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Drug behavior in the bodies of fish is largely influenced by the water temperature. Antimicrobial drugs are needed for the control of bacterial outbreaks in farmed fish including Asian seabass (Lates calcarifer). However, little is known about the temperature effect on appropriate drug uses in this species. The purpose of this study was to investigate the differences in pharmacokinetics (PK), optimal dosages, tissue depletion, and withdrawal time (WDT) of florfenicol (FF) in Asian seabass reared at 25 and 30 °C. In the PK study, the fish were administered with a single oral dose of 10 mg/kg FF. The optimal dosing regimen was determined by the pharmacokinetic-pharmacodynamic (PK-PD) approach. In the tissue depletion and WDT study, FF was administered at the optimal dosages once daily for 5 days and the WDT was determined by linear regression analysis based on the sum of FF and its metabolite florfenicol amine (FFA) in the muscle/skin. When the temperature was increased from 25 to 30 °C, the elimination half-life of FF was significantly decreased from 11.0 to 7.2 h. While the other PK parameters were not changed significantly, the calculated optimal dosages for the target minimum inhibitory concentration (MIC) of 2 µg/mL were 10.9 and 22.0 mg/kg/day, respectively for 25 and 30 °C. The sum of FF + FFA is a preferable marker residue for WDT determination because differential FF metabolism was observed at different temperatures. The depletion half-life of the muscle/skin was shortened from 41.1 to 32.4 h by the 5 °C temperature increase. Despite different absolute amounts of FF given between the two temperature levels, the WDTs were very similar at 6-7 days. Thus, it appears that a single temperature-independent WDT can potentially be assigned when the drug was applied at the optimal dosage. [ABSTRACT FROM AUTHOR]

Asian seabass (Lates calcarifer) is an economically important fish in Asian and Australian markets, but few pharmacokinetic (PK) data of antimicrobial drugs in this species is available. The present study investigated the PK behaviour of florfenicol (FF) through medicated feed in Asian seabass cultured at 25°C. The serum and muscle/skin concentrations of FF and its metabolite florfenicol amine (FFA) were determined by the HPLC‐FLD method and analysed by one‐compartmental model. The optimal dosages were determined by pharmacokinetic‐pharmacodynamic (PK‐PD) approach and the linear regression analysis was used to determine the withdrawal time (WDT). The PK study following a single oral administration of 15 mg/kg FF via medicated feed revealed that the absorption half‐life (t1/2Ka), elimination half‐life (t1/2K), peak concentration (Cmax), area under the concentration‐time curve (AUC), volume of distribution (Vd/F) and clearance (CL/F) were 1.47 h, 8.07 h, 8.61 μg/ml, 146.41 h·μg/ml, 1.19 L/kg and 0.102 L/kg/h, respectively. The muscle/skin concentration‐time profile was similar to that of the serum, suggesting well distribution but only a small fraction of FF was metabolized to FFA. The optimal dosage for a minimum inhibitory concentration of 2 μg/ml was calculated as 13.38 mg/kg/day. The appropriate WDT after multiple oral medications with 15 mg/kg FF once daily for 7 days was determined as 8 days. Information obtained from the current study can potentially be applied for the treatment of bacterial diseases in farming Asian seabass. [ABSTRACT FROM AUTHOR]

Semicarbazide (SEM), a marker residue used to monitor the use of prohibited drug nitrofurazone (NFZ), is commonly found in wild crustaceans, implying the natural origin. However, the difference between endogenous and exogenous SEM has rarely been investigated. So, tissue-bound SEM was determined in samples collected from giant river prawns cultured in an aquaculture farm and in samples from an experiment where giant river prawns were fed twice a day with NFZ at 30 mg/kg for 5 days. At day 10 of drug withdrawal, muscle SEM of the NFZ-fed prawn was 17.78 ng/g and depleted to 1.18 ng/g at day 90 (half-life 20.31 days) which was significantly higher than the control prawn (usually ≤ 0.1 ng/g). In contrast, the average SEM in the shell was independent of NFZ treatment. SEM was not found in the aquaculture farm samples, implying that the SEM in cultured prawn did not originate from SEM contamination. • Investigation of the difference between endogenous and exogenous SEM. • Tissue-bound SEM in the shell was independent of nitrofurazone (NFZ) treatment. • Tissue-bound SEM in the muscle of NFZ-fed prawn was always higher than the control. • The elimination half-life of SEM in the muscle was 20.31 days. • The use of SEM as a marker residue of NFZ should be reconsidered. [ABSTRACT FROM AUTHOR]

Semicarbazide (SEM) has been used as a marker residue of the banned veterinary drug nitrofurazone (NFZ). Although evidence indicates that SEM can be found in some natural crustaceans that have never been exposed to NFZ, such information is limited to a few species. The present study aimed to investigate the natural occurrence of SEM in wild crustaceans in Thailand. A total of 14 species, all economically important food animals, were captured from different regions of Thailand. Tissue-bound SEM and its parent drug NFZ were determined by the UPLC-MS/MS and LC-MS methods, respectively. The results showed that while NFZ was not detected in any samples, the tissue-bound SEM could be found in every natural crustacean species investigated. However, the prevalence and concentration varied greatly. The occurrence of SEM in the freshwater palaemonid Macrobrachium prawns is generally much higher than in the marine penaeid shrimps/prawns. SEM was found in 33% and 80% of the giant river prawn (Macrobrachium rosenbergii) muscles (<0.10–0.46 ng/g) and shells (3.68–13.22 ng/g), respectively. In contrast, SEM was not detected in the muscles of penaeid shrimps/prawns (with few exceptions), but it was occasionally found in the shells at low levels (usually <1 ng/g). The shells of saltwater crabs also contained higher levels of SEM than the muscles. For instance, the highest SEM levels detected in the mud crab (Scylla sp.) muscles and shells were 0.40 and 22.75 ng/g, respectively. However, the situation was reversed for the rice-field crab (Sayamia sp. and Esanthelphusa sp.), in which SEM was not detected in all shells but detected in the muscles (up to 1.46 ng/g). The fact that SEM is often found in wild crustaceans implies a natural origin of this substance. Consequently, using SEM as a marker residue of NFZ is controversial and should be reconsidered. [Display omitted] • Semicarbazide (SEM) occurs naturally in several crustaceans at varying degrees. • The levels of SEM in the shell are usually higher than in the muscle. • SEM in Macrobrachium shrimps is generally higher than in penaeid shrimps. • Tissue-bound SEM in the shells of giant river prawn and mud crab can be > 10 ng/g. • The use of SEM as a marker residue of nitrofurazone should be reconsidered. [ABSTRACT FROM AUTHOR]