Pharmacokinetics

Pharmacokinetics in Special Populations

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.

MAC Bacteremia

Participants who received MYCOBUTIN were one-third to one-half as likely to develop MAC bacteremia as were participants who received placebo. These results were statistically significant (Study 023: p<0.001; Study 027: p = 0.002).

In Study 023, the one-year cumulative incidence of MAC bacteremia, on an intent to treat basis, was 9% for patients randomized to MYCOBUTIN and 22% for patients randomized to placebo. In Study 027, these rates were 13% and 28% for patients receiving MYCOBUTIN and placebo, respectively.

Most cases of MAC bacteremia (approximately 90% in these studies) occurred among participants whose CD4 count at study entry was ≤100 cells/µL. The median and mean CD4 counts at onset of MAC bacteremia were 13 cells/µL and 24 cells/µL, respectively. These studies did not investigate the optimal time to begin MAC prophylaxis.

Clinically Significant Disseminated MAC Disease

In association with the decreased incidence of bacteremia, patients on MYCOBUTIN showed reductions in the signs and symptoms of disseminated MAC disease, including fever, night sweats, weight loss, fatigue, abdominal pain, anemia, and hepatic dysfunction.

Survival

The one-year survival rates in Study 023 were 77% for the group receiving MYCOBUTIN and 77% for the placebo group. In Study 027, the one-year survival rates were 77% for the group receiving MYCOBUTIN and 70% for the placebo group.

These differences were not statistically significant.

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

In vitro susceptibility testing methods and diagnostic products used for determining minimum inhibitory concentration (MIC) values against M. avium complex (MAC) organisms have not been standardized. Breakpoints to determine whether clinical isolates of MAC and other mycobacterial species are susceptible or resistant to rifabutin have not been established.

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 MIC 99 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 MIC 99 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).

Rifabutin in vitro MIC 99 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.

Tuberculosis

MYCOBUTIN Capsules must not be administered for MAC prophylaxis to patients with active tuberculosis. Patients who develop complaints consistent with active tuberculosis while on prophylaxis with MYCOBUTIN should be evaluated immediately, so that those with active disease may be given an effective combination regimen of anti-tuberculosis medications. Administration of MYCOBUTIN as a single agent to patients with active tuberculosis is likely to lead to the development of tuberculosis that is resistant both to MYCOBUTIN and to rifampin.

There is no evidence that MYCOBUTIN is an effective prophylaxis against M. tuberculosis. Patients requiring prophylaxis against both M. tuberculosis and Mycobacterium avium complex may be given isoniazid and MYCOBUTIN concurrently.

Tuberculosis in HIV-positive patients is common and may present with atypical or extrapulmonary findings. Patients are likely to have a nonreactive purified protein derivative (PPD) despite active disease. In addition to chest X-ray and sputum culture, the following studies may be useful in the diagnosis of tuberculosis in the HIV-positive patient: blood culture, urine culture, or biopsy of a suspicious lymph node.

MAC Treatment with Clarithromycin

When MYCOBUTIN is used concomitantly with clarithromycin for MAC treatment, a decreased dose of MYCOBUTIN is recommended due to the increase in plasma concentrations of MYCOBUTIN (see PRECAUTIONS-Drug Interactions, Table 2).

Hypersensitivity and Related Reactions

Hypersensitivity reactions may occur in patients receiving rifamycins. Signs and symptoms of these reactions may include hypotension, urticaria, angioedema, acute bronchospasm, conjunctivitis, thrombocytopenia, neutropenia or flu-like syndrome (weakness, fatigue, muscle pain, nausea, vomiting, headache, fever, chills, aches, rash, itching, sweats, dizziness, shortness of breath, chest pain, cough, syncope, palpitations). There have been reports of anaphylaxis with the use of rifamycins.

Monitor patients receiving MYCOBUTIN therapy for signs and/or symptoms of hypersensitivity reactions. If these symptoms occur, administer supportive measures and discontinue MYCOBUTIN.

Uveitis

Due to the possible occurrence of uveitis, patients should also be carefully monitored when MYCOBUTIN is given in combination with clarithromycin (or other macrolides) and/or fluconazole and related compounds (see PRECAUTIONS-Drug Interactions, Table 2). If uveitis is suspected, the patient should be referred to an ophthalmologist and, if considered necessary, treatment with MYCOBUTIN should be suspended (see also ADVERSE REACTIONS).

Clostridium difficile Associated Diarrhea

Clostridium difficile associated diarrhea (CDAD) has been reported with use of nearly all antibacterial agents, including MYCOBUTIN (rifabutin) Capsules, USP, and may range in severity from mild diarrhea to fatal colitis. Treatment with antibacterial agents alters the normal flora of the colon leading to overgrowth of C. difficile.

C. difficile produces toxins A and B which contribute to the development of CDAD. Hypertoxin producing strains of C. difficile cause increased morbidity and mortality, as these infections can be refractory to antimicrobial therapy and may require colectomy. CDAD must be considered in all patients who present with diarrhea following antibacterial use. Careful medical history is necessary since CDAD has been reported to occur over two months after the administration of antibacterial agents.

If CDAD is suspected or confirmed, ongoing antibacterial use not directed against C. difficile may need to be discontinued. Appropriate fluid and electrolyte management, protein supplementation, antibacterial treatment of C. difficile, and surgical evaluation should be instituted as clinically indicated.

Protease Inhibitor Drug Interaction

Protease inhibitors act as substrates or inhibitors of CYP3A4 mediated metabolism. Therefore, due to significant drug-drug interactions between protease inhibitors and rifabutin, their concomitant use should be based on the overall assessment of the patient and a patient-specific drug profile. The concomitant use of protease inhibitors may require at least a 50% reduction in rifabutin dose, and depending on the protease inhibitor, an adjustment of the antiviral drug dose. Increased monitoring for adverse events is recommended when using these drug combinations (see PRECAUTIONS-Drug Interactions). For further recommendations, please refer to current, official product monographs of the protease inhibitor or contact the specific manufacturer.

General

Because treatment with MYCOBUTIN Capsules may be associated with neutropenia, and more rarely thrombocytopenia, physicians should consider obtaining hematologic studies periodically in patients receiving prophylaxis with MYCOBUTIN.

Information for Patients

Patients should be advised of the signs and symptoms of both MAC and tuberculosis, and should be instructed to consult their physicians if they develop new complaints consistent with either of these diseases. In addition, since MYCOBUTIN may rarely be associated with myositis and uveitis, patients should be advised to notify their physicians if they develop signs or symptoms suggesting either of these disorders.

Urine, feces, saliva, sputum, perspiration, tears, and skin may be colored brown-orange with rifabutin and some of its metabolites. Soft contact lenses may be permanently stained. Patients to be treated with MYCOBUTIN should be made aware of these possibilities.

Diarrhea is a common problem caused by antibacterials which usually ends when the antibacterial is discontinued. Sometimes, after starting treatment with antibacterials, patients can develop watery and bloody stools (with or without stomach cramps and fever) even as late as two or more months after having taken the last dose of the antibacterial. If this occurs, patients should contact their physician as soon as possible.

Drug Interactions

Carcinogenesis, Mutagenesis, Impairment of Fertility

Long-term carcinogenicity studies were conducted with rifabutin in mice and in rats. Rifabutin was not carcinogenic in mice at doses up to 180 mg/kg/day, or approximately 36 times the recommended human daily dose. Rifabutin was not carcinogenic in the rat at doses up to 60 mg/kg/day, about 12 times the recommended human dose.

Rifabutin was not mutagenic in the bacterial mutation assay (Ames Test) using both rifabutin-susceptible and resistant strains. Rifabutin was not mutagenic in Schizosaccharomyces pombe P 1 and was not genotoxic in V-79 Chinese hamster cells, human lymphocytes in vitro, or mouse bone marrow cells in vivo.

Fertility was impaired in male rats given 160 mg/kg (32 times the recommended human daily dose).

Pregnancy

Nursing Mothers

It is not known whether rifabutin is excreted in human milk. Because many drugs are excreted in human milk and because of the potential for serious adverse reactions in nursing infants, a decision should be made whether to discontinue nursing or discontinue the drug, taking into account the importance of the drug to the mother.

Pediatric Use

Safety and effectiveness of rifabutin for prophylaxis of MAC in children have not been established. Limited safety data are available from treatment use in 22 HIV-positive children with MAC who received MYCOBUTIN in combination with at least two other antimycobacterials for periods from 1 to 183 weeks. Mean doses (mg/kg) for these children were: 18.5 (range 15.0 to 25.0) for infants 1 year of age, 8.6 (range 4.4 to 18.8) for children 2 to 10 years of age, and 4.0 (range 2.8 to 5.4) for adolescents 14 to 16 years of age. There is no evidence that doses greater than 5 mg/kg daily are useful. Adverse experiences were similar to those observed in the adult population, and included leukopenia, neutropenia, and rash. In addition, corneal deposits have been observed in some patients during routine ophthalmologic surveillance of HIV-positive pediatric patients receiving MYCOBUTIN as part of a multiple-drug regimen for MAC prophylaxis. These are tiny, almost transparent, asymptomatic peripheral and central corneal deposits which do not impair vision. Doses of MYCOBUTIN may be administered mixed with foods such as applesauce.

Geriatric Use

Clinical studies of MYCOBUTIN did not include sufficient numbers of subjects aged 65 and over to determine whether they respond differently from younger subjects. Other reported clinical experience has not identified differences in responses between the elderly and younger patients. In general, dose selection for an elderly patient should be cautious, usually starting at the low end of the dosing range, reflecting the greater frequency of decreased hepatic, renal, or cardiac function, and of concomitant disease or other drug therapy (see CLINICAL PHARMACOLOGY).

Adverse Reactions from Clinical Trials

MYCOBUTIN Capsules were generally well tolerated in the controlled clinical trials. Discontinuation of therapy due to an adverse event was required in 16% of patients receiving MYCOBUTIN, compared to 8% of patients receiving placebo in these trials. Primary reasons for discontinuation of MYCOBUTIN were rash (4% of treated patients), gastrointestinal intolerance (3%), and neutropenia (2%).

The following table enumerates adverse experiences that occurred at a frequency of 1% or greater, among the patients treated with MYCOBUTIN in studies 023 and 027.

CLINICAL ADVERSE EVENTS REPORTED IN <1% OF PATIENTS WHO RECEIVED MYCOBUTIN

Considering data from the 023 and 027 pivotal trials, and from other clinical studies, MYCOBUTIN appears to be a likely cause of the following adverse events which occurred in less than 1% of treated patients: flu-like syndrome, hepatitis, hemolysis, arthralgia, myositis, chest pressure or pain with dyspnea, skin discoloration, thrombocytopenia, pancytopenia and jaundice.

The following adverse events have occurred in more than one patient receiving MYCOBUTIN, but an etiologic role has not been established: seizure, paresthesia, aphasia, confusion, and non-specific T wave changes on electrocardiogram.

When MYCOBUTIN was administered at doses from 1050 mg/day to 2400 mg/day, generalized arthralgia and uveitis were reported. These adverse experiences abated when MYCOBUTIN was discontinued.

Mild to severe, reversible uveitis has been reported less frequently when MYCOBUTIN is used at 300 mg as monotherapy in MAC prophylaxis versus MYCOBUTIN in combination with clarithromycin for MAC treatment (see also WARNINGS).

Uveitis has been infrequently reported when MYCOBUTIN is used at 300 mg/day as montherapy in MAC prophylaxis of HIV-infected persons, even with the concomitant use of fluconazole and/or macrolide antibacterials. However, if higher doses of MYCOBUTIN are administered in combination with these agents, the incidence of uveitis is higher. FDA proposes moving this paragraph from below with some revisions.

Patients who developed uveitis had mild to severe symptoms that resolved after treatment with corticosteroids and/or mydriatic eye drops; in some severe cases, however, resolution of symptoms occurred after several weeks.

When uveitis occurs, temporary discontinuance of MYCOBUTIN and ophthalmologic evaluation are recommended. In most mild cases, MYCOBUTIN may be restarted; however, if signs or symptoms recur, use of MYCOBUTIN should be discontinued (Morbidity and Mortality Weekly Report, September 9, 1994).

Corneal deposits have been reported during routine ophthalmologic surveillance of some HIV-positive pediatric patients receiving MYCOBUTIN as part of a multiple drug regimen for MAC prophylaxis. The deposits are tiny, almost transparent, asymptomatic peripheral and central corneal deposits, and do not impair vision.

The following table enumerates the changes in laboratory values that were considered as laboratory abnormalities in Studies 023 and 027.

The incidence of neutropenia in patients treated with MYCOBUTIN was significantly greater than in patients treated with placebo (p = 0.03). Although thrombocytopenia was not significantly more common among patients treated with MYCOBUTIN in these trials, MYCOBUTIN has been clearly linked to thrombocytopenia in rare cases. One patient in Study 023 developed thrombotic thrombocytopenic purpura, which was attributed to MYCOBUTIN.

Adverse Reactions from Post-Marketing Experience

Adverse reactions identified through post-marketing surveillance by system organ class (SOC) are listed below:

Blood and lymphatic system disorders: White blood cell disorders (including agranulocytosis, lymphopenia, granulocytopenia, neutropenia, white blood cell count decreased, neutrophil count decreased), platelet count decreased.

Immune system disorders: Hypersensitivity, bronchospasm, rash, and eosinophilia.

Gastrointestinal disorders: Clostridium difficile colitis/ Clostridium difficile associated diarrhea.

Pyrexia, rash and other hypersensitivity reactions such as eosinophilia and bronchospasm might occur, as has been seen with other antibacterials.

A limited number of skin discoloration have been reported.

Treatment

While there is no experience in the treatment of overdose with MYCOBUTIN Capsules, clinical experience with rifamycins suggests that gastric lavage to evacuate gastric contents (within a few hours of overdose), followed by instillation of an activated charcoal slurry into the stomach, may help absorb any remaining drug from the gastrointestinal tract.

Rifabutin is 85% protein bound and distributed extensively into tissues (Vss:8 to 9 L/kg). It is not primarily excreted via the urinary route (less than 10% as unchanged drug); therefore, neither hemodialysis nor forced diuresis is expected to enhance the systemic elimination of unchanged rifabutin from the body in a patient with an overdose of MYCOBUTIN.

Keep tightly closed and dispense in a tight container as defined in the USP. Store at 25°C (77°F); excursions permitted to 15°–30°C (59°–86°F) [see USP Controlled Room Temperature].