Read the article previously published on this topic – “Bronchial Asthma Treatment with Amiodarone Report“.
Amiodarone (courtesy of Dr. A. Curran, Reckitt and Colman Pharmaceutical Division, Sydney) was dissolved in methanol (1 mg/ml) and diluted in assay buffer immediately prior to use. The range of concentrations employed (10 to 1,000 ng/ml) was of the order reported in human plasma after therapeutic doses.
Human pulmonary cells (VA-13 line, American Type Culture, Bethesda, Md) were grown in RPMI 1640 medium supplemented with 10 percent fetal calf serum (Flow Labs) and 2 mM glutamine. To obtain pulmonary membranes for p-adrenergic receptor binding assays, the cells were lysed in 1 mM trimethamine (Tris)-HCl (pH 7.5) for 15 minutes at 4°C and scraped off and centrifuged for ten minutes at 3,400 g. The pellet was resuspended in 50 mM sodium phosphate-4 mM magnesium sulfate buffer (pH 7.4) and stored at — 70°C.
For P-adrenergic binding, pulmonary membranes (500µg/ml) were incubated in a volume of 200µl for 30 minutes at 24°C in polypropylene tubes with buffer alone or buffer with amiodarone (10 or 1,000 µg/ml). Radioactive iodine-labelled cyanopindolol (New England Nuclear; 300,000 µm/ml) with increasing concentrations of unlabelled isoproterenol (final concentrations, 0 to 10,000 nmoles/L) was then added. After 45 minutes at 24°C, membrane-bound radioactivity was separated by rapid filtration and washing on Whatman GF filters and counted in a gamma counter. Nonspecific binding (ie, radioactivity bound in the presence of excess [10µM] isoproterenol) was less than 10 percent of total binding.
For P-adrenergic bioassay, human pulmonary cells (VA13 line) were grown to confluence in 24-well plates. The culture medium was aspirated and replaced with assay buffer (50 mM trimethamine-HCl and 2 mM MgCI2; pH 7.4) alone or with amiodarone. The assay buffer also contained 30µM RO 20-17 24 (Hoffman-LaRoche), a phosphodiesterase inhibitor added to stop cyclic-AMP degradation. After 30 minutes at 37°C, isoproterenol was added to each set in triplicate. After a further 30 minutes at 37°C, the reactions were terminated by the addition of boiling sodium acetate (pH 6.0) to each well. Cells were scraped off, and the suspensions were transferred to polypropylene tubes and boiled for five minutes. Cyclic AMP released was immunoassayed using a commercially available kit (New England Nuclear).
Amiodarone, at concentrations of 10 and 1,000 ng/ml, had no effect on the specific binding of iodine-labelled cyanopindolol to pulmonary membrane B-adrenergic receptors (Fig 1). However, when pulmonary cells were preincubated with amiodarone, their cyclic-AMP response to a submaximum dose of isoproterenol was impaired (Fig 2). In the presence of amiodarone at a concentration of 10 ng/ml, the response curve to isoproterenol was significantly shifted to the right without a change in the maximum response (Fig 3). This decrease in sensitivity to isoproterenol is indicated by the fact that the concentration of isoproterenol required to produce a half-maximum cyclic-AMP response was increased from 12 to 24 nmoles/L.
Therapy with amiodarone is contraindicated in the presence of marked sinus bradycardia and atrioventricular block and should be used with caution in combination with P-adrenergic blocking agents because of possible excessive bradycardia. Pulmonary fibrosis has been reported in a number of patients receiving amiodarone. In 1980, Rot-mensch et al reported the findings of one man who developed hypersensitivity pneumonitis that resolved after treatment with prednisolone and cessation of therapy with amiodarone. Further studies have reported this complication in six of cases reported by Sobol and Rakita and in three of 41 cases reported by Heger and colleagues. In these and in subsequent reports, there were a number of deaths.
Nevertheless, no mention is made in the amiodarone product information literature of the possibility of exacerbating obstruction of airways in patients with asthma, and bronchospasm is not mentioned as a complication or possible contraindication in the larger series reporting clinical experience with amiodarone.
In this report the use of amiodarone resulted in an exacerbation of the symptoms of asthma ( find more information), a slowing of the sinus rate from 85 to 60 beats per minute, and abolition of multiple ventricular ectopic complexes. These effects are all consistent with interference with the action of catecholamines. Laboratory studies to determine the mechanism of this interference revealed that although amiodarone did not alter the binding of radioactive-labelled pindolol to pulmonary-adrenergic receptors, it did reduce the sensitivity of the receptor-mediated response of cyclic AMP to stimulation by isoproterenol. A twofold increase in the concentration of isoproterenol was required to achieve the same (half maximum) response. Since the basal rate of cyclic AMP production was not altered, the site of action of amiodarone would not appear to be adenyl cyclase (which hydrolyzes XP to cyclic AMP) but rather the coupling step between receptor binding and activation of the enzyme. An inference to this effect was drawn from studies of the effect of amiodarone on the chronotropic response of rabbit atrium to isoproterenol. Our findings provide a possible molecular explanation as to how amiodarone might precipitate bronchospasm in susceptible individuals.
It is also interesting to note that in our patient, asthmatic symptoms worsened within a few days of starting amiodarone in both instances and improved very quickly after withdrawal of the drug. This is somewhat surprising, considering the pharmacokinetics of amiodarone. The drug is purported to have an extremely long half-life (up to one month), one of its disadvantages being the long time usually taken to achieve therapeutic levels in the blood with oral administration, even after loading doses. This report suggests that its onset of action is variable and may occur quite rapidly in some patients. Because of the possibility of worsening bronchospasm, amiodarone should be used with caution in patients with chronic obstruction of the airways. survivors had all been treated with prednisolone.
Figure 1. Specific binding of iodine-labelled cyanopindolol to cultured human pulmonary cell membranes as function of increasing concentrations of isoproterenol (isoprenaline), in presence of buffer alone (open circles) or buffer containing amiodarone at either 10 ng/ml (solid circles) or 1,000 ng/ml (triangles). Each point is mean of triplicate determinations.
Figure 2. Effect of amiodarone on cyclic-AMP response (mean ± SD; N = 4) of cultured pulmonary cells to submaximum dose of isoproterenol (isoprenaline).
Figure 3. Isoproterenol (isoprenaline)-cyclic AMP dose-response (mean ± SD; N = 4) of cultured pulmonary cells in absence (control) and presence of amiodarone (10 ng/ml).