Extrapolated withdrawal-interval estimator (EWE) algorithm: a quantitative approach to establishing extralabel withdrawal times.

The extralabel use of drugs can be defined as the use of drugs in a manner inconsistent with their FDA-approved labeling. The passage of the Animal Medicinal Drug Use Clarification Act (AMDUCA) in 1994 and its implementation by the FDA-Center for Veterinary Medicine in 1996 has allowed food animal veterinarians to use drugs legally in an extralabel manner, as long as an appropriate withdrawal period is established. The present study introduces and validates with simulated and experimental data the Extrapolated Withdrawal-Period Estimator (EWE) Algorithm, a procedure aimed at predicting extralabel withdrawal intervals (WDIs) based on the label and pharmacokinetic literature data contained in the Food Animal Residue Avoidance Databank (FARAD). This is the initial and first attempt at consistently obtaining WDI estimates that encompass a reasonable degree of statistical soundness. Data on the determination of withdrawal times after the extralabel use of the antibiotic oxytetracycline were obtained both with simulated disposition data and from the literature. A withdrawal interval was computed using the EWE Algorithm for an extralabel dose of 25 mg/kg (simulation study) and for a dose of 40 mg/kg (literature data). These estimates were compared with the withdrawal times computed with the simulated data and with the literature data, respectively. The EWE estimates of WDP for a simulated extralabel dose of 25 mg/kg was 39 days. The withdrawal time (WDT) obtained for this dose on a tissue depletion study was 39 days. The EWE estimate of WDP for an extralabel intramuscular dose of 40 mg/kg in cattle, based on the kinetic data contained in the FARAD database, was 48 days. The withdrawal time experimentally obtained for similar use of this drug was 49 days. The EWE Algorithm can obtain WDI estimates that encompass the same degree of statistical soundness as the WDT estimates, provided that the assumptions of the approved dosage regimen hold for the extralabel dosage regimen. Population models could be fitted to fragmentary data to predict residue concentrations in tissues, validate the EWE estimates, and obtain WDI estimates.