Effect of administration of moxifloxacin plus rifampin against Mycobacterium tuberculosis for 7 of 7 days versus 5 of 7 days in an in vitro pharmacodynamic system.

UNLABELLED: Some trials administered antituberculosis agents for 5 of 7 days (5/7-day regimen) to optimize adherence. Since moxifloxacin has a longer half-life than rifampin, rifampin concentrations are <1% of the maximum concentration in serum (C(max)) on day 6 and nondetectable on day 7, while concentrations of moxifloxacin remain and are able to induce error-prone replication. We determined if functional moxifloxacin monotherapy for 24 h/week caused resistance. In in vitro pharmacodynamic experiments, Mycobacterium tuberculosis was treated with mean area under the concentration-time curve (AUC) exposures for moxifloxacin and rifampin of 400 and 600 mg/kg/day and exposures equal to 1 standard deviation (SD) above and below the mean values. The drugs were administered on schedules of 7/7 days and 5/7 days. Over the 28-day experiments, bacteria were plated onto antibiotic-free agar to determine the effects of exposure and schedule on the total population. MICs were checked for emergence of resistance. At days 7 and 14, there was a 0.56- to 1.22-log(10)-CFU/ml greater cell kill with the 7/7-day regimen versus the 5/7-day regimen (low exposure). This difference was not seen for the larger exposures at day 21. At day 23, the low-exposure 5/7-day arm had breakthrough resistance, with the total count increasing to >2 log(10) CFU/ml above the low-exposure 7/7-day arm. Pharmacokinetic mismatching of drugs in the therapy of tuberculosis may result in emergence of resistance when a drug holiday is imposed during which there is functional monotherapy and where the remaining agent induces error-prone replication. This is particularly true for the portion of the population where the clearance is higher (1 SD above the mean). IMPORTANCE: Directly observed therapy is a cornerstone of treatment of Mycobacterium tuberculosis. Patients are often given a drug holiday to facilitate the direct observation of therapy. With rifampin and moxifloxacin, there is a discordance between the half-lives of these agents (1.9 versus 6.5 h when employed in combination). In addition, moxifloxacin induces error-prone replication in Mycobacterium tuberculosis. In this experiment, we demonstrate that the drug holiday (5 of 7 days of therapy [5/7-day regimen]) allows the emergence of resistance to moxifloxacin, which was not seen with 7/7-day therapy. If drug holidays are used, it is imperative to better match pharmacokinetics to minimize the risk of emergence of resistance.

Impact of spore biology on the rate of kill and suppression of resistance in Bacillus anthracis.

Bacillus anthracis is complex because of its spore form. The spore is invulnerable to antibiotic action. It also has an impact on the emergence of resistance. We employed the hollow-fiber infection model to study the impacts of different doses and schedules of moxifloxacin on the total-organism population, the spore population, and the subpopulations of vegetative- and spore-phase organisms that were resistant to moxifloxacin. We then generated a mathematical model of the impact of moxifloxacin, administered by continuous infusion or once daily, on vegetative- and spore-phase organisms. The ratio of the rate constant for vegetative-phase cells going to spore phase (K(vs)) to the rate constant for spore-phase cells going to vegetative phase (K(sv)) determines the rate of organism clearance. The continuous-infusion drug profile is more easily sensed as a threat; the K(vs)/K(sv) ratio increases at lower drug exposures (possibly related to quorum sensing). This movement to spore phase protects the organism but makes the emergence of resistance less likely. Suppression of resistance requires a higher level of drug exposure with once-daily administration than with a continuous infusion, a difference that is related to vegetative-to-spore (and back) transitioning. Spore biology has a major impact on drug therapy and resistance suppression. These findings explain why all drugs of different classes have approximately the same rate of organism clearance for Bacillus anthracis.

Use of midazolam urinary metabolic ratios for cytochrome P450 3A (CYP3A) phenotyping.

Midazolam (MDZ) total clearance (ClT) is widely used for cytochrome P450 3A (CYP3A) phenotyping, but requires up to eight blood samples. This study was conducted to compare the use of midazolam ClT to use of a midazolam urinary metabolic ratio for CYP3A phenotyping. Ten male and 10 female subjects received i.v. midazolam 0.025 mg/kg eight times over a 4-month period at approximately 2-week intervals. The first six phenotyping measures were used to estimate baseline CYP3A activity, then subjects received the moderate CYP3A inhibitor fluvoxamine 150 mg/day for the last 4 weeks (two phenotyping visits) of the study. Serial blood samples were obtained for calculation of ClT. Urine was collected for 6 h following each midazolam dose. Midazolam, 1'-hydroxymidazolam (1-OHMDZ), and 4-hydroxymidazolam were measured in plasma and urine by liquid chromatography with tandem mass spectrometry (LC/MS/MS). Analysis of 148 samples from 20 subjects revealed a weak overall correlation between the urinary ratio of 1-OHMDZ/MDZ to midazolam ClT of r(s) = 0.372 (P = 0.0001). There was no correlation when examining either baseline samples or fluvoxamine-inhibited samples alone (r(s) = 0.101, P = 0.289 and r(s) = 0.266, P = 0.123, respectively). The median (range) urinary ratio decreased significantly with fluvoxamine [219 (141-409) versus 127 (50-464); P = 0.005] and to a similar extent to the midazolam ClT (-33.6% versus -42.4%, respectively; P > 0.05). Median urinary recovery of the i.v. midazolam dose varied between 1.4% and 53% and was significantly lower in samples collected while patients were receiving fluvoxamine (34.3% versus 23.1%; P= 0.0004). Based on these results, although this midazolam urinary ratio was not very reflective of baseline CYP3A activity, it may be a useful indicator of CYP3A inhibition.

Effect of fluvoxamine therapy on the activities of CYP1A2, CYP2D6, and CYP3A as determined by phenotyping.

OBJECTIVE: To determine the effect of 150 mg/day fluvoxamine on the activities of CYP1A2, CYP2D6, CYP3A, N-acetyltransferase-2 (NAT2), and xanthine oxidase (XO) by phenotyping with caffeine, dextromethorphan, and midazolam. METHODS: Oral caffeine (2 mg/kg), oral dextromethorphan (30 mg), and intravenous midazolam (0.025 mg/kg) were administered to 10 white male volunteers every 14 days for 4 months and to 10 white premenopausal female volunteers during the midfollicular and midluteal phases of the menstrual cycle for 4 complete cycles (8 total phenotyping measures). The first 6 phenotyping measures were used to establish baseline activity. Subjects were given 150 mg/day fluvoxamine for the fourth month or cycle of the study. Enzyme activity for CYP1A2, CYP2D6, NAT2, and XO was expressed as urinary metabolite ratios. Midazolam plasma clearance was used to express CYP3A activity. RESULTS: No difference between baseline and weeks 2 and 4 of fluvoxamine therapy was observed for NAT2 or XO metabolite ratios. For CYP1A2, CYP2D6, and CYP3A phenotypes, significant differences existed between baseline and fluvoxamine therapy. For CYP1A2, the mean urinary metabolite ratio (+/-SD) was 7.53 +/- 7.44 at baseline and 4.30 +/- 2.82 with fluvoxamine ( P = .012). Mean CYP2D6 molar urinary dextromethorphan ratios before and after fluvoxamine therapy were 0.00780 +/- 0.00694 and 0.0153 +/- 0.0127, respectively (P = .011). Midazolam clearance decreased from 0.0081 +/ 0.0024 L/min/kg at baseline to 0.0054 +/- 0.0021 L/min/kg with therapy (P = .0091). For CYP1A2, CYP2D6, and CYP3A, fluvoxamine therapy changed the phenotyping measures by a median of -44.4%, 123.5%, and -34.4%, respectively. CONCLUSIONS: We concluded that fluvoxamine may cause significant inhibition of CYP1A2, CYP2D6, and CYP3A activity. This metabolic inhibition may have serious implications for a variety medications.

Quantification of 3-month intraindividual variability and the influence of sex and menstrual cycle phase on CYP3A activity as measured by phenotyping with intravenous midazolam.

OBJECTIVE: Intraindividual variability and the effects of sex and menstrual cycle phase on CYP3A activity were evaluated by phenotyping with use of midazolam as the probe drug. METHODS: Midazolam (0.025 mg/kg) was administered intravenously to 10 white male volunteers every 14 days for 3 months and to 10 white premenopausal female volunteers during the midfollicular and midluteal phases of the menstrual cycle for 3 complete cycles. Serum was collected for a 6-hour period, and enzyme activity was represented by midazolam plasma clearance. RESULTS: No difference in clearance was observed during the menstrual cycle phases. Mean +/- SD midazolam clearance was 0.00816 +/- 0.00252 L/min/kg during the midfollicular phase and 0.00818 +/- 0.00224 during the midluteal phase (P = .96). When the menstrual cycle phases were combined, mean midazolam clearance in women was 0.00817 +/- 0.00235 L/min/kg. Mean male midazolam clearance was 0.00766 +/ 0.00167 L/min/kg. There was no difference in midazolam clearance between men and women (P = .68). Coefficients of variation (CV%) for the 6 phenotyping visits were calculated and the median midazolam clearance CV% (25th to 75th percentile) was 9.75% (8.40% to 11.5%). CONCLUSIONS: Because no significant differences in midazolam clearance were noted between menstrual cycle phases or between sexes, pharmacokinetic and clinical investigations of CYP3A activity in adults may not require stratification on the basis of menstrual cycle phase or sex.