The metabolism of sympathomimetic bronchodilator drugs by the isolated perfused dog lung

Absorption of drugs by the lung

Ten patients undergoing diagnostic bronchoscopy had radioisotopically labelled drugs put directly into their bronchi; two received sodium cromoglycate, two salbutamol, three salmefamol and three rimiterol. All four drugs were rapidly absorbed but higher peak plasma levels per unit dose were seen with sodium cromoglycate and salbutamol than with the other two drugs. It is suggested that the lung metabolizes salmefamol and rimiterol but does not metabolize salbutamol or sodium cromoglycate.

Local versus total systemic bioavailability as a means to compare different inhaled formulations of the same substance

For inhaled formulations of a drug substance, the balance between desired local activity and undesired systemic activity can be expressed with an L:T ratio, where L stands for the local bioavailability and T stands for the total systemic bioavailability. The L:T ratio depends not only on the ability of the different devices to divide the delivered dose between the lungs and the oropharynx but also on the inherent difference in gastrointestinal (GI) first-pass metabolism between different substances. The L:T ratio should therefore not be used to make comparisons across different drug substances but only to compare the same drug formulated in different inhalation systems. A high L:T ratio expresses a good targeting ability of the combination of substance and device or a low contribution from the GI tract. A high L:T ratio also reflects a more beneficial balance between wanted and unwanted effects. The L:T ratio was calculated from literature data for a number of salbutamol (albuterol) formulations and for two budesonide and two terbutaline formulations. For salbutamol, values ranged from 0.15 to 0.55 with different devices. For budesonide, the values ranged from 0.66 to 0.85, and for terbutaline, the values ranged from 0.59 to 0.79. The L:T ratio can thus be used to aid in the choice of inhaler.

Salbutamol disposition and dynamics in conscious rabbits: influence of the route of administration and of the dose

This study assessed the influence of dose and route of administration on salbutamol kinetics and hypokaliemic effect. Salbutamol plasma kinetics were studied in a first group of 6 rabbits who received 60, 800, and 60 micrograms/kg by the intravenous (iv), oral (po), and intratracheal (it) routes, respectively, at 1-week intervals. A second group of 6 rabbits received 120, 2400, and 120 micrograms/kg of salbutamol by the same three routes. Multiple blood samples were withdrawn to assay salbutamol and potassium. Following iv salbutamol (60 micrograms/kg), total plasma clearance was 82 +/- 5 ml/min per kg, apparent volume of distribution was 5.0 +/- 0.5 l/kg, and terminal half-life was 41 +/- 2 min. Similar values were estimated when 120 micrograms/kg of salbutamol was administered iv or was given po or it. The bioavailability of po and it salbutamol was approximately 1 and 20%, respectively. For the first group, the maximal decrease in plasma potassium elicited by salbutamol was 0.80 +/- 0.19, 0.48 +/- 0.22, and 0.78 +/- 0.46 mmol/l, and for the second group, maximal decrement was 1.31 +/- 0.37, 0.70 +/- 0.24, and 0.84 +/- 0.17 mmol/l for the iv, po, and it routes, respectively. Compared to salbutamol peak plasma concentrations, maximal decrease in plasma potassium appeared between 60 and 108 min later for the iv route, 90 and 25 min later for po and it routes, and for this reason, the hypokaliemic effect was not associated to salbutamol plasma concentrations. The hypokaliemic effect was dependent upon the route, e.g., po > it > iv.(ABSTRACT TRUNCATED AT 250 WORDS)

A method for determination of the absolute pulmonary bioavailability of inhaled drugs: terbutaline

Terbutaline sulfate (4 X 0.250 mg) was given to 11 healthy volunteers by inhalation from a metered dose inhaler (MDI), with and without oral administration of a charcoal slurry. Before the inhalations, the adsorbing capacity of the charcoal slurry was tested. Deuterated terbutaline, 0.125 mg, was given intravenously at the same time as the test doses. The charcoal slurry adsorbed 97% of an oral dose. The oral contribution to the overall systemic bioavailability after inhalation, when charcoal was coadministered, could thus be neglected. After inhalation of terbutaline, 9.1% of the dose was deposited in the lungs and an additional 6.7% was systemically available via the oral route. The method presented measures the absolute pulmonary bioavailability after inhalation from a MDI. Since a deuterated analogue is given intravenously together with the inhalations, fewer subjects are needed to obtain reliable data.

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.

Isolated perfused rabbit lung as a model for intravascular and intrabronchial administration of bronchodilator drugs I:Isoproterenol

The absorption, uptake, and metabolism of isoproterenol was studied following intravascular, intrabronchial, and aerosol administration of the drug to the isolated perfused rabbit lung. Capacity-limited metabolism of isoproterenol was observed following the addition of five doses, ranging from 10(-7) to 10(-5) moles, directly into the circulation of the lung system. A physiologically based perfusion model was developed to describe the disposition of the drug and metabolite in the isolated lung preparation. This model was also used to analyze data collected following intrabronchial and aerosol administration of isoproterenol.

Contribution of lungs to total body clearance: linear and nonlinear effects

The contribution of the lungs to the total body clearance of drugs is examined in a framework that emphasizes their anatomical position. For intravenous administration, the lung is the only organ other than blood that can account for a total body clearance in excess of the cardiac output. Systemic arterial drug concentration and tissue drug exposure are inversely proportional to total body clearance. Although the role of the lung has been overshadowed by that of the liver, several examples are presented to demonstrate that relatively small amount of pulmonary activity can produce a large reduction in systemic arterial drug concentration. For oral administration, first-pass elimination by the liver and lungs in series results in a synergistic increase in total body clearance. Nonlinear effects caused by saturation of elimination pathways are also examined. Increased emphasis on experimental investigation of the pulmonary contribution is warranted, especially for drugs with high apparent clearance.

The isolated perfused lung--a critical evaluation

Recent refinement and application of the technique of perfusing isolated intact lung preparations from suitable experimental animal has enhanced our knowledge of the non-respiratory functions of the lung. The technique consists of perfusion with whole blood or constituted media via pulmonary artery or artificially ventilated lungs. Such preparations have been known to be especially useful for studies of uptake, metabolism and disposition of exogenous and endogenous substances. Recent advances in unilateral, split-lung perfusion in which left and right lungs are unilaterally perfused simultaneously have enabled investigators to maintain paired controls during perfusion. Such techniques are useful in the study of the interactions of endogenous and exogenous chemicals in the lung tissue. Differences between subcellular or lung slice preparations and intact lung perfusion are to be expected on the basis of distorting natural vascular and extra-vascular barriers in the case of in vitro preparations. Areas in which perfused lungs have not been extensively used include uptake and disposition of gases, solvents and vapors, effects of toxic chemicals on respiratory and non-respiratory functions of the lung and alteration of pulmonary mechanisms and hemodynamics in the presence of interacting chemical or physical stimuli. With suitable modifications, application of isolated perfused lung preparations for these investigations should be technically feasible in the future.

Metabolism of delta1-tetrahydrocannabinol by the isolated perfused dog lung. Comparison with in vitro liver metabolism

The metabolism of (-)-delta1-tetrahydrocannabinol (delta1-THC) has been studied in the isolated perfused dog lung. After intravascular administration of [3H]-delta1-THC there was an overall biotransformation of 12%. Two major metabolites were isolated and identified as 3'-hydroxy-delta1-THC and 4'-hydroxy-delta1-THC. 7-Hydroxy-delta1-THC was also present together with small amounts of 6alpha-hydroxy-delta1-THC and 6beta-hydroxy-delta1-THC. An in vitro experiment using a dog liver microsomal preparation was also carried out and showed that the major metabolites were 6beta-hydroxy-delta1-THC and 6alpha-hydroxy-delta1-THC. 7-Hydroxy-delta1-THC and 1,2-epoxy-hexahydrocannabinol were also isolated together with small amounts of 3'-hydroxy-delta1-THC and 4'-hydroxy-delta1-THC. The side-chain hydroxylated compounds are hitherto undescribed metabolites of delta1-THC.

The physiological disposition of ibuterol, terbutaline and isoproterenol after endotracheal instillation to rats

1. The absorption from the respiratory tract and biotransformation of three bronchodilating drugs, terbutaline, ibuterol (diisobutyrate ester of terbutaline) and isoproterenol, have been studied in rats after endotracheal instillation. 2. At 2-15 min after dosage, ibuterol is more rapidly absorbed from the respiratory tract than terbutaline, but later (15-35 min), absorption of ibuterol is about the same as terbutaline. Absorption of isoproterenol is rapid throughout the experiment. Absorption rates have been related to the physical-chemical properties of the compounds. 3. No significant biotransformation of terbutaline was noted in the rat lung at any time after dosage, but ibuterol is extensively hydrolysed to terbutaline. Extensive biotransformation of isoproterenol to 3-methoxy-isoproterenol and an unidentified metabolite occurred. Metabolites of terbutaline and isoproterenol were found in serum and liver.

Metabolism of isoprenaline after aerosol and direct intrabronchial administration in man and dog

1 Administration of isoprenaline by aerosol inhalation to man results in over 80% being metabolized to the sulphate conjugate.2 The majority of an inhaled dose is probably swallowed since the metabolic pattern resembles that after an oral dose.3 Isoprenaline, administered in aqueous solution directly into the bronchial tree in both man and dog, is rapidly O-methylated to 3-O-methyl isoprenaline, which is subsequently conjugated with sulphate.4 3-O-Methylation is the main metabolic pathway for the small part of an inhaled dose which does enter the bronchial tree.