Metabolite pharmacokinetics: methods for simultaneous estimates of elimination rate constants of a drug and its metabolite. A commentary.
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Acetaminophen↗
Biomedical subjects
K S Pang
Publications and source records attributed to K S Pang.
Dose-dependent kinetics of quinidine in the perfused rat liver preparation. Kinetics of formation of active metabolites.
The disposition of quinidine and the kinetics of metabolite formation were studied in the once-through perfused rat liver preparation by the stepwise increase in quinidine concentration. Dose-dependent kinetics of quinidine were observed; the steady-state hepatic extraction ratio decreased from 0.97 to 0.37 when input quinidine concentration varied from 1.34 to 33.9 micrograms/ml. Moreover, the formation of 3-hydroxyquinidine (detected only in perfusate as unconjugated 3-hydroxyquinidine) remained relatively linear while the formation of O-desmethylquinidine (present in bile and perfusate mostly as conjugates) apparently approached saturation kinetics within the quinidine concentrations used. The dose-dependent character of quinidine elimination cannot be attributed to changes in drug binding, as the blood/plasma ratio (4.07) and the degree of drug unbound in plasma (66.5%) remained unaltered for the quinidine concentrations used.
Animals↗
A commentary: methods and assumptions in the kinetic estimation of metabolite formation.
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Biotransformation↗
Disposition of enalapril and its diacid metabolite, enalaprilat, in a perfused rat liver preparation. Presence of a diffusional barrier for enalaprilat into hepatocytes.
Enalaprilat (MK-422), a new and potent angiotensin- converting enzyme inhibitor, and its monoethyl ester precursor, enalapril, were studied in a single pass perfused rat liver preparation under constant perfusate flow (10 ml/min) at concentrations of 0.29-0.41 microM for 14C-enalapril and 0.01-0.015 microM for 3H- enalaprilat . During their simultaneous delivery to the same rat liver preparation, the steady state hepatic extraction ratio of 14C-enalapril was high (0.861 +/- 0.02) and 14C- enalaprilat appeared rapidly in effluent perfusate plasma. Of the enalapril dose, 22.7 +/- 6.9% appeared in bile. 14C- Enalaprilat accounted for 79% of the total radioactivity in bile (18% of dose) whereas 14C-enalapril was present only as 10% of the total (2.3% of dose). By contrast, the steady state hepatic extraction of 3H- enalaprilat was very low (0.053) and the disappearance was virtually identical to the appearance of 3H- enalaprilat in bile. These findings suggest that diffusional barrier exists for enalaprilat as the preformed metabolite, which hinders penetration into hepatocytes, and therefore, elimination. The precursor, enalapril, effectively brings enalaprilat into hepatocytes were more extensive biliary excretion of the generated metabolite takes place. This account adds to our further understanding of metabolite kinetics; in addition to the uneven distribution of enzyme system and the intrinsic clearance for metabolite formation and elimination, the presence of a diffusional barrier is another important determinant which may cause deviations between the kinetics of a generated and performed metabolite.
Animals↗
Inhibition of acetaminophen sulfation by 2,6-dichloro-4-nitrophenol in the perfused rat liver preparation. Lack of a compensatory increase of glucuronidation.
The effectiveness of 2,6-dichloro-4-nitrophenol ( DCNP ) as an inhibitor of sulfation of acetaminophen, and the effect of DCNP on other conjugation reactions such as glucuronidation and glutathione conjugation were investigated in the once-through perfused rat liver preparation. The formation of glucuronide and glutathione conjugates of acetaminophen was insignificant under conditions when sulfation was suppressed either by the absence of inorganic sulfate or the presence of DCNP , suggesting that acetaminophen was a poor substrate for glucuronidation (high Km) and for the formation of the reactive metabolite leading to the formation of acetaminophen glutathione conjugate. DCNP was most effective in suppressing sulfation at low concentrations of acetaminophen, and the degree of inhibition increased with DCNP concentration, possibly due to a competitive mechanism of inhibition. Subsequent studies with tracer concentration of 3H-acetaminophen and 40 microM DCNP indicated that the steady state hepatic extraction ratio of acetaminophen decreased by 16%, sulfation was reduced by 19%, whereas glucuronidation increased by 28% in the presence of DCNP ; glutathione conjugation was not affected. Because acetaminophen is a poor substrate for glucuronidation and because glucuronidation remains a minor metabolic pathway in the biotransformation of acetaminophen, the changes in glucuronidation of acetaminophen with DCNP (28% above control value) failed to effect gross changes in acetaminophen disposition. Rather, the suppression of acetaminophen sulfation by DCNP is responsible for the decreased hepatic extraction of acetaminophen. This inhibition of acetaminophen sulfation by DCNP is readily reversible.
Acetaminophen↗
Kinetics of meperidine N-demethylation in the perfused rat liver preparation.
Meperidine and normeperidine elimination was studied in the once-through perfused rat liver preparation. Biliary excretion of these bases was minimal, and nonlinear metabolism of meperidine (extraction ratio of 1.0 to 0.89 at 1 to 19 micrograms/ml) and normeperidine (extraction ratio of 0.6 to 0.1 at 1 to 25 micrograms/ml) was observed when input concentration was increased. The rate of efflux of normeperidine in hepatic venous blood represented an increasing proportion of the rate of presentation and the rate of loss of meperidine (34 to 92%) at increasing meperidine input concentration. But when the data were corrected for the nonlinear sequential metabolism of normeperidine, the rate of N-demethylation accounted completely for the rate of metabolism of meperidine. These N-demethylation rates obeyed Michaelis-Menten behavior, and appeared to be saturated at input meperidine concentration greater than 5 micrograms/ml.
Animals↗
Kinetics of procainamide N-acetylation in the rat in vivo and in the perfused rat liver preparation.
The kinetics of procainamide N-acetylation were studied in the rat in vivo and in vitro. For the in vivo studies, first order kinetics for procainamide and N-acetylprocainamide were found among their respective iv doses of 20, 50, and 70 mg/kg, and 10, 20, and 30 mg/kg. The fraction of total body clearance of procainamide that forms N-acetylprocainamide was found to be 0.22, and very insignificant sequential elimination of N-acetylprocainamide occurred during its formation. By contrast, results from once-through liver perfusion indicated that the steady state hepatic extraction ratio of procainamide was highly dependent on the steady state input concentration delivered under constant hepatic blood flow (10 ml/min). The rate of N-acetylation, however, was a constant percentage of the rate of presentation of procainamide at less than or equal to 80 micrograms/min among the preparations and became apparently saturated at higher rates of input of procainamide. Interestingly, the rate of N-acetylation accounted for an increasing proportion of the rate of loss of procainamide at greater than 80 micrograms/min, and suggested that alternate metabolic routes of procainamide are more easily saturable than N-acetylation. The comparative in vivo and in vitro data suggested that a region of nonlinearity existed during the early periods immediately following iv injection of procainamide into the rat in vivo. Because of rapid distribution, the region of nonlinearity was transient, and was not reflected by area under the curve measurements, which is a time-averaged parameter. Total body clearance, which bears a reciprocal relationship with the area under the curve, hence remained constant and was dose-invariant. The trend of nonlinearity may be more evident on chronic dosing of the drug when accumulation sets in.
Acecainide↗
Localization of glutathione conjugation activities toward bromosulfophthalein in perfused rat liver. Studies with the multiple indicator dilution technique.
The plasma binding and conjugation kinetics of bromosulfophthalein (BSP) with glutathione (GSH) were studied in the single-pass in situ perfused rat liver (portal vein perfusion at 10 ml/min); GSH in postmitochondrial fractions of the liver at the end of the experiment was examined as a potential rate-determining factor. BSP was highly bound to 1% albumin with at least two classes of binding sites: one of 0.17 site with an association constant of 1.9 x 10(7) M-1, and one of 7.4 equivalent sites of association constant, 1.3 x 10(5) M-1. Nonlinear binding was observed within between 5-1500 microM BSP. At varying input concentrations (0.4-250 microM) of BSP, the unbound fraction was extremely low (< 0.005) and the hepatic extraction ratio declined from 0.67 to 0.15; loss of BSP was primarily caused by GSH conjugation to form BSP-GSH, which appeared exclusively in bile. Additionally, unchanged BSP and two very minor unidentified metabolites were also excreted in bile. The formation of BSP-GSH proceeded with an apparent Vmax of 22 nmol/min/g and a KM of 0.05 microM, whereas the parameters for BSP excretion were 0.85 nmol/min/g and 0.02 microM, respectively. Within the concentration range of BSP examined, GSH availability did not appear to be rate-limiting in the formation or excretion of BSP-GSH.(ABSTRACT TRUNCATED AT 250 WORDS)
Animals↗