Plasma and saliva concentrations of rifampicin in man after oral administration.

The saliva and plasma concentrations of rifampicin were determined following oral administration of 600 mg of the drug to healthy human subjects. Rifampicin is a zwitterion with PKa's of 1.7 and 7.9. The saliva-plasma concentration ratios calculated on the basis of the PKa of 1.7 was 1.6346. The saliva concentrations were much higher than the MICs of a variety of organisms, indicating the possible usefulness of rifampicin in the treatment of susceptible oropharyngeal and nasopharyngeal pathogens. After 24 hrs, when rifampicin was completely absent in the urine, the saliva and plasma concentrations also had fallen almost to zero.