Photodegradation of 4"-(epimethylamino)-4"-deoxyavermectin Bu as thin films on glass, using artificial light, resulted in the formation of multiple photodegradates. Six primary degradates and two secondary degradates were identified. The primary degradates formed were 8,9-Z-4"-(epimethylamino)-4"deoxyavermectin Bu (a geometric isomer), delta 2,3-4"-(epimethylamino)-4"-deoxyavermectin Bu (a positional isomer), avermectin Bu monosaccharide, 4"-epiamino-4"-deoxyavermectin Bu, 4"-N-formyl-4"-deoxyavermectin Bu, and 4"-iV-methyl-N-formyl-4,,-deoxyavermectin Bu. The secondary degradates formed, both geometric isomers of two primary degradates, were 8,9-Z-4"-epiamino-4"deoxyavermectin Bu and 8,9-Z-4"-JV-formyl-4"-deoxyavermectin Bu. The substitution of the epi-iVmethylamino group for the hydroxyl group at the 4" position of avermectin Bu profoundly affected the formation of photodegradates in thin films reported in earlier studies.

Photodegradation of avermectin B1, thin films under artificial light (above 260 nm) for short periods of time (greater than 60% of parent compound remaining) resulted in the formation of at least 10 primary degradates including two geometric isomers, 7 monooxygenated derivatives, and 1 dealkylated derivative. Prolonged photolysis of avermectin B1, thin films (no parent compound remaining) resulted in a polar residue which appeared to consist of a complex mixture of degradates with few characteristics of the parent molecule.

Flunixin meglumine is used in veterinary medicine as an alternative to narcotic analgesics and as an antiinflammatory agent. Eight Holstein dairy cows were dosed intravenously once daily on three consecutive days with (14)C-flunixin meglumine at approximately 2.2 mg of flunixin free acid/kg of body weight. Milk was collected twice daily to determine the decline of the total radioactive residues (TRR) in milk and to identify or characterize residue components. TRR in milk declined rapidly and averaged 66, 20, and 14 ppb, respectively, for the first, second, and third milkings after administration of the last dose. Milk was extracted, and the extracts were examined for radioactive residues. Mean extractability of milk TRR was always greater than 80%. Flunixin and 5-hydroxyflunixin were identified by coelution with analytical standards using reverse phase HPLC. These two residues were the main radioactive residues found in milk and together accounted for 64, 37, and 44% of the extractable residues, for the first, second, and third milkings, respectively, after administration of the last dose. The presence of 5-OH flunixin in milk was confirmed by HPLC/MS/MS.

Plasma distribution and elimination of florfenicol in channel catfish were investigated after a single dose (10 mg/kg) of intravenous (i.v.) or oral administration in freshwater at a mean water temperature of 25.4 °C. Florfenicol concentrations in plasma were analyzed by means of liquid chromatography with MS/MS detection. After i.v. florfenicol injection, the terminal half-life (t(1/2)), volume of distribution at steady state (V(ss)), and central volume of distribution (V(c)) were 8.25 h, 0.9 and 0.381 L/kg, respectively. After oral administration of florfenicol, the terminal t(1/2), C(max), T(max), and oral bioavailability (F) were 9.11 h, 7.6 μg/mL, 9.2 h, and 1.09, respectively. There was a lag absorption time of 1.67 h in oral dosing. Results from these studies support that 10 mg florfenicol/kg body weight in channel catfish is an efficacious dosage following oral administration.

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