Pharmacokinetics and withdrawal interval calculations for meat and milk of intravenously administered compounded methylene blue in healthy cattle

Objective To characterize the pharmacokinetics in blood and plasma and determine residue profiles of IV-administered methylene blue (MB) to support withdrawal interval (WDI) recommendations. Methods This was a prospective, nonrandomized pharmacokinetic and residue study using noncompartmental analysis. The study took place from May 2023 through December 2023. Eight Holstein-cross cattle (4 steers, 2 heifers, 2 cows; ages, 5.4 to 6 months for steers and heifers and approx 5.5 years for cows; mean weights, 198 to 495 kg) were housed individually with controlled diet and water access. Animals received 6 mg/kg of compounded 2.5% MB IV. Blood and plasma samples were collected for 72 hours. Milk samples were collected twice daily. At 3 and 6 days after dose, tissue samples were collected after euthanasia. Drug concentrations were measured via UPLC-MS-MS assay. Withdrawal interval estimates used modified FDA tolerance limits and European Medicines Agency time-to-safe-concentration methods. Results MB was rapidly eliminated, with plasma concentrations below the limit of quantification by 12 hours and blood by 24 hours. No residues were detected in any tissues at sampling times. Milk concentrations were detected at approximately 12 hours (0.084 ± 0.04 µg/mL). Milk WDI estimates were 89 hours and approximately 5 days based on the modified FDA and European Medicines Agency approaches, respectively. Conclusions IV MB at 6 mg/kg in cattle was well tolerated, eliminated rapidly, and produced no tissue residues at 3 or 6 days. Clinical Relevance Data suggest that compounded MB at this dose and route as an emergency antidote poses no food safety concerns provided WDIs of 5 days for milk and 6 days for meat are recommended to producers.