A population approach to enzyme characterization and identification: application to phenacetin O-deethylation

PURPOSE

To determine the enzyme kinetics (EK) and identify the human cytochrome(s) P450 (CYP) involved in the deethylation of phenacetin to acetaminophen using a population-based method.

METHODS

A sparse data set was generated from incubations containing human liver microsomes (n = 19) with phenacetin. Estimates of the EK parameters were obtained by fitting the concentration-velocity data to Michaelis-Menten models by using nonlinear mixed effects modeling. Relationships between the EK parameters and the CYP activities determined for these liver microsomes were examined.

RESULTS

A two-enzyme kinetic model with a saturated, low KM enzyme and an unsaturated, high KM enzyme capable of forming acetaminophen best fit the data. The population estimates of the EK parameters were Vmax1, 911 pmol/min/mg protein; KM1, 11.3 microM; and Cl(int2), 0.4 microl/min/mg. The coefficients of variation for interliver variability in Vmax1 and residual error of the model were 39% and 15%, respectively. When the selective catalytic activities were examined as potential covariates, 7-ethoxyresorufin O-deethylation (CYP1A2) activity was found to be associated with the low KM enzyme, however, the high KM enzyme(s) could not be identified.

CONCLUSIONS

The population approach characterized the EK parameters and identified the low KM enzyme responsible for phenacetin O-deethylation as CYP1A2. Population modeling of EK provides valuable information on inter- and intraliver variability in CYP dependent activities.

Lack of correlation between in vitro and in vivo studies on the effects of tangeretin and tangerine juice on midazolam hydroxylation

BACKGROUND

Tangeretin is a flavonoid that stimulates the catalytic activity of cytochrome P450 3A4 (CYP3A4) and is found in high levels in tangerine juice.

METHODS

The effect of tangeretin on hydroxylation of midazolam, a CYP3A4 probe, was examined in vitro with human liver microsomes and recombinant CYP3A4. In addition, the effect of tangerine juice on the pharmacokinetics and pharmacodynamics of orally administered midazolam (15 mg) and its active 1'-hydroxymetabolite was studied in a randomized crossover study in eight healthy volunteers.

RESULTS

In microsomes from three human livers, tangeretin (1 to 100 micromol/L) increased 1'-hydroxymidazolam formation (12.5 micromol/L midazolam) by up to 212%. In complementary deoxyribonucleic acid-expressed CYP3A4, a 52% stimulation of midazolam 1'-hydroxylation was reached at 50 micromol/L tangeretin with no effect on midazolam 4-hydroxylation. In the pharmacokinetic-pharmacodynamic study, 200 mL tangerine juice reduced the area under the concentration versus time curve to 1.5 hours [AUC(O-1.5h)] of midazolam and 1'-hydroxymidazolam by 39% and 46%, respectively, and prolonged the time to reach peak concentration (P < .05) without affecting the total AUC values, elimination half-life values, or AUC ratios (1'-hydroxymidazolam/midazolam). These findings are consistent with a small delay in the absorption of midazolam and lack of effect on midazolam 1'-hydroxylation. Accordingly, tangerine juice slightly postponed the maximum pharmacodynamic effects of midazolam (P < .05).

CONCLUSION

Tangeretin is a potent regioselective stimulator of midazolam 1'-hydroxylation by human liver microsomes and complementary deoxyribonucleic acid-expressed CYP3A4. However, tangerine juice is unlikely to have any appreciable effect on CYP3A4 in humans. Further studies are required to assess whether in vitro stimulators of CYP3A4 can influence drug metabolism in vivo.