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K S Pang

Publications and source records attributed to K S Pang.

At least 109 records · Page 6Linked to original sources
PubMed1984

Extrahepatic sulfation and glucuronidation in the rat in vivo. Determination of the hepatic extraction ratio of harmol and the extrahepatic contribution to harmol conjugation.

The phenolic compound, harmol, is metabolized by sulfation and glucuronidation in the rat in vivo. In the present study, various harmol infusion rates into the jugular vein were used to delineate first-order conditions whereby total body clearance was maximal and constant; at low infusion rates the steady state harmol concentration in blood varied proportionally with the infusion rate. At infusion rates of 167 nmole/min and below, the steady state clearance of harmol was 60 ml/min or 200 ml/min/kg. Because this value for total body clearance greatly exceeded the value for hepatic blood flow rate (20 ml/min for a 300 g rat), considerable extrahepatic conjugation of harmol was suggested. At higher harmol infusion rates the total clearance decreased. Since an intraportal infusion of 167 nmole/min to the rat yielded, during steady state, the same arterial harmol blood concentration as a 52 nmole/min jugular infusion, the hepatic extraction ratio of harmol in vivo was 0.7. Extrahepatic clearance, therefore, constituted about 77% of total body clearance (after taking the difference between total body clearance and hepatic clearance). Total sulfation clearance was 52 ml/min, and greatly exceed the value for hepatic clearance (14 ml/min). Extrahepatic clearance for sulfation (at least 38 ml/min) therefore accounted for a major proportion of the sulfation activity. Blood platelets did not seem to contribute to sulfation or glucuronidation in vivo.

Alkaloids↗
PubMed1983

An understanding of the role of enzyme localization of the liver on metabolite kinetics: a computer simulation.

The metabolic sequence of drug, D, to its primary (MI) and terminal (MII) metabolites as mediated by enzymes A and B, respectively, was chosen to illustrate metabolizing activities among hepatocytes in different regions of the liver lobule. Six models of distributions of the hepatocellular activities (intrinsic clearances for A and B) were defined with respect to the flow path in liver, and the concentrations D, MI, and MII in the liver were simulated. The extent of sequential metabolism of the primary metabolite was compared for these six models of enzymic distributions. It was found that when the average hepatic intrinsic clearances of A and B were high (almost complete extraction of both drug and primary metabolite during their single passage through the liver), the distributions of A and B were not important determinants of metabolite kinetics. By contrast, when the average hepatic intrinsic clearances of A and B were both low, the distributions of A and B exerted profound effects on metabolite kinetics. The sensitivity to enzymic distribution in this region, however, was difficult to assess due to difficulties in detecting low levels of MI and MII. The effects of enzymic distributions on metabolite disposition would be better detected in compounds (drug and metabolite) with intermediate extraction ratios.

Animals↗
PubMed1983

Normal and retrograde perfusion to probe the zonal distribution of sulfation and glucuronidation activities of harmol in the perfused rat liver preparation.

The role of zonal distributions of metabolic activities (sulfation and glucuronidation) in the liver on the kinetics of harmol conjugation was investigated. A computer simulation approach was adopted to better understand the effects of distributions of these conjugation activities in competition for a common drug substrate. Several distributions of the sulfation and glucuronidation systems were defined with respect to the flow path of blood; the conjugation activities along the flow path were in turn translated as time elapsed after entry of the substrate via blood into the organ. Realistic values of Km and Vmax for the sulfation and glucuronidation systems were assigned in the simulations. Directional flow, namely, normal vs. retrograde delivery of substrate was used as an additional variable. When the "center" of distribution of the sulfation system was anterior to that for glucuronidation, the steady-state hepatic extraction ratio of harmol (E) would increase, whereas the ratio of the steady-state rates of formation of harmol sulfate to harmol glucuronide (S/G ratio) would decrease when harmol was presented in a reversed direction (via retrograde perfusion), as compared with normal directional flow to the liver. The converse was true for an anterior distribution of the glucuronidation system; E would decrease, whereas S/G would increase with retrograde perfusion. To evaluate such zonal differences experimentally, perfusion studies were conducted in livers of male Wistar rats. A constant concentration of harmol (50 microM) was delivered under constant hepatic flow rate (10 ml/min/liver) by normal and retrograde perfusions to the same rat liver preparation. The steady-state hepatic E was higher (P less than .005) during retrograde perfusion than during normal perfusion, whereas the S/G ratio was significantly decreased (P less than .0005). The observations suggest an intercellular difference in the distribution of the two conjugating systems and are consistent with the view of the center of distribution for the sulfation system being anterior to the center of distribution for the glucuronidation system along the normal flow path of blood in the liver.

Alkaloids↗
PubMed1982

Kinetics of sulfation and glucuronidation of harmol in the perfused rat liver preparation. Disappearance of aberrances in glucuronidation kinetics by inhibition of sulfation.

Harmol is conjugated by glucuronidation and sulfation when it is given to the rat in vivo. In the once-through perfused rat liver preparation glucuronidation of harmol shows kinetic aberrances [Pang et al., J. Pharmac. exp. Ther. 219, 134 (1981)]. In order to further delineate the mechanism behind this, sulfation was inhibited to about 10% of control by 2,6-dichloro-4-nitrophenol. The loss of sulfation was compensated by an increase in the rate of glucuronidation, keeping the total clearance by the liver virtually constant in spite of the loss of sulfation. The inhibition of sulfation eliminated the previously observed lag-phase in the kinetics of glucuronidation; the rate of glucuronidation was now almost linear with the input concentration of the substrate harmol. The constant clearance of harmol in spite of inhibition of sulfation, the occurrence of the lag-phase in glucuronidation in the presence of sulfation, and the disappearance of this lag-phase in the absence of sulfation can be explained by either diffusion-limited metabolism of harmol or a heterogeneous sub-lobular distribution of the sulfating and glucuronidating systems. Activation of glucuronidation by harmol at high concentration can be excluded.

Alkaloids↗
PubMed1982

Selective inhibition of sulfate conjugation in the rat: pharmacokinetics and characterization of the inhibitory effect of 2,6-dichloro-4-nitrophenol.

The pharmacokinetics of 2,6-dichloro-4-nitrophenol (DCNP) have been studied in the rat. Upon i.v. injection the plasma decay curve of DCNP showed a rapid distribution phase. After 30 min the plasma concentration reached a value that was constant for at least 90 min, indicating very slow elimination of DCNP. The volume of distribution was 88 ml/kg and a high degree of binding (over, 99%) of DCNP in vitro to bovine serum albumin was found. The concentration of DCNP in the liver was between 30 and 50% of the plasma values. While in vivo the effect of DCNP persisted for a long time, its action was readily reversible in the single-pass perfused rat liver. In vivo, the effect of the dose of DCNP on the inhibition of sulfation of the phenolic compound harmol was investigated. Upon the i.v. injection of 26 mumole DCNP/kg an instantaneous and complete inhibition of sulfation of harmol was found. Using this property of DCNP, the rate of sulfation of harmol in vivo was evaluated in relation to the dose and the time after injection of the substrate. Saturation of sulfation apparently occurred because the consumption of inorganic sulfate was extremely small.

Animals↗
PubMed1982

Metabolite kinetics: formation of acetaminophen from deuterated and nondeuterated phenacetin and acetanilide on acetaminophen sulfation kinetics in the perfused rat liver preparation.

The role of hepatic intrinsic clearance for metabolite formation from various precursors on subsequent metabolite elimination was was investigated in the once-through perfused rat liver preparation. Two pairs of acetaminophen precursors: [14C] phenacetin-d5 and [3H] phenacetin-do, [14C] acetanilide and [3H] phenacetin were delivered by constant flow (10 ml/min/liver) either by normal or retrograde perfusion to the rat liver preparations. The extents of acetaminophen sulfation were compared within the same preparation. The data showed that the higher the hepatocellular activity (intrinsic clearance) for acetaminophen formation, the greater the extent of subsequent acetaminophen sulfation. The findings were explained on the basis of blood transit time and metabolite "duration time." Because of blood having only a finite transit time in liver, the longer the drug requires for metabolite formation, the less time will remain for metabolite sulfation and the less will be the degree of subsequent sulfation. Conversely, when the drug forms the primary metabolite rapidly, a longer time will remain for the metabolite to be sulfated in liver to result in a greater degree of metabolite sulfation. Finally, the effects of hepatic intrinsic clearances for metabolite formation and zonal distribution of enzyme systems for metabolite formation and elimination in liver are discussed.

Acetaminophen↗
PubMed1981

Metabolite pharmacokinetics: the area under the curve of metabolite and the fractional rate of metabolism of a drug after different routes of administration for renally and hepatically cleared drugs and metabolites.

A model comprised of four compartments, a central and liver compartment for a drug, and a central and liver compartment for a metabolite, is presented to describe the interrelationships between the area under the curve of the metabolite and physiological parameters after intravenous and intraportal administration of the drug. The model includes renal and hepatic eliminatory mechanisms for both drug and metabolite as long as the metabolite is formed only by the liver. It is found that when competing renal eliminatory pathways exist for a drug, the area under the curve for the metabolite will change according to the route of drug administration. Also, the fractional rate of metabolism of a drug to form the metabolite will be underestimated by the normal use of the ratio areas under the curve of the metabolite. Other properties of the model are also discussed.

Animals↗
PubMed1981

Retrograde perfusion to probe the heterogeneous distribution of hepatic drug metabolizing enzymes in rats.

The elimination of [14C]acetaminophen which was formed from [14C]phenacetin was slower than that for the preformed metabolite, [3H]acetaminophen. The observation had been attributed to the uneven distribution of enzyme systems for O-deethylation in the formation of acetaminophen and sulfate-conjugation of acetaminophen in the liver parenchyma. In this study, retrograde perfusion which reversed not only the direction of hepatic flow into the liver but also the location of enzyme systems with respect to the flow path, was used to examine the elimination kinetics of [14C]acetaminophen and [3H]acetaminophen. In the same rat liver preparation, both [14C[phenacetin and [3H]acetaminophen in tracer concentrations were delivered simultaneously into the perfused rat liver in situ preparation with normal directional flow (into the portal vein and out of the hepatic vein). The direction of flow was then reversed to retrograde perfusion (into the hepatic vein and out of the portal vein) which was later reverted back to normal perfusion. The elimination of [14C]acetaminophen was again slower than the elimination of [3H]acetaminophen during normal perfusion, but the elimination kinetics were virtually identical for both metabolite species during retrograde perfusion. This finding confirmed our previous hypothesis that sulfate-conjugation occurred predominantly in the periportal region while O-deethylation occurred preferentially in the centrilobular region of the liver and showed that retrograde perfusion was a useful probe in the investigation of the uneven distribution of hepatic drug metabolizing enzyme systems.

Acetaminophen↗
PubMed1979

High-performance liquid chromatographic assay for acetaminophen and phenacetin in the presence of their metabolites in biological fluids.

We propose a method in which tracer amounts of a radiolabeled compound are used as the internal standard for the same unlabeled compound in high-performance liquid chromatography. The approach is valuable when a response from the internal standard becomes undesirable due to the presence of interference by the metabolites. We tested our approach with phenacetin and its metabolites, acetaminophen, 2-hydroxyphenacetin, N-hydroxyphenacetin, phenetidine, acetaminophen sulfate conjugate and acetaminophen glucuronide conjugate in biological fluids with the use of [14C]phenacetin and [3H]acetaminophen as the internal standards, and were able to quantitate both phenacetin and acetaminophen simultaneously. We also tested the alternative approach in which the unlabeled drug was used as internal standard for tracer amounts of the same radiolabeled compound, with phenacetin and acetaminophen as the internal standards for tracer amounts of [14C]phenacetin and [3H]acetaminophen. Again, we were able to quantitate the two tracer radiolabeled compounds simultaneously.

Acetaminophen↗
PubMed1979

Sequential first-pass elimination of a metabolite derived from a precursor.

We examined data from our previous studies in which we not only delivered perfusate containing tracer concentrations of [14C]phenacetin and its metabolite [3H]acetaminophen under constant perfusate flow (10 ml/min/liver) into the rat liver preparation just once, but also recirculated fresh reservoir perfusate containing a tracer dose of [14C]phenacetin through the same rat liver preparation. From the single-pass studies, estimates of fm, the fractional rate of conversion for [14C]phenacetin to form [14C]acetaminophen, and F(M.P), the apparent availability of [14C]acetaminophen, were obtained by determining the concentrations of [14C]acetaminophen in the perfusate before and after incubation with Glusulase. These estimates were fm = 0.871 +/- 0.16 and F(M.P) = 0.43 +/- 0.10. These and the steady-state clearance values of phenacetin (9.1 +/- 0.8 ml/min) and acetaminophen (6.7 +/- 0.7 ml/min) from the single-pass studies were used to predict the concentrations of [14C]acetaminophen in the reservoir perfusate on recirculation of [14C]phenacetin. We found that the sequential first-pass elimination of the metabolite must be considered when the metabolite is highly extracted by the liver. If we had neglected to take this into account, the fractional rate of conversion of a precursor to form a metabolite and the rate of formation of the metabolite would have been underestimated by the factor F(M.P).

Acetaminophen↗
PubMed1979

Elimination of allopurinol by the perfused rat liver preparation.

The elimination of allopurinol was studied in the perfused rat liver preparation. When tracer input [14C]-allopurinol concentrations (0.04 to 0.17 microgram/ml) were delivered under constant flow rate (10 ml/min/liver) only once through the rat liver, the steady-state hepatic extraction ratios were high (0.87 +/- 0.03). But at higher input allopurinol concentrations (3.3 to 17 micrograms/ml), concentration-dependent elimination of allopurinol was observed. The steady-state hepatic extraction ratio was drastically reduced with increases in input concentrations. At very high input concentrations of allopurinol (17 micrograms/ml) the hepatic extraction ratio was virtually zero. These findings inferred that the rat liver may play only a moderate role in the elimination of allopurinol in the intact animal.

Allopurinol↗
PubMed1978

Theoretical relationships between area under the curve and route of administration of drugs and their precursors for evaluating sites and pathways of metabolism.

The bioavailability of a drug administered extrasystemically is a measure of the initial extraction of a compound by a series of eliminating events involving the intestinal mucosal enzymes, the gut bacterial microflora, the liver, and the lung. A theoretical analysis is presented to differentiate the process of gut wall elimination and hepatic removal of a drug during this first-pass effect. The area under the blood concentration--time curve (AUC) for a drug and its metabolite is shown to be useful in determining the presence of these processes when a drug and its metabolite are administered concomitantly by different routes of administration. Furthermore, the fraction of a precursor transformed to its metabolite also can be determined by pharmacokinetic analysis of the AUC of a drug and its metabolite after administration of both substances.

Biological Availability↗