In a previous study we showed that the disposition of clozapine after a single oral dose is unrelated to either debrisoquine or S-mephenytoin hydroxylation polymorphism. The same 14 healthy subjects studied in that investigation were given 150 mg of caffeine. The reciprocal of plasma clozapine AUC (0,24), was correlated with an index of the N3-demethylation of caffeine (r, = 0.84; P = 0.0024), used as a measure of cytochrome P4501A2 (CYP1A2) activity. Ni-and N7-demethylation indices of caffeine also reflect CYP1A2 activity and were also correlated with clozapine clearance (rs = 0.89 and 0.85; P = 0.0013 and 0.0023; respectively). No significant relationships with xanthine oxidase and N-acetyl transferase activity, also assessed by a caffeine test, were found. This study suggests that clozapine is metabolised by CYP1A2 to a major extent.

Twenty-five healthy volunteers were given 100 mg caffeine orally and several estimates of cytochrome P450 1A2 (CYP1A2) activity were evaluated. The validation was performed by correlation of different parameters in plasma, saliva, and urine to two measures of caffeine clearance, CL(oral) and CL(137X-->17X) that served as standards of reference. Two subjects were excluded because of noncompliance with a caffeine-free diet. In the remaining 23 subjects, both plasma and saliva total clearances of caffeine were highly correlated with each other (r(s) = 0.97, p < 0.0001). The ratio 17X/137X restricted to one sampling point taken 4 hours after dose, showed a high correlation (r(s)) with CL(oral) and CL(137X-->17X) in plasma (0.84/0.83) and saliva (0.82/0.77) (p < 0.0001 for all the correlation values) where 17X is 1,7-dimethylxanthine (paraxanthine) and 137X is 1,3,7-trimethylxanthine (caffeine). Additionally, the ratio (AFMU + 1U + 1X + 17U + 17X)/137X in a 0-24 hours urine sampling showed the highest correlation with CL(137X-->17X) (r(s) = 0.85, p < 0.001) where AFMU is 5-acetylamino-6-formylamino-3-methyluracil, 1U is 1-methyluracil, 1X is 1-methylxanthine, and 17U is 1,7-dimethyluric acid. The major estimates of CYP1A2 activity were significantly less in nonsmoking females, and this probably was related to the use of oral contraceptives in this subpopulation. In summary, among caffeine-based approaches for CYP1A2, the authors recommend either plasma or saliva 17X/137X ratio and the urinary (AFMU + 1U + 1X + 17U + 17X)/137X ratio during a sampling interval of at least 8 hours, starting at time zero since caffeine intake. These indices are simple, reliable, and relatively inexpensive estimates of CYP1A2 activity to be used in the study of human populations.

This study investigated whether the smokinginducible cytochrome P450 (CYP) 1A2 and the polymorphic CYP2D6 play significant roles in the metabolism of olanzapine and its clinical effects at steady-state treatment. Caffeine and debrisoquine were used as measures of CYP1A2 and CYP2D6, respectively. After drug therapy for 15 days, the effect of olanzapine on the activities of CYP1A2 and CYP2D6 was also examined. Seventeen psychiatric patients (9 men and 8 women) were orally administered olanzapine, at a mean +/- standard deviation (SD) dosage of 10 mg/d for all smokers (n = 8) and 7.5 +/- 2.5 mg/d (range, 5-10 mg) for nonsmokers (n = 9;p <0.01). The plasma concentration-to-dose (C:D) ratio was closely correlated to the CYP1A2 activity ( s = -0.89;p <0.0001). The mean urinary caffeine indexes of nonsmokers and smokers were 17 +/- 8 and 101 +/- 44, respectively, indicating that smoking had induced a sixfold higher CYP1A2 activity (p <0.0001). Likewise, the olanzapine plasma C:D ratio (ng.mL.mg) was about fivefold lower in smokers (7.9 +/- 2.6) than in nonsmokers (1.56 +/- 1.1;p <0.0001). On day 15 of the antipsychotic therapy, the percentage decrease in Brief Psychiatric Rating Scale (BPRS) total score relative to the predosing score (in the drug-free period) was higher for nonsmokers than for smokers (30.4 +/- 10% vs. 12.5 +/- 14%;p <0.01). Six nonsmokers and three smokers experienced side effects with olanzapine. After 15 days of drug treatment, olanzapine had caused significant (p <0.0001) and substantial CYP1A2 inhibition (by 50%) in comparison with predosing values, and such inhibition can contribute to adverse drug interactions. In conclusion, smoking-induced increased CYP1A2 activity significantly diminished plasma olanzapine concentrations and the antipsychotic effect of the drug. The performance of a simple caffeine test may assist in individualization of the olanzapine dosage.

Hydroxylation of midazolam (MDZ) is mediated almost exclusively by CYP3A isoforms. The authors describe a high-performance liquid chromatography assay involving MDZ, 1'-hydroxymidazolam, and 4-hydroxymidazolam in plasma. The compounds were eluted on an Ultrasphere ODS, 3-microm particle size, 7.5 cm x 4.6 mm reversed-phase column and monitored by ultraviolet absorbance at 254 nm. The composition of the mobile phase was 35.2% acetonitrile:4.8% methanol:60% buffer acetate (vol/vol/vol), 0.1 M, pH 4.7; the flow rate was 1 ml/minute. Calibration curves were linear (coefficients of correlation > 0.99) within the range of concentrations established (20 to 640 nM). Within- and between-day coefficients of variation were consistently better than 8%. The overall recovery was >90% and the lowest detectable concentration was 8 nM. This approach provides a simple, rapid, and sensitive assessment of MDZ and metabolites in plasma, with a very good accuracy and precision, which enables it as an in vivo marker of CYP3A activity in humans.

No convincing evidence was found that CYP2D6 is an important enzyme for the disposition of fluvoxamine. Other factors seem to be more important. A nontherapeutic oral daily dose of fluvoxamine is sufficient to provide a marked inhibition of both caffeine (CYP1A2) and omeprazole (CYP2C19) metabolism. It was not possible to separate the inhibitory effects of fluvoxamine on these enzymes, even after such a low daily dose such as 10 mg x 1 or x 2 of fluvoxamine.

Both clozapine (CLZ) and caffeine are CYP1A2 substrates. This study raises the hypothesis of whether caffeine withdrawal from the diet alters the metabolism and/or clinical status of patients receiving CLZ. Seven schizophrenic patients (six men and one woman) receiving monotherapy with CLZ at 271+/-102 mg/day (3.73+/-1.4 mg/kg) participated in the study. CLZ, norclozapine (NOR), and clozapine-N-oxide (NOX) were assayed in plasma by high-performance liquid chromatography at three different time points: A, with concomitant intake of caffeine from the diet; B, after caffeine withdrawal for 5 days; and C, after 2 weeks of rechallenge to habitual caffeine intake. The CYP1A2 activity was determined by means of a urinary caffeine test. After a caffeine-free diet for 5 days, CLZ concentrations relative to time point A decreased from 486 to 306 ng/mL (-47%) (p < 0.02), NOX levels decreased from 66 to 49 ng/mL (-31%) (p < 0.03), and the NOR/CLZ ratio significantly increased from 0.47 to 1.04 (185%) (p < 0.02). All parameters returned to initial figures at time point C. The NOR/CLZ ratio was significantly correlated to the CYP1A2 index (rs = 0.96, p < 0.0005). In conclusion, changes in the habitual caffeine intake alter the metabolism of CLZ in schizophrenic patients. Thus, patient intake of caffeine should be medically supervised, and the monitoring of CLZ and metabolite levels may be warranted. Furthermore, in those patients who receive therapy with CLZ, the NOR/CLZ ratio may provide an additional and valuable estimate of CYP1A2 activity.

No abstract

Brain CYP3A activity could be modulated by some neurotransmitters and precursors.