The pharmacokinetic and pharmacodynamic characterization of midazolam continuous infusion in critically ill children and characterization of midazolam metabolism in vitro by cytochrome P-450IIIA
Abstract
The pharmacokinetics and pharmacodynamics of midazolam (M) in five critically ill pediatric patients was investigated. A standardized means of assessing the level of sedation was designed, tested and validated in the intensive care unit and showed good inter-rater agreement (Kappa $\ge$ 0.70). The M infusion rate varied from 0.5-9 $\mu$g/kg/min and the duration of therapy ranged from 48-300 hours. Initial estimates of clearance, volume of distribution at steady state, and elimination half-life ranged from 218-260 ml/min, 0.3-1.5 l/kg, and 0.42-4.5 hours, respectively. Adequate sedation appeared to be associated with serum M concentrations greater than 250 ng/ml. These findings indicate that in critically ill children M pharmacokinetics and possibly pharmacodynamics demonstrate wide variability. The capability of cytochrome P450IIIA4 (CYP3A4), CYP3A5, and CYP3A7 to metabolize midazolam was investigated using human hepatic microsomes and purified CYP3A4 and CYP3A5 proteins. Eighteen human hepatic samples containing CYP3A4 & 5 and six fetal samples containing CYP3A7 showed 30- and 16-fold variations, and 10- and 9-fold variations in the maximal rate of 1-hydroxymethylmidazolam (1$\sp\prime$OH) and 4-hydroxymidazolam (4-OH) formation, respectively. Formation of 4-OH was highly correlated with the formation of 1$\sp\prime$OH at 400 $\mu$M M in adult liver samples (r$\sp2 >$ 0.99) containing only CYP3A4 and fetal liver samples (r$\sp2$ = 0.97), which contained CYP3A7. The formation of 1$\sp\prime$OH and 4-OH at 400 $\mu$M M by adult hepatic microsomal samples containing only CYP3A4 was highly correlated with erythromycin metabolism (r$\sp2 >$ 0.95 and 0.92, respectively) and CYP3A content (r$\sp2$ = 0.89 and 0.86, respectively). Additionally, the formation of 1$\sp\prime$OH and 4-OH was inhibited by erythromycin. These findings indicate that the hydroxylation of midazolam is catalyzed by CYP3A. Purified CYP3A5 in a reconstituted system displayed a 2-fold greater rate of 1$\sp\prime$OH formation and a similar rate of 4-OH formation compared to purified CYP3A4 which confirms the microsomal findings. CYP3A5 containing samples exhibited a significantly greater ratio of 1$\sp\prime$OH/4-OH compared to samples containing only CYP3A4 or CYP3A7 (p $<$ 0.01) at three M concentrations. These results indicate that the tested human CYP3A forms catalyze midazolam hydroxylation and that the 1$\sp\prime$OH/4-OH ratio is indicative (CYP3A5 $>$ CYP3A4 $>$ CYP3A7) of the expressed CYP3A protein(s) present in the microsomal sample.
Degree
Ph.D.
Advisors
Carlstedt, Purdue University.
Subject Area
Pharmacology
Off-Campus Purdue Users:
To access this dissertation, please log in to our
proxy server.