Tolerance and pharmacokinetic interactions of rifabutin and azithromycin.

This multicenter study evaluated the tolerance and potential pharmacokinetic interactions between azithromycin and rifabutin in volunteers with or without human immunodeficiency virus infection. Daily dosing with the combination of azithromycin and rifabutin was poorly tolerated, primarily because of gastrointestinal symptoms and neutropenia. No significant pharmacokinetic interactions were found between these drugs.

Metabolic disposition and pharmacokinetics of [14C]-amprenavir, a human immunodeficiency virus type 1 (HIV-1) protease inhibitor, administered as a single oral dose to healthy male subjects.

The objective of this study was to determine the metabolic profile, routes of elimination, and total recovery of amprenavir and its metabolites after a single oral dose of [14C]-amprenavir. Six healthy male subjects each received a single oral 630 mg dose of amprenavir containing 95.76 microCi of [14C]-amprenavir in this Phase I mass balance study. The metabolic disposition of amprenavir was determined through analyses of radiocarbon in whole blood, plasma, urine, and stool samples, collected for a period of 10 to 17 days postdosing. Cerebral spinal fluid (CSF) sampling was conducted on day 1. The ratio of unchanged amprenavir AUC0-->infinity to plasma radiocarbon was 27%, suggesting that most of the radiocarbon was metabolites. The median total recovery of the administered dose of radiocarbon was 89% (range: 66%-93%), with 75% (range: 56%-80%) recovered in the feces and 14% (range: 10%-17%) in the urine. Most of the recovered radiocarbon in the feces and urine was excreted within 240 and 48 hours postdose, respectively. Of the 75% of the radiocarbon dose recovered in the feces, 62% was identified as a metabolite resulting from dioxidation of the tetrahydrofuran ring (GW549445X) and 32% as a metabolite resulting from subsequent oxidation of the p-aniline sulfonate group (GW549444X). Unchanged amprenavir was below the limit of quantitation in feces and urine. Therefore, approximately 94% of the dose excreted in the feces was accounted for by these two metabolites. Concentrations of radiocarbon in the CSF were below the limit of quantitation in 5 of 6 subjects sampled. In summary, oral amprenavir is extensively metabolized in humans, with concentrations of unchanged drug below the limits of quantitation in urine and feces. The majority (75%) of administered radiocarbon was excreted in feces.

Pharmacokinetic Interaction between amprenavir and rifabutin or rifampin in healthy males.

The objective of this study was to determine if there is a pharmacokinetic interaction when amprenavir is given with rifabutin or rifampin and to determine the effects of these drugs on the erythromycin breath test (ERMBT). Twenty-four healthy male subjects were randomized to one of two cohorts. All subjects received amprenavir (1,200 mg twice a day) for 4 days, followed by a 7-day washout period, followed by either rifabutin (300 mg once a day [QD]) (cohort 1) or rifampin (600 mg QD) (cohort 2) for 14 days. Cohort 1 then received amprenavir plus rifabutin for 10 days, and cohort 2 received amprenavir plus rifampin for 4 days. Serial plasma and urine samples for measurement of amprenavir, rifabutin, and rifampin and their 25-O-desacetyl metabolites, were measured by high-performance liquid chromatography. Rifabutin did not significantly affect amprenavir's pharmacokinetics. Amprenavir significantly increased the area under the curve at steady state (AUC(ss)) of rifabutin by 2.93-fold and the AUC(ss) of 25-O-desacetylrifabutin by 13.3-fold. Rifampin significantly decreased the AUC(ss) of amprenavir by 82%, but amprenavir had no effect on rifampin pharmacokinetics. Amprenavir decreased the results of the ERMBT by 83%. The results of the ERMBT after 2 weeks of rifabutin and rifampin therapy were increased 187 and 156%, respectively. Amprenavir plus rifampin was well tolerated. Amprenavir plus rifabutin was poorly tolerated, and 5 of 11 subjects discontinued therapy. Rifampin markedly increases the metabolic clearance of amprenavir, and coadministration is contraindicated. Amprenavir significantly decreases clearance of rifabutin and 25-O-desacetylrifabutin, and the combination is poorly tolerated. Amprenavir inhibits the ERMBT, and rifampin and rifabutin are equipotent inducers of the ERMBT.

Variability in activity of hepatic CYP3A4 in patients infected with HIV.

STUDY OBJECTIVES: To evaluate hepatic cytochrome P450 (CYP) 3A4 activity in patients infected with the human immunodeficiency virus (HIV) using the erythromycin breath test (ERMBT), and to examine the relationship of the ERMBT to plasma concentrations of indinavir and nelfinavir. DESIGN: Prospective observational study. SETTING: University infectious diseases clinic. SUBJECTS: Thirty-nine HIV-positive patients and 47 healthy controls. INTERVENTION: After the ERMBT in patients and controls, 25 patients received indinavir or nelfinavir. MEASUREMENTS AND MAIN RESULTS: Compared with controls, ERMBT variability was significantly greater in HIV-positive patients, including a subset of 19 patients receiving no concurrent drugs reported to alter CYP3A4 activity. Correlation between the ERMBT and first-dose plasma indinavir concentrations nearly reached statistical significance (p=0.07). CONCLUSION: Variability in hepatic activity of CYP3A4 in HIV-positive patients may be greater than in controls and may explain some between-subject variability in plasma concentrations of indinavir. However, clearance mechanisms for protease inhibitors are complex, and if it is important to assess systemic exposure, the ERMBT is not a substitute for direct measurement of plasma concentrations.

Pharmacokinetic interaction between amprenavir and clarithromycin in healthy male volunteers.

The P450 enzyme, CYP3A4, extensively metabolizes both amprenavir and clarithromycin. To determine if an interaction exists when these two drugs are coadministered, the pharmacokinetics of amprenavir and clarithromycin were investigated in healthy adult male volunteers. This was a Phase I, open-label, randomized, balanced, multiple-dose, three-period crossover study. Fourteen subjects received the following three regimens: amprenavir, 1,200 mg twice daily over 4 days (seven doses); clarithromycin, 500 mg twice daily over 4 days (seven doses); and the combination of the above regimens over 4 days (seven doses of each drug). Twelve subjects completed all treatments and the follow-up period. The erythromycin breath test (ERMBT) was administered at baseline, 2 h after the final dose of each of the three regimens and at the first follow-up visit. Coadministration of clarithromycin and amprenavir significantly increased the mean amprenavir AUC(ss), C(max,ss), and C(min,ss) by 18, 15, and 39%, respectively. Amprenavir had no significant effect on the AUC(ss) of clarithromycin, but the median T(max,ss)for clarithromycin increased by 2.0 h, renal clearance increased by 34%, and the AUC(ss) for 14-(R)-hydroxyclarithromycin decreased by 35% when it was given with amprenavir. Amprenavir and clarithromycin reduced the ERMBT result by 85 and 67%, respectively, and by 87% when the two drugs were coadministered. The baseline ERMBT value did not correlate with clearance of amprenavir or clarithromycin. A pharmacokinetic interaction occurs when amprenavir and clarithromycin are coadministered, but the effects are not likely to be clinically important, and coadministration does not require a dosage adjustment for either drug.

Pharmacokinetics of [(14)C]abacavir, a human immunodeficiency virus type 1 (HIV-1) reverse transcriptase inhibitor, administered in a single oral dose to HIV-1-infected adults: a mass balance study.

Abacavir (1592U89) ((-)-(1S, 4R)-4-[2-amino-6-(cyclopropylamino)-9H-purin-9-yl]-2-cyclopentene- 1-m ethanol) is a 2'-deoxyguanosine analogue with potent activity against human immunodeficiency virus (HIV) type 1. To determine the metabolic profile, routes of elimination, and total recovery of abacavir and metabolites in humans, we undertook a phase I mass balance study in which six HIV-infected male volunteers ingested a single 600-mg oral dose of abacavir including 100 microCi of [(14)C]abacavir. The metabolic disposition of the drug was determined through analyses of whole-blood, plasma, urine, and stool samples, collected for a period of up to 10 days postdosing, and of cerebrospinal fluid (CSF), collected up to 6 h postdosing. The radioactivity from abacavir and its two major metabolites, a 5'-carboxylate (2269W93) and a 5'-glucuronide (361W94), accounted for the majority (92%) of radioactivity detected in plasma. Virtually all of the administered dose of radioactivity (99%) was recovered, with 83% eliminated in urine and 16% eliminated in feces. Of the 83% radioactivity dose eliminated in the urine, 36% was identified as 361W94, 30% was identified as 2269W93, and 1.2% was identified as abacavir; the remaining 15.8% was attributed to numerous trace metabolites, of which <1% of the administered radioactivity was 1144U88, a minor metabolite. The peak concentration of abacavir in CSF ranged from 0.6 to 1.4 microg/ml, which is 8 to 20 times the mean 50% inhibitory concentration for HIV clinical isolates in vitro (0.07 microg/ml). In conclusion, the main route of elimination for oral abacavir in humans is metabolism, with <2% of a dose recovered in urine as unchanged drug. The main route of metabolite excretion is renal, with 83% of a dose recovered in urine. Two major metabolites, the 5'-carboxylate and the 5'-glucuronide, were identified in urine and, combined, accounted for 66% of the dose. Abacavir showed significant penetration into CSF.

Pharmacokinetic interaction between ketoconazole and amprenavir after single doses in healthy men.

STUDY OBJECTIVE: To determine the effects of coadministration of amprenavir and ketoconazole on the pharmacokinetics of both drugs, and to assess the utility of the erythromycin breath test (ERMBT) to predict and explain these effects. DESIGN: Open-label, randomized, balanced, single-dose, three-period crossover study. SETTING: University research center. SUBJECTS: Twelve healthy men. INTERVENTION: Subjects received amprenavir 1200 mg, ketoconazole 400 mg, and amprenavir 1200 mg plus ketoconazole 400 mg. Each treatment was separated by 14 days. MEASUREMENTS AND MAIN RESULTS: Serial plasma samples for amprenavir and ketoconazole concentrations were measured by high-performance liquid chromatography. Coadministration of the drugs increased amprenavir area under the curve extrapolated to infinity (AUCinfinity) by 31% and reduced its maximum concentration (Cmax) by 16%. Amprenavir increased the AUCinfinity of ketoconazole by 44% and increased the drug's half-life and Cmax by 23% and 19%, respectively. Both agents resulted in substantial inhibition of ERMBT. CONCLUSION: Coadministration of ketoconazole and amprenavir results in a statistically significant increase in AUC for both agents, but the changes are not likely to be clinically important.

Drug interaction potential with inhibitors of HIV protease.

We conducted a retrospective chart review to estimate the potential for drug interactions in subjects infected with the human immunodeficiency virus-1 when a protease inhibitor was added to existing therapy. Medical records of 50 patients in each of three immunologic strata (CD4 cell counts/microl < 100, 100-199, 200-500) were randomly selected from records of all patients receiving care at the clinic; 114 records were evaluable. The probabilities of one interaction or more were 31%, 42%, and 77% of patients if treated with indinavir, saquinavir, and ritonavir, respectively, across all CD4 groups; when the CD4 count was below 100 cells/microl, the probabilities were 55%, 63%, and 93%. Many of these interactions, however, resulted from administration of rifabutin, a drug likely to decrease in importance as less toxic alternatives become more widely administered. The potential for drug interactions is high when starting protease inhibitor therapy, especially in patients with low CD4 cell counts who receive ritonavir. Concurrent therapy must be evaluated before treatment, as many agents are either contraindicated or require dosage modification.

Drug interactions of macrolides: emphasis on dirithromycin.

OBJECTIVE: To describe the drug interactions of dirithromycin, a new macrolide, and to compare them with those of other macrolides. DATA SOURCES: A literature search was performed using MEDLINE to identify articles published between January 1980 and July 1995 concerning the drug interactions of macrolides. Published abstracts were also examined. All studies using dirithromycin were performed under the sponsorship of Eli Lilly and Company. DATA SYNTHESIS: Erythromycin, the first macrolide discovered, is metabolized by the cytochrome P450 enzyme system. By decreasing their metabolism, erythromycin can interact with other drugs metabolized by the cytochrome P450 enzymes. The lack of such interactions would be a desirable feature in a newer macrolide. We describe studies performed to detect any interactions of dirithromycin with cyclosporine, theophylline, terfenadine, warfarin, and ethinyl estradiol. The studies showed that dirithromycin, like azithromycin, is much less likely to cause the interactions detected with clarithromycin and erythromycin. A review of the literature showed differences among macrolides in their abilities to inhibit cytochrome P450 enzymes and, thus, to cause drug-drug interactions. Erythromycin and clarithromycin inhibit cytochrome P450 enzymes, and have been implicated in clinically significant interactions. Azithromycin and dirithromycin neither inhibit cytochrome P450 enzymes nor are implicated in clinically significant drug-drug interactions. CONCLUSIONS: Dirithromycin, a new macrolide, does not inhibit the cytochrome P450 enzyme system. The concomitant use of dirithromycin with cyclosporine, theophylline, terfenadine, warfarin, or ethinyl estradiol was studied in pharmacokinetic and pharmacodynamic studies. In vitro, dirithromycin did not bind cytochrome P450. In healthy subjects, erythromycin increases the clearance of cyclosporine by 51%, whereas dirithromycin causes no significant changes in the pharmacokinetics of cyclosporine. In kidney transplant recipients, administration of dirithromycin was associated with a significant (p < 0.003) decrease of 17.4% in the clearance of cyclosporine. In patients taking low-dose estradiol, the administration of dirithromycin caused a significant (p < 0.03) increase of 9.9% in the clearance of ethinyl estradiol; escape ovulation did not occur. Unlike erythromycin and clarithromycin, dirithromycin had no significant effects on the pharmacokinetics of theophylline, terfenadine, or warfarin. The alterations typical of drug interactions that are based on inhibition of the cytochrome P450 system occurring with erythromycin and clarithromycin were not observed with dirithromycin.

Induction of theophylline clearance by rifampin and rifabutin in healthy male volunteers.

Rifampin and rifabutin induce the metabolism of many drugs, which may result in subtherapeutic concentrations and failure of therapy. However, differences between rifabutin and rifampin in potency of induction, and the specific enzymes which are altered, are not clear. This study, involving 12 adult male volunteers, compared the effects of 14-day courses of rifampin and rifabutin on clearance of theophylline, a substrate for the hepatic microsomal enzyme CYP1A2. Subjects were given oral theophylline solution (5 mg/kg of body weight) on day 1 and then randomized to receive daily rifampin (300 mg) or rifabutin (300 mg) on days 3 to 16. Theophylline was readministered as described above on day 15. The first treatment sequence was followed by a 2-week washout period; subjects then received the alternative treatment. Theophylline concentrations were determined for 46 h after each dose, and pharmacokinetic parameters were determined. One subject developed flu-like symptoms while taking rifabutin and withdrew voluntarily. Results from the remaining 11 subjects are reported. Compared with the baseline, the mean area under the concentration-time curve (AUC) (+/- standard deviation) for theophylline declined significantly following rifampin treatment (from 140 +/- 37 to 100 +/- 24 micrograms . h/ml, P <0.001); there was no significant change following rifabutin treatment (136 +/- 48 to 128 +/- 45 micrograms.h/ml). Baseline theophylline AUCs before each treatment phase were not different. A comparison of equal doses of rifampin and rifabutin administered to healthy volunteers for 2 weeks indicates that induction of CYP1A2, as measured by theophylline clearance, is significantly less following rifabutin treatment than it is following rifampin treatment. However, the relative induction potency for other metabolic enzymes remains to be investigated.

Effect of combination therapy with ciprofloxacin and clarithromycin on theophylline pharmacokinetics in healthy volunteers.

Five adults completed this four-way randomized crossover study to compare the effects of oral treatment with ciprofloxacin, clarithromycin, and a combination of the two drugs on theophylline pharmacokinetics. The area under the concentration-time curve for theophylline during combination therapy was not different from that for ciprofloxacin alone. Beta error may explain this finding, but any real effect from combination treatment appears to be clinically unimportant.

Effect of ciprofloxacin on the pharmacokinetics and pharmacodynamics of warfarin.

To determine if ciprofloxacin therapy alters the response to warfarin treatment, 36 adult patients attending three university-affiliated outpatient anticoagulation clinics randomly received a 12-day course of ciprofloxacin (750 mg twice daily) and a 12-day course of placebo; each course was separated by a 2-week washout period. Prothrombin times (PTs), concentrations of S-warfarin and R-warfarin (the isomers of warfarin), and concentrations of clotting factors II and VII were determined three times weekly for 9 weeks. By day 12 of ciprofloxacin therapy, concentrations of S-warfarin remained unchanged compared with those after placebo therapy, but R-warfarin concentrations increased significantly (1.15 times those after placebo therapy; P = .001); concentrations of clotting factors II and VII decreased (0.903 and 0.872 times those after placebo therapy, respectively, P < or = .020). The mean PT ratio after 12 days of ciprofloxacin therapy increased slightly (1.032 times that after placebo therapy; P = .057), but no patient had bleeding or a change in PT that required alteration in warfarin or ciprofloxacin therapy. We conclude that warfarin therapy is not a contraindication to the use of ciprofloxacin.

Effect of fluconazole on pharmacokinetics of 2',3'-dideoxyinosine in persons seropositive for human immunodeficiency virus.

Fluconazole inhibits cytochrome P-450-mediated enzymatic metabolism of several drugs. Since hepatic metabolism is partially responsible for 2',3'-dideoxyinosine (didanosine or ddI) elimination, fluconazole therapy may lead to increased ddI concentrations in serum and subsequent concentration-dependent adverse effects. The purpose of this study was to determine if ddI pharmacokinetics are influenced by a 7-day course of oral fluconazole. Twelve adults with human immunodeficiency virus (HIV) who had received a constant dosage of ddI for at least 2 weeks were investigated. On study day 1, multiple serum samples for determination of ddI concentrations were obtained over 12 h. Then subjects received a 7-day course of oral fluconazole (200 mg every 12 h for two doses and then 200 mg once daily for 6 days) while ddI therapy continued. Following the last dose of fluconazole, serum samples for determination of ddI concentrations were again obtained over 12 h. ddI concentrations in serum were analyzed by radioimmunoassay. In contrast to previously published data, there was marked between-subject variability in ddI areas under the concentration-time curve, even when the dose was normalized for weight. No significant differences were found between mean ddI areas under the concentration-time curve from 0 to 12 h on study day 1 (1,528 +/- 902 ngx.hr/ml) and following fluconazole treatment (1,486 +/- 649 ngx.hr/ml) . There were no significant differences in other pharmacokinetic parameters, such as ddI peak concentrations in serum (971 +/- 509 and 942 +/- 442 ng/ml) or half-lives (80 +/- 32 and 85 +/- 21 min.) before and after fluconazole treatment, respectively. We conclude that a 7-day course of oral fluconazole does not significantly alter ddI pharmacokinetics in adults that are infected with human immunodeficiency virus.

Pharmacokinetic drug interactions with antimicrobial agents.

As new classes of antimicrobial drugs have become available, and new uses found for older drugs, pharmacokinetic drug interactions with antimicrobials have become more common. Macrolides, fluoroquinolones, rifamycins, azoles and other agents can interact adversely with commonly used drugs, usually by altering their hepatic metabolism. The mechanisms by which antimicrobial agents alter the biotransformation of other drugs is increasingly understood to reflect inhibition or induction of specific cytochrome P450 enzymes. Macrolides inhibit cytochrome P450IIIA4 (CYP3A4), which appears to be the most common metabolic enzyme in the human liver and is involved in the metabolism of many drugs, including cyclosporin, warfarin and terfenadine. Some quinolones preferentially inhibit CYP1A2, which is partially responsible for methylxanthine metabolism. Azoles appear to be broad spectrum inhibitors of cytochromes P450. Within each of these antibiotic classes, there is a rank order of inhibitory potency towards specific cytochrome P450 enzymes. By contrast, rifampicin (rifampin) and rifabutin induce several cytochromes P450, including CYP3A4, and hence can enhance the metabolism of many other drugs. By using in vitro preparations of human enzymes it is increasingly possible to predict those antibiotics that will adversely affect the metabolism of other drugs. In addition, between-patient variability in frequency of interaction may relate to differences in the activities of these enzymes. Although the mechanisms and scope of these interactions are becoming well characterised, the remaining challenge is how to best inform the clinician so that the undesirable consequences of interactions may be prevented.

Vancomycin skin tests and prediction of "red man syndrome" in healthy volunteers.

The purpose of the present study was to assess the cutaneous response to intradermally administered vancomycin in healthy adults and to determine whether the magnitude of the cutaneous response correlated to the severity of "red man syndrome" (RMS) following intravenous administration of vancomycin to the same subjects. Eleven healthy males were skin tested with intradermally administered histamine and saline controls and intradermally administered vancomycin at different concentrations. Vancomycin caused a dose-dependent area of flare in all subjects. The sigmoidal maximal flare model was used to fit each dose-response curve, and cutaneous responsiveness to vancomycin was quantified by various methods, including the flare area at each dose, maximum flare area (maximal flare), dose required to produce 50% of maximum flare, dose required to produce a flare area of 400 mm2, and the slope of the dose-response curve. One week after skin testing, subjects received an infusion of vancomycin, 15 mg/kg of body weight over 60 min. For the assessment of the severity of RMS, we used previously described methods. Although all subjects experienced erythema from the intravenously administered vancomycin and 10 subjects had pruritus, there was no significant correlation between vancomycin skin test results and the severity of RMS. We conclude that vancomycin skin tests do not predict the severity of RMS. In addition, vancomycin skin tests may be of no benefit for assessing immunoglobulin E-mediated allergy to vancomycin, since all subjects had a positive reaction at concentrations of > or = 10 micrograms/ml.

The influence of chronic administration of calcium carbonate on the bioavailability of oral ciprofloxacin.

Six healthy male volunteers participated in a two-period, two-treatment study to determine the effect of chronic calcium carbonate administration on ciprofloxacin bioavailability. There was a mean reduction of 40% in Cmax and 43% in AUC when calcium carbonate was administered with ciprofloxacin, compared with ciprofloxacin alone (P < 0.05). There were no changes in either half-life or tmax. It is therefore recommended that patients being treated with ciprofloxacin for serious infections refrain from ingesting calcium supplements. If this is not possible, administration of ciprofloxacin 2 h before ingestion of the supplement is suggested.

Lack of interaction between lomefloxacin and caffeine in normal volunteers.

Sixteen healthy, nonsmoking adult males participated in a randomized, double-blind, placebo-controlled, two-way crossover study to evaluate the influence of chronic lomefloxacin administration on the disposition of caffeine and its major metabolite, paraxanthine, at steady-state conditions. Lomefloxacin (400 mg) or placebo was administered orally once daily for 5 days to xanthine-free volunteers after an overnight fast. Caffeine (200 mg orally) was administered simultaneously with lomefloxacin on days 3 through 5. After a 2-day washout period, subjects were crossed over to the alternate 5-day regimen with caffeine, which was again given on the final 3 days. Blood samples for caffeine, paraxanthine, and lomefloxacin concentration determinations were serially collected for 48 h following the last dose of each regimen. All compounds were analyzed by high-performance liquid chromatography. For the placebo versus lomefloxacin-containing treatments, maximum caffeine concentrations in plasma (4.35 +/- 0.63 versus 4.07 +/- 0.56 micrograms/ml), areas under the concentration-time curve from time zero to 24 h at steady state (30.3 +/- 6.9 versus 29.7 +/- 6.6 micrograms.h/ml), and elimination half-lives of caffeine (4.8 +/- 1.1 versus 4.8 +/- 1.2 h) were not significantly different. In addition, there were no significant changes in the disposition parameters of paraxanthine as a result of lomefloxacin administration. The frequencies of central nervous system-related effects for the two treatments were not statistically different. We conclude that lomefloxacin has no significant effect on the disposition of caffeine in young healthy volunteers.