Pharmacokinetics and oral bioavailability of scopolamine in normal subjects

The pharmacokinetics and bioavailability of scopolamine were evaluated in six healthy male subjects receiving 0.4 mg of the drug by either oral or intravenous administration. Plasma and urine samples were analyzed using a radioreceptor binding assay. After iv administration, scopolamine concentrations in the plasma declined in a biexponential fashion, with a rapid distribution phase and a comparatively slow elimination phase. Mean and SE values for volume of distribution, systemic clearance, and renal clearance were 1.4 +/- 0.3 liters/kg, 65.3 +/- 5.2 liters/hr, and 4.2 +/- 1.4 liters/hr, respectively. Mean peak plasma concentrations were 2909.8 +/- 240.9 pg/ml following iv administration and 528.6 +/- 109.4 pg/ml following oral administration. Elimination half-life of the drug was 4.5 +/- 1.7 hr. Bioavailability of the oral dose was variable among subjects, ranging between 10.7 and 48.2%. The variability in absorption and poor bioavailability of oral scopolamine indicate that this route of administration may not be reliable and effective.