelines are provided on the use of serum theophylline concentrations to individualize the theophylline dose, with an analysis of available techniques to monitor theophylline.

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.

Theophylline and antipyrine disposition in smoking and non-smoking epileptic subjects

Theophylline and antipyrine disposition has been compared in smoking epileptic patients, non-smoking epileptic patients and non-smoking healthy volunteers. Although clear differences in drug clearance and half-life were evident as a result of anticonvulsant drug therapy, no effect of smoking was discernible. Thus, additive effects from induction of the hepatic microsomal monooxygenase system in man by anticonvulsant drugs and polycyclic aromatic hydrocarbons (in cigarette smoke) were not evident.

Longitudinal effects of pregnancy on the pharmacokinetics of theophylline

The effects of pregnancy on the disposition of theophylline were assessed in 10 patients throughout pregnancy and post-partum. The clearance relative to total theophylline concentrations was only slightly affected during the first two trimesters (2.61 +/- 0.63 l/h and 2.85 +/- 1.05 l/h), while a statistically significant reduction was evident late in pregnancy (2.05 +/- 0.49 l/h). Post-partum clearance values (2.16 +/- 2.81 l/h) suggest an ongoing suppression relative to pre-pregnancy levels. A similar pattern was evident with clearance values based on free theophylline plasma concentrations (p = 0.12). Absolute volume of distribution increased in concert with gestation, suggesting that theophylline partitions into the enlarged tissue spaces. In addition, theophylline binding to plasma proteins decreased, albeit insignificantly, during the second (fraction bound = 29%) and third (32%) trimesters compared to post-partum values (41%). Increases in half-life during the third trimester (13.00 +/- 2.31 h vs 9.53 +/- 3.53 h post-partum) were highly significant. This change reflects the net effect of reduced clearance and increased distribution. Breast feeding had no effect on the disposition of theophylline, although the transfer of this compound into breast milk was confirmed.

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.

Polymorphisms of antipyrine and theophylline metabolism in man: molecular and clinical implications

In normal subjects under carefully controlled uniform environmental conditions, evidence was obtained for genetically controlled polymorphisms of antipyrine and theophylline metabolism. The relationship of these polymorphisms to each other and to previously described polymorphisms is discussed in terms of their molecular and clinical implications.

Lack of effect of highly purified subunit influenza vaccination on theophylline metabolism

Plasma concentrations of theophylline, and urinary recovery of theophylline and its metabolites [1, 3-dimethyluric acid, 3-methyl xanthine and 1-methyluric acid] were measured before and after highly purified subunit influenza vaccination in seven healthy subjects, and five subjects with chronic obstructive bronchitis. No cases of theophylline toxicity were seen, and there was no increase in mean plasma theophylline concentration or significant decrease in urinary metabolite concentration after vaccination. An antibody response to vaccination was demonstrated in all subjects. Highly purified subunit influenza vaccination may be given with safety to patients on theophylline.

Polymorphism of theophylline metabolism in man

To determine whether genetic mechanisms control large interindividual variations in theophylline elimination in normal uninduced human subjects, and, if so, to test the possibility that these genetic factors are transmitted as a simple Mendelian trait, theophylline was administered to 79 unrelated adults, six sets of monozygotic twins, six sets of dizygotic twins, and six two-generation families. Thereafter, in urine collected from each subject at regular intervals for 48 h, concentrations of theophylline and its three principal metabolites were measured and rate constants of formation of these metabolites calculated. The twin study, designed to determine the relative contributions of genetic and environmental factors to large interindividual variation in theophylline elimination, revealed predominantly genetic control. Values for this genetic component, designated heritability (H1(2)), of interindividual variation in rate constants of metabolite formation were 0.61, 0.84, and 0.95 for 3-methylxanthine, 1-methyluric acid, and 1,3-dimethyluric acid, respectively. H1(2) for the overall theophylline elimination rate constant (kel) was lower (0.34). In the 79 unrelated adults, each distribution curve for rate constants of formation of each theophylline metabolite appeared to be trimodal. By contrast, the distribution curve for the overall theophylline elimination rate constant appeared to be either unimodal or bimodal. The extent of interindividual variation was fourfold for theophylline kel and 6-8-fold for the three principal metabolites. High correlations among the three rate constants in individual subjects suggested their regulation by a single shared factor. In six families carefully selected to be under near basal environmental conditions so that hepatic theophylline metabolism of each family member would be neither markedly induced nor inhibited, phenotypes for theophylline metabolite rate constants were assigned. This assignment of phenotype was made by the position of each family member's rate constant on the three distribution curves that were generated from the 79 unrelated subjects. In each family, pedigree analysis of the three phenotypes for each rate constant was consistent with their control by two alleles at a single genetic locus and with autosomal codominant transmission. Frequencies of the two alleles at each genetic locus controlling rate constants of formation of theophylline metabolites were similar (p = 0.49, 0.53, and 0.52). In the three families studied with antipyrine (AP) as well as with theophylline, AP k(el) correlated (r approximately 0.7) with each rate constant of theophylline metabolite formation, as well as with theophylline k(el). While these results are compatible with a common regulatory element in the AP and theophylline polymorphisms, other evidence suggests more than a single genetic polymorphism. This additional evidence includes different gene frequencies for the AP (p approximately 0.1) and theophylline (p approximately 0.5) polymorphisms, different genotype assignments in several families for some theophylline metabolites, different distribution curves for theophylline k(el) from those for the three theophylline metabolites in 79 unrelated subjects, and finally low correlations between AP metabolite rate constants and theophylline metabolite rate constants in the three families receiving both drugs.

Impairment of caffeine clearance by chronic use of low-dose oestrogen-containing oral contraceptives

The effect of chronic (greater than 3 months) administration of low-dose oestrogen-containing (less than 50 micrograms oestrogen) oral contraceptives (OCS) on the pharmacokinetics of caffeine has been examined in a treated females matched with 9 non-smoking, drug-free, healthy control females of similar age, weight and ethnic origin. Each subject received 162 mg caffeine base orally after an overnight fast. OCS subjects had a prolonged elimination half-life of caffeine, (mean 7.88 h vs 5.37 h in the controls). This was the result of marked impairment of the plasma clearance of caffeine (1.05 vs 1.75 ml/min/kg, respectively) with no change in apparent volume of distribution (0.685 in OCS vs 0.7501/kg in the control group). The absorption parameters determined were peak plasma caffeine concentration (3.99 vs 4.09 micrograms/ml) and time to peak concentration after drug administration (1.52 vs 0.79), which was moderately prolonged in OCS users. Thus, caffeine clearance, previously reported to be a specific marker of cytochrome P-448 activity in man, is decreased by chronic OCS use. This suggests that OCS may cause significant impairment of this enzyme activity as assessed in vivo. With chronic caffeine consumption, OCS users are predicted to have an increased steady-state plasma caffeine concentration as compared to non-OCS users.

Automated theophylline assay using gas chromatography and a mass-selective detector

An automated gas chromatographic--mass spectrometric assay for theophylline is described. Theophylline is extracted from plasma or urine (50 microliter) and transformed into an N-pentyl derivative. The internal standard used for quantitation is [1,3-15N, 2-13C]theophylline. The detection is performed by monitoring the molecular ions 250 for theophylline and 253 for the internal standard with a quadrupole mass specific detector HP 5790 A. The system has been fully automated: injection, calibration, assay, calculation. The method shows excellent analytical parameters: linearity between 2 and 40 micrograms/ml; day-to-day reproducibility 1.82% for a concentration of 15 micrograms/ml; repeatability 0.75% (15 micrograms/ml) and 0.33% (30 micrograms/ml). Accuracy is also excellent. Due to the use of an internal standard labelled with stable isotopes, the specificity and high analytical quality of the method make it useful as a reference method to compare with routine theophylline assays.

Pharmacokinetic drug interactions with theophylline

Since up to 90% of a theophylline dose is biotransformed, drugs influencing microsomal enzyme systems in the liver may affect the elimination of theophylline. Other integrated mechanisms (e.g. hepatic uptake) may also be altered by concurrent administration of other drugs. Whatever the mechanism, the interaction may be sufficient to necessitate adjustment of the theophylline dosage, preferably guided by plasma theophylline determinations. Comedication with phenobarbitone may require an increase of the theophylline dose by about 30% due to increased clearance resulting from enzyme induction. Similarly, with phenytoin and carbamazepine a dose increase of about 40 to 50% may be required. In the case of rifampicin, isoniazid or sulphinpyrazone comedication, an increase of the theophylline dose by about 20 to 25% may be needed. On the other hand, other drugs decrease theophylline clearance, making a reduction in the dose of concurrent theophylline advisable: with usual doses of erythromycin, propranolol and isoprenaline (isoproterenol), a reduction of about 25% is needed; with cimetidine and oral contraceptives by about 30% or more; and with triacetyloleandomycin (troleandomycin) by about 50%. In high doses, the xanthine oxidase inhibitor allopurinol can also retard theophylline elimination, and a reduction of the theophylline dose by about 20% may be advisable. Conflicting results have been reported on the influence of frusemide (furosemide) and influenza vaccines, while data regarding the effect of corticosteroids, benzodiazepines and verapamil on theophylline kinetics are not yet conclusive. Many drugs, however, appear not to significantly affect theophylline clearance. Some are from the same therapeutic group as the drugs mentioned above and offer clinical alternatives for coadministration with theophylline. Examples of drugs not found to have a significant effect on theophylline pharmacokinetics are ranitidine, josamycin, midecamycin, amoxycillin, tetracycline, cephalexin, cefaclor, orciprenaline, metoprolol, antacids, medroxyprogesterone acetate, metoclopramide and metronidazole. Most of the drugs discussed in this review appear not to affect the volume of distribution of theophylline significantly.

Tobacco smoking and drugs: a clinically important interaction?

Tobacco smoking may interact with the metabolism of a number of drugs. This has been demonstrated clearly in pharmacokinetic studies in animals and man. HOwever, tobacco smoking as a variant has been studied with relatively few clinically essential drugs and these studies do not always demonstrate a consistent effect of smoking. This review therefore not only records what data have emerged from pharmacokinetic or other studies, but it also attempts to determine whether such interactions have clinical significance. The main finding of this review is that, in the majority of examples, there is little evidence that there is a recognizable hazard from the interaction per se. The exceptions to this general conclusion are limited to four drugs: insulin, propoxyphene, propranolol, and theophylline preparations. With these drugs, there is some evidence of clinical importance of an interaction with tobacco smoking.