RIFAMPICIN, ISONIAZID & PYRAZINAMIDE DISPERSIBLE TABLETS 75MG, 50MG AND 150MG

Indications

• Tuberculosis (TB)
• Leprosy
• Other infections caused by susceptible strains of bacteria

Dosage

Children: Refer to the BNF for Children for appropriate dosing recommendations based on the child's age and weight.

Adults: Refer to the BNF for specific dosing guidelines based on the condition being treated.

Mechanism of action

Rifampin works by binding to the beta-subunit of microbial DNA-dependent RNA polymerase (RNAP), inhibiting the enzyme and thus impeding RNA synthesis. It reduces the affinity of RNAP for short RNA transcripts and forms a stable drug-enzyme complex, suppressing the initiation of RNA chain formation. Notably, rifampin has no activity against mammalian RNAP, making it selective for bacterial processes.

Pharmacodynamics

Rifampin is characterized by its broad-spectrum antimicrobial activity and is particularly effective against _Mycobacterium tuberculosis_. It demonstrates bactericidal effects, which means it kills bacteria instead of merely inhibiting their growth. This makes it a critical component in the treatment regimen for tuberculosis and other bacterial infections.

Pharmacokinetics

Rifampin is absorbed well from the gastrointestinal tract, with peak plasma concentrations typically reached within a few hours after oral administration. It is extensively metabolized in the liver, primarily via cytochrome P450 enzymes, and has a relatively long half-life. The drug is excreted mainly in bile, with a small percentage eliminated in urine. Its pharmacokinetic profile allows for once-daily dosing in many cases, aiding patient compliance.

Contra-indications

• Hypersensitivity to rifampicin or any of its components
• Concurrent use with protease inhibitors such as darunavir, indinavir, or nelfinavir
• Severe hepatic impairment

Adverse effects

• Hepatotoxicity
• Gastrointestinal disturbances (nausea, vomiting, diarrhea)
• Rash and other allergic reactions
• Discoloration of body fluids (e.g., urine, sweat, tears)
• Flu-like syndrome
• Thrombocytopenia
• Leukopenia

Interactions

• May reduce the effectiveness of oral contraceptives
• Increases metabolism of drugs metabolized by CYP450 enzymes
• May interact with anticoagulants, antiepileptics, and certain antiretrovirals
• Concomitant use with drugs such as ketoconazole and erythromycin may increase risk of liver toxicity

Precautions

• Monitor liver function tests regularly
• Use with caution in patients with a history of liver disease
• Assess for signs of hypersensitivity reactions
• Consider potential interactions with other medications

Pregnancy

Rifampin is categorized as a pregnancy category C drug. It should be used during pregnancy only if the potential benefit justifies the potential risk to the fetus.

Breast-feeding

Rifampin is excreted in breast milk; caution should be exercised when administering to nursing mothers.

Storage

Store in a cool, dry place, away from direct sunlight and moisture. Keep out of reach of children.

Formulations

• Oral capsules (150 mg, 300 mg)
• Oral suspension (10 mg/mL)
• Injectable solution (rifampicin for injection)

Mechanism of action

Rifampin works by binding to the beta-subunit of microbial DNA-dependent RNA polymerase (RNAP), thereby inhibiting the enzyme and impeding RNA synthesis. It reduces the affinity of RNAP for short RNA transcripts. It has no activity against the mammalian RNAP enzyme. Although rifampin is most active during cell multiplication ... /it/ appears to have some effect on resting cells. Electron microscopy has revealed changes in cytoplasm and disappearance of ribosomes in tubercle bacilli exposed to rifampin, indicating inhibition of DNA-dependent RNA polymerase. Rifampin inhibits DNA-dependent RNA polymerase of mycobacteria and other microorganisms by forming a stable drug-enzyme complex, leading to suppression of initiation of chain formation (but not chain elongation) in RNA synthesis. More specifically, the beta subunit of this complex enzyme is the site of action of the drug, although rifampin binds only to the holoenzyme. Nuclear RNA polymerase from a variety of eukaryotic cells does not bind rifampin, and RNA synthesis is correspondingly unaffected. While rifampin can inhibit RNA synthesis in mammalian mitochondria, considerably higher concentrations of the drug are required than for the inhibition of the bacterial enzyme. High concentrations of rifamycin antibiotics also inhibit viral DNA-dependent RNA polymerases and reverse transcriptases. Rifampin is bactericidal for both intracellular and extracellular microorganisms. Developmental expression of CYPlAl, CYPlA2 and CYP3A6 in the rabbit have been studied. Cytochromes P450IAl, P450IA2 and P450IIIA6 exhibited comparable patterns of developmental expression. Present at low level (less than 0.05 mnol/ng) in the new born animal up to week 3, these proteins sharply accumulated between weeks 3 and 4 to reach a maximum by week 4 (P450IAl, 0.2 nmol/mg; P450IA2, 0.8 nmol/ng; P450IIIA6, 0.12 nmol/mg) and decr in the adult (P450IAl, 0.2 nmol/mg; P450IA2, 0.4 mnol/mg; P450IIIA6, 0.09 nmol/mg). Cytochromes P450IAl and P450IA2 were not expressed in the untreated fetus. Onset of CYP3A6 gene expression occurred at day 30 of gestation and both transcription and mRNA accumulation were transplacentally inducible by rifampicin only shortly before birth, i.e. after treatment of the females between days 28 and 30 of gestation. Both long (1.85 kb) and short (1.7 kb) mRNA transcripts were expressed in untreated or rifampicin treated fetuses. CYP3A6 gene expression was also induced by rifampicin in l week old and 2 week old animals. Developmental expression of CYPlAl and CYPlA2 genes was shown to be closely related to the diet change accompanying weaning which occurs at weeks 3-4. In animals subjected to either delayed (week 6) or early (week 2) weaning, sharp accumulation of messages, proteins and related activities were delayed or anticipated accordingly with respect to normal weaning. Artificially scheduled weaning gave similar results when repeated with biological grade lucern (grown in the absence of chemical fertilizers, pesticides) ... the main constituent of commercial rabbit chow. While CYP3A6 gene expression could be brought forward by early weaning at week 2, both message and protein did not exhibit incr accumulation after delayed weaning at week 6, and remained at the low level of the new born animal. Treatment of l week old and 2 week old animals with triiodothyronine or of 3 week old animals with propylthiouracil, an antithyroid factor, did not modify the normal pattern of developmental expression of genes CYPlAl, CYPlA2 and CYP3A6. ...

Pharmacodynamics

Rifampin is an antimicrobial agent with bactericidal effects and a broad-spectrum activity. It is active against intracellular and extracellular _Mycobacterium tuberculosis_.