Kinetics of Drug Action in Disease States. XXXIX. Effect of Orally Administered Activated Charcoal on the Hypnotic Activity of Phenobarbital and the Neurotoxicity of Theophylline Administered Intravenously to Rats with Renal Failure

The central nervous system (CNS) sensitivity to the hypnotic (general anesthetic) action of pheno-barbital and to the neurotoxic (convulsive) action of theophylline is greater in rats with acute renal failure than in normal animals, consistent with clinical observations. In the case of phenobarbital, this increased sensitivity can be produced in normal rats by infusion of a solution of the lyophilized dialysate of serum from rats with renal failure. It was hypothesized that the relevant constituent(s) of this dialysate may circulate between the blood and the intestinal lumen and that it (they) can be adsorbed by orally administered activated charcoal and thereby removed from the body. If so, treatment of renal failure rats with activated charcoal should partly reverse the increased CNS sensitivity to phenobarbital and to other drugs similarly affected. Accordingly, rats with renal failure produced by bilateral ligation of ureters were given an aqueous suspension of activated charcoal, about 1 g per kg body weight, orally every 8 hr for six doses. Uremic controls received equal volumes of water. About 2 hr after the last dose, the animals were infused i.v. with phenobarbital to onset of loss of righting reflex or with theophylline to onset of maximal seizures. In the phenobarbital study, charcoal treatment partly reversed the hypothermia associated with renal failure and caused a reduction of creatinine and total bilirubin concentrations in serum. The cerebrospinal fluid (CSF) concentration of phenobarbital at onset of loss of the righting reflex was significantly higher in charcoal treated rats than in their controls. In the theophylline experiment, charcoal treatment had no significant effect on the measured biochemical variables but caused a large increase in the dose and concentrations of theophylline required to produce maximal seizures. In both experiments, administration of activated charcoal caused a reversal of the hyperalgesia associated with renal failure, as determined before drug administration by tail flick latency. These results are consistent with the hypothesis that oral administration of activated charcoal can cause a reduction in the concentration of the circulating endogenous substance(s) that alters the pharmacodynamics of certain drugs in renal failure.