Theophylline has a dose-related effect on the airway response to inhaled histamine and methacholine in asthmatics

To study whether theophylline inhibits airway hyperresponsiveness in a dose-dependent fashion, we performed inhalation challenges with histamine and methacholine in 9 asthmatic patients. On 4 separate days, 3 consecutive histamine or methacholine tests were carried out, each of them 20 min after saline (placebo) and after 100, 100, and 200 mg intravenous theophylline ethylenediamine given in a cumulative fashion. Airway responsiveness was expressed as the provocative dosage of histamine or methacholine necessary to increase specific airway resistance by 100% (PD100SRaw). After placebo PD100SRaw for histamine showed a small but significant (p less than 0.01) increase not observed after methacholine. Theophylline markedly attenuated airway reactivity in a dose-dependent manner. At a mean (SD) serum concentration of 6.14 (0.30) mg/L, theophylline increased geometric mean PD100SRaw for histamine from 2.76 to 6.07 units (p less than 0.01) and for methacholine from 1.52 to 2.60 units (p less than 0.05). At a mean (SD) serum concentration of 12.9 (0.70) mg/L, theophylline increased geometric mean PD100SRaw for histamine from 2.70 to 17.1 units (p less than 0.01) and for methacholine from 1.28 to 4.98 units (p less than 0.01). Thus, there was a protective effect of theophylline on histamine and methacholine responsiveness in patients with bronchial asthma at "subtherapeutic" serum theophylline concentrations with increasing efficacy at higher serum theophylline concentrations. These observations may have therapeutic implications in the treatment of patients with mild asthma.

Bronchodilator effect of theophylline preparations and aerosol fenoterol in stable asthma

To compare the acute bronchodilator effect of increasing doses of intravenous theophylline and inhaled beta adrenergic agonists, we administered intravenous theophylline dissolved in ethylenediamine or proxyphylline and diprophylline or placebo in a double blind fashion to nine asthmatics on three different days. At each session, 100 mg theophylline or placebo were given during each of five subsequent periods of 30 minutes' duration and followed by inhalation of 0.4 mg fenoterol. In contrast to placebo, 500 mg theophylline in ethylenediamine or proxyphylline and diprophylline significantly decreased mean specific airway resistance (SRaw in cmH2O.s) from 31.2 to 23.6 or 34.2 to 23.5 at theophylline serum concentrations of 14.4 or 16.6 mg/L, respectively. Fenoterol lowered SRaw to about 40 percent of the respective baseline values independent of theophylline or placebo pretreatment. We conclude that the acute bronchodilator effect of theophylline is weak in comparison to inhaled beta agonists. Furthermore, proxyphylline and diprophylline cause a weak but not significant bronchodilation when compared to ethylenediamine.

Airway response to methacholine during exercise induced refractoriness in asthma

To investigate the mechanisms contributing to refractoriness in exercise induced asthma a methacholine challenge test was performed 30 minutes before and 30 minutes after two exercise tests 45 minutes apart. Exercise was performed by 12 asthmatic patients while they were breathing cold air. There was a smaller airway response to the second exercise test than to the first, though there was wide variation between subjects. The response to the second methacholine challenge was reduced in some patients but showed no significant change overall. Refractoriness to exercise induced asthma positively correlated with a reduced response to methacholine. These data suggest that mediator depletion does not fully explain refractoriness.

[Comparison of the bronchodilator and protective effect of methylxanthines]

In study I, carried out on 9 asthmatics to investigate whether the bronchodilator and protective effect of intravenous theophylline, dissolved in ethylene diamine (T.E.) or proxyphylline and diprophylline (T.P.D.), showed different dose-effect relationships, 100 mg theophylline with T.E., T.P.D. or NaCl (placebo) were infused 5 times at 30 minute intervals. This was followed by inhalation of 0.4 mg fenoterol. Results showed that the bronchodilator effect of 500 mg theophylline with a serum concentration greater than 15 mg/l was only about 60% of that for fenoterol alone. In study II, 11 patients with exercise-induceable respiratory obstruction received 200 and 351 mg theophylline with T.E., 200 mg with T.P.D. or placebo prior to exercise and cold air inhalation. Here theophylline had a dose-dependent protective effect. With a theophylline serum concentration of 5 mg/l there was already a reduction in exercise asthma by more than 60% in comparison to placebo. At comparable serum concentrations the effect of T.P.D. was better than T.E. in both studies although the difference was not statistically significant.

CONCLUSION

Since the bronchodilator effect of methylxanthines is less than the protective effect, the prophylactic use of theophylline should be given more attention.