Lack of effect of concomitant zidovudine on rifabutin kinetics in patients with AIDS-related complex

Antiretroviral treatment in HIV-infected children who require a rifamycin-containing regimen for tuberculosis

INTRODUCTION

In high prevalence settings, tuberculosis and HIV dual infection and co-treatment is frequent. Rifamycins, especially rifampicin, in combination with isoniazid, ethambutol and pyrazinamide are key components of short-course antituberculosis therapy. Areas covered: We reviewed available data, for which articles were identified by a Pubmed search, on rifamycin-antiretroviral interactions in HIV-infected children. Rifamycins have potent inducing effects on phase I and II drug metabolising enzymes and transporters. Antiretroviral medications are often metabolised by the enzymes induced by rifamycins or may suppress specific enzyme activity leading to drug-drug interactions with rifamycins. These may cause significant alterations in their phamacokinetic and pharmacodynamic properties, and sometimes that of the rifamycin. Recommended strategies to adapt to these interactions include avoidance and dose adjustment. Expert opinion: Despite the importance and frequency of tuberculosis as an opportunistic disease in HIV-infected children, current data on the management of co-treated children is based on few studies. We need new strategies to rapidly assess the use of rifamycins, new anti-tuberculosis drugs and antiretroviral drugs together as information on safety and dosing of individual drugs becomes available.

Randomized pharmacokinetic evaluation of different rifabutin doses in African HIV- infected tuberculosis patients on lopinavir/ritonavir-based antiretroviral therapy

BACKGROUND

Pharmacokinetic interactions between rifampicin and protease inhibitors (PIs) complicate the management of HIV-associated tuberculosis. Rifabutin is an alternative rifamycin, for patients requiring PIs. Recently some international guidelines have recommended a higher dose of rifabutin (150 mg daily) in combination with boosted lopinavir (LPV/r), than the previous dose of rifabutin (150 mg three times weekly {tiw}). But there are limited pharmacokinetic data evaluating the higher dose of rifabutin in combination with LPV/r. Sub-optimal dosing can lead to acquired rifamycin resistance (ARR). The plasma concentration of 25-O-desacetylrifabutin (d-RBT), the metabolite of rifabutin, increases in the presence of PIs and may lead to toxicity.

METHODS AND RESULTS

Sixteen patients with TB-HIV co-infection received rifabutin 300 mg QD in combination with tuberculosis chemotherapy (initially pyrazinamide, isoniazid and ethambutol then only isoniazid), and were then randomized to receive isoniazid and LPV/r based ART with rifabutin 150 mg tiw or rifabutin 150 mg daily. The rifabutin dose with ART was switched after 1 month. Serial rifabutin and d-RBT concentrations were measured after 4 weeks of each treatment. The median AUC0-48 and Cmax of rifabutin in patients taking 150 mg rifabutin tiw was significantly reduced compared to the other treatment arms. Geometric mean ratio (90% CI) for AUC0-48 and Cmax was 0.6 (0.5-0.7) and 0.5 (0.4-0.6) for RBT 150 mg tiw compared with RBT 300 mg and 0.4 (0.4-0.4) and 0.5 (0.5-0.6) for RBT 150 mg tiw compared with 150 mg daily. 86% of patients on the tiw rifabutin arm had an AUC0-24 < 4.5 μg.h/mL, which has previously been associated with acquired rifamycin resistance (ARR). Plasma d-RBT concentrations increased 5-fold with tiw rifabutin dosing and 15-fold with daily doses of rifabutin. Rifabutin was well tolerated at all doses and there were no grade 4 laboratory toxicities. One case of uveitis (grade 4), occurred in a patient taking rifabutin 300 mg daily prior to starting ART, and grade 3 neutropenia (asymptomatic) was reported in 4 patients. These events were not associated with increases in rifabutin or metabolite concentrations.

CONCLUSIONS

A daily 150 mg dose of rifabutin in combination with LPV/r safely maintained rifabutin plasma concentrations in line with those shown to prevent ARR.

TRIAL REGISTRATION

ClinicalTrials.gov Identifier: NCT00640887.

Pharmacokinetic drug interactions of antimicrobial drugs: a systematic review on oxazolidinones, rifamycines, macrolides, fluoroquinolones, and Beta-lactams

Like any other drug, antimicrobial drugs are prone to pharmacokinetic drug interactions. These drug interactions are a major concern in clinical practice as they may have an effect on efficacy and toxicity. This article provides an overview of all published pharmacokinetic studies on drug interactions of the commonly prescribed antimicrobial drugs oxazolidinones, rifamycines, macrolides, fluoroquinolones, and beta-lactams, focusing on systematic research. We describe drug-food and drug-drug interaction studies in humans, affecting antimicrobial drugs as well as concomitantly administered drugs. Since knowledge about mechanisms is of paramount importance for adequate management of drug interactions, the most plausible underlying mechanism of the drug interaction is provided when available. This overview can be used in daily practice to support the management of pharmacokinetic drug interactions of antimicrobial drugs.

Mutant Prevention Concentration of Isoniazid, Rifampicin and Rifabutin against Mycobacterium tuberculosis

BACKGROUND

Mutant prevention concentration (MPC) is a new parameter that may be of aid in determining the risk of resistant mutants being selected.

METHODS

The MPCs of 224 Mycobacterium tuberculosis clinical isolates were estimated by plating more than 10(10) cells on drug-containing agar and determining the concentration that allowed no colony growth. Antibiotics used were isoniazid, rifampicin and rifabutin.

RESULTS

The MPC90 of clinical isolates in our setting is 2.4, 2.2 and 0.4 mg/l for isoniazid, rifampicin and rifabutin, respectively.

CONCLUSIONS

Isoniazid and rifampicin are two drugs that present a low risk of selection of resistant mutants when used in monotherapy. However, determination of the MPC of each strain can provide data to minimize this risk and thus enable treatment to be optimized.

Comparative pharmacokinetics and pharmacodynamics of the rifamycin antibacterials

The rifamycin antibacterials, rifampicin (rifampin), rifabutin and rifapentine, are uniquely potent in the treatment of patients with tuberculosis and chronic staphylococcal infections. Absorption is variably affected by food; the maximal concentration of rifampicin is decreased by food, whereas rifapentine absorption is increased in the presence of food. The rifamycins are well-known inducers of enzyme systems involved in the metabolism of many drugs, most notably those metabolised by cytochrome P450 (CYP) 3A. The relative potency of the rifamycins as CYP3A inducers is rifampin > rifapentine > rifabutin; rifabutin is also a CYP3A substrate. The antituberculosis activity of rifampicin is decreased by a modest dose reduction from 600 to 450mg. This somewhat surprising finding may be due to the binding of rifampicin to serum proteins, limiting free, active concentrations of the drug. However, increasing the administration interval (after the first 2 to 8 weeks of therapy) has little effect on the sterilising activity of rifampicin, suggesting that relatively brief exposures to a critical concentration of rifampicin are sufficient to kill intermittently metabolising mycobacterial populations. The high protein binding of rifapentine (97%) may explain the suboptimal efficacy of the currently recommended dose of this drug. The toxicity of rifampicin is related to dose and administration interval, with increasing rates of presumed hypersensitivity with higher doses combined with administration frequency of once weekly or less. Rifabutin toxicity is related to dose and concomitant use of CYP3A inhibitors. The rifamycins illustrate the complexity of predicting the pharmacodynamics of treatment of an intracellular pathogen with the capacity for dormancy.

Drug-Drug interactions of clinical significance in the treatment of patients with Mycobacterium avium complex disease

Therapeutic and prophylactic regimens directed specifically against Mycobacterium avium complex (MAC) are increasingly being used in patients infected with the human immunodeficiency virus (HIV). Several of the drugs used in the management of MAC have been associated with significant drug interactions involving the cytochrome P450 (CYP) enzyme system. This enzyme system is also highly influenced by other drugs used in the management of patients with HIV, particularly the protease inhibitors, non-nucleoside reverse transcriptase inhibitors (NNRTIs) and azole antifungals. This article reviews the published concentrations or subtherapeutic concentrations of other drugs have been described. In particular, concurrent use of rifabutin with clarithromycin or fluconazole has resulted in increased concentrations of rifabutin and an accompanying increase in the incidence of rifabutin toxicities, including uveitis and leucopenia. Similar results have been seen when rifabutin is combined with protease inhibitors or delavirdine. The macrolides, clarithromycin and azithromycin, have also been associated with significant drug interactions. Clarithromycin has a higher affinity for CYP than azithromycin and, thus, is more frequently associated with clinically significant drug interactions. Clarithromycin is an inhibitor of CYP and may result in toxic concentrations of other drugs metabolised by this enzyme system. Such interactions have been described with rifabutin and the statin lipid-lowering agents. In addition, nevirapine and efavirenz have been shown to significantly reduce clarithromycin concentrations, whereas the protease inhibitors and delavirdine may increase clarithromycin concentrations. Other drugs used in the management of patients with MAC are not metabolised by CYP and thus have a lower incidence of interactions, although the absorption of ciprofloxacin may be impaired when it is given with products containing multivalent cations, such as didanosine. However, clinicians must remain vigilant for drug interactions when reviewing a patient's medication profile, keeping in mind both interactions that have been described in the literature and those that may be predicted based upon known pharmacokinetic profiles.