DEPLETION OF RESIDUES FROM MILK AND BLOOD OF COWS DOSED ORALLY AND INTRAVENOUSLY WITH SULFAMETHAZINE

Cows were dosed orally (n = 4) or intravenously (n = 4) with sulfameth azine [sulmet; 4-amino-N-(4,6-dimethyl-2-pyrimidinyl) benzenesuffonami de] for 5 consecutive days (220 mg/kg of body weight on day 1 and 110 mg/kg on days 2-5). The concentrations of sulmet, N4-acetylsulfamethaz ine (Ac-sulmet), and the N4-lactose conjugate of sulfamethazine (lac-s ulmet) were measured in milk and blood collected at 24 h intervals aft er the last doses of sulmet were given. The method of analysis include d (1) spiking of samples with known amounts of C-13(6)-labeled referen ce compounds, (2) resolution of the 3 compounds by reversed-phase chro matography, (3) hydrolysis of lacsulmet, (4) treatment with diazometha ne to yield N1-methyl derivatives, and (5) gas chromatography/mass spe ctrometry. The ratios of intensities of mass spectral ions containing C-12(6) and the corresponding ions containing C-13(6), were used for r esidue quantitation. Sulmet, which was always the most abundant residu e in the blood, decreased to less than 100 ppb 4 days after the last d oses were given and to less than 10 ppb 7 days after the last doses. T he concentrations of sulmet in milk were approximately one fifth the c oncentrations of sulmet in blood. The concentrations of lac-sulmet and Ac-sulmet in milk lower than the concentrations of sulmet in milk.