Acetylation of procainamide in the rat

Effects of chronic ethanol ingestion on pharmacokinetics of procainamide in rats

Blood level studies were carried out in rats to determine the effects of chronic ethanol ingestion on the distribution pharmacokinetic parameters and tissue steady-state partition coefficients of procainamide. The ethanol-treated rats received 4g/kg of ethanol daily for 28 days in Treatment A and 4 g/kg of ethanol for an initial 7 days, followed by 8 g/kg of ethanol for the subsequent 21 days in Treatment B; the control rats received isocaloric sucrose in the respective groups. As determined from two-compartment analysis of the blood level data, both ethanol treatments significantly decreased the distribution clearance (CLd; k12Vdc) and the apparent first-order rate constant for drug transfer from the central compartment to the tissue compartment (k12) of procainamide without affecting the total body clearance of drug (CL) or the apparent volumes of distribution of drug in the body at steady state (Vdss) and at pseudo-equilibrium (Vd beta). Additionally, the apparent volume of distribution of the drug in the central compartment (Vdc) was 57-62% greater due to both ethanol treatments. Furthermore, the steady-state partition coefficients of the drug were found to be significantly lower in heart and kidneys and greater in fat of the ethanol-treated rats (Treatment B) as compared with those in the control rats. Possible mechanisms are proposed to account for these various effects in light of the known effects of chronic ethanol ingestion on the chemical composition of cell membranes of tissues and organs.

Sulfonamide acetylation in isolated rabbit and rat liver cells

Suspensions of isolated liver cells were prepared from rabbit livers perfused with Ca++-free buffer and 0.05% collagenase. Primary cell suspensions (containing both parenchymal and non-parenchymal liver cells) metabolized sulfadimidine and sulfanilamide at first-order kinetics for at least 2-3 hrs. Suspensions of purified rabbit liver parenchymal cells had an equal metabolic capacity, and it could be demonstrated that the metabolic rate of both sufadimidine and sulfanilamide was correlated to the amount of viable parenchymal cells in suspension. Suspensions of non-parenchymal cells were lacking the ability to metabolize both drugs. By means of homogenates of purified rabbit and rat liver cells, it could be demonstrated that the enzyme N-acetyltransferase was located in the cytosolic fraction of the parenchymal cells. It was concluded that the cytosolic fraction of the liver parenchymal cells is the main site of sulfonamide acetylation in both rabbit and rat.

Pharmacokinetics of procainamide and N-acetylprocainamide in rats

The pharmacokinetics of distribution and elimination of procainamide and its major metabolite, N-actylprocainamide, were studied in rats. Eight rats were selected randomly, and each received intravenously 14C-labeled procainamide hydrochloride (75 mg/kg) or 14C-labeled N-acetylprocainamide hydrochloride (86 mg/kg) according to a two-way crossover design. Serial blood samples were withdrawn for 8 hr, and cumulative urine and feces were collected for 48 hr. The plasma concentration-time relationships of procainamide and N-acetylprocainamide were characterized by one- and two-compartment open models, respectively. A pseudo-three-compartment model was necessary to characterize the time course of N-acetylprocainamide in plasma formed after administration of procainamide. The biological half-lives of procainamide and N-acetylprocainamide averaged 0.66 and 2.1 hr, respectively. The urinary excretion profiles of these drugs and the ratio of their biological half-lives in rats were similar to those in humans.