Rifampicin and isoniazid are active bactericidial antituberculosis drugs which are particularly active against the rapidly growing extracellular organisms and also have bactericidal activity intracellularly. Rifampicin has activity against slow- and intermittently-growing M. tuberculosis.
Rifampicin inhibits DNA-dependent RNA polymerase activity in susceptible cells. Specifically, it interacts with bacterial RNA polymerase but does not inhibit the mammalian enzyme. Cross-resistance to rifampicin has only been shown with other rifamycins.
Isoniazid acts against actively growing tubercle bacilli.
Rifampicin is readily absorbed from the stomach and the duodenum. Peak serum concentrations of the order of 10 µg/ml occur about 2-4 hours after a dose of 10mg/kg body weight on an empty stomach.
In normal subjects the biological half-life of rifampicin in serum averages about 3 hours after a 600mg dose and increases to 5.1 hours after a 900mg dose. With repeated administration, the half-life decreases and reaches average values of approximately 2-3 hours. At a dose of up to 600 mg/day, the half-life does not differ in patients with renal failure and consequently, no dosage adjustment is required.
After absorption, rifampicin is rapidly eliminated in the bile and an enterohepatic circulation ensues. During this process, rifampicin undergoes progressive deacetylation, so that nearly all the drug in the bile is in this form in about 6 hours. This metabolite retains essentially complete antibacterial activity. Intestinal reabsorption is reduced by deacetylation and elimination is facilitated. Up to 30% of a dose is excreted in the urine, with about half of this being unchanged drug. Absorption of rifampicin is reduced when the drug is ingested with food.
Rifampicin is widely distributed throughout the body. It is present in effective concentrations in many organs and body fluids, including cerebrospinal fluid. Rifampicin is about 80% protein bound. Most of the unbound fraction is not ionized and therefore is diffused freely in tissues.
After oral administration isoniazid produces peak blood levels within 1 to 2 hours which decline to 50% or less within 6 hours. Ingestion of isoniazid with food may reduce its absorption. It diffuses readily into all body fluids (cerebrospinal, pleural and ascitic fluids), tissues, organs and excreta (saliva, sputum and faeces). From 50 to 70% of a dose of isoniazid is excreted in the urine in 24 hours.
Isoniazid is metabolised primarily by acetylation and dehydrazination.. The rate of acetylation is genetically determined.
Pharmacokinetic studies in normal volunteers have been shown that the two ingredients in rifampicin/isoniazid combination tablet have comparable bioavailability whether they are given together as individual dose forms or as combination.
No further data.
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