Pharmacokinetics of a new sustained-release formulation of theophylline sodium glycerinate in healthy subjects with a new asymmetric dosage regimen

Pharmacokinetics of a new sustained-release formulation of theophylline sodium glycerinate in healthy subjects was studied. In this study, a new asymmetric dosage regimen was presented to achieve a better accordance with the chronotherapy of asthma. Each of 10 subjects was administered one tablet (equivalent to 0.1 g anhydrous theophylline) in the morning and four tablets in the evening for a consecutive 6 days and blood samples were collected at the predetermined time and analyzed by a validated HPLC method. This new regimen produced a steady and effective level of theophylline in plasma for the whole day, especially in the evening. A lower dose in the morning could reach the effective level (C(min)4.97 +/- 1.60 microg/mL and C(max)10.68 +/- 1.80 micro g/mL over the a.m. dosing interval) and a higher dose in the evening did not result in toxic levels but led to a reasonable concentration range (C(max)9.72 +/- 1.56 microg/mL over p.m. dosing interval), which could maintain a higher plasma theophylline concentration without the risk of serious adverse events and control asthmatic symptoms probably occurring during the night or early in the morning. The results suggested that the proposed asymmetric regimen was necessary, practicable and safe for twice daily sustained-release tablets of theophylline sodium glycerinate and also provides the basis for the clinical dosage regimen of other theophylline formulated products.

Therapeutic drug monitoring of theophylline in frail elderly patients: oral compared with nasogastric tube administration

SUMMARY

Treating debilitated elderly patients through nasogastric tube (NGT) can change the pharmacokinetic characteristics of drugs, mainly those that are slow released (SR). The purpose of this study was to compare pharmacokinetic parameters between patients who receive SR theophylline orally and those who receive it through NGT.

PATIENTS AND METHODS

The authors studied elderly patients in the geriatric ward receiving SR theophylline for chronic obstructive lung disease. In 17 patients fed by NGT (group I), theophylline was removed from the capsule and mixed with 10 mL of water. Group II included 15 patients who swallowed the drug orally. Theophylline blood levels were measured, as were peak concentration (C(max) ), time to peak (T(max) ), and area under the serum concentration-time curves (AUC).

RESULTS

The mean daily dose was not statistically different between the two groups: 320 +/- 130 (200-500) mg/d in group I and 360 +/- 85 (200-500) mg/d in group II, given twice daily. All pharmacokinetic measurements were lower in group I as compared with group II: trough theophylline blood levels were 3.78 +/- 3.2 (0.5-10.77) microg/mL versus 8.63 +/- 4.6 (0-15.61) microg/mL ( P= 0.002); C(max) was 6.53 +/- 4.1 (1.3-13.33) microg/mL versus 10.51 +/- 3.30 (4.3-16.28) microg/mL (P = 0.0058), and AUC was 50.04 +/- 38.59 (11-112) microg/h/mL versus 80.37 +/- 28.8 (23-148) microg/h/ml (P = 0.024). CONCLUSIONS Patients receiving the drug through NGT had variability and unexpectedly low blood levels. Therefore, the pharmacokinetic parameters of SR preparations should be evaluated before their administration through NGT.

Severe 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) intoxication: insights into the measurement of hepatic cytochrome P450 1A2 induction

OBJECTIVE

The correct in vivo quantification of aryl hydrocarbon receptor-mediated induction of cytochrome P450 1A2 (CYP1A2) in humans is a long-standing question. We compared the performance of several modifications of the caffeine test for measurement of CYP1A2 activity in subjects with exceptionally high, low, or absent enzyme induction.

METHODS

CYP1A2 activity was measured in 2 women highly exposed to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), in 1 man moderately exposed, and in 50 control subjects (30 nonsmokers and 20 heavy smokers). After the application of a test dose, caffeine demethylation was detected with the carbon 13 breath test, the total clearance, and several serum and urinary metabolite ratios.

RESULTS

In the highly TCDD-exposed persons, results of the breath test (cumulative 15-minute dose), the total caffeine clearance, the serum metabolic ratio paraxanthine/caffeine (30 and 120 minutes after application), and the urinary metabolic ratio sum of 5-acetylamino-6-formylamino-3-methyluracil (AFMU), 1-methyluric acid (1U), and 1-methylxanthine (1X) over 1,7-dimethyluric acid (17U) showed a CYP1A2 activity 8 to 10 times higher than the mean of nonsmokers. In contrast, two caffeine urinary metabolic ratios with the parent substance in the denominator did not reflect the CYP1A2 enzyme induction. These ratios strongly depended on urine flow. For the breath test, only results evaluated for a short sampling period (eg, 15 minutes after application) revealed the high induction. Compared with nonsmokers, higher mean values (maximally 1.8 times) were observed in smokers with all tests.

CONCLUSION

After high TCDD exposure, hepatic CYP1A2 activity is inducible at least 10 times in humans. Moderate TCDD exposure (up to 1000 ppt in blood fat) does not cause a CYP1A2 induction that can be measured to differentiate from background exposure individually. Therefore direct quantification of such toxins is more specific and sensitive.

Theophylline blocks ethanol withdrawal-induced hyperalgesia

AIMS

This study examined the effects of theophylline on the hyperalgesia produced by ethanol withdrawal using a radiant heat tail-flick assay.

METHODS

Chronic effects of ethanol were tested in four groups of rats which received 10 days exposure to a liquid diet [ethanol alone or with theophylline [0.5 and 1.0 mg/kg, twice daily, intraperitoneally (i.p.)], and dextrin control diet]. Ethanol withdrawal was tested 12 h after removal of the liquid diet. Effects of cumulative doses of the non-selective adenosine agonist 2-chloroadenosine (2-CADO; 0.6-10 mg/kg, i.p.) were tested during withdrawal in the ethanol-treated groups.

RESULTS

Chronic exposure to ethanol produced antinociception, and hyperalgesia was seen during withdrawal. Subchronic administration of theophylline (0.5-1.0 mg/kg, twice daily, i.p.) dose-dependently prevented the ethanol-withdrawal-induced hyperalgesia. During ethanol withdrawal, 2-CADO was less potent than when given to non-dependent rats and this effect was prevented by subchronic administration of theophylline (1.0 mg/kg).

CONCLUSIONS

These findings provide behavioural evidence in agreement with earlier work on the role of adenosine in the development of ethanol tolerance and withdrawal, and suggest that adenosine receptors play an important role in the development of hyperalgesia during ethanol withdrawal.

Differences in caffeine and paraxanthine metabolism between human and murine CYP1A2

For the characterisation of murine models of CYP1A2 mediated metabolism in humans we compared the metabolism of caffeine and paraxanthine in human liver microsomes (LM) (two samples) and in LM from CYP1A2-null and wild-type mice. Inhibition experiments were carried out with the quinolones norfloxacin and pefloxacin and the substrate, caffeine. Additionally, in vivo pharmacokinetics of paraxanthine was determined in CYP1A2-null and wild-type mice. All LM produced the primary metabolites of caffeine and paraxanthine. In human LM, the main metabolite of caffeine was paraxanthine (K(M) 0.4 and 0.5 mmol L(-1)). In wild-type and CYP1A2-null mice LM, the main caffeine metabolite was 1,3,7-trimethylurate, but formation was not saturable. Apparent K(M) for paraxanthine formation from caffeine in wild-type and CYP1A2-null murine LM were 0.2 and 4.9 mmol L(-1), respectively. The main metabolite of paraxanthine was 1-methylxanthine in human (K(M) 0.13 and 0.2 mmol L(-1)) and in wild-type mice LM (K(M) 0.53 mmol L(-1)). In CYP1A2-null murine LM, the main paraxanthine metabolite was 7-methylxanthine. The quinolones competitively inhibited caffeine metabolism in human but not in wild-type or CYP1A2-null murine LM. No obvious differences were seen for blood pharmacokinetics and urinary metabolite excretion of paraxanthine between CYP1A2-null and wild-type mice. Thus, for paraxanthine, norfloxacin and pefloxacin interaction with CYP1A2 there were clear differences between mice and man. Our results suggest that an interspecies comparison is required for the metabolism of individual xenobiotics interacting with CYP1A2 prior to the use of mice models to predict its toxicity and/or pharmacological activity in man.

Theophylline blood levels in Sri Lankan asthmatics: comparison of two methods of assay

OBJECTIVES

To assay theophylline blood levels in a sample of Sri Lankan chronic asthmatics taking oral theophylline, and to evaluate a simple and cost effective ultraviolet spectrophotometric assay for theophylline levels in blood.

SETTING

Chronic asthmatics taking oral theophylline attending medical clinics at the National Hospital of Sri Lanka (NHSL) were recruited for the study. Blood samples were collected from recruited patients on their subsequent clinic visit.

DESIGN AND METHODS

A cross-sectional study of theophylline blood levels. Blood samples were assayed for trough theophylline levels using two methods: an automated homogeneous enzyme immunoassay (EMIT), and a low cost ultraviolet spectrophotometric method.

RESULTS

Only 2 patients of the 24 had theophylline blood levels in the accepted therapeutic range (10 to 20 micrograms/ml) (3.4); 19 patients had levels under 5 micrograms/ml. A correlation coefficient of 0.99 was obtained in the statistical comparison of the two methods, indicating that the spectrophotometric method has similar accuracy as the reference EMIT assay.

CONCLUSIONS

The results signal a need for monitoring of theophylline in asthmatics when accepted clinical indications are present. The ultraviolet spectrophotometric method is ideal to initiate therapeutic drug monitoring (TDM) in the country because of its low cost (about Rs. 55 per assay), requiring only a UV recording spectrophotometer.

A quantitative thin layer chromatography method for determination of theophylline in plasma

A simple assay method for theophylline in plasma using thin layer chromatography (TLC) was developed. The method involves extraction of the drug and internal standard (acetaminophen) by chloroform-isopropanol (75:25) followed by separation on TLC silica plates using a mixture of acetic acid, isopropanol, toluene (1: 12: 6), as the eluting solvent. Both peak height ratios and peak are ratios showed high correlation coefficient (r>0.98, p<0.001). However we used peak heights for the determinations. Within-day and between-day coefficients of variation were less than 4.4% and 7.8% respectively. The assay proved inexpensive, accurate and reproducible with a limit of detection of 100 ng/ml that makes it suitable for bioavailability studies.

Lack of renoprotective effect of theophylline during aortocoronary bypass surgery

BACKGROUND

The incidence of acute renal failure (ARF) after cardiac surgery remains high, despite improvements in surgical techniques and perioperative care, and is associated with an unacceptably high mortality. The adenosine receptor antagonist theophylline has been shown to confer some benefit in experimental and clinical ARF due to ischaemia, contrast media and various nephrotoxic agents.

METHODS

In a double-blind, randomized, placebo-controlled trial, the effectiveness of theophylline for prevention of renal impairment after elective coronary artery bypass grafting (CABG) was evaluated. Fifty-six patients with normal renal function received a bolus of 4 mg/kg and a subsequent continuous infusion of 0.25 mg/kg/h theophylline (n=28) or isotonic saline (n=28) for up to 96 h. Serum creatinine concentrations were measured preoperatively and daily until day 5 after surgery, and the glomerular filtration rate (GFR) ([(51)Cr]EDTA-clearance) was determined preoperatively, and at days 1, 3 and 5 after surgery.

RESULTS

Serum creatinine and GFR were the same in both groups. The number of patients with increases of serum creatinine > or =0.4 mg/dl were five in the theophylline group and four in the placebo group. Volumes of infused fluid and urine volumes were not different between groups, both ranging from approximately 7.5 to 8 l during the first 24 h after surgery. The number of patients with termination of study medication due to presumed side effects was not different between placebo and theophylline groups.

CONCLUSIONS

Theophylline administration for renal protection after CABG appears to be ineffective in a pilot study in well-hydrated patients. However, the statistical power of our study was not sufficient to exclude a possible protective effect of theophylline. The present study demonstrated the feasability of a larger trial with theophylline or one of the new specific adenosine A1 receptor antagonists in the setting of ARF after cardiac surgery.

Pharmacokinetic analysis of theophylline to assess noncompliance in therapy

OBJECTIVE

To report a case showing patient noncompliance, supported by outcomes of pharmacokinetic analysis of theophylline as a surrogate drug.

CASE SUMMARY

A 45-year-old woman with severe hypertension was treated with a variety of oral antihypertensive drugs, but there was no improvement in her elevated blood pressure. Since we suspected that her intestinal drug absorption capacity was impaired, a theophylline absorption test was performed. When oral theophylline was given to the patient in tablet form, the apparent bioavailability was only 20%, which agreed with our hypothesis. However, the bioavailability of theophylline given in liquid form was almost 100%, and theophylline in tablet form was confirmed to be fully bioavailable when the test was performed under supervision by medical staff.

DISCUSSION

The almost complete bioavailability of oral theophylline indicated that there was no impairment in intestinal absorption capacity. The low bioavailability of theophylline observed after tablet administration in the first trial was apparently a result of noncompliance, because the staff did not supervise administration to ensure that tablets were swallowed. Thus, the low response to antihypertensive therapy was attributed to patient noncompliance in taking the drugs, despite her insistence to the contrary.

CONCLUSIONS

The use of theophylline was a novel approach to evaluating the absorbability of orally administered drugs in a patient suspected of poor compliance with therapy. A great difference in theophylline bioavailability between the supervised and unsupervised trials was strongly indicative of patient noncompliance. The possibility of impaired absorption was virtually ruled out.

Selective solid-phase extraction sorbent for caffeine made by molecular imprinting

A molecularly imprinted polymer (MIP) was prepared with caffeine as the template molecule. Thermal polymerisation (60 degrees C) was optimised, varying ratios of monomer, cross linker and template. The polymer was used as a solid-phase extraction (SPE) sorbent, for selective trapping and pre-concentration of caffeine. Caffeine was loaded on the MIP-SPE cartridge using different loading conditions (solvents, pH value). Washing and elution of the caffeine bound to the MIP was studied utilising different protocols. The extraction protocol was successfully applied to the direct extraction of caffeine from beverages and spiked human plasma.

Effect of pazufloxacin mesilate on the serum concentration of theophylline

We studied the effect of pazufloxacin mesilate (T-3762), a new fluoroquinolone for intravenous administration, on the serum concentration of theophylline. Evaluation consisted of comparisons of serum levels of theophylline (when it was given alone, as a control), with serum levels of theophylline when T-3762 was given concomitantly. We measured the serum concentrations and the urinary excretion rates of theophylline in healthy adult male volunteers who were given theophylline for 5 days, followed by an i.v. infusion of T-3762. Blood and urine samples were investigated on the third and fifth days after the concomitant dosing with T-3762, to compare the serum levels and urinary concentrations of theophylline with the control values. We found that the serum concentration and the urinary excretion rates of theophylline on the fifth day after concomitant dosing with T-3762 were significantly increased compared with the levels when the volunteers had been given theophylline alone.

Prediction of plasma concentration-time curve of orally administered theophylline based on a scintigraphic monitoring of gastrointestinal transit in human volunteers

The plasma concentration-time profile of theophylline after oral administration in human volunteers was predicted using the individual gastrointestinal (GI) transit data monitored by a gamma scintigraphic technique. Theophylline was administered as aminophylline under fasted and fed condition, along with 99mTc-labeled diethylenetriamine-pentaacetic acid (DTPA), an unabsorbable marker to evaluate the GI transit by a gamma scintigraphic technique. Two healthy male volunteers participated under fasted and fed conditions in a crossover study. The GI transit was evaluated by dividing the GI tract to four segments, stomach, jejunum, ileum and cecum/colon. Under the fed condition, the GI transit pattern for each segment was confirmed to alter considerably, causing a delay in the gastric emptying mainly. Further, the plasma concentration curves of theophylline after oral administration were predicted using the GI-Transit-Absorption Model on the basis of individual GI transit parameters calculated by the fitting of the observed data to the GI-Transit Kinetic Model. The absorption rate constant in each segment and the pharmacokinetic parameters after intravenous administration used for the prediction were the values extrapolated from the data in rats and the ones normalized from the values in literatures, respectively. The plasma concentration-time curves for theophylline were well predicted using obtained individual GI transit parameters. The analysis using this method could estimate the variable absorption behavior governed by the GI transit in detail.

The pharmacokinetics, metabolism and urinary detection time of etamiphylline in camels after intramuscular administration

The pharmacokinetics of etamiphylline were determined after an intramuscular (i.m.) dose of 3.5 mg/kg body weight in six healthy camels. Furthermore, the metabolites and drug detection time were evaluated. The data obtained median and (range) were as follows: the terminal elimination half-life (t(1/2 beta), h) was 3.04 (2.03-3.62); apparent total body clearance (Cl/F, L/h/kg) was 1.27 (0.74-2.99); the apparent volume of distribution at steady state (V(ss)/F, L/kg) was 4.94 (3.57-12.54); and renal clearance (Cl(r), L/h/kg) determined in two camels was 0.005 and 0.004, respectively. The detection time of etamiphylline in urine after an i.m. dose of 3.5 mg/kg body weight ranged between 12 and 13 days. Three etamiphylline metabolites were tentatively identified in camels urine: The first one desethyletamiphylline was the main metabolite and resulted from N-deethylation of etamiphylline had a molecular weight of 251, and was detected in urine for about 13-14 days. Theophylline (molecular weight 180) was the second metabolite and resulted from ring N-dealkylation of etamiphylline. It was present in small amounts and was detected for about 5 h after drug administration in urine. The third metabolite, possibly resulted from demethylation of etamiphylline, had a molecular weight of m/z 265, and was present in small amounts and was detected in urine for about 5 h after drug administration.

Pulmonary function changes and immunomodulation of Th 2 cytokine expression induced by aminophylline after sensitization and allergen challenge in brown Norway rats

BACKGROUND AND OBJECTIVE

Evidence has shown that aminophylline has bronchoprotective, anti-inflammatory, and immunomodulatory effects. Our purpose was to evaluate the effect of different doses of aminophylline on the late-phase reaction, bronchial hyperresponsiveness (BHR) and T cell-related cytokine mRNA expression in brown Norway rats induced by ovalbumin (OA) sensitization.

METHODS

Forty rats were equally divided into four groups. Groups I, II, and III animals were sensitized and subsequently provoked with OA. Aminophylline 25 mg/kg was given intraperitoneally to the group I animals and 5 mg/kg to group II animals. Group III animals received intraperitoneal normal saline. Group IV breathed aerosolized saline as a control. After OA provocation, the animals were anesthetized. Pulmonary function tests were performed at baseline and after varying doses of acetylcholine. Thereafter, bronchoalveolar lavage was performed and the lungs were examined histologically. Total RNA was extracted from lung tissue and reverse transcriptase-polymerase chain reaction was performed using primers for interleukin (IL)-2, IL-4, IL-5, IL-6, IL-10, interferon-gamma, inducible nitric oxide synthase, and beta-actin.

RESULTS

Group III had worse pulmonary function tests, more severe BHR, and more severe lung inflammation, higher IL-4 and IL-10 cytokine levels in bronchoalveolar lavage fluid, and higher IL-4, IL-5, IL-6, IL-10, tumor necrosis factor-alpha and inducible nitric oxide synthase mRNA expression than the other three groups. Expression of IL-2 and interferon-gamma was significantly reduced in group III.

CONCLUSIONS

Both low and high dose aminophylline are effective in preventing late-phase bronchoconstriction, BHR, and an inflammatory response. Aminopylline decreases T helper cell 2-related cytokine mRNA expression but increases T helper cell 1-related cytokines mRNA expression.

Maternal serum caffeine metabolites and small-for-gestational age birth

To determine whether the third-trimester maternal serum concentration of paraxanthine, caffeine's primary metabolite, is associated with delivery of a small-for-gestational age infant (birth weight less than the 10th percentile for gestational age, gender, and ethnicity) and whether this association differs by smoking, the authors studied 2,515 women who participated in the Collaborative Perinatal Project from 1959 to 1966. The women provided a third-trimester serum sample and had been controls for a nested case-control study of spontaneous abortion. The mean serum paraxanthine concentration was greater in women who gave birth to small-for-gestational age infants (754 ng/ml) than to appropriately grown infants (653 ng/ml, p = 0.02). However, the linear trend for increasing serum paraxanthine concentration to be associated with increasing risk of small-for-gestational age birth was confined to women who also smoked (p = 0.03). There was no association between paraxanthine and fetal growth in nonsmokers (p = 0.48). Adjustment for maternal age, pre-pregnant weight, education, parity, ethnicity, and the number of cigarettes smoked per day did not alter the results substantially, although the p value for trend among smokers increased to 0.07. The authors conclude that maternal third-trimester serum paraxanthine concentration, which reflects caffeine consumption, was associated with a higher risk of reduced fetal growth, particularly among women who smoked.

An evaluation of theophylline dosing. Suggested by the Saudi national protocol for children with severe asthma

OBJECTIVE

To evaluate the impact of Saudi national protocol on the diagnosis and management of asthma for optimizing theophylline use in children with severe asthma. To also review theophylline clearance and provide guidelines for individualization of doses.

METHODS

Theophylline level was estimated in blood samples of 66 children with severe asthma who were admitted to King Abdulaziz University Hospital during the period 1998-1999. The theophylline doses given to these patients was reviewed and patients were categorized into 2 groups: Group one had received doses recommended by Saudi national protocol +/- 10%, group 2 had received doses <90% of that recommended by Saudi national protocol. The distribution of theophylline levels in blood samples of each group was estimated. Theophylline steady state level was used to estimate theophylline clearance using a standard pharmacokinetic equation.

RESULTS

Out of the total samples from group one, 70% of theophylline levels were within therapeutic range, while only 10% of the total samples of group 2 were within therapeutic range. The mean theophylline clearance in children (1-8 years) was estimated as 0.092 +/- 0.023 and was found significantly higher than the mean theophylline clearance (0.069 +/- 0.014) which is observed in older children (9-13 years).

CONCLUSION

Saudi national protocol theophylline dose guidelines had a favorable impact on the optimization of theophylline use in children with severe asthma. Guidelines that ensure accurate adjustment of doses on individual basis in view of drug level were suggested.

Physiologic hyperinsulinemia enhances human skeletal muscle perfusion by capillary recruitment

Despite intensive study, the relation between insulin's action on blood flow and glucose metabolism remains unclear. Insulin-induced changes in microvascular perfusion, independent from effects on total blood flow, could be an important variable contributing to insulin's metabolic action. We hypothesized that modest, physiologic increments in plasma insulin concentration alter microvascular perfusion in human skeletal muscle and that these changes can be assessed using contrast-enhanced ultrasound (CEU), a validated method for quantifying flow by measurement of microvascular blood volume (MBV) and microvascular flow velocity (MFV). In the first protocol, 10 healthy, fasting adults received insulin (0.05 mU. kg(-1). min(-1)) via a brachial artery for 4 h under euglycemic conditions. At baseline and after insulin infusion, MBV and MFV were measured by CEU during continuous intravenous infusion of albumin microbubbles with intermittent harmonic ultrasound imaging of the forearm deep flexor muscles. In the second protocol, 17 healthy, fasting adults received a 4-h infusion of either insulin (0.1 mU. kg(-1). min(-1), n = 9) or saline (n = 8) via a brachial artery. Microvascular volume was assessed in these subjects by an alternate CEU technique using an intra-arterial bolus injection of albumin microbubbles at baseline and after the 4-h infusion. With both protocols, muscle glucose uptake, plasma insulin concentration, and total blood flow to the forearm were measured at each stage. In protocol 2 subjects, tissue extraction of 1-methylxanthine (1-MX) was measured as an index of perfused capillary volume. Caffeine, which produces 1-MX as a metabolite, was administered to these subjects before the study to raise plasma 1-MX levels. In protocol 1 subjects, insulin increased muscle glucose uptake (180%, P < 0.05) and MBV (54%, P < 0.01) and decreased MFV (-42%, P = 0.07) in the absence of significant changes in total forearm blood flow. In protocol 2 subjects, insulin increased glucose uptake (220%, P < 0.01) and microvascular volume (45%, P < 0.05) with an associated moderate increase in total forearm blood flow (P < 0.05). Using forearm 1-MX extraction, we observed a trend, though not significant, toward increasing capillary volume in the insulin-treated subjects. In conclusion, modest physiologic increments in plasma insulin concentration increased microvascular blood volume, indicating altered microvascular perfusion consistent with a mechanism of capillary recruitment. The increases in microvascular (capillary) volume (despite unchanged total blood flow) indicate that the relation between insulin's vascular and metabolic actions cannot be fully understood using measurements of bulk blood flow alone.

Comparative pharmacokinetics of theophylline in camels (Camelus dromedarius) and goats (Caprus hircus)

A comparative randomized crossover study was conducted to determine the pharmacokinetics of theophylline in male and female camels (Camelus dromedarius) and goats (Caprus hircus). Theophylline is an established 'probe drug' to evaluate the drug metabolizing enzyme activity of animals. It was administered by the intravenous (i.v.) route and then intramuscularly (i.m.) at a dose of 2 mg/kg. The concentration of the drug in plasma was measured using a high-performance liquid chromatography (HPLC) technique on samples collected at frequent intervals after administration. Following i.v. injection, the overall elimination rate constant (lambda z,) in goats was 0.006 +/- 0.00076/min and in camels was 0.0046 +/- 0.0008/min (P < 0.01). The elimination half-life (t 1/2 lambda z) in goats (112 .7 min) was lower than in camels (154.7 min) (P < 0.01). The apparent volume of distribution (Vz) and the total body clearance (Cl) in goats were 1440.1 +/- 166.6 ml/kg and 8.9 +/- 1.4 ml/min/kg, respectively. The corresponding values in camels were 1720.3 +/- 345.3 ml/kg and 6.1 +/- 1.0 ml/min/kg, respectively. After i.m. administration, theophylline reached a peak plasma concentration (Cmax) of 1.8 +/- 0.1 and 1.7 +/- 0.2 microg/ml at a post-injection time (Tmax) of 67.5 +/- 8.6 and 122.3 +/- 6.7 min in goats and camels, respectively. The mean bioavailability (T) in both goats and camels was 0.9 +/- 0.2. The above data suggest that camels eliminate theophylline at a slower rate than goats.

Effect of alosetron on theophylline pharmacokinetics

AIMS

To examine the potential for alosetron to alter the pharmacokinetics of theophylline by inhibiting its metabolism, as suggested by in vitro and in vivo effects on CYP1A2 activity.

METHODS

Ten healthy female volunteers received theophylline 200 mg twice daily alone for 8 days and with alosetron 1 mg twice daily for 15 days in this randomized, placebo-controlled, two-way-crossover study.

RESULTS

Alosetron had no significant effect on theophylline plasma concentrations (Cmax approximately 9 microg ml(-1), AUC approximately 90 microg ml(-1) h) or oral formation clearance of three major metabolites produced via CYP1A2: 3-methylxanthine, 1-methylurate and 1,3-dimethylurate (5, 7 and 16 ml min(-1), respectively). Concomitant administration of alosetron and theophylline was well tolerated.

CONCLUSIONS

The absence of a clinical drug interaction involving inhibition of theophylline metabolism by alosetron was not predicted by in vitro and in vivo metabolic probe data.

Variability of cytochrome P450 1A2 activity over time in young and elderly healthy volunteers

AIMS

To assess the age-associated changes over time of plasma paraxanthine/caffeine (PAX/CAF) ratios used as a probe for CYP1A2 activity.

METHODS

Intraindividual and interindividual variabilities in PAX/CAF ratio were compared by phenotyping with caffeine, 16 young and 16 elderly healthy subjects on five occasions.

RESULTS

PAX/CAF ratio variability was comparable regardless of age (intraindividual CV: 17.6 +/- 6% and 16.2 +/- 5.9%, interindividual CV: 48.1 +/- 2.9% and 42.7 +/- 3.6% in young and elderly, respectively). The PAX/CAF ratio was lower in elderly than in young subjects (95% CI for the difference: 0.004, 0.32) but the difference was not significant in nonsmokers compared separately.

CONCLUSIONS

The variability over time of the PAX/CAF ratio is not influenced by age.

Comparative effect of theophylline and aminophylline on theophylline blood concentrations and peripheral blood eosinophils in patients with asthma

The comparative effects of a new theophylline preparation (Theodrip) and aminophylline on blood concentrations of theophylline were examined in 74 patients with asthma. Subjects were intravenously administered 200 mg of Theodrip or 250 mg of aminophylline for 1 h. The mean increases in blood theophylline concentration after Theodrip or aminophylline administration were 8.80 +/- 1.80 mg/l and 8.81 +/- 2.15 mg/l, respectively. In addition, these patients were divided into four groups based on baseline theophylline concentrations before infusion of Theodrip or aminophylline: i) naïve patients (not administered theophylline); ii) those with a baseline theophylline concentration of 0-5 mg/l; iii) those with a baseline theophylline concentration of 5-10 mg/l; iv) those with a baseline theophylline concentration of 10-15 mg/l. Mean increases in blood theophylline concentration after administration of Theodrip in each group were similar to those after aminophylline administration. We found no significant differences between Theodrip and aminophylline. However, when the comparative effects of Theodrip and aminophylline on peripheral blood eosinophil counts were examined, Theodrip, but not aminophylline, reduced blood eosinophil counts. With acute exacerbations of bronchial asthma, it is expected that Theodrip, but not aminophylline, may have an antiinflammatory effect. In conclusion, it is suggested that Theodrip is a more useful drug than aminophylline in patients with acute exacerbations of bronchial asthma.

Effect of passive smoking on theophylline clearance in children

The author investigated the effects of passive tobacco smoking on the metabolism of theophylline in a pediatric population. In a retrospective analysis of 201 children admitted to a pediatric unit for asthma, 31 were identified with an acute exacerbation of asthma of noninfectious origin in which environmental exposure to tobacco smoke could be established. The parents were known smokers with a minimum 1-pack-per-day habit. An age-and gender-matched control population of children was then identified who had an acute exacerbation of asthma without any environmental exposure to tobacco smoke. In addition, the patients in both groups received the same dose of intravenous aminophylline for a minimum of 48 hours to ensure steady-state conditions. Total body clearance of theophylline was significantly elevated in the children exposed to environmental tobacco smoke (1.36 +/- 0.09 vs. 0.90 +/- 0.04 mL/min per kg, p < 0.0001). Steady-state serum levels were significantly lower in the passive smoking group (55.3 +/- 2.8 vs. 73.2 +/- 3.3 p < 0.00001) for those receiving nearly identical intravenous doses. The length of hospital stay was longer in the group exposed to passive smoke (4.4 +/- 2.6 vs. 2.9 +/- 1.3 days, p < 0.05). The clearance of theophylline is greater in asthmatic children exposed to passive tobacco smoke than in asthmatic children not exposed to passive tobacco smoke. These findings suggest that passive smoking may increase the clearance of other drugs metabolized in a manner similar to theophylline.

Bisoprolol: studies of potential interactions with theophylline and warfarin in healthy volunteers

Many patients receiving bisoprolol treatment might also require warfarin or theophylline therapy. Two studies were carried out in healthy volunteers to investigate the possibility that bisoprolol might interact with warfarin or theophylline, both of which have low therapeutic ratios. In a balanced, two-way, crossover study, eight men and four women took bisoprolol 10 mg daily for 14 days. They were divided into two groups of six volunteers. On the tenth day of treatment, six subjects received theophylline 375 mg (Nuelin tablets) by mouth and the pharmacokinetics of theophylline were determined. After a washout period of 7 days, they also underwent determination of single-dose theophylline kinetics. The other six subjects had single-dose theophylline kinetics determined 7 days before the first dose of bisoprolol, and then again after 10 days of bisoprolol treatment. Concurrent administration of bisoprolol did not influence the tolerability of theophylline and had no significant effect on any of the pharmacokinetic parameters for theophylline. In a separate study, 12 healthy men received warfarin daily until their prothrombin times were about 1.5 x control. They then received bisoprolol 10 mg daily p.o. for 10 days; warfarin was continued until 5 days after the end of bisoprolol treatment. Bisoprolol and warfarin treatments were well tolerated, and there was no effect on prothrombin times of either starting or stopping bisoprolol treatment. These studies show no effect of bisoprolol treatment upon theophylline pharmacokinetics, and no evidence of an effect of bisoprolol upon low-level anticoagulation with warfarin. Nevertheless, care should always be taken when changes are made in the medication of patients receiving theophylline or warfarin.

Drug interactions with clinafloxacin

Many fluoroquinolone antibiotics are inhibitors of cytochrome P450 enzyme systems and may produce potentially important drug interactions when administered with other drugs. Studies were conducted to determine the effect of clinafloxacin on the pharmacokinetics of theophylline, caffeine, warfarin, and phenytoin, as well as the effect of phenytoin on the pharmacokinetics of clinafloxacin. Concomitant administration of 200 or 400 mg of clinafloxacin reduces mean theophylline clearance by approximately 50 and 70%, respectively, and reduces mean caffeine clearance by 84%. (R)-Warfarin concentrations in plasma during clinafloxacin administration are 32% higher and (S)-warfarin concentrations do not change during clinafloxacin treatment. An observed late pharmacodynamic effect was most likely due to gut flora changes. Phenytoin has no effect on clinafloxacin pharmacokinetics, while phenytoin clearance is 15% lower during clinafloxacin administration.

Therapeutic drug monitoring in drug overdose

The treatment of poisoned patients is still largely defined by history, clinical assessment and interpretation of ancillary investigations. Measurement of drug concentrations is clinically important for relatively few compounds. Most measurements form an adjunct to and should not be considered a substitute for clinical assessment. Drug concentrations are particularly important for those compounds where the concentration is predictive of serious toxicity in an otherwise asymptomatic patient.