Isoniazid Pharmacokinetics in the Presence of Ofloxacin and Norfloxacin Antibiotics.

The in vivo effects of norfloxacin (NXC) and ofloxacin (OXC) on isoniazid (INH) pharmacokinetics were investigated in 5 apparently healthy volunteers aged 18-39 years after an informed consent. The study was carried out in 3 phases with an interval drug wash out period of at least 1 week in between the phases. In phase 1 (INH alone), subject received 300 mg (usual adult dose) of INH. In phase 2 (INH + OXC), 300 mg of INH was coadministered with 200 mg of OXC, and in phase 3 (INH + NXC) each received 300 mg of INH together with 400 mg of NXC after 1-week drug wash period. Drugs were taken orally with 350 mL of water after an overnight fast, and the subject fasted 3 hours after drug. Plasma, saliva, and urine concentration of INH were predetermined at zero hour, then hourly until the eighth hour, 12 hours, 24 hours, and finally at 48 hours. The urine samples were further collected at 72 hours after drug(s) administration using validated methods. Various pharmacokinetics parameters were calculated. Various pharmacokinetic parameters of INH significantly differed when administered alone or in combination with OXC or with NXC. The mean saliva to plasma ratio of INH concentration was 0.14. The bioavailability indices of INH in the saliva and plasma were similar in all the groups. NXC and OXC reduced the extent and rate of absorption of INH. The determination of INH levels in saliva may be useful in therapeutic drug monitoring and pharmacokinetic studies.