Source: Medicines & Healthcare Products Regulatory Agency (GB) Revision Year: 2018 Publisher: Fannin (UK) Ltd., DCC Vital, Westminster Industrial Estate, Repton Road, Measham, Swadlincote, Derbyshire, DE12 7DT, England
ATC Code: J04AK02 – Other drugs for treatment of tuberculosis
Ethambutol is bacteriostatic. It is effective against Mycobacterium tuberculosi and M.bovis with an MIC of 0.5–8µg per ml. The exact mechanism of action is unknown. While it has activity against some atypical mycobacteria including M.Kansasii, activity against other micro-organisms has not yet been reported.
It is effective against tubercle bacilli resistant to other tuberculostatics.
Cross-resistance has not yet been reported. Primary resistance to ethambutol is uncommon but resistant strains of M.tuberculosis are readily produced if ethambutol is used alone.
Ethambutol is readily absorbed after oral administration and this absorption is not significantly impaired by food.
After a single oral dose of 25 mg/kg bodyweight, within 4 hours peak plasma concentrations of up to 5µg/ml are obtained, by 24 hours the concentration decreases to less than 1µg/ml. Ethambutol readily diffuses into red blood cells and into the cerebrospinal fluid when the meninges are inflamed. It has also been reported to cross the placenta.
Most of a dose is excreted unchanged in the urine and up to 20% in the faeces, within 48 hours. From 8–15% of a dose appears in the urine as inactive metabolites.
Ethambutol has been shown to be teratogenic in pregnant mice and rabbits when given in high doses. When pregnant mice or rabbits were treated with high doses of ethambutol hydrochloride, fetal mortality was slightly but not significantly (P>0.05) increased. Female rats treated with ethambutol hydrochloride displayed slight but insignificant (>0.05) decreases in fertility and litter size. In foetuses born of mice treated with high doses of ethambutol hydrochloride during pregnancy, a low incidence of cleft palate, exencephaly and abnormality of the vertebral column were observed. Minor abnormalities of the cervical vertebra were seen in the newborn of rats treated with high doses of ethambutol hydrochloride during pregnancy. Rabbits receiving high doses of ethambutol hydrochloride during pregnancy gave birth to two foetuses with monophthalmia, one with a shortened right forearm accompanied by bilateral wrist-joint contracture and one with hare lip and cleft palate.
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