Following the oral administration of either chlorambucil/prednisolone or prednimustine to patients, the plasma levels of free chlorambucil and phenylacetic acid mustard, the beta-oxidation product of chlorambucil, were measured using a new high-performance liquid chromatographic (HPLC) assay. This assay permitted the simultaneous detection of the analyzed compounds with a lower limit of detection of 30 ng/ml. The pharmacokinetics of chlorambucil and phenylacetic acid mustard were found to be entirely different when prednimustine was administered as opposed to its components chlorambucil and prednisolone together. After the ingestion of the conjugate, the plasma concentration-time curves of chlorambucil and phenylacetic acid mustard showed a "delayed" pattern compared with those obtained after the administration of the components. The mean area under the concentration-time curves (AUCs) of prednimustine-derived chlorambucil and phenylacetic acid mustard were 25% and 40%, respectively, of the areas obtained after a stoichiometrically equivalent dose of chlorambucil. Free plasma prednimustine could not be detected at any time. This different pharmacokinetic behavior might offer an explanation for the superior therapeutic effects of prednimustine demonstrated by clinical studies.

The pharmacokinetics of chlorambucil, phenylacetic acid mustard (the β-oxidation product of chlorambucil), and prednisolone were investigated in a cross-over study after oral administration of chlorambucil (30 mg) + prednisolone (50 mg) versus prednimustine (300 mg), the ester of chlorambucil and prednisolone. Intact prednimustine could not be detected in plasma at any time. After administration of prednimustine, the plasma concentration-time curves of chlorambucil, phenylacetic acid mustard, and prednisolone showed a retarded profile compared to the administration of the single components. The mean bioavailability was 14% for chlorambucil, 21% for phenylacetic acid mustard, and 22% for prednisolone, when given as prednimustine, compared to the administration of free compounds in stoichiometrically equivalent doses. When given in the oral dosages mentioned above, the average dose-intensity was 62% for chlorambucil, 95% for phenylacetic acid mustard, and 72% for prednisolone, indicating sufficient therapeutic concentrations of the detectable agents.