Abstract

Gentamicin was administered IV (4 mg/kg), IM (4 mg/kg), and PO (40 mg/kg) to catheterized, restrained, adult channel catfish. One ml blood samples were taken by catheter for 96 hr following drug administration, and were analyzed for gentamicin concentrations by RIA. Following IV administration, the following pharmacokinetic parameters were calculated: t1/2(a) = 0.39 hr, t1/2(b) = 20.9 hr, Vd(area) = 0.116 L/kg, Vd(ss) = 0.08 L/kg, CIB = 0.065 L/kg/min, and MRT = 20.5 hr. Following IM injection, the following parameters were calculated: t1/2 (abs) = 0.88 hr, T (lag) = 0.2 hr, and F ~ 0.87. Kinetic parameters could not be calculated following oral administration due to very poor absorption of gentamicin (F = 0.029). Ninety-six hr after IM administration, liver, posterior kidney, and muscle were sampled to determine tissue residue concentrations of gentamicin. The following concentrations were determined: liver 7.01 µg/g, kidney 105.7 µg/g and muscle 0.13 µg/g.

Introduction

Gentamicin is a broad-spectrum aminoglycoside antibiotic which is effective against many gram-negative and gram-positive organisms, especially Pseudomonas.^1-3 Blood serum concentrations of 5 mcg/ml will inhibit most organisms, and 0.5 mcg/ml has inhibited E. coli isolated from calves.4 The frequency and route of administration of gentamicin is based partially on the duration of therapeutic blood levels and partially on the severity of the disease. Intramuscularly-administered gentamicin will produce peak blood levels in 0.5 to 1 hour.⁵

In the absence of controlled experiments to establish the pharmacokinetics of antibiotics in fish, drug therapy has often resulted from personal experience and case studies. In treating infections in large volume operations such as the cultured channel catfish industry, antibiotics are generally administered by addition to feed or water due to ease and efficiency of administration. One distinct disadvantage to dosing in food and water is inconsistent delivery of a therapeutically effective dose of antibiotic which could lead to treatment failures or the enhancement of bacterial resistance development.

Due to its possible use in the catfish industry, the Consortium for Research on Minor Use Animal Drugs, a grant funded by the Food and Drug Administration, chose gentamicin as one of the model compounds for comparative pharmacokinetic investigation in minor use food animal species including goats, turkeys, rabbits, and ducks as well as channel catfish. Specifically, this investigation was designed to establish pharmacokinetic values following IV and oral administration.

Materials and Methods

Adult channel catfish of either sex, weighing approximately 1 kg each were used. The fish were acclimated for at least 14 days prior to use in the study. Water temperature was maintained at 22°C and oxygen concentration was regulated at 7 ppm throughout the experimental period.

The catfish were catheterized for chronic blood sampling by a technique developed in our laboratory.6 Briefly, the catfish were anesthetized in an 80L aquarium using 3-aminobenzoic acid ethyl ester (MS-222) at a concentration of 1:10,000. When anesthetized, a catheter was placed in the dorsal aorta using a cather-inside-needle arrangement. After removing the needle, the catheter was sutured in place. The fish were placed in custom designed plexiglas chambers supplied with temperature and oxygen controlled water in a flow-through design. The blood collection catheter was passed through a rubber stopper to the outside of the chamber for sampling with minimal stress to the fish.

Gentamicin sulfate was administered IV or IM at a dosage rate of 4 mg/kg. One ml blood samples were taken into heparinized collection tubes at 0, 0.25, 0.5, 1, 2, 4, 8, 12, 24, 36, 48, 72, 84, and 96 hr after administration of the drug. The plasma was harvested and frozen at -70C following centrifugation (1100 xg). At the termination of the study, the catfish were sacrificed by administering an overdose of 3-aminobenzoic acid ethyl ester and liver, posterior kidney, and muscle samples were collected for residue concentration determination. The concentrations of gentamicin in plasma and tissues were determined by a commercially available radioimmunoassay technique. (Antibodies Inc., Davis, CA.)

Pharmacokinetic calculations were based on a 2-compartment open model for the IV study, and first-order absorption following IM administration. A regression technique weighted by the factor 1/Y² was used to determine alpha and beta values for the curve of best fit. Individual values of A, B, alpha, and beta were calculated, meaned, and reported as mean +/- SID. Other pharmacokinetic parameters including VD(area), Vd(ss), CIB, MIRT, tl/2(abs), and t(lag) were determined using standard calculations.⁷

Results

Pharmacokinetic values for the IV and IM studies are presented in Table 1. A 2­-compartment open model was used to best describe the resulting plasma concentration versus time values. Values are reported as mean ± SID where appropriate.

Table 1. Pharmacokinetic values for IV and IM administration of gentamicin (4 mg/kg)

The tissue concentrations of gentamicin in liver, posterior kidney, and muscle at the end of the 96-hour study are presented in Table 2. These values are reported as mean ug/g concentration ± SID.

Table 2. Tissue concentrations of gentamicin 96 hrs after IM administration of 4 mg/kg (ug/g)

Discussion

To perform accurate pharmacokinetic modeling of a drug, nonstressful scheduled blood sampling must be done to ensure reliability of the kinetic model chosen. In this study, the cannulation and confinement methods utilized prevented excess movement while permitting unrestricted gill movement. The cannulation technique permitted blood sampling from outside the chamber so that the fish did not have to be stressed by capture at each sampling time. Also to minimize variability, water temperature and dissolved oxygen concentrations were carefully controlled in the conditioning tank (2 weeks prior to experimental technique) and the experimental chamber. Studies in other species of fish have shown variation in body fluid volumes following water temperature changes.(8)

The elimination half-life is approximately 50 times greater than the distribution half-life. This indicates very slow elimination of gentamicin from the channel catfish. The elimination half-life of approximately 21 hr is very long when compared to other species such as calves (1 day - 2.5 hr; 5 days - 2 hr; 15 days - 1.8 hr)9, cattle (1.2 hr)(9), pigs (1.7 hr)(10), and horses (2.2 - 3 hr).(11) In another study, the elimination half-life in channel catfish was estimated at about 13 hours.(12) However, in the Setser study, the fish were not confined, and injections and samplings were performed by repeated intracardiac injections.

Ninety-six hours after IM injection of gentamicin, residues of drug could still be detected in kidney, liver, and muscle tissue. Since gentamicin is known to accumulate in renal parenchymal tissue in other species of animals, it was no surprise to find the highest concentration of gentamicin in fish tissue was in the posterior kidney. Gentamicin residues in kidney tissue were 15 times greater than liver, and about 1000 times greater than in muscle. Therefore, posterior kidney would be the tissue of choice for residue sampling it gentamicin had been used to treat the channel catfish.

References

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