Ocular pharmacokinetics of tobramycin. A comparative radioimmunoassay and microbiological assay

The ocular pharmacokinetics of tobramycin were studied in human and rabbit eyes following topical and subconjunctival administration in humans and following intravitreal administration in rabbit eyes. Each aqueous and vitreous sample was analysed by radioimmunoassay and microbiological assay. Topical instillation of 0.3% eyedrops by a loading dose schedule provided rapid penetration of the drug into the aqueous humour with a peak titre of 3.24 mg/l (RIA) at 2-hour intervals and therapeutic levels up to 6 hours. A subconjunctivally administered standard dose of 20 mg/0.5 ml of the drug also showed rapid penetration in therapeutically effective concentration with a peak titre of 22.34 mg/l (RIA) and 16.80 mg/l (MBA) at 2-hour intervals and therapeutic levels of the drug remained up to 20 hours later. A study of vitreous clearance following the administration of a single intravitreal injection in rabbit eyes showed bioavailability of the drug at therapeutically effective levels for more than 72 hours. Mostly two, but sometimes only one sample at each time interval, were studied. Radioimmunoassay and microbiological assay results showed excellent correlation, with correlation coefficient > 0.90 at 1.0% level of probability.