Induction of fluconazole metabolism by rifampin: in vivo study in humans

Assessment of Pharmacokinetic Interaction Between Letermovir and Fluconazole in Healthy Participants

Letermovir is a prophylactic agent for cytomegalovirus infection and disease in adult cytomegalovirus-seropositive recipients of allogeneic hematopoietic stem cell transplant. As the antifungal agent fluconazole is administered frequently in transplant recipients, a drug-drug interaction study was conducted between oral letermovir and oral fluconazole. A phase 1 open-label, fixed-sequence study was performed in healthy females (N = 14, 19-55 years). In Period 1, a single dose of fluconazole 400 mg was administered. Following a 14-day washout, a single dose of letermovir 480 mg was administered (Period 2), and after a 7-day washout, single doses of fluconazole 400 mg and letermovir 480 mg were coadministered in Period 3. Pharmacokinetics and safety were evaluated. The pharmacokinetics of fluconazole and letermovir were not meaningfully changed following coadministration. Fluconazole geometric mean ratio (GMR; 90% confidence interval [CI]) with letermovir for area under the concentration-versus-time curve from time 0 to infinity (AUC_0-∞ ) was 1.03 (0.99-1.08); maximum concentration (C_max ) was 0.95 (0.92-0.99). Letermovir AUC_0-∞ GMR (90%CI) was 1.11 (1.01-1.23), and C_max was 1.06 (0.93-1.21) following coadministration with fluconazole. Coadministration of fluconazole and letermovir was generally well tolerated.

Pharmacokinetics of antifungal drugs: practical implications for optimized treatment of patients

Introduction

Because of the high mortality of invasive fungal infections (IFIs), appropriate exposure to antifungals appears to be crucial for therapeutic efficacy and safety.

Materials and methods

This review summarises published pharmacokinetic data on systemically administered antifungals focusing on co-morbidities, target-site penetration, and combination antifungal therapy.

Conclusions and discussion

Amphotericin B is eliminated unchanged via urine and faeces. Flucytosine and fluconazole display low protein binding and are eliminated by the kidney. Itraconazole, voriconazole, posaconazole and isavuconazole are metabolised in the liver. Azoles are substrates and inhibitors of cytochrome P450 (CYP) isoenzymes and are therefore involved in numerous drug–drug interactions. Anidulafungin is spontaneously degraded in the plasma. Caspofungin and micafungin undergo enzymatic metabolism in the liver, which is independent of CYP. Although several drug–drug interactions occur during caspofungin and micafungin treatment, echinocandins display a lower potential for drug–drug interactions. Flucytosine and azoles penetrate into most of relevant tissues. Amphotericin B accumulates in the liver and in the spleen. Its concentrations in lung and kidney are intermediate and relatively low myocardium and brain. Tissue distribution of echinocandins is similar to that of amphotericin. Combination antifungal therapy is established for cryptococcosis but controversial in other IFIs such as invasive aspergillosis and mucormycosis.

Factors Influencing the Magnitude and Clinical Significance of Drug Interactions Between Azole Antifungals and Select Immunosuppressants

The magnitude of drug interactions between azole antifungals and immunosuppressants is drug and patient specific and depends on the potency of the azole inhibitor involved, the resulting plasma concentrations of each drug, the drug formulation, and interpatient variability. Many factors contribute to variability in the magnitude and clinical significance of drug interactions between an immunosuppressant such as cyclosporine, tacrolimus, or sirolimus and an antifungal agent such as ketoconazole, fluconazole, itraconazole, voriconazole, or posaconazole. By bringing similarities and differences among these agents and their potential interactions to clinicians' attention, they can appreciate and apply these findings in a individualized patient approach rather than follow only the one-size-fits-all dosing recommendations suggested in many tertiary references. Differences in metabolism and in the inhibitory potency of cytochrome P450 3A4 and P-glycoprotein influence the onset, magnitude, and resolution of drug interactions and their potential effect on clinical outcomes. Important issues are the route of administration and the decision to preemptively adjust dosages versus intensive monitoring with subsequent dosage adjustments. We provide recommendations for the concomitant use of these agents, including suggestions regarding contraindicated combinations, those best avoided, and those requiring close monitoring of drug dosages and plasma concentrations.

Symptomatic Relapse of HIV‐Associated Cryptococcal Meningitis after Initial Fluconazole Monotherapy: The Role of Fluconazole Resistance and Immune Reconstitution

BACKGROUND

Cryptococcal meningitis (CM) in South Africa is often treated with fluconazole as initial therapy. Surveillance data suggest that the prevalence of fluconazole-resistant CM is increasing, and expanding access to antiretroviral therapy is resulting in increasing recognition of immune reconstitution inflammatory syndrome. Therefore, we conducted a study to assess the contribution of these factors to CM relapse in this context.

METHODS

Patients with symptomatic relapse of CM were prospectively identified at 2 hospitals in Cape Town, South Africa, during the period of 2003-2005. Patients met the following criteria: (1) a previous laboratory-confirmed episode of CM, with resolution of symptoms after treatment; (2) reported adherence to fluconazole treatment; (3) recurrence of typical CM symptoms; (4) cerebrospinal fluid antigen test and/or culture positive for Cryptococcus neoformans; and (5) no alternative diagnosis. Data on patients' human immunodeficiency virus (HIV) and CM infections and treatment were collected and analyzed.

RESULTS

Thirty-two episodes of relapse occurred among 27 patients. Episodes were classified into 3 groups: culture-positive episodes in antiretroviral therapy-naive patients (6 episodes), culture-positive episodes in patients receiving antiretroviral therapy (15 episodes), and culture-negative episodes in patients receiving antiretroviral therapy (11 episodes). Seventy-six percent of culture-positive relapses were associated with isolates that had reduced susceptibility to fluconazole. Drug-resistant cases required prolonged intravenous therapy with amphotericin B, and despite this treatment, the mortality rate was high (54% at a median of 6 months of follow-up). Despite a long interval between initiation of antifungal therapy and initiation of antiretroviral therapy (median interval, 144 days), immune reconstitution inflammatory syndrome contributed to at least one-third of relapses.

CONCLUSIONS

After initial treatment with fluconazole, relapses of symptomatic CM are often associated with fluconazole resistance and immune reconstitution inflammatory syndrome. These data add to concern about the efficacy of fluconazole, compared with amphotericin B, for initial treatment of HIV-associated CM.

Pharmacokinetics of ruboxistaurin are significantly altered by rifampicin-mediated CYP3A4 induction

AIMS

The aim of this study was to evaluate the effect of rifampicin co-administration on the pharmacokinetics of ruboxistaurin and its active metabolite, N-desmethyl ruboxistaurin and, in addition, to compare the changes in pharmacokinetics of ruboxistaurin and N-desmethyl ruboxistaurin with the urinary 6beta-hydroxycortisol : cortisol ratio. Ruboxistaurin is a specific protein-kinase-C beta inhibitor in clinical development for the treatment of diabetic microvascular complications.

METHODS

This was a two-period, one-sequence study. Sixteen healthy male subjects completed both study periods. In period one, a single 64 mg oral dose of ruboxistaurin was administered. In period two, 600 mg rifampicin was administered daily for 9 days, during which another single 64 mg ruboxistaurin dose was administered on day 7. Blood samples were collected and assayed for ruboxistaurin and N-desmethyl ruboxistaurin. CYP3A4 induction was assessed by ratios of urinary 6beta-hydroxycortisol : cortisol (6beta-OHC : C) obtained via 24 h and morning-spot sampling techniques. Results Following repeated doses of rifampicin, both the mean C(max) and AUC(0,infinity) of ruboxistaurin were significantly reduced by approximately 95% (P < or = 0.001). For the metabolite, the mean C(max) decreased by 68% (P < or = 0.001), and AUC(0,infinity) decreased by 77% (P < or = 0.001). The t(max) values did not appear affected. The 6beta-OHC : C ratios from both 24 h and morning spot methods increased significantly, consistent with CYP3A4 induction.

CONCLUSIONS

The effect of rifampicin co-administration on the exposure of ruboxistaurin is consistent with ruboxistaurin being a substrate of CYP3A4. Therefore, co-administration with known CYP3A4 inducing agents (rifampicin, carbamazepine, phenobarbital, etc.) may decrease the concentrations of ruboxistaurin and N-desmethyl-ruboxistaurin. In this study, 6beta OHC : C ratios substantially underestimated the impact of rifampicin on ruboxistaurin.

A New, Rapid, Fully Automated Method for Determination of Fluconazole in Serum by Column-Switching Liquid Chromatography

A sensitive and rapid HPLC assay for the determination of fluconazole in serum is described. HPLC-integrated sample preparation allows direct injection of serum samples without any pretreatment. The in-line extraction technique is carried out by automatically switching from the extraction column (Lichrospher ADS C8) to the analytic column (Nucleosil C18). After 6 minutes the matrix passes the extraction column, and the retained analyte is quantitatively transferred to the analytic column, where separation by isocratic HPLC is performed. The extraction eluent is sodium dihydrogen phosphate buffer, pH 5.0 (50 mM), and the analytic eluent is acetonitrile/sodium dihydrogen phosphate buffer, pH 5.0 (50 mM) (26.8/73.2, vol/vol). Fluconazole is detected according to its absorption maximum at 210 nm. The lower limit of quantification (LLOQ) is 0.65 microg/mL, the limit of detection (LOD) is 0.2 microg/mL, and the quantification range is 0.65-23.3 microg/mL. The assay was precise with a between-run coefficient of variation of < or = 5.59%. The within-run accuracy was 99.8% and 103.4%, and the between-run accuracy was 99.2% and 99.7%, respectively, for the concentrations 23.3 microg/mL and 1.3 microg/mL. The recovery was 78%. The described procedure allows sample cleanup and determination within 20 minutes, thereby facilitating drug monitoring in clinical routine. The method was applied successfully.

Effect of Rifampicin on the Pharmacokinetics of Fluconazole in Patients with AIDS

OBJECTIVE

To study the effect of rifampicin on the pharmacokinetics of fluconazole and on clinical outcomes of fluconazole treatment in patients with AIDS-related cryptococcal meningitis.

PATIENTS

Forty Thai patients with AIDS and cryptococcal meningitis, of whom 20 had been receiving oral rifampicin for at least 2 weeks to treat tuberculosis.

METHODS

Patients were treated for cryptococcal meningitis with amphotericin 0.7 mg/kg/day for 14 days followed by fluconazole 400 mg/day, which was reduced to 200 mg/day once culture of cerebrospinal fluid (CSF) became negative. Patients with tuberculosis received oral rifampicin 600 mg/day at night. Blood samples were collected from the first 12 patients in each group and pharmacokinetic parameters for fluconazole were calculated. CSF samples were collected by lumbar puncture and monitored for eradication of Cryptococcus neoformans.

RESULTS

Concomitant administration of rifampicin with fluconazole resulted in significant changes in the pharmacokinetic parameters of fluconazole, including a 39% increase in elimination rate constant, 28% shorter elimination half-life, 22% decrease in area under the concentration-time curve, 17% decrease in maximum concentration and 30% increase in clearance (p < 0.05). Different fluconazole regimens did not affect the extent of change in the elimination rate constant. Although serum concentrations of fluconazole during the time that patients received rifampicin concomitantly with fluconazole 200 mg/day were generally lower than the minimum inhibitory concentration for C. neoformans, there were no significant differences in clinical outcomes between the two groups to date (p = 0.792).

CONCLUSIONS

Coadministration of rifampicin with fluconazole caused significant changes in the pharmacokinetic parameters of fluconazole. Long-term monitoring for recurrence rates of cryptococcal meningitis is required to assess the clinical significance of this interaction.

Enteral fluconazole population pharmacokinetics in patients in the surgical intensive care unit

STUDY OBJECTIVE

To determine the population pharmacokinetic parameters of enterally administered fluconazole in patients in a surgical intensive care unit (SICU).

DESIGN

Population pharmacokinetics component of a prospective, randomized clinical study.

SETTING

The SICU at a university hospital.

PATIENTS

One hundred ten patients with an expected length of stay in the SICU of 3 or more days and a need for intubation, in whom at least one fluconazole plasma concentration-time measurement was available. Intervention. Patients received fluconazole as an 800-mg loading dose and as a 200- or 400-mg (depending on renal function) daily maintenance dose. Fluconazole suspension was administered enterally followed by a 30-ml free water flush.

MEASUREMENTS AND MAIN RESULTS

Plasma samples were collected, and population pharmacokinetic analysis was performed with NONMEM software; a one-compartment pharmacokinetic model was used. Fluconazole clearance was dependent on creatinine clearance, and volume of distribution was dependent on body weight and age. In patients with creatinine clearance values greater than 80 ml/minute, between 30 and 80 ml/minute, and less than 30 ml/minute, geometric mean (percentage coefficient of variation) fluconazole clearance was 14.39 ml/minute (21%), 10.53 ml/minute (28%), and 5.47 ml/minute (30%), respectively. The geometric mean (percentage coefficient of variation) volume of distribution in all patients was 1.27 L/kg (28%) and decreased with increasing age.

CONCLUSIONS

Fluconazole clearance values in patients in the SICU who had normal renal function and in those with renal impairment were in agreement with previously reported data. Fluconazole volume of distribution was larger and half-life was longer in the SICU population than in healthy subjects.

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.

Systemic antifungal therapy

Systemic antifungal therapy for superficial mycoses has advanced greatly since the introduction of griseofulvin in 1958. The discovery of the azole antifungal compounds, ketoconazole, itraconazole, and fluconazole, allowed for a broader spectrum of treatment and a shorter treatment duration. Terbinafine, through a unique mechanism of action, has a fungicidal power not seen previously in the other antifungals. It is important to use our knowledge of the pharmacology in combination with clinical experience and cost of therapy in order to select the proper drug. The search to identify new oral antifungal agents should continue, since none of the five currently used drugs fulfill the criteria of the "ideal" antifungal.

Effects of the antifungal agents on oxidative drug metabolism: clinical relevance

This article reviews the metabolic pharmacokinetic drug-drug interactions with the systemic antifungal agents: the azoles ketoconazole, miconazole, itraconazole and fluconazole, the allylamine terbinafine and the sulfonamide sulfamethoxazole. The majority of these interactions are metabolic and are caused by inhibition of cytochrome P450 (CYP)-mediated hepatic and/or small intestinal metabolism of coadministered drugs. Human liver microsomal studies in vitro, clinical case reports and controlled pharmacokinetic interaction studies in patients or healthy volunteers are reviewed. A brief overview of the CYP system and the contrasting effects of the antifungal agents on the different human drug-metabolising CYP isoforms is followed by discussion of the role of P-glycoprotein in presystemic extraction and the modulation of its function by the antifungal agents. Methods used for in vitro drug interaction studies and in vitro-in vivo scaling are then discussed, with specific emphasis on the azole antifungals. Ketoconazole and itraconazole are potent inhibitors of the major drug-metabolising CYP isoform in humans, CYP3A4. Coadministration of these drugs with CYP3A substrates such as cyclosporin, tacrolimus, alprazolam, triazolam, midazolam, nifedipine, felodipine, simvastatin, lovastatin, vincristine, terfenadine or astemizole can result in clinically significant drug interactions, some of which can be life-threatening. The interactions of ketoconazole with cyclosporin and tacrolimus have been applied for therapeutic purposes to allow a lower dosage and cost of the immunosuppressant and a reduced risk of fungal infections. The potency of fluconazole as a CYP3A4 inhibitor is much lower. Thus, clinical interactions of CYP3A substrates with this azole derivative are of lesser magnitude, and are generally observed only with fluconazole dosages of > or =200 mg/day. Fluconazole, miconazole and sulfamethoxazole are potent inhibitors of CYP2C9. Coadministration of phenytoin, warfarin, sulfamethoxazole and losartan with fluconazole results in clinically significant drug interactions. Fluconazole is a potent inhibitor of CYP2C19 in vitro, although the clinical significance of this has not been investigated. No clinically significant drug interactions have been predicted or documented between the azoles and drugs that are primarily metabolised by CYP1A2, 2D6 or 2E1. Terbinafine is a potent inhibitor of CYP2D6 and may cause clinically significant interactions with coadministered substrates of this isoform, such as nortriptyline, desipramine, perphenazine, metoprolol, encainide and propafenone. On the basis of the existing in vitro and in vivo data, drug interactions of terbinafine with substrates of other CYP isoforms are unlikely.

Drug interactions with itraconazole, fluconazole, and terbinafine and their management

A drug interaction develops when the effect of a drug is increased or decreased or when a new effect is produced by the prior, concurrent, or subsequent administration of the other. Before prescribing a drug, it is important to obtain a thorough drug history of the prescription and nonprescription medications taken by the patient. The nonprescription medications may include items such as nutritional supplements and herbal medications. The risk of side effects is an inevitable consequence of drug use. The frequency of adverse reactions is increased in those patients receiving multiple medications. Drug interactions reported in animal or in vitro studies may not necessarily develop in humans. When drug interactions are observed with a particular agent, it cannot be automatically assumed that all closely related drugs will necessarily produce the same interaction. However, caution is advised until sufficient experience accrues. The prescriber should not overestimate or underestimate the potential for a given drug interaction on the basis of personal experience alone. Drug interactions will not necessarily occur in every patient who is given a particular combination of drugs known to produce an interaction. For a clinically significant drug interaction to be manifest, several other factors may be relevant other than just using the two drugs. In many instances drug interactions can be predicted and therefore avoided if the pharmacodynamic effects, the pharmacokinetic properties, and the mechanisms of action of the 2 drugs in question are known. In the case of contraindicated drugs, it may be possible to use an alternative agent.

Fluconazole bioequivalence study: quantification by tandem mass spectrometry

To develop a new method for quantifying fluoconazole in human plasma and to compare the bioavailability of two fluconazole capsule formulations, an open, randomized, two-period crossover study with a one-week washout interval was conducted in 24 healthy volunteers. Plasma samples were obtained up to 168 hours after drug administration and the serum fluconazole concentrations were analyzed using electrospray tandem mass spectrometry coupled to liquid chromatography using multiple reaction monitoring mode. The pharmacokinetic parameters obtained for fluconazole after the administration of each formulation included the Area under the curve (AUC)(0-168h), AUC(0-infinity), Cmax, Cmax/AUC(0-168h), Tmax, elimination rate constant (Ke), and half-life (T1/2). Within- and between-run imprecision was less than 2.3% and 8.2%, respectively. Inaccuracy within and between runs was -1.5% and -9.7%, respectively. The pharmacokinetic parameters for bioequivalence showed a normal distribution, and the variance of AUC(0-168h), AUC(0-infinity), and Cmax were homoscedastic. The geometric mean for the Fluconal/Zoltec (Fluconal; Libbs Farmacêutica Ltda, São Paulo, Brazil; Zoltec; Laboratórios Pfizer Ltda., São Paulo, Brazil) individual percent ratio was 94.9% for AUC(0-168h), 94.7% for AUC(0-infinity), 80.1% for Cmax, 102.6% for Ke, 97.5% for T1/2, and 0.93 for Tmax (arithmetic mean of individual differences). We have developed a method in which liquid chromatography is coupled with electrospray tandem mass spectrometry to improve the pharmacokinetic analysis of fluconazole. Because the 90% CI AUC is within the interval proposed for the Food and Drug Administration, we concluded that Fluconal is bioequivalent to Zoltec in terms of absorption. The CV was 27.5% for the Cmax parameter, indicating that fluconazole's absorption rate is highly variable. The European Union Regulatory Agency accepts an interval of 70-143%, and because the 90% CI for Cmax is within the interval proposed for the European Union agency, we conclude that Fluconal is bioequivalent to Zoltec for the rate of absorption.

Update on drug interactions with azole antifungal agents

OBJECTIVE

To review and update the incidence, mechanism, and clinical relevance of drug interactions with itraconazole, ketoconazole, and fluconazole.

DATA SOURCES

Literature was identified by MEDLINE search (from January 1990 to May 1997) using the name of each antifungal and the term "interaction" as MeSH headings. Abstracts were identified by literature citation and by review of Interscience Conference on Antimicrobial Agents and Chemotherapy from 1995 to 1996.

STUDY SELECTION

Randomized, controlled, double-blind studies were emphasized; however, uncontrolled studies and case reports were also included. In vitro data were selected from literature review and citations.

DATA EXTRACTION

Data were evaluated with respect to study design, clinical relevance, magnitude of interaction, and recommendations provided.

DATA SYNTHESIS

The incidence of fungal infections and consequent azole antifungal usage continues to increase. By virtue of their antifungal mechanism (i.e., inhibition of cytochrome P450 fungal enzyme systems), azoles have been investigated and implicated in several drug interactions. The magnitude of interactions can vary from trivial to potentially fatal, and also vary with specific azole and interactant.

CONCLUSIONS

The azole antifungal agents represent a commonly used class of agents with a broad range of potential interactions. Recent data have increased our understanding of drug--drug interactions with azoles. Pharmacists are in a unique position to identify these interactions and to intervene to decrease their morbidity and improve patient care.

Current issues in onychomycosis

OBJECTIVE

To review the epidemiology, mycology, clinical features and diagnosis, current pharmacotherapy, and pharmacoeconomics of onychomycosis.

DATA SOURCES

We conducted a MEDLINE search from 1966 to May 1997. References from these articles, manufacturers of the discussed antimycotics, and relevant abstracts from recent dermatology conferences were used to collect pertinent data.

DATA EXTRACTION

Data were obtained from published controlled studies and case reports. In the pharmacotherapy section, the most weight was placed on fully reported, randomized, controlled comparative trials, but abstracts and case series were included when well-controlled studies were unavailable.

DATA SYNTHESIS

Onychomycosis is a common nail disorder that has a substantial impact on patients' quality of life. It is most commonly caused by dermatophytes, but yeasts and molds can also be involved. Diagnosis is made through clinical presentation, potassium hydroxide preparations, and culture of tissue/nail samples. Griseofulvin was the drug of choice for many years, but its low cure rates and the development of newer, more effective drugs made it fall out of favor. Current therapeutic alternatives include fluconazole, itraconazole, and terbinafine. Data on the use of fluconazole are limited to case series and reports. Continuous dosing of itraconazole and terbinafine are well-proven therapies. New data are becoming available on the use of pulse itraconazole dosing, which has recently been approved by the Food and Drug Administration for fingernail infections. These drugs are well tolerated, but attention to drug interactions is necessary with the azoles.

CONCLUSIONS

Currently, continuous terbinafine appears to be the most cost-effective drug for dermatophyte onychomycosis.

Management of drug interactions in patients with HIV

OBJECTIVE

To provide a complication of relevant information on drug interactions to assist healthcare practitioners in managing complex HIV-related pharmacotherapy.

DATA SOURCES

Information was retrieved via a MEDLINE search (January 1966-December 1996) using MeSH headings "human immunodeficiency virus," "drug interactions," and names of medications commonly prescribed for the management of HIV infection and related opportunistic infections. Abstracts of international and national conferences, review articles, textbooks, and references of all articles were also searched.

STUDY SELECTION AND DATA EXTRACTION

All literature on pharmacokinetic or pharmacodynamic interactions was considered for inclusion. Pertinent information, as assessed by the authors, was selected and summarized for discussion.

DATA SYNTHESIS

Drug disposition and/or pharmacologic effect may be affected either by HIV-related physiologic changes or by the presence of concomitant drug therapy. Modifications in drug selection, dosage, dosing regimen, or route of administration may be needed to avoid or manage drug-disease, drug-drug, or drug-food interactions. Management options may depend on the mechanism and the clinical significance of the interaction, the availability of therapeutic alternatives, patient convenience, and cost restrictions. In the absence of specific data, consideration of pharmacokinetic and pharmacodynamic characteristics to assist practitioners in predicting the likelihood of possible interactions was included.

RESULTS

A comprehensive table of clinically significant drug interactions is provided. Drug interaction principles and practical management strategies are also discussed.

CONCLUSIONS

The potential for drug interactions is extremely common, given the increasing complexity of managing patients infected with HIV. To avoid compromising therapeutic efficacy or increasing drug toxicity, practitioners need to be aware of potential interactions and are encouraged to use a systematic approach when managing patient drug therapy.

Oral antifungal drug interactions

The recent approval of itraconazole and terbafine for the treatment of onychomycosis has launched a new era in the therapeutic management of this previously resistant form of dermatomycoses. These agents represent safe and effective treatments. The clinician should, however, become knowledgeable with the potential drug interactions that are discussed in this article.

Pharmacokinetic interaction between itraconazole and rifampin in Yucatan miniature pigs

The objective of this study was to examine the effects of rifampin on itraconazole pharmacokinetics, at steady state, in three Yucatan miniature pigs. Daily for 3 weeks, the pigs received 200 mg of itraconazole orally at the beginning of each meal, and for the following 2 weeks they received itraconazole orally combined with intravenous administration of rifampin at 10 mg/kg/day. Coadministration of rifampin resulted in an 18-fold decrease in the maximum concentration of itraconazole in serum, from 113.0 (standard deviation [SD] 17.2) to 6.2 (SD, 3.9) ng/ml and a 22-fold decrease in the area under the concentration-time curve, from 1,652.7 (SD, 297.7) to 75.6 (SD, 30.0) ng.h/ml. The active metabolite of itraconazole, hydroxyitraconazole, was undetectable. This study demonstrates that rifampin affects itraconazole kinetics considerably at steady state in this miniature-pig model, probably by inducing hepatic metabolism of itraconazole.

Rifampin-fluconazole interaction in critically ill patients

OBJECTIVE

To report the influence of rifampin coadministration on the pharmacokinetics of fluconazole in 2 critically ill patients.

CASE SUMMARIES

The pharmacokinetics of fluconazole are reported in 5 patients in the intensive care unit (ICU), 2 of whom received rifampin and 3 who received only fluconazole. Patient 1 was a 52 year-old man with bilateral pneumonia who received rifampin for 9 days in addition to other antibiotics when fluconazole was added for suspected fungal superinfection. Patient 2, a 39-year-old man with steroid-dependent asthma was admitted to the ICU with a right middle lobe pneumonia and ventilatory insufficiency. Because of rapid clinical deterioration, intravenous rifampin 600 mg q12h and fluconazole 100 mg q24h were added to conventional antibacterial therapy. Patients 3-5 received intravenous fluconazole therapy, but were never administered rifampin prior to their antifungal therapy.

DISCUSSION

Fluconazole has gained wide use as an antifungal agent because of its efficacy, limited toxicity, and the paucity of reported drug interactions. In some clinical situations, however, the drug must be coadministered with rifampin. Limited data in healthy volunteers suggest that the coadministration of rifampin and fluconazole results in a 23% reduction in the fluconazole area under the concentration-time curve (AUC). In this report, we found a statistically significant lowering of the AUC (52%) and a 93% higher total body clearance of fluconazole in patients treated with rifampin.

CONCLUSIONS

Although limited data are available describing the magnitude of the interaction between fluconazole and rifampin in patients, our data suggest a more significant interaction than previously reported. If the concurrent administration of the 2 drugs in unavoidable, the patient's clinical response to treatment should be monitored closely, as the unexpectedly large reduction in fluconazole serum concentrations may lead to poor treatment outcomes.

Fluconazole. An update of its antimicrobial activity, pharmacokinetic properties, and therapeutic use in vaginal candidiasis

Fluconazole is a bis-triazole antifungal drug which has a pharmacokinetic profile characterised by its high water solubility, low affinity for plasma proteins, and metabolic stability. After a single 150 mg oral dose, therapeutic concentrations in vaginal secretions are rapidly achieved and are sustained for a duration sufficient to produce high clinical and mycological responses in nonimmunocompromised patients with vaginal candidiasis (candidosis). At this dosage, clinical and mycological responses have compared favourably with responses achieved after multiple dose regimens of other oral and intravaginal antifungal agents. Clinical efficacy rates have ranged between 92 and 99% at short term evaluation (5 days post-treatment). At 80 to 100 days post-treatment clinical efficacy rates of 91% have been reported. In addition, limited data indicate that fluconazole is more effective than placebo as prophylactic treatment of frequently recurring vaginal candidiasis. Single oral doses of fluconazole 150 mg are well tolerated. Most frequently observed adverse events are gastrointestinal symptoms, which are generally mild and transient in nature. Thus, fluconazole is a valuable alternative to established systemic and intravaginal azole antifungal drugs which are used to treat vaginal candidiasis. Moreover, in view of its favourable patient acceptability and compliance profile compared with alternative treatments, single-dose oral fluconazole should be considered as a first-line therapeutic choice for the treatment of women with vaginal candidiasis.

Fluconazole-induced symptomatic phenytoin toxicity

OBJECTIVE

To report two cases of fluconazole-induced symptomatic phenytoin toxicity and review literature related to this interaction.

DATA SOURCES

Case reports and review articles identified by a computerized (MEDLINE) and manual (Index Medicus) search.

DATA SYNTHESIS

Fluconazole is a broad-spectrum triazole antifungal agent primarily eliminated by renal mechanisms, although hepatic cytochrome P-450 inhibition and hepatotoxicity have been observed. We report two cases of fluconazole-induced symptomatic phenytoin toxicity. Both patients received high doses of the drug; one patient developed phenytoin toxicity only after long-term coadministration. Previously reported cases have occurred primarily with high-dose fluconazole and short-term coadministration.

CONCLUSIONS

Fluconazole can increase phenytoin serum concentrations leading to toxicity. Constant and continuous monitoring of serum phenytoin concentrations with fluconazole doses as low as 200 mg/d is warranted.

Oral azole drugs as systemic antifungal therapy

The oral azole drugs--ketoconazole, fluconazole, and itraconazole--represent a major advance in systemic antifungal therapy. Among the three, fluconazole has the most attractive pharmacologic profile, including the capacity to produce high concentrations of active drug in cerebrospinal fluid and urine. Ketoconazole, the first oral azole to be introduced, is less well tolerated than either fluconazole or itraconazole and is associated with more clinically important toxic effects, including hepatitis and inhibition of steroid hormone synthesis. However, ketoconazole is less expensive than fluconazole and itraconazole--an especially important consideration for patients receiving long-term therapy. All three drugs are effective alternatives to amphotericin B and flucytosine as therapy for selected systemic mycoses. Ketoconazole and itraconazole are effective in patients with the chronic, indolent forms of the endemic mycoses, including blastomycosis, coccidioidomycosis, and histoplasmosis; itraconazole is also effective in patients with sporotrichosis. Fluconazole is useful in the common forms of fungal meningitis--namely, coccidioidal and cryptococcal meningitis. In addition, fluconazole is effective for selected patients with serious candida syndromes such as candidemia, and itraconazole is the most effective of the azoles for the treatment of aspergillosis.

Pharmacokinetic drug interactions with rifampicin

Rifampicin, an antituberculosis drug, is usually administered for 4 to 12 months with other antituberculosis drugs or medications from other classes. A potential for drug interactions often exists because rifampicin is a potent inducer of hepatic drug metabolism, as evidenced by a proliferation of smooth endoplasmic reticulum and an increase in the cytochrome P450 content in the liver. The induction is a highly selective process and not every drug metabolised via oxidation is affected. Case reports and studies have demonstrated enhanced metabolism of several drugs; most of these interactions are clinically important. At the start of rifampicin treatment, and again at the end, clinicians must check the dosages of any accompanying medications with which rifampicin may potentially interact. Monitoring of clinical response and blood drug concentrations is essential to adjust the drug dosage during rifampicin therapy. Rifampicin also interacts with cholephils such as bilirubin and bromosulphthalein. Its pharmacokinetics are reported to be altered by ethambutol, p-aminosalicylic acid (through its excipient component), ketoconazole, cyclosporin, clofazimine, probenecid and phenobarbital through one or other of the following mechanisms--impaired absorption of rifampicin, competition between the drug and rifampicin for hepatic uptake and altered hepatic metabolism of rifampicin. Most interactions affecting rifampicin have been relatively minor or are not expected to alter its therapeutic efficacy.