Pharmacokinetics of gentamicin in perilymph and endolymph of the rat as determined by radioimmunoassay.

The kinetics of gentamicin in the inner ear fluids of the rat were studied with a sensitive radioimmunoassay. Continuous infusion over a broad range (7.5--150 micrograms/min) was used to obtain equilibrium. The lowest rate of infusion produced plasma concentrations after 45 min that were within the range of peak serum levels desirable in humans. The purity of perilymph and endolymph was ensured by measurement of sodium and potassium concentrations. The concentration of gentamicin in perilymph was linearly related to its concentration in plasma, which depended on the rate of infusion. After six days of constant infusion of gentamicin at 15 microgram/min, the concentrations in plasma, perilymph, and endolymph averaged 25.8, 5.1, and 1.2 micrograms/ml, respectively, for five of 11 rats. Gentamicin cleared from the perilymph with a half-life of 3 hr.