Impact of cytochrome P-450 inhibition by cimetidine and induction by carbamazepine on the kinetics of hypericin and pseudohypericin in healthy volunteers.

This study evaluated the influence of cimetidine and carbamazepine on the pharmacokinetics of the St. John's wort (SJW) ingredients hypericin and pseudohypericin. In a placebo-controlled, double blind study, 33 healthy volunteers were randomized into three treatment groups that received SJW extract (LI160) with different comedications (placebo, cimetidine, and carbamazepine) for 7 days after a run-in period of 11 days with SJW alone. Hypericin and pseudohypericin pharmacokinetics were measured on days 10 and 17. Between-group comparisons showed no statistically significant differences in AUC(0-24), C(max), and t(max) values for hypericin and pseudohypericin. Within-group comparisons, however, revealed a statistically significant increase in hypericin AUC(0-24) from a median of 119 (range 82-163 microg h/l) to 149 microg h/l (61-202 microg h/l) with cimetidine comedication and a decrease in pseudohypericin AUC(0-24) from a median of 51.0 (16.4-102.9 microg h/l) to 36.4 microg h/l (14.0-102.0 microg h/l) with carbamazepine comedication compared to the baseline pharmacokinetics in each group. Hypericin and pseudohypericin pharmacokinetics were only marginally influenced by comedication with the enzyme inhibitors and inducers cimetidine and carbamazepine.

The impact of the CYP2D6 polymorphism on haloperidol pharmacokinetics and on the outcome of haloperidol treatment.

OBJECTIVES: The genetically polymorphic enzyme cytochrome P450 (CYP) 2D6 contributes to the biotransformation of the antipsychotic drug haloperidol. The impact of the polymorphism on haloperidol pharmacokinetics, adverse events, and efficacy was prospectively evaluated under naturalistic conditions in 172 unselected psychiatric inpatients with acute psychotic symptoms. METHODS: Serum trough levels of haloperidol and reduced haloperidol of patients receiving clinically adjusted doses were analyzed on days 3, 14, and 28 after hospital admission. Adverse events such as extrapyramidal symptoms were assessed by standardized rating scales. Efficacy was documented by recording the change in positive and negative schizophrenic symptoms. These parameters were correlated with the CYP2D6 genotype determined by polymerase chain reaction analysis for alleles *1 to *15 and *17. RESULTS: The serum concentrations showed wide interindividual variation. Reduced haloperidol trough levels and haloperidol total clearance correlated significantly with the number of active CYP2D6 genes. In addition, body weight and smoking had significant effects on haloperidol kinetics, whereas age, gender, and comedication showed only slight effects. The ratings for pseudoparkinsonism were significantly higher in poor metabolizers of substrates of CYP2D6. On the other hand, there was a trend toward lower therapeutic efficacy with increasing number of active CYP2D6 genes. CONCLUSIONS: Treatment with haloperidol should be avoided in extremely slow and extremely rapid metabolizers of CYP2D6 substrates. Both genotyping and blood concentration measurement explained only a fraction of the adverse events; about 20 patients would have to be genotyped to achieve a significant benefit in 1 patient. It is interesting that genotyping was at least as good a predictor of adverse events as the measured drug concentrations.

Decreased plasma levels of amitriptyline and its metabolites on comedication with an extract from St. John's wort ( Hypericum perforatum ).

Extracts of St. John's wort ( Hypericum perforatum ) became increasingly popular as easily available remedies for mild to moderate depression. Comedication with hypericum extract was recently shown to drastically reduce plasma concentration of ciclosporin, digoxin, and indinavir. We investigated the possible interaction of hypericum extract LI160 with amitriptyline. Both antidepressants have a high probability of concomitant use. Twelve patients requiring amitriptyline treatment received a single dose of hypericum extract (900 mg) at day 1, continued by a 12-to 14-day treatment with retarded amitriptyline (75 mg twice daily). Then hypericum (900 mg/day) was added for another 14 to 16 days. Steady-state pharmacokinetics of amitriptyline were compared before and after multiple-dose treatment with hypericum extract. Furthermore, comparisons were made for single-dose kinetics of hypericum-extract ingredients hypericin, pseudohypericin, and hyperforin between the first day of concomitant treatment and LI160 alone. Multiple-dose comedication with LI160 led to a statistically significant decrease in the area under the plasma concentration-time curve within one dosing interval of amitriptyline by 22% ( p = 0.03) and nortriptyline by 41% ( p = 0.002), as well as of all hydroxylated metabolites, except for 10-E-hydroxynortriptyline. Plasma levels of amitriptyline and hydroxylated metabolites gradually decreased, whereas nortriptyline concentrations were already markedly decreased after 3 days of cotreatment with hypericum. Cumulative urinary amounts of amitriptyline and metabolites decreased to the same extent as plasma concentrations upon hypericum comedication. Induction of cytochrome P-450 enzymes or drug transporters (P-glycoprotein) by St. John's wort extract may explain this pharmacokinetic interaction. Physicians should be aware of this interaction when treating patients with amitriptyline.