Two drugs that are often used concurrently in patients infected with HIV are rifabutin, for the prevention of M. avium complex bacteremia [4], and fluconazole, for the prevention of fungal infections [6]. Rifabutin is an antimicrobial agent similar in structure to rifampin. Fluconazole, which is used to treat cryptococcal meningitis and oropharyngeal and esophageal candidiasis [7], has been reported to be effective for the primary prevention of deep and superficial fungal infections in HIV-infected patients whose CD4 lymphocyte counts are less than 50 cells/mm³ [6]. Fluconazole and a related azole, ketoconazole, are potent inhibitors of hepatic microsomal enzymes, especially the cytochrome p450 3A group [8]. Inhibition of these enzymes has, in turn, been shown to cause clinically significant increases in circulating levels of concomitant drugs that are metabolized by these enzymes [9-15].

Our study was designed to assess a possible mechanism for the changes observed in the toxicity and efficacy of rifabutin with concomitant fluconazole therapy. We report the results of a steady-state pharmacokinetic and safety study of rifabutin and fluconazole during concurrent zidovudine therapy in HIV-infected persons.

Methods

Top Methods Results Discussion Author & Article Info References

Study Design

This was a phase 1, open-label pharmacokinetic and safety study of 13 persons infected with HIV who were receiving maintenance therapy with zidovudine, 100 mg five times per day. The study enrolled HIV-infected adults who had CD4 lymphocyte counts between 200 and 500 cells/mm³, had no active disease by chest radiograph, had no clinically significant hepatic or renal impairment, and were receiving no other antiretroviral therapy or concomitant medications known to substantially modulate hepatic or renal function. Persons were excluded from participation if they had a history of known hypersensitivity to the study medications, had previously received treatment with cytolytic agents or radiation therapy, had had blood transfusion within 1 week of study entry, had received treatment with rifabutin or rifampin within 3 months of study entry, or had received treatment with fluconazole or other azole drugs within 4 weeks of study entry. Pregnant or lactating women were also excluded. Whenever possible, other concomitant medications were maintained at constant doses throughout the study. The study was approved by the Georgetown University Medical Center Institutional Review Board; each participant gave written informed consent before study entry.

A medical evaluation was done within 1 week of study entry. Fluconazole, 200 mg, was administered orally every 24 hours beginning on day 3. On day 16, when fluconazole had reached steady state, participants returned to the outpatient clinic, where blood was drawn just before the morning doses of fluconazole and zidovudine were given. Blood and urine samples were collected during the 24 hours after drug administration.

Beginning on day 17, oral rifabutin, 300 mg/d, was added to the fluconazole-zidovudine regimen. All study medications were administered concurrently. On day 30, when rifabutin had reached steady state, study participants returned to the clinic for serial blood and urine collections. Finally, fluconazole therapy was discontinued on day 31; rifabutin and zidovudine were continued for the remaining 2 weeks of the study. On day 44, participants returned to the clinic and again had serial blood and urine collections.

Study participants received medical evaluations with routine laboratory testing to evaluate the safety of these therapies on days 16, 30, and 44. Each participant also returned to the outpatient clinic on the mornings of days 1, 15, 29, and 43 to provide an additional blood sample before receiving medication to estimate within-person variation in the trough concentrations of the appropriate study drugs.

Drug Supply and Analysis

Rifabutin was supplied as 150-mg capsules by Pharmacia, Inc. (Columbus, Ohio); fluconazole was supplied as 200-mg tablets by Pfizer, Inc. (Groton, Connecticut); and zidovudine was supplied as 100-mg capsules by Burroughs-Wellcome Company (Research Triangle Park, North Carolina).

We collected all blood samples in heparinized tubes and promptly centrifuged them to separate the plasma. Plasma specimens were frozen at –20 °C until they were assayed. Urine collection bottles were kept on ice or were refrigerated during the collection periods. A 10-mL aliquot of urine was placed in cryotubes and kept frozen at –20 °C until it was assayed. Plasma and urine concentrations of fluconazole, rifabutin, and the 25-desacetyl metabolite of rifabutin, LM565, were determined by Harris Laboratories (Lincoln, Nebraska) using a validated high-performance liquid chromatography method as previously described [16, 17]. The respective interday precision (expressed as a percentage of relative standard deviation) and inaccuracy estimates for the quantity of fluconazole were 8% or less and ±5%, respectively; those for rifabutin and LM565 were 10% or less and ±5%, respectively.

Pharmacokinetic Analysis

Pharmacokinetic variables were estimated by noncompartmental analyses [18]. Steady-state estimates of the area under the concentration-time curve for rifabutin and fluconazole were obtained by using linear trapezoidal integration over a dosing interval. Renal clearance was estimated by dividing the amount excreted in the urine by the area under the plasma drug concentration-time curve.

Statistical Analysis

Statistical analyses were done using Statistical Analysis Systems software, version 6.06 (SAS Institute, Cary, North Carolina). Estimates of pharmacokinetic variables from the 12 evaluable participants in the presence or absence of the study drugs were compared using a paired, two-tailed t-test. Values are given as mean ±SD.

Results

Top Methods Results Discussion Author & Article Info References

Thirteen persons were enrolled in the study. Data from 12 participants were considered evaluable for data analysis; studies were discontinued in 1 participant before completion because of the development of a diffuse maculopapular rash 12 days after rifabutin therapy began. Data were not stratified for sex (9 men, 4 women) or race (9 white participants, 4 black participants) because of the small sample sizes within each group. Other demographic information included age (35.8 ± 7.2 years), body weight (89 ± 20 kg), CD4 lymphocyte count (369.3 ± 63.1 cells/mm³), aspartate aminotransferase level (0.56 ± 0.16 µkat/L), and serum creatinine level (106 ± 8 µmol/L).

No clinically significant changes in the results of physical examinations or laboratory evaluations were seen during the study in any of the evaluable study participants. Figure 1 shows the plasma concentrations of rifabutin and LM565 as a function of time from study days 30 (rifabutin and fluconazole) and 44 (rifabutin alone). Rifabutin levels were significantly higher during concurrent fluconazole treatment; the steady-state estimate of the area under the concentration-time curve increased 82% (5442 ± 2404 ng · h/mL compared with 3025 ± 1117 ng · h/mL; P 0.05). The area under the LM565 concentration-time curve over the 24-hour dosing interval increased 216% (959 ± 529 ng · h/mL compared with 244 ± 141 ng · h/mL; P 0.05). This finding was consistent among all study participants (Figure 2). Urinary excretion of rifabutin and LM565 also increased during concurrent fluconazole treatment. The amounts of rifabutin and LM565 excreted on day 30 compared with day 44—expressed as a percentage of the rifabutin dose—were 2.5% ± 1.5% compared with 6.2% ± 2.0% (P < 0.01) and 0.8% ± 0.4% compared with 2.3% ± 0.9% (P < 0.01), respectively. The renal clearance of rifabutin, however, was unchanged (0.0502 ± 0.0199 L/h · kg^–1 and 0.0446 ± 0.0248 L/h · kg^–1).

View larger version (12K): [in this window] [in a new window]   Figure 1. Steady-state concentration compared with time curves of the plasma concentrations (mean ±SD) of rifabutin and its 25-desacetyl metabolite, LM565, over one dosing interval. The dashed line indicates rifabutin alone; the solid line indicates rifabutin plus fluconazole.

View larger version (18K): [in this window] [in a new window]   Figure 2. Area under the plasma concentration-time curve for rifabutin and its 25-desacetyl metabolite, LM565, when rifabutin is administered alone and in combination with fluconazole. Bars and error bars represent mean ±SD. Data on individual participants are joined by a solid line.

The steady-state fluconazole plasma concentration did not change (area under the concentration-time curve over a dosing interval without rifabutin, 201.0 ± 36.2 µg · h/mL; with rifabutin, 196.8 ± 44.7 µg · h/mL), and rifabutin did not affect urinary excretion of fluconazole (percentage of fluconazole dose excreted without rifabutin, 73.7% ± 18.6%; with rifabutin, 68.8% ± 15.3%).

Discussion

Top Methods Results Discussion Author & Article Info References

Our data indicate that concurrent fluconazole administration markedly increases the steady-state plasma concentrations of both rifabutin and its equiactive 25-desacetyl metabolite, LM565, in HIV-infected persons receiving maintenance therapy with zidovudine. This is consistent with fluconazole's inhibition of cytochrome p450 3A [7]. Renal clearance of rifabutin was unchanged with fluconazole. This further supports our hypothesis that the increased rifabutin concentrations are due to the inhibition of metabolism. Interestingly, LM565 concentrations increased 2.5-fold higher than rifabutin with concurrent fluconazole, which may represent the inhibition of further metabolism of this metabolite [19].

Although study participants were receiving other concurrent medications, these were minimized, and persons who were receiving medications known to alter drug disposition were excluded from participation. Because we also excluded persons with impaired renal or hepatic function, the significance of the drug interaction in question in patients with renal or hepatic impairment is unknown. The order of fluconazole and rifabutin administration was fixed for all study participants to ensure that the previously reported inhibitory effects of fluconazole on cytochrome p450 3A would be maximal during assessment of rifabutin disposition and, at the same time, to limit the study duration.

Our results suggest that higher rifabutin and LM565 concentrations may contribute to the enhanced rifabutin efficacy for the prophylaxis of disseminated M. avium complex disease reported in patients with the acquired immunodeficiency syndrome [15] and uveitis seen in patients receiving other drugs that may inhibit the metabolism of rifabutin [11-14]. Thus, although our study shows that modifications of the fluconazole dose are not needed when rifabutin is added to a treatment regimen, patients receiving rifabutin in combination with other drugs that may inhibit its metabolism should be followed closely. However, assessment of the clinical effect of these pharmacokinetic changes is difficult, given that the exposure of rifabutin necessary to maximize the drug's efficacy and minimize its side effects is currently unknown. In fact, this relation is not known for most drugs. A complete understanding of the effect of covariates, such as drug exposure, may be helpful in the proper assessment of the outcomes of interest in large clinical trials, in dosing decisions made during drug development, and in the therapeutic use of drugs.

Dr. Narang: Pharmacia, Inc., PO Box 16529, Columbus, OH 43216-6529.

Dr. Li: Department of Pharmacy, Faculty of Medicine, The Chinese University of Hong Kong, Hong Kong.

Dr. Lavelle: 4825 Butterworth Place, NW, Washington, DC 20016.