Metabolism, Elimination, and Pharmacokinetics of the Fish Anesthetic Benzocaine

Abstract

Benzocaine (ethyl para-aminobenzoate), a mammalian local anesthetic, is an effective general anesthetic agent for fish when administered by bath exposure. Its proposed use in aquaculture is to sedate fish and reduce mortality during transport and/or spawning operations in hatcheries or during stocking operations. Benzocaine is metabolized into at least three compounds by acetylation and hydrolysis. Benzocaine and its acetylated metabolite are rapidly eliminated across the gills while the more polar de-ethylated and de-ethylated-acetylated metabolites are excreted at slower rates primarily in the urine. Benzocaine and the metabolite, acetylbenzocaine, are rapidly lost from the edible fillet tissue after prolonged exposures to solutions of benzocaine. During an exposure, benzocaine is rapidly distributed into the highly perfused tissues and rapidly eliminated. Elimination of benzocaine from plasma is characterized by a very rapid initial phase followed by a relatively slow terminal phase. During the initial elimination phase, the majority of benzocaine is lost from the fish while only a small portion of the benzocaine is lost during the terminal elimination phase. Benzocaine pharmacokinetic parameters of uptake, clearance, and metabolic clearance are influenced by temperature, i.e., the pharmacokinetic parameters increase with increasing water temperature. The research presented in this manuscript indicates that benzocaine has the characteristics appropriate for a fish anesthetic with a short withdrawal time.