Intravenous formulations of the enantiomers of thalidomide: pharmacokinetic and initial pharmacodynamic characterization in man

Thalidomide, a racemate, is coming into clinical use as an immunomodulating and antiinflammatory drug. These effects may chiefly be exerted by S-thalidomide, but the enantiomers are interconverted in-vivo. Thalidomide is given orally, although parenteral administration would be desirable in some clinical situations. The aim of this study was to prepare solutions of the enantiomers of thalidomide for intravenous administration and to investigate their pharmacokinetics and sedative effects following infusion in man. Solubility and stability of the enantiomers in 5% glucose solution was investigated. After a dose-determination experiment in one subject, six healthy male volunteers received R- and S-thalidomide separately by 1-h infusions in a randomized double-blind cross-over study. Blood was sampled over 22h and sedative effects were recorded. Blood concentrations of the enantiomers were determined by stereospecific HPLC. A four-compartment model consisting of a two-compartment model for each enantiomer, with elimination from both compartments, connected by rate constants for chiral inversion was fitted to the concentration data, while the sedative effects were correlated with the blood concentrations of R- and S-thalidomide by means of logistic regression. The enantiomers of thalidomide were chemically stable in solution for at least a week at room temperature. The infusions were well tolerated. Sedation, which was the only observed effect, was related to the blood concentration of R-thalidomide. Inter-individual variation in the disposition of the enantiomers was modest (e.g. terminal half-lives ranged between 3.9 and 5.3h). Pharmacokinetic modelling predicted that varying the infusion time of a fixed dose of S-thalidomide between 10 min and 6h would have little influence on the maximal blood concentration of formed R-thalidomide. To our knowledge this is the first time that thalidomide has been administered intravenously.