The efficacy of orally administered drugs in cattle is thought to be slow because of the anatomical and physiological features of their forestomach. Thus, parenteral routes are mainly preferred to administer drugs. However, the effect of some drugs with unique physicochemical properties was promptly obtained even after oral administration in clinically ill cattle. Therefore, the present study aimed to investigate pharmacokinetically the usefulness of the oral route in cattle by comparing the oral pharmacokinetic properties of two sulfonamides with different physicochemical properties. Sulfadiazine (SDZ) and sulfamonomethoxine (SMM) were administered by intravenous and oral route to four female Holstein cows with a 4-weeks washout period. Blood samples were collected over time, and SDZ and SMM concentrations in plasma were analyzed by HPLC. Data obtained from the same animal after intravenous and oral administration were simultaneously analyzed with the one compartment model, and kinetic parameters were calculated. The Tmax (mean±SD) of SMM (2.75±0.96 hr) was significantly achieved earlier than that of SDZ (5.00±1.15 hr). Further, the mean absorption time of SMM (5.24±0.69 hr) was significantly shorter than that of SDZ (5.92±1.11 hr). Also, the half-life of absorption of SMM (3.91±0.51 hr) was significantly shorter than that of SDZ (4.51±0.82 hr). These data suggest that the absorption rates of highly unionized drugs (such as SMM) from the forestomach of cattle may be markedly higher than less unionized ones (such as SDZ).
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