WHAT IS ALREADY KNOWN ABOUT THIS SUBJECT• We have suggested that 4b-hydroxycholesterol may be used as an endogenous marker of CYP3A activity.• Recently, we found unexpectedly that the plasma concentration of 4b-hydroxycholesterol continued to increase for several weeks after complete induction of CYP3A4/5 by carbamazepine.• In the present study we investigated the time course of elimination of 4b-hydroxycholesterol from the circulation following CYP3A induction with rifampicin. WHAT THIS STUDY ADDS• 4b-Hydroxycholesterol is eliminated very slowly from the circulation with an apparent half-life of 17 days.• The long half-life results in a low variation in plasma concentration with time, but excludes 4b-hydroxycholesterol as a marker for rapid changes in CYP3A activity. AIMSThe oxysterol 4b-hydroxycholesterol has been suggested as a marker for CYP3A4/5 activity. We have previously shown that plasma 4b-hydroxycholesterol continues to increase for several weeks after maximal induction of CYP3A4/5 by carbamazepine at the dose given. In the present study we aimed to determine the time course of the decrease in plasma 4b-hydroxycholesterol after termination of induction of CYP3A4/5 by rifampicin. An additional aim was to determine the variation in plasma level of 4b-hydroxycholesterol with time in 12 untreated healthy volunteers. METHODSTwenty-four healthy subjects were allocated into three study groups of equal sizes. The volunteers were treated with rifampicin (either 20 mg day -1 , 100 mg day -1 or 500 mg day -1 ) for 2 weeks. Blood samples were taken before, during and after rifampicin treatment. In another group of 12 untreated volunteers blood samples were collected at different time points in order to determine the intraindividual variations in plasma 4b-hydroxycholesterol concentrations. Plasma levels of 4b-hydroxycholesterol were determined by isotope-dilution gas chromatography-mass spectrometry. RESULTSRifampicin treatment increased plasma 4b-hydroxycholesterol levels. After termination of rifampicin treatment plasma levels of 4b-hydroxycholesterol decreased slowly with an apparent half-life of 17 days. The intraindividual variation in plasma levels of 4b-hydroxycholesterol in untreated subjects was low, with coefficients of variation of between 4.8 and 13.2% over a period of 3 months. CONCLUSIONSAfter termination of induction of CYP3A4/5, plasma 4b-hydroxycholesterol levels decreased slowly during 8 weeks. The half-life of elimination (17 days) resembled that of cholesterol rather than other oxysterols. The long half-life results in stable plasma concentrations with time.

The Karolinska cocktail, comprising caffeine, losartan, omeprazole, and quinine, was given before and after administration of rifampicin (20, 100, or 500 mg daily) to measure induction of cytochrome P450 (P450) enzymes. Rifampicin was given for 14 days to eight healthy subjects (all of whom possessed at least one wild-type CYP2C9 and one wild-type CYP2C19 gene) in each dose group. 4beta-hydroxycholesterol was assessed as an endogenous marker of CYP3A4 induction. A fourfold induction of CYP3A4 was seen at the highest dose by both quinine:3'-hydroxyquinine and 4beta-hydroxycholesterol measurements (P < 0.001). CYP3A4 was also induced at the two lower doses of rifampicin when measured by these two markers (P < 0.01 or P < 0.001). CYP1A2, CYP2C9, and CYP2C19 were induced after 500 mg rifampicin daily (1.2-fold, P < 0.05; 1.4-fold, P < 0.05; and 4.2-fold, P < 0.01, respectively). In conclusion, we have shown that the Karolinska cocktail and 4beta-hydroxycholesterol can be used for an initial screening of the induction properties of a drug candidate.

INTRODUCTIONOmeprazole, the ®rst proton pump (H + , K + -ATPase) inhibitor, has been used for over a decade in the treatment of acid-related diseases. Like subsequent proton pump inhibitors, omeprazole is metabolized primarily by a polymorphically expressed enzyme within cytochrome P450 (CYP), CYP2C19.1 Omeprazole is a racemic composition of its two optical isomers, S-omeprazole (esomeprazole) and R-omeprazole, which have demonstrated stereoselective metabolisms. 2±4Esomeprazole is metabolized to a lesser degree than R-omeprazole by CYP2C19. Esomeprazole has also been shown to be metabolized at a lower rate, resulting in higher plasma levels, 4 which, because the area under the curve (AUC) directly correlates to the antisecretory effect, 5 promotes more effective acid control. Thus, esomeprazole's inhibitory effect on gastric acid secretion is greater than that of both omeprazole and its R-isomer. We investigated the relations between dose/concentration and response for esomeprazole and omeprazole after single and repeated doses. MATERIALS AND METHODS SubjectsTwelve healthy males, mean age 24 years (range, 20±30 years) and mean weight 75 kg (range, SUMMARY Background: Esomeprazole, the S-isomer of omeprazole, is the ®rst proton pump inhibitor developed as a single isomer for the treatment of acid-related diseases. Aim: To examine the pharmacokinetics and pharmacodynamics of esomeprazole. Methods: In a crossover study, 12 healthy males received 5, 10 or 20 mg of esomeprazole, or 20 mg of omeprazole, once daily over 5 days. The pharmacokinetics and effects on pentagastrin-stimulated peak acid output of esomeprazole and omeprazole were studied on days 1 and 5. Results: The area under the curve (AUC) of both esomeprazole and omeprazole increased from day 1 to

CYP3A4, considered the most important enzyme in drug metabolism, is often involved in drug-drug interactions. When developing new drugs, appropriate markers for detecting CYP3A4 induction are needed. Our study compared endogenously formed 4β-hydroxycholesterol with the midazolam clearance in plasma and the 6β-hydroxycortisol/cortisol ratio in urine as markers for CYP3A4 induction. To this end, we performed a clinical trial in which 24 healthy subjects were randomized to 10, 20, or 100 mg daily doses of rifampicin for 14 days (n = 8 in each group) to achieve a low and moderate CYP3A4 induction. The CYP3A4 induction could be detected even at the lowest dose of rifampicin (10 mg) via the estimated midazolam clearance, the 4β-hydroxycholesterol ratio (both P < 0.01), and the 6β-hydroxycortisol ratio (P < 0.05). For the three dosing groups (10, 20, and 100 mg), the median fold induction from baseline was 2.0, 2.6, and 4.0 for the estimated midazolam clearance; 1.3, 1.6, and 2.5 for the 4β-hydroxycholesterol/cholesterol ratio; and 1.7, 2.9, and 3.1 for the 6β-hydroxycortisol/cortisol ratio. In conclusion, the 4β-hydroxycholesterol ratio is comparable to midazolam clearance as a marker of CYP3A4 induction, and each may be used to evaluate CYP3A4 induction in clinical trials evaluating drug-drug interactions for new drugs.