1-Aminobenzotriazole (ABT) is widely used as a non-specific inhibitor of animal cytochrome P450 (CYP). In the present study, the inhibitory effect of ABT was investigated on drug oxidations catalyzed by human CYP isoforms. This inhibitory effect was compared with that of SKF-525A, another non-specific inhibitor, and ketoconazole, a potent inhibitor of CYP3A. Bacurovirus-expressed recombinant human CYP isoforms were used as an enzyme source. The specific activities for human CYP isoforms are: phenacetin O-deethylation, for CYP1A2; diclofenac 4'-hydroxylation, for CYP2C9; S-mephenytoin 4'-hydroxylation, for CYP2C19; bufuralol 1'-hydroxylation, for CYP2D6; chlorzoxazone 6-hydroxylation, for CYP2E1; testosterone 6beta-hydroxylation, nifedipine oxidation, and midazolam 1'-hydroxylation, for CYP3A4. ABT inhibited both CYP1A2-dependent activity (Ki=330 microM) and CYP2E1-dependent activity (Ki=8.7 microM). In contrast, SKF-525A weakly inhibited CYP1A2-dependent activities (46% inhibition at 1200 microM) and CYP2E1-dependent activities (65% inhibition at 1000 microM). ABT exhibited the highest Ki value for CYP2C9-dependent diclofenac 4'-hydroxylation among those determined by this assay (Ki=3500 microM). Moreover, SKF-525A showed strong inhibition of CYP2D6-dependent bufuralol 1'-hydroxylation (Ki=0.043 microM). Ketoconazole inhibited all tested drug oxidations, however, its inhibitory effect on CYP1A2-dependent activities was very weak (50% inhibition at 120 microM). ABT, SKF-525A, and ketoconazole showed different selectivity and had a wide range of Ki values for the drug oxidations catalyzed by human CYP enzymes. Therefore, we conclude that inhibitory studies designed to predict the contribution of CYP enzymes to the metabolism of certain compounds should be performed using multiple CYP inhibitors, such as ABT, SKF-525A, and ketoconazole.

1-Aminobenzotriazole (ABT) is extensively used as a non-specific cytochrome P450 (CYP) inhibitor. In this study, the inhibitory effect of ABT on CYP-dependent drug oxidations was investigated in human liver microsomes (HLM) and compared with that of SKF-525A, another non-specific inhibitor. The following probe activities for human CYP isoforms were determined using pooled HLM: phenacetin O-deethylation (CYP1A2); diclofenac 4'-hydroxylation (CYP2C9); S-mephenytoin 4'-hydroxylation, (CYP2C19); bufuralol 1'-hydroxylation (CYP2D6); chlorzoxazone 6-hydroxylation (CYP2E1); midazolam 1'-hydroxylation, nifedipine oxidation, and testosterone 6beta-hydroxylation (CYP3A). ABT had the strongest inhibitory effect on the CYP3A-dependent drug oxidations and the weakest effect on the diclofenac 4'-hydroxylation. SKF-525A potently inhibited the bufuralol 1'-hydroxylation, but weakly inhibited chlorzoxazone 6-hydroxylation. The inhibitory effects of ABT and SKF-525A were increased by preincubation in some probe reactions, and this preincubation effect was greater in ABT than in SKF-525A. The remarkable IC50 shift (> 10 times) by preincubation with ABT was observed on the phenacetin O-deethylation, chlorzoxazone 6-hydroxylation, and midazolam 1'-hydroxylation. In conclusion, ABT and SKF-525A had a wide range of IC50 values in inhibiting the drug oxidations by HLM with and without preincubation.

The purpose of this study was to evaluate a telemetry system for examining QT evaluation in the conscious free-moving guinea pig using 10 reference compounds whose effects on human QT interval are well established: 8 positive references (bepridil, terfenadine, cisapride, haloperidol, pimozide, quinidine, E-4031 and thioridazine), and 2 negative references (propranolol and nifedipine). Pharmacokinetic experiments were also performed for the 8 positive references. Telemetry transmitters were implanted subcutaneously in male Hartley guinea pigs, and the RR and QT intervals were measured. All 8 positive references prolonged QTc (QTc = k x QT/RR(1/2)) 10% or more during the 60 min observation period. When the values of the QTc changes were plotted against the serum concentrations, the resulting curves exhibited an anticlockwise hysteresis loop for all 8 references. In guinea pigs treated with haloperidol, changes of the T-wave shape from positive to flat were observed. The 2 negative references did not prolong the QTc. These findings suggest that the present telemetry guinea pig model is useful for QT evaluation in the early stages of drug development, because of the small body size of guinea pigs and their action potential configuration, which is similar to that of humans.

We performed a prospective evaluation of pharmacokinetics of fluconazole administered for prophylactic purposes to 19 patients after cytotoxic chemotherapy for hematological malignancies. On days 7 and 15, we obtained 5 ml of blood from each patient. If flucona-zole was administered orally, blood samples were drawn 2, 8, and 24 hr after ingestion of the drug. If it was administered intravenously, blood samples were drawn 1, 8, and 24 hr post-injection. Serum fluconazole levels were analyzed by HPLC with ultraviolet light detection. In patients receiving 200 or 400 mg of fluconazole per day, maximal serum levels were 7.9 and 15.6 mg/l and minimum levels were 5.0 and 10.3 mg/l, respectively. There was no significant difference in serum fluconazole levels comparing the levels after oral and intravenous administration, and pharmacokinetic parameters of fluconazole were comparable at each time point within one dose level. However, considerable variation in serum fluconazole levels was noted in this study, as the maximal serum levels ranged from 4.0 to 13.3 mg/l and from 8.7 to 26.9 mg/l in patients receiving 200 and 400 mg of fluconazole orally, respectively. These variations may be associated with prophy-lactic failures for patients with insufficient fluconazole concentrations. Multiple regression analysis showed significant correlation between serum fluconazole levels and some variables including dose of fluconazole, age, serum aspartate aminotransferase levels and blood urea nitrogen levels. These variations may be associated with disturbance of body water balance, such as massive hemorrhage and dehydration. Am. J. Hematol. 66:85-91, 2001.

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