Abstract: The pharmacokinetics of enrofloxacin soluble powder in the drinking water were investigated in chicks of various ages and the PK/PD model combined with Monte Carlo simulation (MCS) was used to evaluate the rationality of enrofloxacin soluble powder administration by the recommended regimen. Chicks were given 75 mg·L^-1 of enrofloxacin soluble powder (calculated by enrofloxacin) at 1, 7 and 14 days of age. At predefined time intervals, blood samples were obtained, the plasma concentration of enrofloxacin was measured by HPLC, and pharmacokinetic parameters were calculated. PK/PD parameters were calculated by combining the pharmacokinetic parameters with the breakpoint (0.25 μg·mL^-1) of enrofloxacin against Escherichia coli in poultry obtained by Clinical and Laboratory Standards Institute (CLSI), and the target attainment rate (TAR) of the current administration regimen under different MIC ranges of Escherichia coli was obtained by MCS. The results showed that C_max of chicks at 1, 7 and 14 days of age were 0.561, 0.564 and 0.550 μg·mL^-1 on day 0 of administration, and AUC_0-24 were 8.85, 9.85 and 9.27 μg·h·mL^-1, respectively. C_max of chicks at 1, 7, and 14 days of age on day 4 of administration were 0.599, 0.550 and 0.487 μg·mL^-1, C_avg were 0.513, 0.493 and 0.432 μg·mL^-1; AUC_0-24were 12.31, 11.82and 10.37 μg·h·mL^-1; C_max/MIC were 2.40, 2.20 and 1.95; C_avg/MIC were 2.05, 1.97 and 1.73; AUC_0-24/MIC were 49.24, 47.28 and 41.48, respectively. When the MIC of enrofloxacin against clinical isolates of Escherichia coli within 0.25 μg·mL^-1, TAR varied from 75.01 percent to 76.11 percent at 1, 7, and 14 days of age. The results revealed that enrofloxacin soluble powder failed to achieve the PK/PD parameter ratio and TAR of sensitive bacteria as expected by the currently suggested regimen, which not only limited bactericidal effect, but also easy to induce bacterial drug resistance.

Key words: enrofloxacin, PK/PD model, Monte Carlo simulation, chick