A pilot study assessing pharmacokinetics and tolerability of oral and intravenous baclofen in healthy adult volunteers.

Our objective was to characterize baclofen pharmacokinetics and safety given orally and intravenously. Twelve healthy subjects were enrolled in a randomized, open-label, crossover study and received single doses of baclofen: 3 or 5 mg given intravenously and 5 or 10 mg taken orally with a 48-hour washout. Blood samples for baclofen analysis were collected pre-dose and at regular intervals up to 24 hours post-dose. Clinical response was assessed by sedation scores, ataxia, and nystagmus. Mean absolute bioavailability of oral baclofen was 74%. Dose-adjusted areas under the curve between the oral and intravenous arms were statistically different (P = .0024), whereas area under the curve variability was similar (coefficient of variation: 18%-24%). Adverse effects were mild in severity and not related to either dose or route of administration. Three- and 5-mg intravenous doses of baclofen were well tolerated. Seventy-four percent oral bioavailability indicates that smaller doses of intravenous baclofen are needed to attain comparable total drug exposures.