Ethambutol Hydrochloride

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Ethambutol is authorised in the world under the following brand names: Dadibutol, Diambutol, Etibi, Myambutol, Tibutol.

Drug Nomenclature

INNs in main languages (French, Latin, Russian, and Spanish):

Pharmacopoeias. In China, Europe, International, Japan, US, and Vietnam. European Pharmacopoeia, 6th ed., 2008 and Supplements 6.1 and 6.2 (Ethambutol Hydrochloride). A white or almost white, crystalline powder. Freely soluble in water; soluble in alcohol. A 2% solution in water has a pH of 3.7 to 4.0. Store in airtight containers. The United States Pharmacopeia 31, 2008 (Ethambutol Hydrochloride). A white crystalline powder. Freely soluble in water; soluble in alcohol and in methyl alcohol; slightly soluble in chloroform and in ether.

Adverse Effects and Treatment

The most important adverse effect of ethambutol is ret-robulbar neuritis with a reduction in visual acuity, constriction of visual field, central or peripheral scotoma, and green-red colour blindness. One or both eyes may be affected. The degree of visual impairment appears to depend on the dose and duration of therapy; toxicity is observed most frequently at daily doses of 25 mg/kg or more and after at least 2 months of therapy.

Recovery of vision usually takes place over a period of a few weeks or months, but in rare cases it may take up to a year or more or the effect may be permanent. Retinal haemorrhage has occurred rarely. Renal clearance of urate may be reduced and acute gout has been precipitated rarely. Hypersensitivity reactions including skin rashes, pruritus, leucopenia, fever, and joint pains have occurred but appear to be rare with ethambutol.

Other adverse effects which have been reported include confusion, disorientation, hallucinations, headache, dizziness, malaise, j aundice or transient liver dysfunction, peripheral neuropathy, thrombocytopenia, pulmonary infiltrates, eosinophilia, and gastrointestinal disturbances such as nausea, vomiting, anorexia, and abdominal pain. Teratogenicity has been observed in animals (but see also Precautions, below). Blood concentrations of ethambutol after overdosage may be reduced by haemodialysis or peritoneal dialysis.

Effects on the blood

Neutropenia has been reported in a patient on ethambutol, isoniazid, and rifampicin. Each drug induced neutropenia individually on rechallenge. In another patient also receiving mixed antituberculous therapy, eosinophilia and neutropenia were associated with ethambutol; the effects recurred only on rechallenge with this drug. Skin rash, blood eosinophilia, and pulmonary infiltrates occurred in a patient after 8 weeks of multidrug therapy for miliary tuberculosis. Rechallenge again attributed the adverse event to ethambutol. Thrombocytopenia attributable to ethambutol has been reported in 2 patients.

Effects on the CNS

A 40-year-old man with advanced HIV infection taking oral ethambutol for Mycobacterium avium complex infection had rapid cognitive decline, hallucinations, and delusions within 2 weeks of starting ethambutol treatment; symptoms resolved on stopping treatment.

Effects on the eyes

A review on the ocular toxicity of ethambutol reported that when ethambutol is taken for more than 2 months the incidence of retrobulbar neuritis is about 18% in patients receiving a daily dose of more than 35 mg/kg, reducing to 5 to 6% with a daily dose of 25 mg/kg, and less than 1 % with a daily dose of 15 mg/kg. An earlier study reported ophthalmic effects in 10 of 2184 patients receiving ethambutol in doses of 25 mg/kg or less daily, although few of the 10 patients complained of symptoms. In 9 of the 10 patients, ocular changes occurred after the second month of treatment. In the 928 patients who only received 2 months of ethambutol therapy, ocular toxicity was not reported.

A prospective study of 229 patients taking ethambutol for Mycobacterium avium complex lung disease reported that ocular toxicity was more common in patients given daily doses rather than intermittent (3 times a week) therapy. While short-term use of ethambutol is usually safe, deterioration of vision leading to long-term blindness has been reported after only a few doses of ethambutol; it was suspected that this was an idiosyncratic reaction. Rapid onset reversible ocular toxicity has also occurred. Visual defects occurring with ethambutol generally resolve when the drug is stopped.

Effects on the kidneys

Interstitial nephritis has been reported in 5 patients receiving ethambutol and isoniazid; 3 were also receiving additional antimycobacterials. In another patient, acute renal failure occurred secondary to interstitial nephritis which was thought to have been induced by ethambutol.

Effects on the liver

Although transient abnormalities in liver function commonly occur during the early stages of antitubercu-losis treatment, drugs other than ethambutol are generally considered responsible. Ethambutol has generated fewer reports of hepatotoxicity to the UK CSM than rifampicin, isoniazid, or pyrazinamide, and the use of regimens containing ethambutol has been recommended for patients unable to tolerate standard regimens due to hepatotoxicity

Effects on the skin

Toxic epidermal necrolysis and lichenoid and erythema multiforme-type drug eruptions have been associated with the use of ethambutol. Delayed hypersensitivity reactions have also been reported. Licensed product information notes that Stevens-Johnson syndrome and dermatitis have also occurred.

Hyperuricaemia

In a controlled study of 71 patients receiving ethambutol 20 mg/kg daily orally with other antimycobacterials, serum-uric acid concentrations increased in 66, mainly in the first 2 weeks of treatment. One patient experienced arthralgia and another acute gouty arthritis. Serum-uric acid concentrations did not change in 60 control patients receiving other antimycobacterials.

Precautions

Ethambutol is generally contra-indicated in patients with optic neuritis. It should be used with great care in patients with visual defects, the elderly, and in children in whom evaluation of changes in visual acuity may be difficult (see also Children, below). Ocular examination is recommended before treatment with ethambutol and some consider thatregular examinations are necessary during treatment, especially in children.

Patients should be advised to report visual disturbances immediately and to stop ethambutol pending visual evaluation. Ethambutol should be given in reduced dosage to patients with renal impairment and dosage adjustments may need to be made according to serum concentrations. The BNF recommends peak concentrations of 2 to 6 mg/L and trough concentrations of less than 1 mg/L. Ethambutol may precipitate attacks of gout.

Although ethambutol crosses the placenta and may be teratogenic in animals, problems in humans have not been documented. It is generally considered that the benefits of ethambutol in the treatment of tuberculosis outweigh any potential risks in pregnancy.

Breast feeding

Ethambutol distributes into breast milk to produce concentrations similar to those in plasma. However, no adverse effects have been seen in breast-fed infants whose mothers were receiving ethambutol, and the American Academy of Pediatrics considers that it is therefore usually compatible with breastfeeding.

Children

Due to the possible difficulty of evaluating changes in visual acuity that may be induced in children receiving ethambutol, the BNFC advises that it should be used with caution in children under 5 years of age and unable to report visual changes accurately, whereas in the United States of America (US and USA) licensed product information has advised against use in those under 13 years of age.

The authors of a review of the use of ethambutol in children concluded that no extra precautions were necessary in children aged 5 years or more, and that it could also be used in younger children without undue fear of adverse effects. Another review suggested that visual toxicity is not a particular problem except perhaps when CNS infection is involved. A literature review on the use of ethambutol in children reported almost no ocular toxicity at daily doses of 15 to 30mg/kg. Ethambutol is therefore considered safe in children of all ages at a daily dose of 20 mg/kg (range 15 to 25 mg/kg) or a three times weekly dose of 30 mg/kg.

Antimicrobial Action

Ethambutol is active against Mycobacterium tuberculosis and some other mycobacteria. Resistant strains of M. tuberculosis are readily produced if ethambutol is used alone.

Pharmacokinetics

About 80% of an oral dose of ethambutol is absorbed from the gastrointestinal tract. Absorption is not significantly impaired by food (but see also Bioavailability below). After a single dose of 25 mg/kg peak plasma concentrations of up to 5 mg/L appear within 4 hours, and are less than 1 mg/L by 24 hours. Ethambutol is distributed to most tissues, including the lungs, kidneys, and erythrocytes. About 10 to 50% may diffuse into the CSF when the meninges are inflamed. It has been reported to cross the placenta and is distributed into breast milk. The elimination half-life after oral doses is about 3 to 4 hours. Ethambutol is partially metabolised in the liver to the aldehyde and dicarboxylic acid derivatives which are inactive and then excreted in the urine. Most of a dose appears in the urine within 24 hours as unchanged drug and 8 to 15% as the inactive metabolites. About 20% of the dose is excreted unchanged in the faeces.

Bioavailability

Although the absorption of ethambutol is not generally regarded as being impaired by food, a study in 14 healthy subjects suggested that giving it with a high-fat meal or an antacid could delay absorption and reduce the maximum plasma concentration.

HlV-infected patients

Malabsorption of ethambutol and other antituberculous drugs may occur in patients with HIV infection and tuberculosis, and may contribute to acquired drug resistance and reduced efficacy of tuberculosis treatment. For further information on the absorption of antituberculous drugs in HIV-infected patients see under Rifampicin.

Pregnancy and breast feeding

Ethambutol crosses the placenta and is present in fetal tissue in amounts of at least 74.5% of the maternal serum concentration. Ethambutol distributes into breast milk to produce concentrations similar to those in plasma.

Uses

Tuberculosis

Active Tuberculosis

Ethambutol is used in conjunction with other antituberculosis agents in the treatment of clinical tuberculosis.

The American Thoracic Society (ATS), US Centers for Disease Control and Prevention (CDC), and Infectious Diseases Society of America (IDSA) currently recommend several possible multiple-drug regimens for the treatment of culture-positive pulmonary tuberculosis. These regimens have a minimum duration of 6 months (26 weeks), and consist of an initial intensive phase (2 months) and a continuation phase (usually either 4 or 7 months).

Ethambutol is considered a first-line antituberculosis agent for use in the initial phase of these regimens. In patients with previously untreated pulmonary tuberculosis, ethambutol usually is included in the initial phase of treatment in conjunction with isoniazid and rifampin (with or without pyrazinamide). In patients receiving a daily treatment regimen for the initial phase, the ATS, CDC, and IDSA state that ethambutol usually can be discontinued when in vitro susceptibility tests indicate that the strain of Mycobacterium tuberculosis is susceptible to isoniazid and rifampin.

Ethambutol also is considered a first-line agent for use in multiple-drug regimens for the management of patients with drug-resistant pulmonary tuberculosis.

Myobacterium avium Complex (MAC) Infections

Treatment of MAC Infections

Ethambutol is used in conjunction with other antituberculosis agents in the treatment of Mycobacterium avium complex (MAC) infections. The ATS currently recommends that therapy for MAC pulmonary infections in HIV-negative patients consist of at least 3 drugs, including clarithromycin (500 mg twice daily) or azithromycin (250 mg daily or 500 mg 3 times weekly), rifabutin (300 mg daily) or rifampin (600 mg daily), and ethambutol (25 mg/kg daily for 2 months, then 15 mg/kg daily). In addition, the ATS recommends therapy that includes either clarithromycin or azithromycin combined with ethambutol and rifabutin for the treatment of disseminated MAC infection in HIV-infected patients. The choice of the drug regimen should be made in consultation with an expert.

Prevention of Recurrence

To prevent recurrence of MAC infections, maintenance therapy (secondary prophylaxis) is recommended for all HIV-infected adults, adolescents, and children who have previously been treated for disseminated MAC infection. Unless there is clinical or laboratory evidence of macrolide resistance, the Prevention of Opportunistic Infections Working Group of the US Public Health Service and Infectious Diseases Society of America (USPHS/IDSA) recommends a regimen of clarithromycin (or azithromycin) in conjunction with ethambutol with or without rifabutin for secondary MAC prophylaxis.

Secondary MAC prophylaxis after disseminated MAC infection in HIV-infected adults and adolescents generally is recommended for life, unless there is immune recovery in response to highly active antiretroviral therapy (HAART). There is some evidence that the risk for recurrence of MAC is low in adults and adolescents who have completed at least 12 months of MAC therapy, have remained asymptomatic with respect to MAC signs and symptoms, and have a sustained (e.g., for 6 months or longer) increase in CD4+ T-cell counts to greater than 100/mm3 as the result of potent antiretroviral therapy. Based on these data and more extensive cumulative data regarding the safety of discontinuing secondary prophylaxis for other opportunistic infections, the USPHS/IDSA states that it is reasonable to consider discontinuance of secondary MAC prophylaxis in adults and adolescents meeting these criteria. To substantiate that the disease is no longer active, some experts would obtain a blood culture for MAC (even in asymptomatic patients) prior to discontinuing secondary MAC prophylaxis. The USPHS/IDSA recommends that secondary MAC prophylaxis be reinitiated if CD4+ T-cell counts subsequently decrease to less than 100/mm3.

The safety of discontinuing secondary MAC prophylaxis in HIV-infected children whose CD4+ T-cell count has increased in response to HAART has not been studied. Therefore, the USPHS/IDSA recommends that HIV-infected children with a history of disseminated MAC infection receive lifelong secondary MAC prophylaxis.

Administration

Ethambutol hydrochloride is administered orally.

Dosage

Active Tuberculosis

In the treatment of clinical tuberculosis, ethambutol should not be given alone. The drug is considered a first-line agent for use in patients with tuberculosis. Therapy for tuberculosis should be continued long enough to prevent relapse. The minimum duration of treatment currently recommended for patients with culture-positive pulmonary tuberculosis is 6 months (26 weeks), and recommended regimens consist of an initial intensive phase (2 months) and a continuation phase (usually either 4 or 7 months). However, completion of treatment is determined more accurately by the total number of doses and is not based solely on the duration of therapy.

Adult Dosage

The manufacturer states that the usual adult dosage of ethambutol hydrochloride for use in conjunction with other antituberculosis agents in previously untreated patients is 15 mg/kg once daily. In adults who have received previous antituberculosis therapy, the usual dosage recommended by the manufacturer for use in conjunction with other antituberculosis agents is 25 mg/kg daily for 60 days or until bacteriologic smears and cultures become negative, followed by 15 mg/kg daily.

The American Thoracic Society (ATS), US Centers for Disease Control and Prevention (CDC), and Infectious Diseases Society of America (IDSA) recommend that when ethambutol hydrochloride is used in an initial regimen that involves daily administration of antituberculosis agents, adults and children 15 years of age or older weighing 40-55 kg should receive a dosage of 800 mg, those weighing 56-75 kg should receive 1.2 g, and those weighing 76-90 kg should receive 1.6 g. The maximum daily dosage recommended is 1.6 g regardless of weight.

If ethambutol hydrochloride is used in an intermittent regimen that involves twice-weekly administration of antituberculosis agents, the ATS, CDC, and IDSA recommend that adults and children 15 years of age or older weighing 40-55 kg should receive a dosage of 2 g, those weighing 56-75 kg should receive 2.8 g, and those weighing 76-90 kg should receive 4 g; the maximum dosage for this regimen is 2.4 g regardless of weight. If a 3-times weekly regimen is used, adults weighing 40-55 kg should receive a dosage of 1.2 g, those weighing 56-75 kg should receive 2 g, and those weighing 76-90 kg should receive 2.4 g; the maximum dosage for this regimen is 4 g regardless of weight.

Pediatric Dosage

The manufacturer states that ethambutol hydrochloride is not recommended for use in children younger than 13 years of age. The ATS, CDC, and IDSA state that ethambutol should be used with caution in children in whom it may be difficult to monitor visual acuity (e.g., those younger than 5 years of age). If ethambutol hydrochloride is used in pediatric patients, the ATS, CDC, and IDSA recommend a dosage of 15-20 mg/kg daily (up to 1 g) or a dosage of 50 mg/kg twice weekly (up to 2.5 g). The American Academy of Pediatrics recommends a pediatric dosage of 15-25 mg/kg daily or 50 mg/kg twice weekly (up to 2.5 g).

Mycobacterium avium Complex (MAC) Infections

Adult Dosage

When ethambutol hydrochloride is used in conjunction with other antituberculosis agents for the treatment of pulmonary Mycobacterium avium complex (MAC) infections in adults not infected with the human immunodeficiency virus (HIV), a dosage of 25 mg/kg daily for 2 months, followed by 15 mg/kg daily thereafter, has been recommended by ATS. Therapy should continue until the patient has been culture negative for 1 year. When ethambutol hydrochloride is used in conjunction with other antituberculosis agents for the treatment of disseminated MAC infection, a dosage of 15 mg/kg daily has been recommended by ATS.

For maintenance therapy to prevent recurrence of disseminated MAC infection (secondary prophylaxis), the Prevention of Opportunistic Infections Working Group of the US Public Health Service and the Infectious Diseases Society of America (USPHS/IDSA) recommends that adults or adolescents receive clarithromycin (500 mg twice daily) or azithromycin (500 mg once daily) in conjunction with ethambutol hydrochloride (15 mg/kg once daily), with or without rifabutin (300 mg once daily).

Pediatric Dosage

For maintenance therapy to prevent recurrence of disseminated MAC infection (secondary prophylaxis), the USPHS/IDSA recommends that infants and children receive clarithromycin (7. mg/kg [maximum 500 mg] daily) or azithromycin (5 mg/kg [maximum 250 mg] once daily) in conjunction with ethambutol hydrochloride (15 mg/kg [maximum 900 mg] once daily), with or without rifabutin (5 mg/kg [maximum 300 mg] once daily).

Dosage in Renal Impairment

In patients with impaired renal function, doses and/or frequency of administration of ethambutol hydrochloride should be modified in response to the degree of renal impairment. Some clinicians suggest that if the creatinine clearance is 70-100 mL/minute, dosage of ethambutol hydrochloride should not exceed 15 mg/kg daily, and if the creatinine clearance is less than 70 mL/minute, dosage should be further reduced. Other clinicians have suggested that the usual dose be administered every 24-36 hours in patients with creatinine clearances of 10-50 mL/minute and every 48 hours in patients with creatinine clearances less than 10 mL/minute.

Cautions

Ocular Effects

The most important adverse effect of ethambutol is optic neuritis with decreases in visual acuity, constriction of visual fields, central and peripheral scotomas, and loss of red-green color discrimination. The extent of ocular toxicity appears to be related to the dose and duration of ethambutol therapy. However, such toxicity also has been reported rarely after only a few days of therapy with the drug, and may represent an idiosyncratic reaction. Tests for visual acuity should be performed using a Snellen eye chart prior to and periodically during ethambutol therapy.

Changes in visual acuity may be unilateral or bilateral; therefore, each eye must be tested separately and both eyes tested together. During 1-2 years of ethambutol therapy, some patients may develop a refractive error which must be corrected in order to obtain accurate test results; testing visual acuity through a pinhole eliminates the refractive error.

When ocular toxicity is detected early and ethambutol is discontinued promptly, the visual effects are generally reversible over a period of weeks or months. Patients have then received ethambutol again without recurrence of loss of visual acuity. Rarely, depending on the degree of impairment, recovery may be delayed for up to 1 year or more, or the effect may be irreversible. Care should be taken to be certain that variations in vision are not caused by underlying pathologic conditions (e.g., optic neuritis from other causes, cataracts, recurrent ocular inflammatory conditions, diabetic retinopathy).

Other Adverse Effects

Other adverse effects of ethambutol include dermatitis, pruritus, headache, malaise, dizziness, fever, mental confusion, disorientation, possible hallucinations, joint pain, and rarely anaphylactoid reactions. GI upset, abdominal pain, nausea, vomiting, and anorexia have also occurred occasionally with ethambutol. Peripheral neuritis, with numbness and tingling of the extremities, has been reported infrequently. Increased serum uric acid concentrations and precipitation of acute gout have occurred occasionally in patients receiving ethambutol and are probably the result of decreased renal clearance of urate. Transient impairment of liver function, as indicated by abnormal liver function test results, has also occurred. Cholestatic jaundice, which appeared to be caused by ethambutol, has been reported in at least one patient who received the drug both alone and in conjunction with streptomycin.

Precautions and Contraindications

Visual testing should be performed prior to initiating ethambutol therapy and then periodically during therapy with the drug. Testing should be done monthly in patients receiving more than 15 mg/kg daily. Examinations should include ophthalmoscopy, finger perimetry, and testing of color discrimination.

Patients developing adverse ocular effects during ethambutol therapy may show subjective visual symptoms either before or simultaneously with decreases in visual acuity.

All patients receiving the drug should be questioned periodically about blurred vision and other subjective visual symptoms and should be instructed to report to their physicians any such changes as soon as they are noticed. If substantial changes in visual acuity occur, ethambutol should be discontinued immediately.

Renal, hepatic, and hematopoietic tests should be performed periodically during long-term ethambutol therapy.

Ethambutol should be used with caution and in reduced dosage in patients with impaired renal function. The drug should also be used with caution in patients with ocular defects (e.g., cataracts, recurrent ocular inflammatory conditions, diabetic retinopathy) that make visual changes difficult to detect or evaluate; consideration should be given to whether the benefits of ethambutol therapy justify the possible ocular effects in these patients.

Ethambutol is contraindicated in patients with optic neuritis unless clinical judgment deems it necessary that the drug be used.

Ethambutol is also contraindicated in patients with known hypersensitivity to the drug.

Pediatric Precautions

The manufacturer states that ethambutol should not be used in children younger than 13 years of age. The American Thoracic Society (ATS), US Centers for Disease Control and Prevention (CDC), and Infectious Diseases Society of America (IDSA) state that ethambutol should be used with caution in children in whom it may be difficult to monitor visual acuity (e.g., those younger than 5 years of age). These experts state that ethambutol usually is recommended for use in such children only when the strain of Mycobacterium tuberculosis involved is known or suspected to be resistant to isoniazid or rifampin or when the child has adult-type (upper lobe infiltration, cavity formation) tuberculosis.

Pregnancy

Ethambutol has caused teratogenic effects in animals when used in high doses. Although safe use of the drugs during pregnancy has not been definitely established, ethambutol (combined with isoniazid or with isoniazid and rifampin) has been used to treat clinical tuberculosis in pregnant women and has not been reported to date to cause adverse effects on the fetus The manufacturer states that ethambutol should be used during pregnancy only when the potential benefits justify the possible risks to the fetus. The ATS, CDC, and IDSA state that ethambutol is considered safe for use in pregnant women.

Mechanism of Action

Ethambutol is bacteriostatic in action. Although the exact mechanism of action has not been fully elucidated, the drug appears to inhibit the synthesis of one or more metabolites in susceptible bacteria resulting in impairment of cellular metabolism, arrest of multiplication, and cell death. Ethambutol is active against susceptible bacteria only when they are undergoing cell division.

Spectrum

Ethambutol is a highly specific agent and is active only against organisms of the genus Mycobacterium. The drug is active in vitro and in vivo against M. tuberculosis, M. bovis, M. marinum, and some strains of M. kansasii, M. avium, M. fortuitum, and M. intracellulare. In vitro, the minimum inhibitory concentration (MIC) of ethambutol for most susceptible mycobacteria is 1-8 mcg/mL, depending on the culture media used.

Resistance

Natural and acquired resistance to ethambutol have been demonstrated in vitro and in vivo in strains of M. tuberculosis. In vitro, resistance to ethambutol appears to occur in a stepwise manner. Resistant strains of initially susceptible M. tuberculosis develop rapidly if ethambutol is used alone in the treatment of clinical tuberculosis. When ethambutol is combined with other antituberculosis agents in the treatment of the disease, emergence of resistant strains may be delayed or prevented. There is no evidence of cross-resistance between ethambutol and other antituberculosis agents currently available in the US.

Pharmacokinetics

Absorption

Approximately 75-80% of an oral dose of ethambutol hydrochloride is rapidly absorbed from the GI tract. Absorption is not substantially affected when the drug is administered with food. Following a single oral ethambutol hydrochloride dose of 25 mg/kg, peak serum ethambutol concentrations of 2-5 mcg/mL are attained within 2-4 hours; serum concentrations of the drug are undetectable 24 hours after the dose. There is no evidence that accumulation of the drug occurs when ethambutol doses of 25 mg/kg are given once daily in patients with normal renal function. Serum concentrations of the drug are higher and accumulation may occur when ethambutol is used in patients with impaired renal function.

An agar-diffusion microbiologic assay may be used to determine serum and urine concentrations of ethambutol. Although the technique of this assay has not been published, it is based on inhibition of M. smegmatis (ATCC 607) and specific information can be obtained from the manufacturer (Lederle).

Distribution

Ethambutol is widely distributed into most body tissues and fluids. Highest concentrations of the drug are found in erythrocytes, kidneys, lungs, and saliva; lower drug concentrations are found in ascitic fluid, pleural fluid, brain, and CSF. Peak intracellular concentrations of ethambutol in erythrocytes are about twice peak plasma concentrations and maintain this ratio for at least 24 hours after a single oral dose. In patients with meningitis, administration of an oral ethambutol hydrochloride dose of 25 mg/kg has produced peak CSF concentrations of the drug ranging from 0.15-2.0 mcg/mL.

Ethambutol crosses the placenta and is distributed into cord blood and amniotic fluid. Ethambutol is distributed into milk in concentrations approximately equal to plasma concentrations of the drug.

At a concentration of a 4.8 mcg/mL, ethambutol is approximately 8% bound to plasma proteins; at a concentration of 1.3 mcg/mL, the drug is approximately 22% bound to plasma proteins.

Elimination

The plasma half-life of ethambutol is approximately 3.3 hours in patients with normal renal function. The half-life is prolonged in patients with impaired renal or hepatic function. In patients with renal failure, the half-life may be 7 hours or longer.

Ethambutol is partially inactivated in the liver by oxidation to an aldehyde intermediate, 2,2-(ethylenediimino)-di-butyraldehyde, which is converted to the decarboxylic acid derivative, 2,2-(ethylenediimino)-di-butyric acid. Within 24 hours, approximately 50% of an oral dose of ethambutol hydrochloride is excreted in urine as unchanged drug, and 8-15% is excreted as inactive metabolites. Unabsorbed ethambutol is excreted in feces as unchanged drug. Approximately 20-22% of an oral dose of the drug is excreted in feces as unchanged drug.

Ethambutol is removed by peritoneal dialysis and to a lesser extent by hemodialysis.

Chemistry and Stability

Chemistry

Ethambutol hydrochloride is a synthetic antituberculosis agent. The drug occurs as a white, crystalline powder and is freely soluble in water and soluble in alcohol. The drug has pKas of 6.1 and 9.2.

Stability

Ethambutol hydrochloride tablets should be protected from light, moisture, and excessive heat and should be stored in well-closed containers at 15-30°C.

Proprietary Preparations

Australia: Myambutol; Austria; Etibi; Myambutol; Belgium: Myambutol; Canada: Etibi; Czech Republic: Sural; Denmark: Myambutol; Finland: Oributol; France: Myambutol; Germany; EMB; Myambutol; Greece: Dexambutol; Myambutol; Hong Kong; EMB; Myambutol; Hungary: Sural; India: Combutol; Myambutol; Mycobact; Mycobutol; Rifacom E-Z; Themibutol; Tibitol; Indonesia: Arsitam; Bacbutol; Cetabutol; Corsabutol; ETH Ciba; Parabutol; Santibi; Tibigon; Tibitol; Italy: Etapiam; Miambutol; Mexico: Apo-Probutol; Dovalem; Myambutol; Tambutec; The Netherlands: Myambutol; New Zealand: Myambutol; Philippines: Danbutol; Odetol; Portugal; Turresis; Russia: Ebutol; Ethambusin; Up-butol; South Africa: Purderal; Singapore: E-Butol; Spain: Myambutol; Sweden: Myambutol; Switzerland: Myambutol; Thailand: Conbutol; Etham; Ethbutol; Lambutol; Myambutol; Myrin; Myrin; Servambutol; Tobutol; Turkey: Miambutol; United States of America (US and USA): Myambutol.

Multi-ingredient

Austria; Myambutol-INH; Denmark: Rimstar; Finland: Rimstar; Germany; EMB-INH; Myambutol-INH; India: Akt-3; Akt-4; Bicox-E; Combunex; Coxina-3; Coxina-4; Cx-4; Cx-5; Gocox-4; Inabutol Forte; Myconex; RHZ-Plus; Rifa E; Wokex-3; Wokex-4; Xeed-3E; Xeed-4; Indonesia: bacbutlNH; Erabutol Plus; Meditam-6; Mycothambin-INH; Niazitol; Pulna; Rimstar; Santibi Plus; Italy: Etanicozid B6; Miazide B6; Mexico: Myambutol-INH; Philippines: 4D; Continukit; Continukit Plus; Econokt; Econokit-MDR; Ethamizid; Ethi 400; Fixcom3; Fixcom4; Myrin; Myrin; Quadtab; Rimstar; Sthamizide; SVM-Polypac-A; Tres; Tritab; Viper; Russia: Isocomb; Lomecomb; Phthizoetham; Protiocomb; Repin B; Rimstar 4-FDC; South Africa: Myrin Plus; Myrin; Rifafour; Rimstar; Spain: Rimstar; Sweden: Rimstar; Switzerland: Myambutol-INH; Thailand: Rifafour; Rimstar

Synonyms of Ethambutol:

Aethambutolum, D-Ethambutol, EMB, Etambutol [INN-Spanish], Etambutolo [DCIT], Ethambutol Dihydrochloride, Ethambutol HCL, Ethambutol Hydrochloride, Ethambutol, Racemic Mixture, Ethambutolum [INN-Latin]

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Therapeutic classes of Ethambutol:

Antitubercular Agents

Delivery

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