Bioavailability of oral dexamethasone during high dose steroid therapy in neurological patients.

The pharmacokinetics and oral biovailability of dexamethasone were studied in 6 patients with neurological disease being treated with high dosages of the drug. A specific high performance liquid chromatographic assay was used to measure dexamethasone concentrations. Unlike the previously published mean figure of 0.78 for the oral bioavailability of the drug given in single doses to healthy volunteers, the mean bioavailability of dexamethasone in the patients studied was 0.53 +/- SD 0.40. It appeared more likely that this incomplete bioavailability was due to presystemic elimination than to poor absorption. The intravenous clearance of the drug was relatively high (0.4902 +/- SD 2291 1 kg-1, approximately 65% of expected hepatic plasma flow), the oral clearance higher (2.5804 +/- SD 3.2181 1 kg-1 h-1) while the absorption rate constant (4.8729 +/- 8.4998 h-1), suggested rapid absorption after oral administration. Prior phenytoin and possibly prior dexamethasone therapy is likely to have contributed to the higher clearance values of the drug in these patients than the values reported in healthy volunteers after single dose studies.