The clinical pharmacology of salmefamol

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.

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.

Dopamine receptors do not influence beta2-sympathomimetic-mediated inhibition of gastric acid secretion in the dog

The effect of dopamine blockade by haloperidol and metoclopramide on beta2-adrenergic-induced changes of acid secretion and pulse rate was studied in conscious gastric fistula dogs. Neither haloperidol (0.15 mg/kg and 0.60 mg/kg) nor metoclopramide (0.25 mg/kg and 1.0 mg/kg), given intravenously, had a significant influence on beta2-adrenergic-induced inhibition of gastric acid secretion or tachycardia. Dopamine blockade had no influence on pentagastrin-stimulated gastric acid secretion, but it increased pulse rate. It is concluded that dopamine receptors are not involved in the inhibiton of acid output or tachycardia induced by beta2-sympathomimetics, and dopamine is probably not a neurotransmitter for these effects.

Effect of a beta 2-sympathomimetic on gastrin release, acid secretion, and blood glucose during basal conditions and in response to insulin, 2-deoxy-D-glucose, and feeding in the dog

The effect of a selective beta 2-adrenoceptor agonist on basal volume and on insulin-, 2-deoxy-D-glucose (2-DG)-, and food-induced gastrin release was studied in conscious gastric fistula dogs. Acid output and blood glucose changes were also studied, except in the food experiments. Basal acid secretion and serum gastrin were unchanged after beta 2-sympathetic infusion, whereas a slight increase in blood and glucose was found. The beta 2-agonist almost prevented acid output and gastrin release after insulin hypoglycaemia. However, the hypoglycaemia was also inhibited. Gastric acid secretion stimulated by 2-DG was inhibited, as was probably the gastrin release. 2-DG increased the blood glucose level, and no significant differences were found after beta 2 infusion. After feeding, gastrin release was initially decreased for one of five doses of the beta 2-agonist, and higher doses of the beta 2-agonist prevented the subsequent fall in serum gastrin after the initial peak value. This pattern was also found for the histamine H2-blocker cimetidine in a dose that blocks acid output. The beta 2-agonist and 2-DG increased pulse rate. It is concluded that beta 2-sympathetic stimulation inhibits acid output and gastrin release after insulin and 2-DG stimulation, but one should be cautions in drawing conclusions from the insulin experiments. The effect on gastrin release is small compared with the effect on the acid secretion, and it is unlikely that the inhibition of acid secretion acts through a change in gastrin release.

Effect of a beta2-sympathomimetic on urecholine-stimulated gastric acid secretion in dogs

The effect of a selective beta2-adrenoceptor agonist on urecholine-stimulated gastric acid secretion was studied in conscious gastric fistula dogs. A dose-dependent inhibition was found, most pronounced for the highest doses of urecholine. The inhibition was primarily on volume, whereas the acidity showed a decreasing tendency only. The pulse rate was increased by the beta2-agonist and by urecholine in high doses. Propranolol prevented both the inhibition of acid secretion and the increase in pulse rate. Practolol had no effect on the inhibition of acid secretion and the increase in pulse rate. Practolol had no effect on the inhibition of acid secretion but prevented the increase in pulse rate. Dose-response experiments with five doses of urecholine and two doses of the beta2-agonist showed a decrease in maximal response, and transformation of the curve suggested an unchanged D50. It is concluded that the beta2-agonist inhibits urecholine-stimulated acid secretion in the dog to the same degree as pentagastrin-induced acid secretion, and much stronger than found for histamine-induced acid secretion in earlier experiments. The inhibition seems to follow a noncompetitive kinetic, and beta2-receptors are probable involved.

The absorption and elimination of ICI 74,917 in man

1 The pharmacokinetics of ICI 74,917 were studied in both asthmatic patients and normal volunteers. 2 The tritiated compound was administered to the lungs by inhalation from an aerosol and a bronchoscope, and by intravenous, oral and buccal routes. Radioactivity was measured in plasma, urine, faeces, sputum and exhaled air. 3 After bronchoscopic administration 63% of the available dose was absorbed; after aerosol administration 8% was absorbed from the lung and more than 50% swallowed. 5 Intravenous studies indicated that the drug is excreted in the bile and urine in the ratio 2:1. 5 Minimal oral and no buccal absorption occurred. 6 There was no evidence of tritium exchange or drug metabolism. 7 The mean terminal half-life following administration by all route was 16.1 hours. However, the majority of the dose was rapidly excreted. 8 Aerosol administration is the method of choice for the clinical use of ICI 74,917.

Salmefamol orally in asthmatics - two doses compared

The responses of twelve patients with chronic asthma to salmefamol 1 mg and 2 mg, taken orally, were compared in a double-blind cross-over study. Both produced a rise of 40-50% in PEFR and FEV1. Statistically significant improvements were maintained for three to four hours, and 20% improvements for four to six hours. There was no significantly different effect on ventilatory capacity between the two doses. After the 2 mg dose there was a statistically significant fall in diastolic blood pressure at 1 and 1 1/2 hours. Four patients experienced tremor and this was the only side-effect noted. The possible reasons for failure to demonstrate a greater effect with the higher dose are discussed.

Comparison of salmefamol and salbutamol in patients with chronic airways obstruction

1 Inhaled salmefamol, in doses of 100 mug and 200 mug has been compared with inhaled salbutamol, in a dose of 200 mug, and with placebo in patients with airways obstruction. 2 Both salmefamol and salbutamol are potent bronchodilators with a signficantly superior action over placebo at all times up to 8 h after treatment. 3 The mean peak percentage increases in FEV produced by the three active preparations were similar. The decline from peak values was significantly slower with salmefamol than with salbutamol. Neither drug produced tachycardia.

Comparison of the effects of (--)-isoprenaline, orciprenaline terbutaline, and Me506 on heart rate, soleus muscle contractility and pulmonary resistance of anaesthetized cats

1. Three resorcinol derivatives with N-isopropyl (orciprenaline), N-t-butyl (terbutaline) and N-p-hydroxypheny-t-butyl (Me506) amine substituents have been compared with (--)-isoprenaline for their ability to produce beta-receptor mediated reductions in serotonin-induced increases in pulmonary resistance, decreases in soleus muscle contractility and increases in heart rate in anaesthetized cats. 2. For all parameters studied the four compounds produced similar maximal responses and dose-response curves were close to parallel. From the graphs doses of the compounds producing 50% of the maximal response (ED50) were interpolated, and from these dose-ratios with respect to (--)-isoprenaline [drug ED50:(--)-isoprenaline ED50] were calculated on a molar basis. 3. Increasing the size of the amine substituent from N-isopropyl to N-t-butyl led to an increase in beta-receptor stimulant activity in bronchial and skeletal muscle, but not in the heart. The change from N-t-butyl to N-p-hydroxyphenyl-t-butyl did not further affect stimulant activity in any of the parameters studied. 4. Calculation of selectivity ratios [molar dose-ratio (heart): molar dose-ratio (pulmonary resistance)] showed that orciprenaline was non-selective, and that terbutaline and Me506 showed a similar degree of selectivity for beta2- as opposed to beta1-receptor mediated actions.