The acetylation-deacetylation equilibrium of sulfadimidine in ruminant calves

Plasma disappearance curves of sulfadimidine (SDM) in calves show at high doses a pattern resembling that of capacity-limited elimination. The half-life of the first part of the elimination phase of SDM when administered at high doses ranged between 6.4 and 11.5 h, while that of the terminal end of the plasma concentration-time curves was similar to that obtained at a low level application, ranging between 2.5 and 6.0 h. The percentage of N4-acetylsulfadimidine (N4-SDM) in plasma was low, viz. 2.2 to 5.8% of the total sulfadimidine concentration measured. The acetylation-deacetylation equilibrium was established within 3 h p.i. The N4-SDM plasma concentration-time curves were parallel to those of SDM beyond 3 h p.i. At high doses (66-235 mg/kg) the percentage of N4-SDM was slightly higher than that found at the low dose level. A small proportion of N4-acetylsulfadimidine, injected as the parent compound, was deacetylated to SDM. The intrinsic elimination half-life of N4-SDM was 0.9 h. It may be concluded that ultra-trace concentrations of N4-SDM, left in edible tissues of ruminants at slaughter, have in case of negative sulfonamide-sensitive bioassays no significance for the public health.