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

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

The Effect of Food on the Absorption of Controlled-Release Theophylline in Mini-Swine

The effect of differing fat contents of food on the bioavailability of theophylline following a 400-mg single dose of Theo-24 was studied in mini-swine. The pharmacokinetics of theophylline, following the intravenous administration of aminophylline equivalent to 5 mg/kg as a single dose, were also studied in the same animals. The terminal plasma half-life of theophylline following an i.v. dose was found to be approximately 24 hr. The volume of distribution, V _dext, and clearance following the i.v. dose were approximately 0.7 liter/kg and 0.023 liter/hr/kg, respectively. The terminal half-life of theophylline following the administration of theophylline capsules under fasting conditions was 21 hr. The average bioavailability under fasting conditions was approximately 80% compared to the i.v. dose. Food appeared to have decreased the rate of absorption but no significant effect on the extent of absorption.     

Continuous Blood Withdrawal as a Rapid Screening Method for Determining Clearance and Oral Bioavailability in Rats

Purpose. To develop a methodology for continuous blood withdrawal in rats suitable for drug discovery screening purposes and perform limited validation studies with a series of test compounds. Methods. A reliable methodology for continuous blood withdrawal in rats was developed. The method is dependent on continuous heparin infusion during withdrawal and the minimization of constrictive, thrombogenic sites. Plasma drug concentrations from either intermittent sampling or continuous withdrawal experiments were determined with HPLC analysis. Results. The continuous withdrawal method was successfully adapted to rats such that blood samples could be reliably collected over a 6-hr experiment. The clearance and oral bioavailability values for theophylline, atenolol, propranolol, warfarin. BMS-182874 and BMS-A were determined from continuous withdrawal or intermittent sampling experiments. The results from the two methods were comparable, with each compound reliably placed in the same low, medium or high category based on clearance or oral bioavailability characteristics. Conclusions. The continuous withdrawal method proved to be a viable alternative to the classic intermittent sampling technique. The method should prove useful in drug discovery screening, where the evaluation of large numbers of compounds for systemic clearance or oral bioavailability is often necessary.     

The Beagle Dog as an Animal Model for a Bioavailability Study of Controlled-Release Theophylline Under the Influence of Food

Beagle dogs were evaluated as an animal model to study the effect of food on the bioavailability of two commercially available oral controlled-release theophylline products. The products were administered with and without food in single doses, and the bioavailability parameters were compared with those following an i.v. aminophylline dose. The total plasma theophylline clearance in dogs following an i.v. dose was 0.128 liter/hr/kg and the volume of distribution was 0.8 liter/kg using a one-compartment model. The absolute bioavailabilities of these two products under fasting conditions were 31 and 48%, respectively. The food increased the bioavailability of one product and decreased the bioavailability of the other. The overall trends in relative bioavailability of these two products with and without food appeared to be similar to those reported in humans.     

Lack of influence of an intensive antacid regimen on theophylline bioavailability

We examined the influence of a large-volume, therapeutic antacid regimen, administered for three full days, on the steady-state bioavailability of a conventional-release and sustained-release theophylline product, Aminophyllin and Theodur, respectively. Nine stable asthmatics voluntarily completed a four-phase investigation requiring a total stay of 12 days in the Clinical Research Unit. The treatments consisted of administration of the formulations mentioned with and without antacids to each patient in a randomized sequence. Four patients participated in an additional phase where antacids were administered q2h around the clock for three days. After coadministration of theophylline plus antacids for two days, theophylline therapy was discontinued while numerous blood samples were obtained over 22 hr and analyzed for theophylline content via radioimmunoassay. Antacids had no predictable, consistent influence on theophylline absorption rate as determined by the absorption rate constant, the time to maximal theophylline concentration, or the lag time for theophylline absorption. Antacids had no detectable influence on theophylline elimination half-life and had no consistent, statistically significant effect on the extent of theophylline bioavailability, according to measurements of maximal concentration, AUCmeasured over the appropriate steady-state dosing interval, or elimination-rate adjusted AUC.The substantial intraindividual changes for all parameters of theophylline bioavailability that occurred for control and treatment phases likely represent spontaneous, random between-day variability in theophylline disposition independent of antacid administration, as evidenced by the comparability of the percent coefficient of variation for parameters of biovailability across all phases. Our data demonstrate that therapeutic antacid administration has no effect on steady-state theophylline bioavailability and does not alter the intrinsic variability in theophylline absorption. Based on the results of our data, it is unlikely that a clinically significant (>20%) decrease in theophylline absorption would occur in any patient treated intensively with antacids concurrently.Presented in part at the American College of Clinical Pharmacy, Third Annual Meeting, Kansas City, Missouri, June, 1982. Supported by Clinical Research Unit Grant # MO-1-RR00080-21, The American Lung Association of Northern Ohio and Affiliates, and in part by the Hoffman LaRoche Hospital Pharmacy Research Grant Program.     

Absolute bioavailability of theophylline from a sustained-release formulation using different intravenous reference infusions

Summary The effect of different intravenous infusions on the absolute bioavailability of theophylline from a sustained-release formulation has been investigated. Oral administration of 750 mg theophylline (2 capsules Euphylong 375) was referenced to intravenous aminophylline infusions corresponding to 506 mg theophylline over 8 h (63 mg·h^–1) in Study 1, and to 749 mg theophylline over 14 h in Study 2. A reduction in the infusion rate from 69 to 33 mg·h^–1 was made in Study 2 after 8 h in order to mimic the concentration/time profile of the oral formulation as closely as possible. The absolute bioavailability was 100 (89, 115) % in Study 1 and 88 (73, 105) % in Study 2.The lower clearance values and, as a consequence, the lower bioavailability ratios observed with the higher intravenous dose, although not significant, indicate that the absolute bioavailability of theophylline might appear to depend on the choice of the intravenous reference standard.     

Effect of hepatic cirrhosis on the pharmacokinetics of theophylline in rats

The experimental hepatic cirrhosis was induced either by bile duct ligation (BDL) or by pretreatment with dimethylnitrosamine (DMNA). The pharmacokinetics of theophylline were studied after a single intravenous or a single oral administration. Using the ultrafiltration method, protein-drug binding experiments were also carried out. The bilirubin level was several-fold increased by BDL, but not by DMNA treatment. The albumin content was decreased in both cirrhotic groups. The total clearance (Clt, ml/kg/hr) of theophylline in both hepatic cirrhosis groups significantly decreased and the terminal half-life (t_1/2) in the cirrhotic rats was increased about two-fold after intravenous and oral administration. The volume of distribution at steady state (Vdss, ml/kg) was increased slightly in the cirrhotic groups. Protein binding in BDL (8.67±4.85%) decreased about four-folds, but in DMNA (73.00±9.85%) similar result, was observed as compared with the control. Increased free fraction of theophylline did not increase the volume of distribution in BDL. Therefore decreased total body clearance of theophylline was mainly due to decreased intrinsic clearance of theophylline in the liver. The absolute bioavailability of theophylline in these experiments was between 63.8 and 72.8%(66.1% in BDL, 63.8% in Sham operated and Control, 72.8% in DMNA). These results suggest that in the experimental hepatic cirrhosis model, administration route does not affect the disposition of theophylline.     

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

Bioavailability studies are commonly undertaken, and most, because they involve subjects taking repeated doses of a drug, contain information on intraindividual variability in pharmacokinetics. However, because in such studies bioavailability itself is unknown, it is difficult to resolve which pharmacokinetic parameters vary within individuals. A mathematical model is presented which permits estimation of variability in clearance and in volume of distribution. When applied to pooled data arising from five theophylline bioavailability studies, this model has given statistical evidence that clearance of theophylline is inherently more variable within individuals (coefficient of variation, 13%) than volume of distribution (8%). As a result, use of the measurement AUC · rather than AUCas a more precise index of bioavailability is justified in studies where is measured with reasonable precision. The model could be applied to estimation of withinbatch within-person variability in bioavailability.Deceased, April 4th, 1981.     

Evaluation of passively absorbed saliva for determination of oral slow-release theophylline bioavailability in children

Assessment in young children of the bioavailability of slow-release theophylline formulations is hampered by the requirement for frequent blood sampling. Calculations of bioavailability from serial serum and passively absorbed saliva samples were therefore compared in six 9- to 12-year-old asthmatic children receiving multiple doses of Theo-Dur Sprinkle every 12 hours, using Theo-Dur tablets, a previously characterized formulation, as a reference. Results indicated 85 +/- 5 percent and 82 +/- 8 percent (mean +/- SEM) relative bioavailability based on serum and salivary measurements, respectively. Correlation coefficient for serum and passively absorbed saliva bioavailabilities was 0.90. Passively absorbed saliva provides an acceptably accurate, noninvasive method for theophylline bioavailability assessment and may be a useful alternative for bioavailability studies in young children.     

The Effect of Gastric pH on the Absorption of Controlled-Release Theophylline Dosage Forms in Humans

The bioavailability of three marketed controlled-release dosage forms and a reference solution of theophylline was studied in eight subjects with normal gastric fluid acidity and seven subjects who were achlorhydric. Gastric pH was monitored with a Heidelberg capsule. One of the controlled-release dosage forms dissolved more rapidly in vitro when exposed to acid conditions, one dissolved more rapidly in pH 7.5 media, and the third dissolved at a rate independent of pH. Using a crossover design, each subject received each dosage form twice. Blood was sampled for up to 47 hr after each dose, and serum was assayed for theophylline by HPLC. The product which dissolved more rapidly under acid conditions in vitro exhibited a 3 hr longer T _max in the achlorhydrics compared to the normal subjects. The product which dissolved more rapidly in the pH 7.5 media exhibited a relatively higher AUC(0–) in the achlorhydric subjects than in normal subjects after the AUC data were normalized for clearance differences between the two subject groups. The in vivo bioavailability of these dosage forms could be related to the in vitro dissolution characteristics for some parameters. However, with the exception of the mean T _max values, the mean bioavailability parameters differed by less than 20% between the two subject groups.     

Evaluation of the effect of food on the absorption of sustained-release theophylline and comparison of two methods for serum theophylline analysis

Fifteen healthy volunteers took part in a study to investigate the effect of food on the bioavailability of a slow-release formulation of theophylline. Serum theophylline levels were measured every two hours for ten hours after a single oral dose of 500 mg of theophylline. Serum levels were significantly higher after the dose was taken on an empty stomach; however, serum levels were significantly higher 10 hours later when the dose was taken after a standard meal. Despite these differences, eating had no overall effect on theophylline bioavailability. Two analytic methods for measuring serum levels of theophylline were compared, and it was found that fluorometry could measure lower levels and was therefore more precise than an enzyme immunoassay method, which is probably due to the complete automation and reduced interference of bilirubin and hemoglobin.     

The chimpanzee (Pan troglodytes) as a pharmacokinetic model for selection of drug candidates: model characterization and application.

The chimpanzee (CHP) was evaluated as a pharmacokinetic model for humans (HUMs) using propranolol, verapamil, theophylline, and 12 proprietary compounds. Species differences were observed in the systemic clearance of theophylline (approximately 5-fold higher in CHPs), a low clearance compound, and the bioavailability of propranolol and verapamil (lower in CHPs), both high clearance compounds. The systemic clearance of propranolol (approximately 1.53 l/h/kg) suggested that the hepatic blood flow in CHPs is comparable to that in humans. No substantial differences were observed in the in vitro protein binding. A preliminary attempt was made to characterize cytochrome P450 (P450) activities in CHP and HUM liver microsomes. Testosterone 6beta-hydroxylation and tolbutamide methylhydroxylation activities were comparable in CHP and HUM liver microsomes. In contrast, dextromethorphan O-demethylation and phenacetin O-deethylation activities were approximately 10-fold higher (per mg protein) in CHP liver microsomes. Intrinsic clearance estimates in CHP liver microsomes were higher for propranolol (approximately 10-fold) and theophylline (approximately 5-fold) and similar for verapamil. Of the 12 proprietary compounds, 3 had oral clearances that differed in the two species by more than 3-fold, an acceptable range for biological variability. Most of the observed differences are consistent with species differences in P450 enzyme activity. Oral clearances of proprietary compounds in HUMs were significantly correlated to those from CHPs (r = 0.68; p = 0.015), but not to estimates from rat, dog, and monkey. In summary, the chimpanzee serves as a valuable surrogate model for human pharmacokinetics, especially when species differences in P450 enzyme activity are considered.     

Pharmacokinetics and relative bioavailability of oral theophylline capsules

The oral bioavailability of liquid-filled theophylline capsules relative to a nonalcoholic aminophylline solution was determined in normal volunteers. In addition, theophylline absorption and elimination kinetics were reexamined. There were no statistically significant differences between the bioavailability of capsules and liquid as measured by the area under the curve (AUC) from time 0 leads to infinity (p greater than 0.05). The bioavailability parameters of Cmax, tmax, and AUC were determined from actual serum theophylline concentration-time data and from a nonlinear least-squares fit of the serum concentration-time data. Theophylline absorption from the capsules was noticeably faster than from the liquid in most subjects, although the differences in absorption rates were not significantly different (p greater than 0.05). The determined apparent volume of distribution, elimination half-life, and plasma clearance of theophylline were similar to values reported by other investigators. Marked inter- and intraindividual variations in the elimination half-life were noted.     

Adaptive control of theophylline therapy: Importance of blood sampling times

A two-observation protocol for estimating theophylline clearance during a constant-rate intravenous infusion is used to examine the importance of blood sampling schedules with regard to the information content of resulting concentration data. Guided by a theory for calculating maximally informative sample times, population simulations are used to assess the effect of specific sampling times on the precision of resulting clearance estimates and subsequent predictions of theophylline plasma concentrations. The simulations incorporated noise terms for intersubject variability, dosing errors, sample collection errors, and assay error. Clearance was estimated using Chiou 's method, least squares, and a Bayesian estimation procedure. The results of these simulations suggest that clinically significant estimation and prediction errors may result when using the above two-point protocol for estimating theophylline clearance if the time separating the two blood samples is less than one population mean elimination half-life.     

Theophylline. Pooled Michaelis-Menten parameters (Vmax and Km) and implications

The literature on theophylline is confusing since in the same dose range one article will report linear kinetics while another will report non-linear kinetics. Single dose clearances and lower steady-state clearances of theophylline, recently reported in the literature, were used to estimate pooled Vmax and Km values of the Michaelis-Menten equation for 10 normal subjects. The mean Vmax was 1960 mg/day and the mean Km was 24.1 mg/L. These values were then utilised to: explain another set of different oral clearances following doses of 2 and 6 mg/kg reported in the literature; estimate relative effects of dose rate and type of input on absolute bioavailability; estimate AUC (0-infinity) as a function of single dose over the range 0 to 1500 mg; estimate the average steady-state serum concentration of theophylline (Cssav and steady-state oral clearance (CLsspo) as a function of dose rate in mg/day; illustrate how Michaelis-Menten kinetics alters the apparent first-order elimination rate constant and the half-life estimated from terminal log-linear plots at concentrations appreciably lower than the Km value; and illustrate how Michaelis-Menten kinetics affects the estimation of a zero-order absorption rate constant using the Wagner-Nelson method.     

Method for the rapid estimation of the total body drug clearance and adjustment of dosage regimens in patients during a constant-rate intravenous infusion

A simple method is proposed to estimate the total body clearance and biological half-life of drugs in patients by determining two blood or plasma levels of drugs separated by an appropriate interval during the constant-rate intravenous infusion. The method is based on the assumptions that the drug disposition in the body can be adequately described by the linear onecompartment open model and its apparent volume of distribution can be estimated with a reasonable degree of accuracy from the literature data. The validity of this newly proposed method was demonstrated in three rabbits using theophylline as a test drug. Results of theoretical error analyses indicate that the method is potentially useful in providing an early guide for dosing adjustments for those potent drugs whose elimination rates have been shown to vary greatly in patients. The method may be particularly useful in patients with changing elimination kinetics of drugs. Effects of the accuracy in V_d estimate, plasma level assay, and sampling time interval on the error of total body clearance estimation are discussed.     

Accuracy of 11 methods for predicting theophylline dose

Eleven methods for pharmacokinetic determination of theophylline dose were compared, based on the ability of each method to predict theophylline serum concentration and clearance for adults with asthma. Predictions by each method were compared with actual serum theophylline concentrations before and after an aminophylline loading dose was administered to treat bronchospasm in 22 patients. Follow-up serum theophylline concentrations were obtained after maintenance therapy with i.v. aminophylline in 6 patients and an oral sustained-release theophylline product (Theo-Dur, Key) in 16 patients; maintenance doses administered to the patients were calculated by the method of Chiou et al. Two variations of the method of Chiou et al., seven Bayesian methods using one or more measured serum theophylline concentrations, FDA's standardized clearance estimate, and a population-based clearance-estimation method were compared. A one-compartment, open model with first-order elimination was assumed for all calculations. All 11 methods predicted steady-state concentration with minimal bias and good precision. The Chiou and Bayesian methods performed similarly, with the highest precision found for the Bayesian method that incorporated four measured concentrations. The population-based method had the highest correlation coefficient and the lowest mean error for predicting steady-state concentration. Decreasing the number of measured concentrations had a minimal effect on the various Bayesian methods. All methods evaluated are sufficiently accurate for clinical application in patients with stable, uncomplicated asthma.     

Update on quinolone drug interactions

Concurrent administration of both ciprofloxacin and norfloxacin with sucralfate leads to a decrease in quinolone bioavailability. It is unknown whether this decrease is clinically significant because studies have focused primarily on pharmacokinetics and not therapeutic outcomes. A reasonable recommendation may be to avoid using sucralfate and norfloxacin concurrently, or avoid administration of norfloxacin and ciprofloxacin within two hours of sucralfate administration. Magnesium- and aluminum-containing antacids may also interfere with quinolone absorption. Calcium carbonate and H2 receptor antagonists do not appear to interact with quinolones and may be considered as an alternative to sucralfate or magnesium- and aluminum-containing antacids when quinolones are administered. Concurrent administration of ciprofloxacin and theophylline may precipitate theophylline toxicity if not monitored carefully. Some clinicians recommend a 30% empiric reduction in theophylline dosage when ciprofloxacin therapy is initiated. Because the drug interaction is not completely predictable, the patient's theophylline levels should be monitored and signs and symptoms of toxicity noted, adjusting the dose as needed. Decreased theophylline clearance may persist for as long as five days following discontinuation of ciprofloxacin. Some potential for slight increases in serum theophylline concentrations secondary to norfloxacin administration may exist. However, it is unlikely to be clinically significant, based on currently available information.     

Longitudinal effects of pregnancy on the pharmacokinetics of theophylline

Summary 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.     

Theoretical considerations in the calculation of bioavailability of drugs exhibiting Michaelis-Menten elimination kinetics

Two approaches used for bioavailability determination of drugs with Michaelis-Menten elimination kinetics were examined by computer simulation. The first method involved treating the drug as though its clearance remained constant during elimination, and the conventional method of taking the ratio of areas under the curve resulting from the oral and intravenous doses was used to calculate bioavailability. The second approach involved using the Michaelis parameters, V_max and K_m,to determine concentration dependent clearance values, but based these calculations on peripheral drug concentrations rather than on concentrations entering or in the liver. We have developed a simulation method that was used to test the accuracy of the above two methods. In the simulations described, V_max, K_m,and hepatic blood flow were chosen to represent a drug with an extraction ratio of 0.9 under linear conditions, but with Michaelis-Menten kinetics occurring at the doses given. Absorption was assumed to be first-order, and metabolism was assumed to occur only in the liver. These simulations showed that the most accurate determination of bioavailability requires knowledge of the direct contribution of oral absorption to the concentration of drug entering the liver. Unexpectedly, the results also showed that if a drug has a large volume of distribution or a large absorption rate constant, or both, use of the much simpler conventional method of bioavailability determination may be appropriate even in cases where the degree of saturation is substantial.This work was supported in part by grant GM26556 from the Institute of General Medical Sciences of the National Institutes of Health.