Absorption and first pass metabolism of ¹⁴C-gamma hydroxybutyric acid

The recovery of radioactivity from plasma, urine and feces was determined in rats after administration of oral and intravenous doses (200 mg/kg) of ¹⁴C labeled sodium gamma hydroxybutyric acid. Very small portions of the radioactive dose were recovered in the urine (5.5%, oral; 7.1%, intravenous) and feces (1.5%, oral; 0.6%, intravenous) collected between 0-48 hours after drug administration. Considerable levels of radioactivity were found in the plasma after oral dosing. The area under the plasma radioactivity time curve after an oral dose was found to be 65% of that observed after an equivalent intravenous dose. This value is much larger than the relative area value (8%) calculated on the basis of free gamma hydroxybutyric acid. Results of this study strongly indicate that first pass metabolism, rather than lack of absorption, is responsible for the apparently poor oral bioavailability of gamma hydroxybutyric acid.