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MYCOBUTIN® (rifabutin)

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Clinical Pharmacology

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CLINICAL PHARMACOLOGY

Pharmacokinetics

Absorption

Following a single oral dose of 300 mg to nine healthy adult volunteers, rifabutin was readily absorbed from the gastrointestinal tract with mean (±SD) peak plasma levels (Cmax) of 375 (±267) ng/mL (range: 141 to 1033 ng/mL) attained in 3.3 (±0.9) hours (Tmax range: 2 to 4 hours). Absolute bioavailability assessed in five HIV-positive patients, who received both oral and intravenous doses, averaged 20%. Total recovery of radioactivity in the urine indicates that at least 53% of the orally administered rifabutin dose is absorbed from the gastrointestinal tract. The bioavailability of rifabutin from the capsule dosage form, relative to an oral solution, was 85% in 12 healthy adult volunteers. High-fat meals slow the rate without influencing the extent of absorption from the capsule dosage form. Plasma concentrations post-Cmax declined in an apparent biphasic manner. Pharmacokinetic dose-proportionality was established over the 300 mg to 600 mg dose range in nine healthy adult volunteers (crossover design) and in 16 early symptomatic human immunodeficiency virus (HIV)-positive patients over a 300 mg to 900 mg dose range.

Distribution

Due to its high lipophilicity, rifabutin demonstrates a high propensity for distribution and intracellular tissue uptake. Following intravenous dosing, estimates of apparent steady-state distribution volume (9.3 ± 1.5 L/kg) in five HIV-positive patients exceeded total body water by approximately 15-fold. Substantially higher intracellular tissue levels than those seen in plasma have been observed in both rat and man. The lung-to-plasma concentration ratio, obtained at 12 hours, was approximately 6.5 in four surgical patients who received an oral dose. Mean rifabutin steady-state trough levels (Cp,minss; 24-hour post-dose) ranged from 50 to 65 ng/mL in HIV-positive patients and in healthy adult volunteers. About 85% of the drug is bound in a concentration-independent manner to plasma proteins over a concentration range of 0.05 to 1 µg/mL. Binding does not appear to be influenced by renal or hepatic dysfunction. Rifabutin was slowly eliminated from plasma in seven healthy adult volunteers, presumably because of distribution-limited elimination, with a mean terminal half-life of 45 (±17) hours (range: 16 to 69 hours). Although the systemic levels of rifabutin following multiple dosing decreased by 38%, its terminal half-life remained unchanged.

Metabolism

Of the five metabolites that have been identified, 25-O-desacetyl and 31-hydroxy are the most predominant, and show a plasma metabolite:parent area under the curve ratio of 0.10 and 0.07, respectively. The former has an activity equal to the parent drug and contributes up to 10% to the total antimicrobial activity.

Excretion

A mass-balance study in three healthy adult volunteers with 14C-labeled rifabutin showed that 53% of the oral dose was excreted in the urine, primarily as metabolites. About 30% of the dose is excreted in the feces. Mean systemic clearance (CLs/F) in healthy adult volunteers following a single oral dose was 0.69 (±0.32) L/hr/kg (range: 0.46 to 1.34 L/hr/kg). Renal and biliary clearance of unchanged drug each contribute approximately 5% to CLs/F.

Pharmacokinetics in Special Populations

Geriatric

Compared to healthy volunteers, steady-state kinetics of MYCOBUTIN are more variable in elderly patients (>70 years).

Pediatric

The pharmacokinetics of MYCOBUTIN have not been studied in subjects under 18 years of age.

Renal Impairment

The disposition of rifabutin (300 mg) was studied in 18 patients with varying degrees of renal function. Area under plasma concentration time curve (AUC) increased by about 71% in patients with severe renal impairment (creatinine clearance below 30 mL/min) compared to patients with creatinine clearance (Crcl) between 61–74 mL/min. In patients with mild to moderate renal impairment (Crcl between 30–61 mL/min), the AUC increased by about 41%. In patients with severe renal impairment, carefully monitor for rifabutin associated adverse events. A reduction in the dosage of rifabutin is recommended for patients with Crcl <30 mL/min if toxicity is suspected (see DOSAGE AND ADMINISTRATION).

Hepatic Impairment

Mild hepatic impairment does not require a dose modification. The pharmacokinetics of rifabutin in patients with moderate and severe hepatic impairment is not known.

Malabsorption in HIV-Infected Patients

Alterations in gastric pH due to progressing HIV disease has been linked with malabsorption of some drugs used in HIV-positive patients (e.g., rifampin, isoniazid). Drug serum concentrations data from AIDS patients with varying disease severity (based on CD4+ counts) suggests that rifabutin absorption is not influenced by progressing HIV disease.

Drug-Drug Interactions

(see also PRECAUTIONS-Drug Interactions)

Multiple dosing of rifabutin has been associated with induction of hepatic metabolic enzymes of the CYP3A subfamily. Rifabutin's predominant metabolite (25-desacetyl rifabutin: LM565), may also contribute to this effect. Metabolic induction due to rifabutin is likely to produce a decrease in plasma concentrations of concomitantly administered drugs that are primarily metabolized by the CYP3A enzymes. Similarly concomitant medications that competitively inhibit the CYP3A activity may increase plasma concentrations of rifabutin.


Microbiology

Mechanism of Action

Rifabutin inhibits DNA-dependent RNA polymerase in susceptible strains of Escherichia coli and Bacillus subtilis but not in mammalian cells. In resistant strains of E. coli, rifabutin, like rifampin, did not inhibit this enzyme. It is not known whether rifabutin inhibits DNA-dependent RNA polymerase in Mycobacterium avium or in M. intracellulare which comprise M. avium complex (MAC).

Susceptibility Testing

For specific information regarding susceptibility test interpretive criteria and associated test methods and quality control standards recognized by FDA for this drug, please see: https://www.fda.gov/STIC.

In Vitro Studies

Rifabutin has demonstrated in vitro activity against M. avium complex (MAC) organisms isolated from both HIV-positive and HIV-negative people. While-gene probe techniques may be used to identify these two organisms, many reported studies did not distinguish between these two species. The vast majority of isolates from MAC-infected, HIV-positive people are M. avium, whereas in HIV-negative people, about 40% of the MAC isolates are M. intracellulare.

Various in vitro methodologies employing broth or solid media, with and without polysorbate 80 (Tween 80), have been used to determine rifabutin MIC values for mycobacterial species. In general, MIC values determined in broth are several fold lower than that observed with methods employing solid media. Utilization of Tween 80 in these assays has been shown to further lower MIC values.

However, MIC values were substantially higher for egg-based compared to agar-based solid media.

Rifabutin activity against 211 MAC isolates from HIV-positive people was evaluated in vitro utilizing a radiometric broth and an agar dilution method. Results showed that 78% and 82% of these isolates had MIC99 values of ≤0.25 µg/mL and ≤1.0 µg/mL, respectively, when evaluated by these two methods. Rifabutin was also shown to be active against phagocytized, M. avium complex in a mouse macrophage cell culture model.

Rifabutin has in vitro activity against many strains of Mycobacterium tuberculosis. In one study, utilizing the radiometric broth method, each of 17 and 20 rifampin-naive clinical isolates tested from the United States and Taiwan, respectively, were shown to be susceptible to rifabutin concentrations of ≤0.125 µg/mL.

Cross-resistance between rifampin and rifabutin is commonly observed with M. tuberculosis and M. avium complex isolates. Isolates of M. tuberculosis resistant to rifampin are likely to be resistant to rifabutin. Rifampicin and rifabutin MIC99 values against 523 isolates of M. avium complex were determined utilizing the agar dilution method (Heifets, Leonid B. and Iseman, Michael D. Determination of in vitro susceptibility of Mycobacteria to Ansamycin. Am. Rev. Respir. Dis. 1985; 132(3):710–711).

Table 1 Susceptibility of M. Avium Complex Strains to Rifampin and Rifabutin
% of Strains Susceptible/Resistant to Different Concentrations of Rifabutin (μg/mL)
Susceptibility to Rifampin
(µg/mL)
Number of Strains Susceptible to 0.5 Resistant to 0.5 only Resistant to 1.0 Resistant to 2.0
Susceptible to 1.0 30 100.0 0.0 0.0 0.0
Resistant to 1.0 only 163 88.3 11.7 0.0 0.0
Resistant to 5.0 105 38.0 57.1 2.9 2.0
Resistant to 10.0 225 20.0 50.2 19.6 10.2
TOTAL 523 49.5 36.7 9.0 4.8

Rifabutin in vitro MIC99 values of ≤0.5 µg/mL, determined by the agar dilution method, for M. kansasii, M. gordonae and M. marinum have been reported; however, the clinical significance of these results is unknown.

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