Kikuo Iwamoto

Gastrointestinal and hepatic first-pass metabolism of aspirin in rats.

The first‐pass effect of aspirin was measured in male Wistar rats by comparing the plasma concentration after intravenous, oral or intraportal administration (10 mg kg−1) of the drug. Approximately 88 and 86% of the dose was excreted mostly as salicylic acid and its conjugated forms, glucuronide and sulphate, in urine within 48 h of i.v. or oral administration, respectively. This suggests that the gastrointestinal absorption of aspirin was essentially complete in rats. On the average, the area under the plasma concentration‐time curve for unchanged aspirin following oral dosing (AUCo) was 0.35 of that obtained following i.v. administration (AUCi.v) and 0.53 of that following intraportal administration (AUCp). Therefore, orally administered aspirin is subject to first‐pass metabolism both in the gut and in the liver of rats. The gastrointestinal first‐pass effect is estimated to be relatively more important than the hepatic effect.

Dose-dependent presystemic elimination of propranolol due to hepatic first-pass metabolism in rats

Gastrointestinal first‐pass elimination of propranolol and the effect of dose (1.0, 2.5, 5.0 and 10.0 mg kg−1) on its systemic availability were studied in male Wistar rats which received the drug intravenously, orally or intraportally. The plasma elimination half‐life was not altered either by the route of administration or the dose. There was no gastrointestinal first‐pass metabolism of propranolol, since the same systemic availability was obtained after oral and intraportal administration. Hepatic clearance was estimated to be constant at any dose. In contrast, the hepatic intrinsic clearance was found to be largely dependent on the portal dose.

Effect of age on the hepatic clearance of propranolol in rats

The effect of age on hepatic clearance of propranolol was investigated in male Wistar rats (3 to 104 weeks old). Pharmacokinetic analysis of the plasma propranolol data obtained after i.v. dosage (1.0 mg kg−1) indicated that both the elimination rate constant and the volume of distribution decreased with age between weeks 5 and 11, after when the distribution volume remained almost constant. The elimination rate constant was reduced consistently with age after 15 weeks. Total body clearance of the drug was reduced extensively with age between weeks 5 and 11 (94 to 43 ml min−1 kg−1) and decreased gradually thereafter. The plasma free fraction of propranolol also decreased with age, falling from more than 20% in 3 to 5 weeks‐old rats to about 10% in rats of 52 to 104 weeks. In immature rats the renal clearance was approximately 5 to 12% of the total body clearance. Intrinsic hepatic clearance for unbound propranolol also decreased with age between weeks 7 and 24. As expected from the evidence that propranolol has a high extraction ratio, the extent of its hepatic clearance was significantly dependent on the liver blood flow. These data suggest that the age‐dependent decrease in hepatic clearance of propranolol is largely due to a reduction of the elimination rate that might be accompanied by the age‐dependent decrease in liver blood flow.

Gastrointestinal and hepatic first-pass effects of salicylamide in rats.

m e orally administered drug that exhibits reduced systemic availability even after being completely absorbed from the gastrointestinal tract is. subject to first-pass metabolism. To identify where this occurs a drug is either directly administered into the hepatoportal vein, whereby the extraction and/or metabolism component in the gut is bypassed (Harris & Riegelman 1969; Iwamoto & Klaassen 1977a, b; Back et a1 1978; Iwamoto et a1 1982), or a portal systemic shunt that bypasses the liver is created (Gugler et al 1975). Salicylamide has been recently excluded from several National Pharmacopoeias because of its lower efficacy by mouth compared with aspirin. Salicylamide was reported to have the first-pass effect during intestinal absorption in rabbits (Barr & Riegelman 1970a, b) and to have the effect both in the gut and in the liver of dogs (Gugler et a1 1975). Shibazaki et a1 (1981) have also reported the relative importance of the intestine and the liver in the first-pass metabolism of salicylamide in rabbits and rats the intestinal segments of which were perfused in-situ. However, neither the intestinal or hepatic effects have been evaluated either completely or directly in intact animals. We have examined the ettects 01 routes or administration upon the pharmacokinetics of salicylamide in rats, to estimate the relative importance of the gut and the liver in producing the first-pass effect, to compare this effect in rats with those reported in dogs (Gugler et a1 1975) and in rabbits (Shibazaki et a1 1981) and to compare the first-pass metabolism of salicylamide with that of aspirin in rats (Iwamoto et al 1982).

Age-dependent propranolol clearance in perfused rat liver☆

The effect of age on the hepatic clearance of propranolol was studied by perfusing the liver isolated from 3- to 104-week-old rats. Propranolol levels in the recirculating perfusate declined biexponentially with time in all age groups. When the liver isolated from 7-week-old rats was perfused with propranolol (1 microgram/ml, 100 ml), hepatic clearance of this drug by the perfused liver (CLperf) increased from 0.589 to 1.14 ml X min-1 X (g liver)-1 with the increase of the perfusion flow rate from 1.0 to 2.0 ml X min-1 X (g liver)-1, confirming evidence of perfusion-limited hepatic clearance for this drug. Furthermore, there was no initial concentration(dose)-dependence in CLperf up to 2.5 micrograms/ml (i.e. 250 micrograms/organ). The effect of age on CLperf was then investigated by perfusing the isolated liver with 1.0 micrograms/ml propranolol at 2.0 ml X min-1 X (g liver)-1. Elimination of this drug from the perfusion medium was relatively rapid in 5- to 7-week-old rats, yielding the highest CLperf in these relatively young rats [approximately 1.0 to 1.1 ml X min-1 X (g liver)-1]. In contrast, CLperf values in both immature and older rats were 0.5 ml X min-1 X (g liver)-1 or less. The in vitro intrinsic hepatic clearance estimated in 5- and 7-week-old rats was about ten times as high as that in 104-week-old rats.

Difference in hepatic uptake kinetics of aspirin and salicylamide in rats

Immediately after intraportal administration to rats, the ratio of liver to plasma concentrations for total aspirin was close to unity, whereas that for total salicylamide ranged from about 3 to 7. The hepatic accumulation of salicylamide appeared to be capacity-limited because the ratio decreased with increases in the dose. In vitro experiments with isolated hepatocytes indicated that aspirin was slowly transported into the hepatocytes by an apparently linear process only, while salicylamide was taken up very rapidly by both saturable and apparently linear transport processes. The cell to medium concentration ratio estimated for the initially net transported component of the unchanged drug was significantly larger with salicylamide, which give ratios from 3.5 to 19, than with aspirin which gave an almost constant value lower than 2 despite wide variations in the initial concentration. For the capacity-limited uptake process of salicylamide, the kinetic parameters were estimated as Vmax = 0.325 nmole X (mg cellular protein)-1 X sec-1 and Km = 201 microM. Among various metabolic inhibitors, 2,4-dinitrophenol (50 microM) inhibited the uptake of salicylamide most extensively. The present comparison of the in vivo and in vitro data for aspirin with those for salicylamide confirmed the previously reported difference in the hepatic first-pass effect of these two drugs.

Limited capacity of jejunal segments to effect glucuronidation and sulphation of aspirin and salicylamide in rats

We have previously demonstrated that both aspirin and salicylamide, after oral administration to rats, are subject to the first-pass effect in the gastrointestinal tract as well as in the liver and that gastrointestinal first-pass effect of aspirin is relatively large compared with the hepatic effect whilst with salicylamide this was reversed (Iwamoto et a1 1982,1983). It has been implied that in man (Rowland et al 1967) and dogs (Harris & Riegelman 1969), most of the hydrolysis of aspirin occurs in the liver the remainder taking place across the intestinal mucosa. The site at which salicylamide is conjugated with ethereal sulphate has not been clarified in man (Levy & Matsuzawa 1967), but it has been shown that rabbit ileum is able to effect the glucuronidation of salicylamide (Barr & Riegelman 1970). This approach to the determination of conjugated metabolites immediately after their formation by continuous collection of maenteric venous blood samples from rabbits (Barr & Riegelman 1970) has been recently applied to rats, indicating that both morphine and nalorphine are subject to capacity-limited jejunal glucuronidation invivo (Iwamoto & Klaassen 1977) and that salicylamide is exclusively glucuronated in the in-situ ileal portion (Shibazaki et al 1981). The effect of dose on in-vivo intestinal conjugation of aspirin and salicylamide is now reported in this communication which describes and compares for the first time the capacity-limited conjugation (glucuronidation and sulphation) of aspirin and salicylamide in rats by direct mesurement of the amounts of both unchanged and conjugated drugs in the mesenteric venous blood immediately after absorption.

Propranolol uptake with high capacity by rat perfused lung

Abstract— Lung isolated from 7‐week‐old rats was perfused with pH 7.4 Krebs‐Ringer bicarbonate buffer solution (35 mL) containing 1 to 100 μg mL−1 of propranolol and 3% BSA at the recirculation rate of 8 mL min−1. Almost parallel bi‐exponential drug concentration‐time curves were obtained at the initial load lower than 10 μg mL−1, whereas relatively slow, mono‐exponential decline was found after perfusion at 100 μg mL−1. Pharmacokinetic analysis for the perfusate propranolol concentration‐time curves when loaded at 1 to 10 μg mL−1 yielded almost comparable values for the pulmonary perfusion clearance (0.387 ± 0.092 to 0.486 ± 0.095 mL min−1 g−1). In contrast, this parameter was significantly reduced at 100 μg mL−1 (0.113 ± 0.042 mL min−1 g−1). The present findings suggest a trend towards saturation kinetics in the in‐vitro pulmonary clearance of propranolol.

Mandibular and parotid salivary levels of indomethacin following intravenous administration to rabbits

Indomethacin was measured in mandibular and parotid saliva, obtained from separate cannulas in the salivary ducts, after bolus intravenous administration (15 mg/kg) to male white rabbits that were stimulated for salivation with pilocarpine given subcutaneously. There was a significant correlation between each salivary drug concentration and plasma drug concentration. Saliva to plasma drug concentration ratio (S/P ratio) and pH were higher in mandibular saliva than in parotid saliva. These gland specific differences were in contrast with the previously reported differences in dogs. Matins equation was found to predict approximately the mean observed S/P ratio of indomethacin for each saliva sample.

Specific age-dependence in capacity-limited uptake of propranolol by isolated rat lung.

To investigate the effect of age on the pulmonary uptake of propranolol, minced tissue (0·4 g) of lungs isolated from 3‐ to 104‐week‐old rats was incubated with the drug (1 to 500 ng μL−1) prepared in oxygenated, pH 7·4 Krebs‐Ringer bicarbonate buffer solution (20 mL) containing 3% BS A for 60 min at 37°C. In any age‐group the metabolism of propranolol was not significant (i.e. less than 0·6% of any initial load) under the present in‐vitro conditions. The extent of uptake after the incubation with 2·5 μ mL−1 of the drug was largest in the 7 weeks (i.e. 82% of the initial load) and relatively small in the 3(64%), 24(61 %), 52(51%) and 104(48%) week old rats. Similar, specific age‐dependence was observed in the tissue‐to‐medium concentration ratio of the drug. In any age‐group, the initial uptake rate obtained in the first 5 min of the incubation was found to be a combination of apparently linear transport and saturable (capacity‐limited) processes. There was a marked, specific age‐dependence in the capacity‐limited uptake rate. Although Km′ value was almost equivalent in any age‐group (i.e. 24·4 to 25·4 μg mL1), Vmax exhibited a specific age‐dependence by yielding the highest value in 7 weeks (0·726 ± 0·101 mg g−1 min−1) and relatively low values in 3 (0·501 ±0·082 mg g−1 min−1), 52 (0·410 ± 0·088 mg−1 g−1 min1) and 104 (0·397 ± 0·074 mg g−1 min−1) weeks.