The effects of a beta-2 selective adrenergic agonist and a beta-nonselective antagonist on theophylline clearance

Evaluation of fasted and fed state simulated and human intestinal fluids as solvent system in the Ussing chambers model to explore food effects on intestinal permeability

The Ussing chambers model is almost exclusively used in the presence of plain aqueous phosphate buffers as solvent system. In an attempt to further elucidate the effect of luminal ingredients and postprandial conditions on intestinal permeability, pooled fasted and fed state human intestinal fluids (FaHIFpool, FeHIFpool) were used. In addition, simulated intestinal fluids of both nutritional states (FaSSIF, FeSSIF) were evaluated as possible surrogate media for HIF. The use of FaHIFpool generated a broad range of Papp values for a series of 16 model drugs, ranging from 0.03×10(-6)cm/s (carvedilol) to 33.8×10(-6)cm/s (naproxen). A linear correlation was observed between Papp values using FaSSIF and FaHIFpool as solvent system (R=0.990), justifying the use of FaSSIF as surrogate medium for FaHIF in the Ussing chambers. In exclusion of the outlier carvedilol, a strong sigmoidal relationship was found between Papp and fahuman of 15 model drugs, illustrated by correlation coefficients of 0.961 and 0.936 for FaHIFpool and FaSSIF, respectively. When addressing food effects on intestinal permeability, the use of FeHIFpool resulted in a significantly lower Papp value for nine out of sixteen compounds compared to fasting conditions. FeSSIF as solvent system significantly overestimated Papp values in FeHIFpool. To conclude, the optimized Ussing chambers model using biorelevant media as apical solvent system holds great potential to investigate food effects in a more integrative approach, taking into account drug solubilisation, supersaturation and formulation effects.

Evaluation of fasted state human intestinal fluid as apical solvent system in the Caco-2 absorption model and comparison with FaSSIF

To date, the Caco-2 model is considered as the gold standard to predict intestinal drug absorption. Often, aqueous phosphate buffers are used as apical medium. The purpose of this study was to use fasted state human intestinal fluid (FaHIF) as apical solvent system to generate biorelevant permeability values for a series of 16 model drugs that can be used as reference data to critically evaluate fasted state simulated intestinal fluid (FaSSIF) as possible substitute medium. Caco-2 compatibility with FaHIF was achieved when 50mg/ml mucus was applied on top of the cells before adding the apical medium. The use of FaHIF as solvent system generated a broad range of apparent permeability values (Papp) for the series of model compounds. When Papp values obtained with FaHIF were compared to those obtained with FaSSIF, a strong correlation was observed (R=0.951). The use of FaSSIF in the absence of mucus did not significantly alter this correlation. For FaHIF, FaSSIF and reference phosphate buffer blank FaSSIF, a strong sigmoidal relationship was found between Papp and fahuman, illustrated by correlation coefficients of 0.961, 0.893 and 0.868, respectively. In terms of inter-subject variability, the use of FaHIF from different volunteers originating from two distinct age groups (18-25 years; 65-72 years) exhibited an average coefficient of variance (CV) of 30%. However, no age dependency in permeability could be observed. In conclusion, the data generated in this article justify the use of FaSSIF as biorelevant apical medium in the Caco-2 assay to accurately predict in vivo drug absorption. Also, the optimized mucus-containing Caco-2 model can be used in combination with intestinal fluid samples aspirated after drug administration to further investigate intraluminal drug and formulation behavior.

Enhancement of theophylline clearance by oral albuterol

The possible effect of albuterol on theophylline clearance was studied in ten adult volunteers. Subjects received intravenous aminophylline loading dose (5.6 mg/kg), with oral albuterol (4 mg every 6 h), or inhaled albuterol (200 micrograms every 6 h), or alone (control). Theophylline levels were determined for 12-h periods. Theophylline clearance and elimination t 1/2 were calculated. Theophylline clearance was significantly higher when given with oral albuterol, in comparison with control (0.83 +/- 0.05 vs 0.73 +/- 0.06 ml/kg/min, p less than 0.02). Theophylline elimination t 1/2 was shorter with the coadministration of oral albuterol, compared with control (7.1 +/- 0.3 vs. 8.1 +/- 0.6 h, p less than 0.02). These alterations in theophylline clearance and elimination were greater in subjects who had lower control theophylline clearance. Theophylline clearance and elimination t 1/2 recorded with inhaled albuterol were not significantly different from control values. Coadministration of oral albuterol and theophylline resulted in enhancement of theophylline clearance, particularly in subjects with initially slow theophylline elimination. Such patients may require theophylline dosage adjustment as a result of this interaction.

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.

Systemic drug interactions with topical glaucoma medications

Topically administered ophthalmic medications are capable of attaining clinically important serum concentrations, as evidenced by the variety of systemic side effects they can produce. These drugs are therefore also capable of interacting with other drugs administered orally and intravenously. This is especially important when medications are administered to an elderly population, who are often receiving multiple medications. The Physician's Desk Reference and the United States Pharmacopeia Drug Information directory contain numerous warnings of potential interactions between topical glaucoma medications and systemically administered drugs. Unfortunately, literature supporting such interactions is limited, both in quantity and quality, being anecdotal in many cases. This report summarizes available information regarding the nonocular interactions between topical ophthalmic glaucoma medications and other systemically administered medications. Clinical evidence of these interactions is lacking in many cases.

Clinical pharmacokinetics of beta-agonists

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

Lack of effect of atenolol and nadolol on the metabolism of theophylline

The effects of 7 days of pretreatment with atenolol, 150 mg day-1, or nadolol, 80 mg day-1, on the pharmacokinetics and metabolism of theophylline were determined in six male smokers. Theophylline clearance, volume of distribution, half-life and the urinary excretion of theophylline and its metabolites were unchanged during either treatment compared with control.

Lack of effect of atenolol on the pharmacokinetics of theophylline

The effects of 3 days of pretreatment with cardioselective doses of atenolol on theophylline pharmacokinetics were determined. Nine healthy nonsmoking male volunteers received 6 mg kg-1 i.v. aminophylline under baseline conditions and after both 50 mg day-1 and 100 mg day-1 atenolol. Theophylline clearance, volume of distribution and half-life were not influenced by atenolol pretreatment. These data indicate that cardioselective doses of atenolol do not alter the pharmacokinetics of theophylline.