New sodium hydroxide digestion method for measurement of renal tobramycin concentrations

Because of the known propensity of the cationic aminoglycosides to interact with a variety of anionic tissue constituents, it was postulated that present elution methods fail to detect a substantial proportion of the aminoglycoside that accumulates in the kidney. After preliminary experiments documented the pH and temperature stability of tobramycin, a new sodium hydroxide (NaOH) tissue digestion method was applied to renal tissue collected from rats given tobramycin at 80 mg/kg (body weight) per day in two divided doses for 3, 7, 10, 14, and 17 days. Compared with elution with buffer from the renal homogenate, digestion with 1.0 N NaOH at 70 degrees C for 15 min significantly increased the amount of assayable tobramycin (P less than 0.001). The absolute increase varied between 37 and 96%, depending on the number of days of in vivo drug administration. A single trichloroacetic acid (TCA) treatment of the homogenate altered the amount of assayable tobramycin over a range which varied from a decrease of 6% to an increase of 32%. After TCA treatment, it was possible to increase the amount of measurable tobramycin by 17 to 22% by treatment of the TCA-induced precipitate with 1.0 N NaOH. Digestion of renal homogenates with 1.0 N NaOH significantly increases the amount of tobramycin detectable by immunoassay.