Pharmacokinetics and pharmacodynamics of 4-aminopyridine in anesthetized dogs

The pharmacokinetics and pharmacodynamics of 4-aminopyridine (4-AP), a drug which antagonizes nondepolarizing neuromuscular blockade, were studied in seven anesthetized dogs. Using a constant infusion of pancuronium, force of the anterior tibialis contraction in response to stimulation of the sciatic nerve was depressed to 10% of the control tension (90% depression of twitch tension). After 20 min of steady state, 4-AP (1.0 mg/kg) was administered i.v. Serum, urine and bile samples were analyzed for 4-AP concentration at several intervals for 10 hr after administration of 4-AP, using a sensitive high-performance liquid chromatographic assay (1 ng/ml). Serum data best fit a three-compartment pharmacokinetic model. The volume of the central compartment was 412 +/- 352 ml/kg (mean +/- S.D.) and the volume of distribution at steady state was 2517 +/- 363 ml/kg. Initial half-lives were 1.1 +/- 0.7 and 25.4 +/- 11 min. The terminal elimination half-life was 125 +/- 23 min and total clearance was 21 +/- 4 ml/kg/min. Of the injected dose, 60 +/- 9% was recovered in the urine and only 0.01 +/- 0.01% of the dose was recovered in the bile in 10 hr. Inasmuch as renal clearance of 4-AP exceeded glomerular filtration rate we conclude that 4-AP undergoes tubular secretion into the urine. The pharmacodynamic results included an onset time of 14 +/- 8 min, peak effect (maximum percentage of antagonism of twitch tension depression) 97 +/- 27% and duration of action 219 +/- 54 min. We conclude that 4-AP has a longer serum elimination half-life and a longer and more variable duration of action than other antagonists (i.e., neostigmine and pyridostigmine) of nondepolarizing neuromuscular blockade.