In vivo effect of clarithromycin on multiple cytochrome P450s.

The in vivo effects of oral clarithromycin administration on the in vivo activity of cytochrome P450 1A2, 2C9, and 2D6 were determined. The cytochrome P450 probes caffeine (CYP1A2), tolbutamide (CYP2C9), and dextromethorphan (CYP2D6) were administered as an oral cocktail prior to and 7 days after oral clarithromycin (500 mg twice daily) administration to 12 healthy male subjects. Blood and urine samples were collected and assayed for each of the compounds and their metabolites using high-performance liquid chromatography. The CYP1A2 indices, oral caffeine clearance (6.2 +/- 3.3 l/h before and 5.7 +/- 4.2 l/h after, p > 0.05) and the 6-h paraxanthine to caffeine serum concentration ratio (0.49 +/- 0.3 before and 0.44 +/- 0.3 after, p > 0.05), were unchanged following clarithromycin dosing. Neither the tolbutamide oral clearance (0.77 +/- 0.28 l/h before and 0.72 +/-0.24 l/h after, p > 0.05) nor the tolbutamide urinary metabolic ratio (779 +/- 294 before and 681 +/- 416 after, p > 0.05) indices of CYP2C9 were altered by clarithromycin administration. In the case of CYP2D6, the dextromethorphan to dextrorphan urinary ratio was not significantly different before (0.021 +/- 0.04) and after (0.024 +/- 0.06) clarithromycin dosing. In conclusion, clarithromycin does not appear to alter the in vivo catalytic activity of CYP1A2, CYP2C9, and CYP2D6 in healthy individuals as assessed by caffeine, tolbutamide, and dextromethorphan, respectively.

The effect of rifampin administration on the disposition of fexofenadine.

OBJECTIVE: Our objective was to assess the effect of rifampin (INN, rifampicin) on the pharmacokinetics of fexofenadine and to assess the influence of advanced age and sex. METHODS: Twelve young volunteers (6 men and 6 women; age range, 22 to 35 years) and twelve elderly volunteers (6 men and 6 women; age range, 65 to 76 years) received a 60-mg oral dose of fexofenadine before and after treatment with 600 mg of oral rifampin for 6 days. Blood and urine were collected for 48 hours and assayed for fexofenadine, azacyclonol, and rifampin by HPLC with either fluorescence or mass spectrometry detection. RESULTS: All of the groups had a significant increase (P <.05) in the oral clearance of fexofenadine after rifampin treatment: young men, 2955 +/- 1516 versus 5524 +/- 3410 mL/min; young women, 2632 +/- 996 versus 7091 +/- 5,379 mL/min; elderly men, 1760 +/- 850 versus 4608 +/- 1159 mL/min; and elderly women, 2210 +/- 554 versus 4845 +/- 1600 mL/min. The peak serum concentration of fexofenadine was also significantly reduced (P <.05) by rifampin treatment: young men, 77 +/- 31 versus 52 +/- 17 ng/mL; young women, 72 +/- 19 versus 36 +/- 14 ng/mL; elderly men, 106 +/- 42 versus 52 +/- 14 ng/mL; elderly women, 76 +/- 23 versus 46 +/- 19 ng/mL. Half-life (150 to 230 minutes), time to maximum concentration (130 to 205 minutes), renal clearance (95 to 153 mL/min), and fraction unbound (2.9% to 3.7%) of fexofenadine showed no significant difference between control and treatment. The amount of azacyclonol, a CYP3A4 mediated metabolite of fexofenadine, eliminated renally increased on average 2-fold after rifampin dosing; however, this pathway accounted for less than 0.5% of the dose. No effect of age or sex on fexofenadine disposition or serum trough rifampin concentration (0.2 microg/mL to 1.8 microg/mL) was observed before or after rifampin treatment. CONCLUSION: This study showed that rifampin effectively increased fexofenadine oral clearance and that this effect was independent of age and sex. We conclude that the cause of the increased oral clearance of fexofenadine is a reduced bioavailability caused by induction of intestinal P-glycoprotein.

The effect of hormone replacement therapy on CYP3A activity.

BACKGROUND: The effect of menopause and hormone replacement therapy on hepatic and intestinal wall CYP3A activity is poorly defined. This study was therefore designed to determine the effect of menopause and estrogen replacement therapy on hepatic and intestinal CYP3A activity with a specific CYP3A substrate, midazolam. METHODS: Twelve young women (27 +/- 5 years), 10 elderly women receiving estrogen replacement therapy (71 +/- 6 years), and 14 elderly women not receiving estrogen replacement therapy (71 +/- 5 years) received simultaneous intravenous (0.05 mg/kg over 30 minutes) and oral (3 to 4 mg of a stable isotope, 15N3-midazolam) doses of midazolam. Serum and urine samples were assayed for midazolam, 15N3-midazolam, and metabolites by use of gas chromatography-mass spectrometry. RESULTS: No significant (P > .05) differences were observed in systemic clearance and oral clearance between the three groups. Likewise, no differences were observed in oral, hepatic, or intestinal availability. A significant correlation was observed between oral and intestinal availability and not hepatic availability. CONCLUSION: Neither menopause nor menopause with estrogen replacement therapy altered intestinal or hepatic CYP3A activity relative to that in a control group of young women.

Stereoselective sulfoxidation of sulindac sulfide by flavin-containing monooxygenases. Comparison of human liver and kidney microsomes and mammalian enzymes.

The stereoselective sulfoxidation of the pharmacologically active metabolite of sulindac, sulindac sulfide, was characterized in human liver, kidney, and cDNA-expressed enzymes. Kinetic parameter estimates (pH = 7.4) for sulindac sulfoxide formation in human liver microsomes (N = 4) for R- and S-sulindac sulfoxide were V(max) = 1.5 +/- 0.50 nmol/min/mg, K(m) = 15 +/- 5.1 microM; and V(max) = 1.1 +/- 0.36 nmol/min/mg, K(m) = 16 +/- 6.1 microM, respectively. Kidney microsomes (N = 3) produced parameter estimates (pH = 7.4) of V(max) = 0.9 +/- 0.29 nmol/min/mg, K(m) = 15 +/- 2.9 microM; V(max) = 0.5 +/- 0.21 nmol/min/mg, K(m) = 22 +/- 1.9 microM for R- and S-sulindac sulfoxide, respectively. In human liver and flavin-containing monooxygenase 3 (FMO3) the V(max) for R-sulindac sulfoxide increased 60-70% at pH = 8.5, but for S-sulindac sulfoxide was unchanged. In fourteen liver microsomal preparations, significant correlations occurred between R-sulindac sulfoxide formation and either immunoquantified FMO or nicotine N-oxidation (r = 0.88 and 0.83; P < 0.01). The R- and S-sulindac sulfoxide formation rate also correlated significantly (r = 0.85 and 0.75; P < 0.01) with immunoquantified FMO in thirteen kidney microsomal samples. Mild heat deactivation of microsomes reduced activity by 30-60%, and a loss in stereoselectivity was observed. Methimazole was a potent and nonstereoselective inhibitor of sulfoxidation in liver and kidney microsomes. n-Octylamine and membrane solubilization with lubrol were potent and selective inhibitors of S-sulindac sulfoxide formation. cDNA-expressed CYPs failed to appreciably sulfoxidate sulindac sulfide, and CYP inhibitors were ineffective in suppressing catalytic activity. Purified mini-pig liver FMO1, rabbit lung FMO2, and human cDNA-expressed FMO3 efficiently oxidized sulindac sulfide with a high degree of stereoselectivity towards the R-isomer, but FMO5 lacked catalytic activity. The biotransformation of the sulfide to the sulfoxide is catalyzed predominately by FMOs and may prove to be useful in characterizing FMO activity.

The contribution of intestinal and hepatic CYP3A to the interaction between midazolam and clarithromycin.

OBJECTIVE: To assess the relative contribution of intestinal and hepatic CYP3A inhibition to the interaction between the prototypic CYP3A substrates midazolam and clarithromycin. METHODS: On day 1, 16 volunteers (eight men and eight women; age range, 20 to 40 years; weight range, 45 to 100 kg) received simultaneous doses of midazolam intravenously (0.05 mg/kg over 30 minutes) and orally (4 mg of a stable isotope, 15N3-midazolam). Starting on day 2, 500 mg clarithromycin was administered orally twice daily for 7 days. On day 8, intravenous and oral doses of midazolam were administered 2 hours after the final clarithromycin dose. Blood and urine samples were assayed for midazolam, 15N3-midazolam, and metabolites by gas chromatography-mass spectrometry. RESULTS: There was no significant (p > 0.05) difference in the urinary excretion of 1'-hydroxymidazolam after intravenous and oral dosing on day 1 or day 8, indicating that the oral dose was completely absorbed into the gut wall. The oral clearance of midazolam was found to be significantly greater in female subjects (1.9 +/- 1.0 versus 1.0 +/- 0.3 L/hr/kg; p < 0.05) than in male subjects but not systemic clearance (0.35 +/- 0.1 versus 0.44 +/- 0.1 L/hr/kg). For women not receiving oral contraceptives (n = 6) a significant gender-related difference was observed for systemic and oral clearance and for area under the curve and elimination half-life after oral administration. A significant (p < 0.05) reduction in the systemic clearance of midazolam from 28 +/- 9 L/hr to 10 +/- 3 L/hr occurred after clarithromycin administration. Oral midazolam availability was significantly increased from 0.31 +/- 0.1 to 0.75 +/- 0.2 after clarithromycin dosing. Likewise, intestinal and oral availability were significantly increased from 0.42 +/- 0.2 to 0.83 +/- 0.2 and from 0.74 +/- 0.1 to 0.90 +/- 0.04, respectively. A significant correlation was observed between intestinal and oral availability (n = 32, r = 0.98, p < 0.05). After clarithromycin administration, a significant correlation was observed between the initial hepatic or intestinal availability and the relative increase in hepatic or intestinal availability, respectively. Female subjects exhibited a greater extent of interaction after oral and intravenous dosing than male subjects (p < 0.05). CONCLUSION: These data indicate that in addition to the liver, the intestine is a major site of the interaction between oral midazolam and clarithromycin. Interindividual variability in first-pass extraction of high-affinity CYP3A substrates such as midazolam is primarily a function of intestinal enzyme activity.