Role of Deacetylation in the Nonlinear Pharmacokinetics of Sulfamonomethoxine in Pigs

Abstract

The role of deacetylation in the pharmacokinetics of sulfamonomethoxine (SMM) in pigs was studied using 6 Goettingen minipigs and a pig from a commercial breed. The rapid decrease of plasma concentration of the parent compound followed by the rapid increase of plasma concentration of the deacetylated metabolite, SMM, was observed after an i.v. injection of N⁴-acetylsulfamonomethoxine (AcSMM ; 10mg/kg). The concentration of the metabolite, SMM, was greater than that of AcSMM and after reaching peak, both compounds decreased in parallel on a semilogarithmic graph. On the other hand, the acetylated compound of SMM (AcSMM) appeared in the plasma and reached peak after an i.v. injection of SMM. After reaching the peak, both SMM and AcSMM decreased in parallel. The slopes of the terminal phases of SMM and AcSMM after both injections showed no significant difference. After the i.v. injection of a high dose of SMM (100mg/kg), a nonlinear pharmacokinetics profile with capacity limited elimination type of SMM was observed. The concentration of plasma AcSMM increased rapidly and the parallel decrease of SMM and AcSMM was observed in both nonlinear and linear phases, while the renal excretion of AcSMM was saturated in the nonlinear phase. The results suggest that the deacetylation in pigs is so strong that it largely affected the pharmacokinetics of SMM in both high and low doses. In spite of the saturation in renal excretion of the main metabolite, AcSMM, after the high dose, the profile of the parallel decrease remained unchanged, which may be due to the rapid conversion of the excess AcSMM to SMM. The converted SMM was added to the existing plasma SMM. As a result, the nonlinear pharmacokinetics of SMM may have occurred after a high dose of SMM in pigs.