The contribution of the swallowed fraction of an inhaled dose of salmeterol to it systemic effects

Salmeterol is approximately eight times as potent as salbutamol for systemic effects. This may be because the drug is eight times more potent on receptors or there may be differences in systemic bioavailability. The systemic effects of salbutamol are limited by its fairly high first-pass metabolism, but the oral bioavailability of salmeterol is unknown. The contribution of the swallowed fraction of an inhaled dose of salmeterol to its systemic effects were analysed in a randomized, double-blind, crossover study. Twelve healthy subjects were given inhaled salmeterol 400 microg, inhaled salmeterol 400 microg plus oral activated charcoal or inhaled placebo plus oral activated charcoal on three separate days. Cardiac frequency (fC), Q-T interval corrected for heart rate (QTc), plasma potassium and glucose concentrations were measured for 4 h following the inhaled drug. Salmeterol with and without oral charcoal produced significant changes for all measures compared to placebo. The magnitude of effect following salmeterol alone was significantly greater than that following salmeterol plus charcoal for fC and glucose (mean (95% confidence interval) differences 8 (2-13) beats x min(-1), 0.59 (0.04, 1.13) mmol x L(-1), respectively) and nonsignificantly greater for QTc interval and potassium concentration. The differences between salmeterol given with and without charcoal suggest that 28-36% of the systemic response to salmeterol administered from a metered-dose inhaler are due to drug absorbed from the gastrointestinal tract. Thus, most of the systemic effects are due to the inhaled fraction of the drug.

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