title: Bioavailability of rifampicin following concomitant administration of ethambutol or isoniazid or pyrazinamide or a combination of the three drugs.
abstract: OBJECTIVE
  Poor bioavailability of rifampicin (R) in combination with other anti-tuberculosis drugs such as isoniazid (H), pyrazinamide (Z), and ethambutol (E) is a subject of much concern for the last few decades. This could be due to an interaction between R and other drugs. An investigation was therefore undertaken to examine the bioavailability of R in the presence of H, Z and E or a combination of the three drugs.
  
  
  METHODS
  The study included eight healthy volunteers, each being investigated on four occasions at weekly intervals once with R alone and with three of the four combinations on the three remaining occasions. A partially balanced incomplete block design was employed and the allocation of R or the drug combinations was random. Plasma concentrations of R at intervals up to 12 h were determined by microbiological assay using Staphylococcus aureus as the test organism. The proportion (%) dose of R as R plus desacetyl R (DR) in urine excreted over the periods 0-8 and 8-12 h was also determined. Bioavailability was expressed as an index (BI) of area under time concentration curve (AUC) calculated from the plasma concentrations or proportion of dose of R excreted as R plus DR in urine with the combinations to that with R alone.
  
  
  RESULTS
  The bioavailability indices based on AUC were 0.96 with RE, 0.76 with RH, 1.08 with RZ and 0.65 with REHZ. The indices based on urine estimations (0-8 h) were similar, the values being 0.94, 0.84, 0.94 and 0.75, respectively. A second investigation revealed that the decrease of bioavailability of R with H was not due to the excipients present in H tablets.
  
  
  CONCLUSIONS
  Isoniazid alone or in combination with E and Z reduces the bioavailability of R. Urinary excretion data offer a simple and non invasive method for the assessment of bioavailability of R.
authors.affiliation: Tuberculosis Research Centre (ICMR), Chennai, India.
authors.forename: Chandra
authors.initials: C
authors.lastname: Immanuel
authors.forename: Prema
authors.initials: P
authors.lastname: Gurumurthy
authors.forename: Geetha
authors.initials: G
authors.lastname: Ramachandran
authors.forename: P
authors.initials: P
authors.lastname: Venkatesan
authors.forename: V
authors.initials: V
authors.lastname: Chandrasekaran
authors.forename: R
authors.initials: R
authors.lastname: Prabhakar
mesh_terms: D000293:Adolescent; D000328:Adult; D000904:Antibiotics, Antitubercular / Q000493:pharmacokinetics*; D019540:Area Under Curve; D001835:Body Weight; D004977:Ethambutol / Q000494:pharmacology*; D006801:Humans; D007538:Isoniazid / Q000494:pharmacology*; D008297:Male; D008875:Middle Aged; D011718:Pyrazinamide / Q000494:pharmacology*; D012293:Rifampin / Q000493:pharmacokinetics*; D013211:Staphylococcus aureus / Q000378:metabolism; D013997:Time Factors
chemical_list: D000904:Antibiotics, Antitubercular; D011718:Pyrazinamide; D004977:Ethambutol; D007538:Isoniazid; D012293:Rifampin