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Gabapentin [1-(aminomethyl)cyclohexaneacetic acid, Neurontin], is a new anticonvulsant used as adjunctive therapy in the treatment of partial seizures in humans not controlled with standard antiseizure drugs, and it has also been used in veterinary medicine. In performance horses, gabapentin is listed as a class 3 performance-enhancing substance by the Association of Racing Commissioners International, and thus is considered to have the potential to influence the outcome of races. Therefore, we developed and validated a sensitive gas chromatographic-mass spectrometric (GC-MS) method for gabapentin detection. Gamma-aminobutyric acid-d(2) (GABA-d(2)) was used as an internal standard during solid-phase extraction; lacking the cyclohexyl ring of gabapentin, GABA-d(2) formed a lactam structure to only a minor extent. Gabapentin, on the other hand, readily formed a lactam on thermal exposure during trimethylsilyl-derivatization and/or GC analysis; electrospray-ionization MS was employed to verify that the original compound was present as the expected 171 m.w. compound. Extraction efficiency for the assay was about 60%, and a curvilinear standard curve ranging from 50 ng/mL to 3000 ng/mL provided excellent within-run and between-run coefficients of variation and accuracies over a range of low, medium, and high values. The limit of detection, defined as the concentration calculated from the mean response at zero concentration plus two times the standard deviation, was calculated at 7.6 ng/mL; the limit of quantitation, defined as the concentration calculated from the mean of the zero responses plus five times the standard deviation, was calculated at 17 ng/mL. This method will enable accurate quantification of gabapentin in equine biological fluids for use in both pharmacokinetic and forensic studies.

The pharmacokinetics of levamisole was studied in 20 broiler breeder chickens (chickens that give eggs to breed broilers). A single dose of levamisole (40 mg/kg) was administered orally or intravenously to chickens before the onset of egg production, prelay (age = 22 weeks), and repeated at the peak of egg production (age = 32 weeks). A high-pressure liquid chromatographic with ultraviolet detection method (HPLC-UV) was used for quantification of levamisole in plasma. Using compartmental analysis, levamisole followed a three-compartmental open model with mean values of alpha = 0.1224 and 0.4968, beta = 0.01663 and 0.01813, gamma = 0.002 and 0.002/min at the prelay and at the peak of egg production periods, respectively. The mean values for volume of distribution at steady state (V(ss)), determined by compartmental analysis, were significantly different for prelay and peak of egg production (8.358 and 13.581 mL/kg), respectively.

Levamisole is currently being used to treat capillaria infection in chickens even though there is no published withdrawal information available for levamisole in chickens. Tissue residue withdrawal of levamisole in chickens was studied in 32 healthy broiler breeder chickens at the age of 32 wk (peak of egg production). Levamisole residues in chicken tissues, eggs, and plasma were determined by HPLC with ultraviolet (UV) detection at 225 nm. The highest level of residue and longest withdrawal after oral administration of 40 mg/kg levamisole to chickens was in the liver. On d 3 the level of levamisole were undetectable in the plasma. On d 9, levamisole residue in eggs was 0.096 microg/g and on d 18 it was 0.06 microg/g or less in all the analyzed chicken tissues. Those levels were lower than the recommended maximum residue limit (MRL). The withdrawal time for levamisole in chickens was longer than for other species tested, which is due in part to a larger dose of levamisole being recommended for chickens. In conclusion from this research, 9 d are needed for levamisole in eggs to be less than the MRL, and 18 d of withdrawal are needed before medicated birds are slaughtered if their tissues are to be safe for human consumption.

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