The Japanese eel (Anguilla japonica) is a major aquaculture species in South Korea. However, a limited number of officially approved antimicrobial agents have led to frequent off-label drug use, raising concerns about therapeutic efficacy, food safety, and regulatory compliance. Flumequine (FQ), a first-generation quinolone effective against gram-negative pathogens, has been used in eels without species-specific pharmacokinetic or residue data. This study evaluated the pharmacokinetics, therapeutic efficacy, and residue depletion of FQ in the Japanese eel. The oral administration of 20 mg/kg significantly protected against Edwardsiella piscicida, with a relative survival rate exceeding 80

This study evaluated the therapeutic efficacy of amoxicillin (AMX) or lincomycin (LCM) in starry flounders (Platichthys stellatus) infected with Streptococcus parauberis. Seventeen S. parauberis strains isolated from various regions of Korea were tested to determine the minimum inhibitory concentrations (MICs) of AMX and LCM. A representative strain (HFTC0091) was used for the experiments. Following bacterial infection, starry flounders were treated with either oral AMX (40 or 80 mg/kg/d for 7 days) or a single intramuscular (IM) injection of LCM (10 or 20 mg/kg). High-dose treatments with both antibiotics resulted in significantly improved survival rates (96%-100% for AMX and 76%-96% for LCM) compared to nearly 100% mortality in the untreated controls. Serum biochemical analyses (glutamic oxaloacetic transaminase [GOT], glutamic pyruvic transaminase [GPT], and total protein [TP]) showed no significant toxicity; however, GOT and GPT levels were slightly decreased in the LCM-treated group, indicating a potential anti-inflammatory role of LCM, which warrants further investigation. This study demonstrates the effectiveness and safety of AMX and LCM in treating streptococcal infections in starry flounders and highlights the potential of the antibiotics as therapeutic agents in aquaculture. Future studies should investigate whether LCM modulates inflammatory pathways and elucidates the underlying molecular mechanisms in fish.

Background: Amoxicillin (AMOX) is widely used in aquaculture for bacterial infections due to its efficacy and safety. Despite official approval for select species, off-label use is common. This study evaluated the antibacterial efficacy and residue depletion of AMOX in five aquaculture species: olive flounder (Paralichthys olivaceus), rainbow trout (Oncorhynchus mykiss), Japanese eel (Anguilla japonica), black rockfish (Sebastes schlegelii), and Israeli carp (Cyprinus carpio). Methods: Fish were administered AMOX orally at 40 mg/kg and 80 mg/kg for seven days. Antibacterial efficacy was assessed by bacterial load reduction and survival rates following artificial infection. Residue depletion was analyzed using HPLC–MS/MS to determine the time required for AMOX levels to fall below the maximum residue limit (MRL, 0.05 mg/kg). Results: AMOX, at 40 mg/kg, significantly reduced bacterial loads in olive flounder, rainbow trout, and Japanese eel (p < 0.05), while Israeli carp exhibited a limited response (p = 0.54). Black rockfish showed moderate efficacy (RPS 72.7%) but increased mortality at 80 mg/kg. Residue levels fell below the MRL within 10 days for all species except Israeli carp (~30 days). Conclusions: These findings highlight species-specific differences in AMOX efficacy and residue depletion rates, emphasizing the necessity of tailored dosing regimens and withdrawal periods to ensure optimal therapeutic outcomes and food safety compliance in aquaculture. Further pharmacokinetic studies are needed to refine dosing strategies, particularly for species with extended residue retention and potential dose-dependent adverse effects.

This study investigated the pharmacokinetics of enrofloxacin (ENR) and its primary metabolite, ciprofloxacin (CIP), in black rockfish (Sebastes schlegelii) following a single oral administration of ENR (10 mg/kg) under two water temperature conditions (13°C and 22°C). Serum samples were collected up to 168 h post-dosing and analyzed using a validated HPLC-MS/MS method. Contrary to conventional expectations, ENR absorption was delayed and elimination was slower at 22°C compared to 13°C, while the plasma concentrations of CIP were higher at the elevated temperature. The elimination half-life of ENR increased from 27.38 h at 13°C to 42.01 h at 22°C, despite enhanced hepatic metabolism. These findings suggest that the primary route of ENR elimination may shift from passive diffusion via the gills at lower temperatures to hepatic metabolism at higher temperatures, likely due to functional impairment of the gill tissues under thermal stress. Notably, the AUC values of ENR remained comparable between the two groups, indicating consistent drug exposure despite differing pharmacokinetic dynamics. PK/PD analysis revealed that single-dose administration may not achieve optimal therapeutic indices (AUC0-24h/MIC ≥ 125, Cmax/MIC ≥ 10) against certain bacterial pathogens, underscoring the need for repeated dosing. Overall, this study provides novel insights into the temperature-modulated pharmacokinetics of ENR and highlights the importance of temperature-specific dosing strategies for effective antibiotic therapy in aquaculture species like black rockfish.

This study comprehensively evaluated the in vitro and in vivo anthelmintic efficacy and pharmacokinetics of four benzimidazole compounds: febantel (FB), fenbendazole (FBZ), albendazole (ABZ), and mebendazole (MBZ) against the gill fluke Microcotyle sebastis in cultured black rockfish (Sebastes schlegelii). In vitro tests with isolated adult parasites demonstrated that ABZ exhibited the highest efficacy, followed by FBZ, while MBZ showed minimal activity and FB remained largely inactive at lower concentrations. At the highest concentration tested (200 mg/L), FB showed only weak, nonspecific activity. In in vivo experiments, ABZ was the most effective drug when orally administered at 50 mg/kg for three consecutive days, while FB demonstrated greater efficacy than FBZ, and MBZ exhibited the lowest efficacy under the tested conditions. Pharmacokinetic analysis revealed marked variation among the drugs, with FB achieving the highest active metabolite levels at 100 mg/kg. ABZ, at 50 mg/kg, achieved comparable systemic exposure to FBZ at 100 mg/kg through its active metabolite albendazole sulfoxide (ABZSO). These findings highlight ABZ as the most efficacious compound, with FB showing promise through metabolic conversion to FBZ, and MBZ demonstrating limited efficacy due to poor absorption and rapid inactivation. Overall, this study provides an integrated evaluation of both efficacy and pharmacokinetics, indicating that ABZ and FB are effective candidates for managing M. sebastis infections in aquaculture.

Oxolinic acid (OA) is a quinolone class antimicrobial agent widely used in aquaculture, yet its off-label application via bath administration in Japanese eel (Anguilla japonica) has raised concerns regarding tissue residue persistence and appropriate withdrawal periods (WTs). This study aimed to investigate the residue depletion kinetics and calculate the WTs of OA in A. japonica following 12 h bath treatment under different water temperatures (20 °C and 28 °C) and exposure concentrations (10 and 20 mg/L). Laboratory trials were conducted under controlled conditions using recirculating systems, and additional field validation was performed at a commercial aquaculture farm. Residue concentrations in muscle plus skin were quantified using validated LC–MS/MS, and depletion data were modeled using first-order exponential regression. The calculated depletion half-lives ranged from 5.46 to 18.24 days, showing temperature- and dose-dependent variation. Corresponding WTs, derived from 95/95 statistical tolerance limits, were 78.8 and 115.7 days at 20 °C, and 27.4 and 55.9 days at 28 °C for the 10 and 20 mg/L groups, respectively. The WT estimated under farm conditions (47.3 days) closely matched the laboratory-derived value, demonstrating that laboratory-based models can reasonably predict field outcomes under similar conditions. All calculated WTs markedly exceeded the currently approved 25 days WT for oral administration of OA in eels, suggesting that extended WTs may be warranted following immersion exposure. These findings support the establishment of temperature- and concentration-dependent WTs under the conditions studied, which may help improve regulatory compliance and consumer protection in eel aquaculture.

The objective of this study was to investigate the pharmacokinetic (PK) properties of oxolinic acid (OA) in black rockfish (Sebastes schlegelii) following a single oral administration of 30 mg/kg at 13°C and 22°C. Serum and tissue (muscle, liver and kidney) samples were collected at pre-determined time points and analysed using high-performance liquid chromatography coupled with tandem mass spectrometry (HPLC-MS/MS). The results demonstrated that temperature significantly influenced the PK parameters of OA. The terminal half-life (t1/2λz) was prolonged at 22°C across all tissues, while the total body clearance (CL/F) was reduced, suggesting a slower drug elimination at higher temperatures. The peak concentration (Cmax) and the area under the concentration-time curve (AUC) values increased with temperature, indicating enhanced systemic exposure at 22°C. The volume of distribution (Vz/F) decreased from 13°C to 22°C, indicating reduced tissue distribution of OA at the higher temperature. PK/pharmacodynamic (PK/PD) analysis revealed that a single oral dose of 30 mg/kg was insufficient to achieve optimal antibacterial efficacy, implying that higher doses may be necessary. These findings highlight the necessity of temperature-adjusted dosing strategies for OA treatment in black rockfish aquaculture to optimise therapeutic efficacy while minimising antimicrobial resistance risks.

The body color state is an important determinant of the value of golden severum (Heros efasciatus)—a popular ornamental fish. The use of dietary supplements to improve the color development and health of this species is unexplored. Herein, the effects of marigold extract (MG) and carophyll red (CR) are examined on the growth, body color development, antioxidant properties, and innate immunity in golden severum. Fish were maintained under controlled water quality conditions (pH, temperature, and dissolved oxygen) and fed six experimental diets containing either 0% MG and CR, 1% MG, 2% MG, 5% MG, 0.5% CR, or 2% CR for five weeks. Both MG and CR significantly decreased lipid peroxide levels in hepatic tissues. In contrast, only MG enhanced the activities of reactive oxygen species (ROS)-scavenging enzymes (superoxide dismutase and catalase). Although MG and CR decreased the respiratory burst activity of splenic leukocytes, other innate immune parameters remained unchanged. Additionally, MG and CR stimulated body color development patterns in golden severum that reflect their unique coloring principles. The ROS-scavenging abilities of MG and CR appear to be related to their antioxidant activity. Hence, MG and CR at the optimal levels of 1.0% and 0.5%, respectively, can improve the body color of golden severum and protect against oxidative stress.

Although enrofloxacin (ENR) is an important pharmaceutical in aquaculture for its excellent efficacy, it is mostly utilized through an off‐label prescription system. Thus, the optimal clinical use regimen is not well‐established. A series of studies was performed to obtain pharmacological and safety data needed for the use of ENR, a synthetic fluoroquinolone, in treating bacterial diseases occurring in black rockfish, Sebastes schlegelii. Minimal inhibitory concentrations showed that ENR exerts a potent in vitro efficacy inhibiting the growth of several pathogenic bacteria at concentrations less than 1 mg/L. ENR with oral doses of 5 mg/kg and up for 1 week significantly prevented mortality of black rockfish infected with Streptococcus iniae and Aeromonas salmonicida, demonstrating in vivo efficacy. Complete protection was obtained against these two bacteria by 10 mg/kg ENR in this in vivo infection trial. Black rockfish did not exhibit any severe abnormal signs by ENR up to 20 mg/kg when assessed by basic haematoclinical and histopathological parameters. Efficacy and toxicity study results collectively suggest that oral dose of 10 mg/kg ENR is optimal to treat bacterial infections. Through muscle residue analyses, the required withdrawal period for safe fish consumption was found to be approximately 90 days by measuring clearance of ENR and its metabolite ciprofloxacin (CIP). This study provides necessary information for the use of ENR to treat two frequently‐occurring bacterial infections, Streptococcus iniae and Aeromonas salmonicida in black rockfish. Obtained data can be the basis for an official guideline for aquacultural ENR use.

The Korean government regulates pharmaceutical parameters and withdrawal times for antimicrobials in aquaculture. Tylosin tartrate (TT) is a bacteriostatic antibacterial agent mainly used in veterinary medicine for the prevention and treatment of gram-positive bacterial infections. However, pharmacokinetic (PK) and drug withdrawal reports in aquatic animals, including olive flounder (Paralichthys olivaceus), are limited. This study aimed to determine the PK profile and drug withdrawal time following oral TT administration (10 or 20 mg/kg) in olive flounder. The concentrations of tylosin in the serum and muscle plus skin were analyzed using liquid chromatography and tandem mass spectrometry. PK parameters were calculated for serum samples using the PKSolver software based on a non-compartmental model. The PK parameters after a single oral dose of 10 (or 20) mg/kg were as follows: Cmax, 0.7 (1.49) μg/mL; Tmax, 12 h; t1/2, 22.83 (19.44) h; area under the concentration curve, 10.58 (33.17) μg/mL h; and mean residence time, 20.53 (25.62) h. Oral administration of TT at a dose of 10 mg/kg at 24–48 h intervals may help achieve efficacy comparable to that obtained with antimicrobials at minimum inhibitory concentration values of ≤ 1 μg/mL. After a 5-day oral administration of TT in olive flounder (water temperature, 22 ℃), a withdrawal time of 17.8 (rounded up to 18 days) days was required to achieve muscle plus skin TT residue levels below 100 μg/kg, which is the European limit; the corresponding value at a lower water temperature (13 ℃) was 20 days. Overall, this study not only serves as a reference for authorizing the clinical use of TT in olive flounder but also provides valuable data for evaluating the safety of food derived from aquatic animals.

The application of injectable antibiotics has not been actively adopted in aquaculture despite some clear advantages over the oral administration under certain circumstances. This study aimed to assess the applicability of intramuscularly (i.m.) injectable ceftiofur sodium (CS), a cephalosporin, to treat streptococcal infection in olive flounder (Paralichthys olivaceus). The efficacy of CS was estimated in vitro with 48 different clinical isolates ofStreptococcus parauberis: it was found that growth of 32 isolates (66.7%) was inhibited by 0.25 mu g/ml and 47 isolates (97.9%) by 8 mu g/ml CS. The in vivo laboratory efficacy showed that 20-40 mg/kg single CS (i.m.) injection completely prevented olive flounder from death while streptococcal infection led to 100% mortality without CS. In a field study, CS was administered at 10 or 20 mg/kg i.m. to naturally infected olive flounder in culture farms, and significant mortality prevention was observed by either dose. Plasma and muscle concentrations of CS were monitored in the laboratory following single CS injections at 10 and 20 mg/kg (i.m.) to estimate pharmacokinetic parameters. Peak plasma concentrations (C-max) were 7.61 and 13.34 mu g/ml, respectively, after the two doses leading to largeC(max)/MIC(50)andT > MIC50(time period stayed at above MIC50) values againstS. parauberis. Analyses of residue depletion from the muscle demonstrated legitimate withdrawal times of 4.15 days after 10 mg/kg and 4.32 days after 20 mg/kg. These results collectively indicate that a single i.m. injection of CS above 10 mg/kg is acceptably efficacious with reasonable withdrawal periods in streptococcal infection of olive flounder.

The striatin-interacting phosphatase and kinase (STRIPAK) complex is a large, multisubunit protein phosphatase 2A (PP2A) assembly that integrates diverse cellular signals in the Hippo pathway to regulate cell proliferation and survival. The architecture and assembly mechanism of this critical complex are poorly understood. Using cryo-EM, we determine the structure of the human STRIPAK core comprising PP2AA, PP2AC, STRN3, STRIP1, and MOB4 at 3.2-Å resolution. Unlike the canonical trimeric PP2A holoenzyme, STRIPAK contains four copies of STRN3 and one copy of each the PP2AA–C heterodimer, STRIP1, and MOB4. The STRN3 coiled-coil domains form an elongated homotetrameric scaffold that links the complex together. An inositol hexakisphosphate (IP6) is identified as a structural cofactor of STRIP1. Mutations of key residues at subunit interfaces disrupt the integrity of STRIPAK, causing aberrant Hippo pathway activation. Thus, STRIPAK is established as a noncanonical PP2A complex with four copies of regulatory STRN3 for enhanced signal integration. A cryo-EM structure of the striatin-interacting phosphatase and kinase (STRIPAK) complex reveals the overall architecture of this large, multisubunit assembly that broadly regulates different signaling pathways.

Butaphosphan (BTP) has recently been introduced into the Korean aquaculture sector as a stress-attenuating agent. In this study, a sensitive chemical analytical method was established for the detection of BTP in the olive flounder (Paralichthys olivaceus) tissues. Utilizing a method employing liquid chromatography coupled with tandem mass spectrometry (LC-MS/MS), detection sensitivity, specificity, and precision were satisfactorily established. Temporal changes in the BTP plasma and muscle concentrations were assessed after a single intramuscular injection of BTP (50 and 150 mg/kg) to the olive flounder maintained at 13 °C or 22 °C. High BTP plasma levels were achieved immediately after the injection, and the drug was rapidly eliminated. Additionally, plasma BTP levels were markedly dependent on the elimination rate, which, in turn, seemed dependent on the water temperature, with the drug elimination half-life and mean residence time significantly shorter at 22 °C than 13 °C. Overall, muscle BTP levels were markedly lower than the plasma levels. Notably, muscle levels were not influenced by water temperatures. Muscle BTP concentrations were used to estimate the necessary withdrawal period for drugs used in food fish, with BTP levels maintained far below the possible hazardous limit. In conclusion, the established LC-MS/MS method can be used for BTP residue detection with high sensitivity and reproducibility.

The MST-LATS kinase cascade is central to the Hippo pathway that controls tissue homeostasis, development, and organ size. The PP2A complex STRIPAK(SLMAP) blocks MST1/2 activation. The GCKIII family kinases associate with STRIPAK, but the functions of these phosphatase-associated kinases remain elusive. We previously showed that the scaffolding protein SAV1 promotes Hippo signaling by counteracting STRIPAK (Bae et al., 2017). Here, we show that the GCKIII kinase STK25 promotes STRIPAK-mediated inhibition of MST2 in human cells. Depletion of STK25 enhances MST2 activation without affecting the integrity of STRIPAK(SLMAP). STK25 directly phosphorylates SAV1 and diminishes the ability of SAV1 to inhibit STRIPAK. Thus, STK25 as the kinase component of STRIPAK can inhibit the function of the STRIPAK inhibitor SAV1. This mutual antagonism between STRIPAK and SAV1 controls the initiation of Hippo signaling.

Effects on glutathione-related antioxidant parameters were examined after a chronic exposure of olive flounder, Paralichthys olivaceus to dietary 4-tert-octylphenol (4-tert-OP). Fish were fed diets containing 4-tert-OP at 0, 1, 5 and 10 mg/kg diet for 6 weeks. Antioxidant parameters examined were reduced glutathione (GSH) contents and enzyme activities of glutathione reductase (GR), glutathione S-transferase (GST) and glutathione peroxidase (GPx) in tissue homogenates of the liver, kidney and gill. It was observed that all parameters examined increased although there were some differences in dose responses and temporal patterns in the increase. GSH contents increased after exposure to 4-tert-OP in the three organs examined. However, the GSH increase was evident only after 4 weeks in the liver whereas it was elevated after 2 weeks in the kidney and gill. GR activity exhibited a significant increase in response to 4-tert-OP at 1 mg/kg in all three organs, however, its activity returned to control levels when exposed to 5 and 10 mg/kg. Hepatic GST activity showed an earlier increase at week 2 in contrast to the kidney and gill where they increased after 4 weeks of 4-tert-OP exposure. Temporal patterns in GPx activity changes to 4-tert-OP exposure were dissimilar among the organs: hepatic activity increased from week 2 through week 6; renal activity increased transiently at week 2; gill levels were higher through weeks 4 - 6. The results suggest that elevation of several GSH-related antioxidant parameters can be considered as evaluation criteria for 4-tert- OP-induced oxidative stress in a fish.

Butaphosphan {[1-(butylamino)-1-methylethyl]-phosphonic acid (BTP)}-cyanocobalamin [vitamin B12 (C)], a combination drug comprising an organic source of phosphorus and a synthetic form of vitamin B12, is used to attenuate stress responses in livestock. This study was performed to examine the effects of BTP-C in olive flounder Paralichthys olivaceus under normal and stressful conditions. P. olivaceus was intramuscularly injected with BTP-C (50–300 mg BTP-C/kg body weight) under normal conditions and changes in muscle adenosine nucleotides examined. BTP-C increased adenosine 5′-triphosphate (ATP) levels; this effect was more pronounced at 22 °C than at 13 °C. Serum transaminase levels were also elevated at a dose of 300 mg BTP-C/kg, which suggested possible hepatotoxicity of the drug. Effects of BTP-C on stress were examined by injecting it at a rate of 50 mg/kg. BTP-C partially prevented hypothermia- and hypoxia-induced reduction in feeding. Serum immunoglobulin M levels were increased by BTP-C in these stress tests. Although BTP-C did not affect hypothermia-induced cortisol elevation or lysozyme reduction, changes in these parameters due to hypoxia were alleviated by the drug. Stress-attenuating effects of BTP-C were also confirmed in fish farms following injection of an antibiotic. These results indicate that BTP-C may be useful against various forms of stress in fish. Accelerated ATP production following the administration of BTP-C is one possible mechanism resulting in the attenuation of stress effects. The clinically effective dose of BTP-C, 50 mg/kg, did not cause any marked toxicity in P. olivaceus.