Comparative single-dose kinetics of oxazolam, prazepam, and clorazepate: three precursors of desmethyldiazepam.

Twelve healthy volunteers received a single 40-mg oral dose of the benzodiazepine derivative oxazolam, which serves primarily as a precursor of the active substance desmethyldiazepam (DMDZ). Concentrations of DMDZ were measured in multiple serum samples drawn for up to two weeks after the dose. Peak serum DMDZ concentrations averaged 115 ng/ml, measured at 8.6 hours after dosage. Mean DMDZ elimination half-life averaged 61 hours. Three of the subjects also received 40 mg each of prazepam and clorazepate, two other DMDZ precursors, on separate occasions. Although DMDZ elimination half-life was similar, total area under the curve (AUC) for DMDZ was larger for clorazepate, known to be completely transformed into DMDZ, than for oxazolam or prazepam the extent of whose conversion to DMDZ has not been previously established. After correcting for the different molar equivalent of DMDZ available from each preparation, the DMDZ ratio averaged 0.22 for oxazolam vs. clorazepate and 0.51 for prazepam vs. clorazepate. Thus, both oxazolam and prazepam lead to slow appearance of DMDZ in the systemic circulation. Furthermore the extent of DMDZ formation from oxazolam and prazepam is either incomplete or the drugs are incompletely absorbed. Equivalent doses of oxazolam, prazepam, and clorazepate should not be interchanged in clinical practice.