Alprazolam as a probe for CYP3A using a single blood sample:: pharmacokinetics of parent drug, and of α- and 4-hydroxy metabolites in healthy subjects

Objectives: ( 1) To determine the pharmacokinetic parameters of alprazolam and its two metabolites in plasma from healthy volunteers; ( 2) to identify a suitable single time point to take a plasma sample for CYP3A phenotyping. Methods: Twelve healthy Swedish volunteers received a single oral dose of 1 mg alprazolam. Blood samples were collected before drug intake and frequently up to 72 h thereafter. A liquid-chromatography/mass-spectrometry (LC/MS) method was used for the quanti. cation of alprazolam, and 4- and alpha-hydroxyalprazolam. Results: The interindividual variation in the area under the concentration - time curve (AUC) was two, three and fourfold for alprazolam, 4- hydroxyalprazolam and alpha-hydroxyalprazolam, respectively. Plasma concentration ratios collected between 1 h and 48 h for both alprazolam/4- hydroxyalprazolam and alprazolam/alpha-hydroxyalprazolam correlated significantly to the corresponding AUC((0 - infinity)) ratios. Conclusions: The metabolic ratios of alprazolam to respective metabolite in a single plasma sample at 3 - 24 h are suggested to reflect the alprazolam 4- and alpha-hydroxylation activities. In future, it will be important to study these activities in populations where CYP3A5, in addition to CYP3A4, is expressed at a high frequency and to clarify the relative importance of the two enzymatic pathways for in vivo clearance of alprazolam.