Effects of 2 quinolone antibacterials, temafloxacin and enoxacin, on theophylline pharmacokinetics

Study on the daily dose and serum concentration of clozapine in psychiatric patients and possible influencing factors of serum concentration

BACKGROUND

Clozapine is the most effective drug for treatment-resistant schizophrenia, and the dosage and concentration of clozapine in the treatment of mental illness vary greatly in different populations and are affected by many factors.

METHODS

The serum clozapine concentration of 3734 psychiatric patients was detected, and data on daily dose, sex, age and other medical records were collected for statistical analysis.

RESULTS

The mean daily dose, mean serum concentration and mean C/D (concentration/dose) ratio of clozapine were 191.02 ± 113.47 mg/day, 326.15 ± 235.66 ng/mL and 1.94 ± 1.25 ng/mL per mg/day, respectively. There was difference in daily dose between sexes, and females had higher daily dose (p <0.01), higher serum clozapine concentrations (p < 0.01) and higher C/D ratios (p < 0.01). There were significant differences in daily dose (p < 0.001), serum drug concentration (p < 0.001) and C/D ratio (p < 0.001) among different age groups. The daily dose decreased with age (p for trend < 0.001), and the C/D ratio increased with age (p for trend < 0.001). Inpatients and outpatients had no difference in daily dose, but inpatients had higher serum concentration (p < 0.001) and C/D ratio (p < 0.001). There was no difference in daily dose among different occupations, but there were significant differences in serum concentration (p < 0.001) and C/D ratio (p < 0.001), and unemployed patients may have higher serum concentration and C/D ratio. Duration of disease, comorbidity, marital status, and psychotic type may influence the daily dose and serum concentration.

CONCLUSIONS

The effective daily dose and serum concentration of clozapine in the study area may be lower than recommended levels, and women have higher serum concentrations and slower metabolic rates. With increasing age, the daily dose decreases, and the metabolic rate slows. Inpatient status and occupation of patients may influence the serum concentration and metabolic rate of clozapine.

Pharmacokinetic aspects of treating infections in the intensive care unit: focus on drug interactions

Pharmacokinetic interactions involving anti-infective drugs may be important in the intensive care unit (ICU). Although some interactions involve absorption or distribution, the most clinically relevant interactions during anti-infective treatment involve the elimination phase. Cytochrome P450 (CYP) 1A2, 2C9, 2C19, 2D6 and 3A4 are the major isoforms responsible for oxidative metabolism of drugs. Macrolides (especially troleandomycin and erythromycin versus CYP3A4), fluoroquinolones (especially enoxacin, ciprofloxacin and norfloxacin versus CYP1A2) and azole antifungals (especially fluconazole versus CYP2C9 and CYP2C19, and ketoconazole and itraconazole versus CYP3A4) are all inhibitors of CYP-mediated metabolism and may therefore be responsible for toxicity of other coadministered drugs by decreasing their clearance. On the other hand, rifampicin is a nonspecific inducer of CYP-mediated metabolism (especially of CYP2C9, CYP2C19 and CYP3A4) and may therefore cause therapeutic failure of other coadministered drugs by increasing their clearance. Drugs frequently used in the ICU that are at risk of clinically relevant pharrmacokinetic interactions with anti-infective agents include some benzodiazepines (especially midazolam and triazolam), immunosuppressive agents (cyclosporin, tacrolimus), antiasthmatic agents (theophylline), opioid analgesics (alfentanil), anticonvulsants (phenytoin, carbamazepine), calcium antagonists (verapamil, nifedipine, felodipine) and anticoagulants (warfarin). Some lipophilic anti-infective agents inhibit (clarithromycin, itraconazole) or induce (rifampicin) the transmembrane transporter P-glycoprotein, which promotes excretion from renal tubular and intestinal cells. This results in a decrease or increase, respectively, in the clearance of P-glycoprotein substrates at the renal level and an increase or decrease, respectively, of their oral bioavailability at the intestinal level. Hydrophilic anti-infective agents are often eliminated unchanged by renal glomerular filtration and tubular secretion, and are therefore involved in competition for excretion. Beta-lactams are known to compete with other drugs for renal tubular secretion mediated by the organic anion transport system, but this is frequently not of major concern, given their wide therapeutic index. However, there is a risk of nephrotoxicity and neurotoxicity with some cephalosporins and carbapenems. Therapeutic failure with these hydrophilic compounds may be due to haemodynamically active coadministered drugs, such as dopamine, dobutamine and furosemide, which increase their renal clearance by means of enhanced cardiac output and/or renal blood flow. Therefore, coadministration of some drugs should be avoided, or at least careful therapeutic drug monitoring should be performed when available. Monitoring may be especially helpful when there is some coexisting pathophysiological condition affecting drug disposition, for example malabsorption or marked instability of the systemic circulation or of renal or hepatic function.

Absence of effect of rufloxacin on theophylline pharmacokinetics in steady state

Several quinolone antibacterial agents are known to inhibit the metabolism of theophylline, with the potential to cause adverse events due to raised theophylline concentrations during coadministration. A randomized crossover study was therefore conducted with 12 healthy male volunteers (ages, 23 to 34 years; body weight, 64 to 101 kg) to evaluate a possible interaction between rufloxacin and theophylline. Both drugs were administered at steady state. Following the administration of an oral loading dose of 400 mg on day 1, rufloxacin was given orally at 200 mg once daily on days 2 to 7 during one period only. During both periods, 146 mg of theophylline was administered orally twice daily for 3 days (which were days 4 to 6 of the rufloxacin coadministration period) and intravenously once the next morning to test for an interaction. Theophylline and rufloxacin concentrations were measured by reversed-phase high-pressure liquid chromatography, the pharmacokinetics of theophylline at steady state following administration of the last dose were calculated by compartment-model-independent methods. To compare the treatments, analysis of variance-based point estimates and 90% confidence intervals (given in parentheses) were calculated for the mean ratios of the pharmacokinetic parameters from the test (rufloxacin coadministration) over those from the reference (theophylline without rufloxacin) period. These were as follows: maximum concentration at steady state, 1.01 (0.96 to 1.07); area under the concentration-time curve from 0 to 12 h, 0.98 (0.94 to 1.02); half-life, 0.99 (0.95 to 1.03); total clearance at steady state, 1. 02 (0.99 to 1.06); and volume of distribution in the elimination phase, 1.01 (0.97 to 1.05). In conclusion, rufloxacin did not affect theophylline pharmacokinetics at steady state. Therefore, therapeutic coadministration of rufloxacin and theophylline is not expected to cause an increased incidence of theophylline-related adverse events.

Effect of zileuton on theophylline pharmacokinetics

In controlled trials involving asthma patients, zileuton - a selective 5-lipoxygenase inhibitor - has significantly improved pulmonary function and reduced symptoms. Since theophylline is frequently prescribed for asthma, we designed a placebo-controlled randomised crossover trial to examine the influence of zileuton on theophylline pharmacokinetics. 16 healthy adult males were given theophylline (Slo-Phyllin) 200mg 4 times daily for 5 days and either zileuton 800mg twice daily or a matching placebo. After a 15-day washout period, theophylline was resumed and the other study drugs reversed. During coadministration with zileuton, mean peak theophylline levels rose from 12.14 to 20.99 mg/L (p < 0.001), while the apparent plasma clearance dropped from 3.74 to 1.91 L/h (p < 0.001). The time to the peak theophylline concentration was delayed by 0.5 hours and the half-life was significantly prolonged by 1.5 hours. 14 volunteers reported 44 mild or moderately severe adverse events, possibly or probably related to coadministration of zileuton, and 8 volunteers reported 8 such events with placebo coadministration. Three volunteers receiving theophylline plus zileuton withdrew from the trial prematurely. Thus, a pharmacokinetic interaction that may produce theophylline toxicity exists between zileuton and theophylline. Accordingly, theophylline dosages in patients receiving zileuton should be adjusted to maintain levels within the therapeutic range. Upon initiation of zileuton, the typical asthma patient may require dosage reductions of one-half, and monitoring of plasma theophylline concentrations is recommended.

Relationship between enoxacin and ciprofloxacin plasma concentrations and theophylline disposition

Certain fluoroquinolone antibiotics affect theophylline (THEO) disposition by inhibition of its metabolism, yet no studies to date have investigated the relationship between fluoroquinolone plasma concentration and THEO pharmacokinetics. The effects of two fluoroquinolones, enoxacin (ENOX) and ciprofloxacin (CIPRO), have been studied in male Sprague-Dawley rats (n = 33-46) at steady state plasma concentrations of 0-33 mg.l-1, achieved by supplementing an intravenous bolus dose with a constant rate infusion. The effects of steady state ENOX and CIPRO plasma concentrations on the clearance of THEO determined after an intravenous bolus dose of 6 mg.kg-1 were described using a competitive inhibition model. The model consisted of two components, one describing a residual component of THEO clearance, which was unaffected by fluoroquinolone, the other describing the non-linear reduction of THEO clearance by fluoroquinolone. The residual clearance estimated from the model was comparable to renal clearance for THEO in the rat. The potency of each fluoroquinolone was characterised by a Ki value, the concentration reducing THEO clearance by 50% of the maximum change. These values were 4.7 microM and 16.3 microM for ENOX and CIPRO, respectively. Thus, in this study, ENOX was found to be a more potent inhibitor of THEO clearance than CIPRO. The method allowed direct in vivo comparison of potency between different fluoroquinolones, as pharmacokinetic differences, such as clearance, volume of distribution and bioavailability, were 'designed out.'

Pharmacokinetics of temafloxacin after multiple oral administration

Four multiple dose studies involving 102 healthy volunteers were reviewed to determine the sources of intersubject variability in the pharmacokinetics of temafloxacin. As a result of the preferential distribution of temafloxacin into muscle, liver and kidney compared with adipose tissue, the best anthropometric explanatory variable was found to be lean body mass, rather than total body mass or body surface area. Single dose studies have confirmed that the distribution volume of temafloxacin is smaller in subjects in whom the lean body mass:total body mass ratio is low, notably in females, the elderly, and those with hepatic impairment. Average creatinine clearance for the volunteers included in this analysis was normal (109 +/- 29 ml/min), and renal function was only a marginally significant covariate; however, renal clearance accounts for 60 to 70% of total clearance (CLT) and has been shown to be a major factor in the elderly, as well as in patients with renal or hepatic impairment. Since temafloxacin is partially reabsorbed renally, urine flow rate was found to be a potentially important secondary factor. Overall, the pharmacokinetics of temafloxacin are essentially linear, with steady-state plasma concentrations at trough (Cssmin) and 2h postdose (Css2h) averaging slightly more than 0.5 and 1.0 mg/L per 100mg administered every (q) 12h. For example, mean Cssmin and peak steady-state plasma concentration (Cssmax) values after administration of temafloxacin 600mg q12h were 3.3 +/- 1.1 and 6.2 +/- 1.8 mg/L, respectively. Total apparent clearance (CLT/F) and the terminal elimination half-life (t1/2) averaged 11 L/h (184 ml/min) per 55kg lean body mass and 8.4h, respectively. Considering that the data reviewed came from 4 studies with doses ranging from 100 to 800mg q12h, the intersubject coefficients of variation were low for an orally administered drug, ranging from 15.4% for t1/2, 20.6% for Css2h, 21.2% for CLT/F and 25% for Cssmin.

Effects of temafloxacin and ciprofloxacin on the pharmacokinetics of caffeine

A number of quinolone antibacterial agents, particularly enoxacin, pefloxacin, pipemidic acid and ciprofloxacin, are known to decrease the clearance of methylxanthines. The effects of temafloxacin and ciprofloxacin on the pharmacokinetics of caffeine were therefore compared in a 3-way crossover study in 12 healthy young volunteers. Each volunteer received 183mg once-daily doses of caffeine in conjunction with twice-daily placebo, temafloxacin 600mg and ciprofloxacin 750mg in 3 separate phases according to a randomised sequence. A doubling of the area under the plasma concentration-time curve (77.8 vs 31.8 mg/L.h) and terminal-phase half-life (9.7 vs 4.5h) of caffeine were observed in the presence of ciprofloxacin. The magnitude of the reduction in the intrinsic clearance of caffeine produced by ciprofloxacin was greater than that described in the literature for ciprofloxacin and theophylline. This may partly be explained by intertrial differences in dosage and study design. Coadministration of temafloxacin did not have any effect on the pharmacokinetics of caffeine, confirming results of other studies suggesting that this agent does not affect methylxanthine clearance. Accordingly, it appears that restriction of caffeine intake during temafloxacin therapy is not necessary.

The quinolones. An overview of their pharmacology

The fluoroquinolones represent a relatively new class of antibiotics with outstanding therapeutic potential, attributable to their broad spectrum of antimicrobial activity and favourable tissue distribution. They are highly active against most Gram-negative pathogens, as well as Staphylococcus aureus and coagulase-negative staphylococci. In addition, the fluoroquinolones have useful pharmacokinetic properties: they are orally active, and their lipophilicity and low degree of plasma protein binding allow for excellent tissue penetration and concentrations, as reflected in their particularly large apparent volumes of distribution. Infections due to aerobic Gram-negative pathogens are considered those most susceptible to the quinolones. Disease indications in which these agents appear to offer the greatest therapeutic advantage over currently available alternatives include the following: complicated urinary tract infections (particularly those caused by Pseudomonas aeruginosa or resistant Gram-negative microorganisms); suspected bacterial gastroenteritis; eradication of Salmonella typhi from the faeces in known carriers; P. aeruginosa-associated respiratory exacerbation in patients with cystic fibrosis; and chronic Gram-negative bacterial osteomyelitis. Direct comparisons of the various quinolones are too limited to date to provide clear therapeutic options. Nevertheless, this class of compounds is likely to play a major role in providing effective oral therapy for conditions that have previously required prolonged parenteral treatment.