Repeated oral doses of activated charcoal and the clearance of tobramycin, a non-absorbable drug

Factors which determine the ability of activated charcoal to increase systemic drug clearance include adsorption characteristics, the extent of back diffusion, and biliary excretion into the gut. Orally absorbed drugs diffuse back into the gut, but it is not known whether non-absorbed agents also diffuse back. Tobramycin was studied with a highly activated charcoal to determine whether this occurs. Five volunteers received a single IV dose of tobramycin on two occasions. Using a randomized, crossover design, subjects received 10 g of activated charcoal (as Super-Char^R suspension) 2 hr prior, and at 0, 2, 6, and 8 hr after tobramycin administration during one of the study days. In vitro, tobramycin adsorbed to charcoal at pH 5.6, but not at pH 2.6. A 20:1 charcoal:tobramycin ratio resulted in 34.9% of tobramycin adsorbed to activated charcoal. Blood (0-12 hr) and urine (0-24 hr) were collected and tobramycin concentrations determined. Urinary tobramycin recovery, renal and total drug clearance, half-life, and volume of distribution were calculated. There were no differences in these parameters between the two groups. We conclude that activated charcoal has no effect on tobramycin distribution or elimination in normal volunteers, and that back diffusion does not occur with this drug.