Enoxolone pharmacokinetics, tissue distribution, and residue depletion in largemouth bass (Micropterus salmoides)

Enoxolone, a hydrolyzed component of Chinese herbal medicine, has shown efficacy against Flavobacterium columnare in largemouth bass (Micropterus salmoides). However, its pharmacokinetics and muscle residue depletion in M. salmoides are not well understood. This study analyzed plasma and tissue samples using High-performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS) after exposure to 16 mg/L enoxolone. Results indicated a bimodal plasma absorption pattern, with the first peak at 2 h and the highest concentration (Cmax = 44.67 mg/L) at 24 h. The plasma AUC0−24 h was 813.40 mg·h/L, and the MRT was 36 h. Significant accumulation occurred in the kidneys (Cmax = 2223.33 mg/kg, AUC0−24 h = 7268.967 mg·h/L) which is more than other organs. But the Tmax (72 h) and MRT (51 h) is the longest. Muscle residues declined to 0.0164 mg/kg by day 11 post-immersion and the withdrawal time is at least 15 d. Enoxolone concentrations in plasma and tissues reached the MIC within 4–8 h. Acute toxicity tests showed a 96 h LC50 of 12.17 mg/L, with no mortality at concentrations ≤ 50 mg/L within 24 h. Based on these findings, a 4 h immersion at 16 mg/L is recommended for effective F. columnare control, followed by an 15 d withdrawal period to meet food safety standards. This study offers crucial data for optimizing enoxolone use in aquaculture.