L.W. Whitehouse

Effect of a concomitant single dose of ethanol on the hepatotoxicity and metabolism of acetaminophen in mice

A concomitant single dose of ethanol (1 g/kg) protected mice from hepatic injury induced by acetaminophen (250 mg/kg) as evidenced by the lowering of plasma transaminases. Pharmacokinetic studies with [14C]acetaminophen indicated that ethanol enhanced the initial blood concentrations of radiolabel and its rate of elimination. A tissue distribution study suggested that these effects were probably due to an ethanol-induced inhibition of the biliary clearance of acetaminophen from the blood. Examination of the urinary and biliary metabolites indicated that ethanol inhibited the excretion of the degradation products derived from the glutathione-deactivated hepatotoxic acetaminophen intermediate. The decrease in acetaminophen induced hepatotoxicity was therefore attributed to an inhibitory effect of ethanol on the biotransformation of acetaminophen to the toxic intermediate.

Pathways of disposition of acetaminophen conjugates in the mouse.

After a single dose of [14C]acetaminophen (50 mg/kg) was administered orally to bile duct cannulated mice, 13.9% of the radioactivity was recovered in the bile while 41.2% was found in the urine in the first 3 h after administration. Analyses of biles revealed that the major biliary metabolite was acetaminophen glutathione (AG) conjugate which was derived from the hepatotoxic acetaminophen intermediate. Examination of urines showed that they contained mostly glucuronide and sulfate conjugates with no AG or its degradation products (cysteine and mercapturate). Analysis of urines collected from non-cannulated animals at 4 h showed that they contained glucuronide, sulfate, cysteine and mercapturate metabolites. Our results suggest that after formation in the liver, the majority of the glucuronide and sulfate conjugates were directly eliminated by the kidney. On the other hand, the pathway for the disposition of the glutathione conjugate was first into the bile, then reabsorption, and finally disposition into the urine as cysteine and mercapturate metabolites.

Effect of acetylsalicylic acid on a toxic dose of acetaminophen in the mouse

Abstract The effect of acetylsalicylic acid (ASA) on the toxicity and metabolism of [14C]acetaminophen was studied in the mouse. Pretreatment with ASA did not affect lethality, but hepatotoxicity as determined by plasma transaminases was reduced by ASA pretreatment. The blood concentration profiles of radioactivity were altered by ASA following po and ip administration. It was suggested that ASA reduced the rate of acetaminophen absorption and inhibited elimination. Paper chromatography of urine showed that following ip dosing ASA reduced the excretion of sulfate conjugate but increased the excretion of total catabolites of the glutathione conjugate. In the po study a similar inhibition of sulfation was observed, but excretion of total glutathione degradation products was not altered statistically even though excretion of mercapturate was statistically elevated by ASA pretreatment. An attempt was made to correlate the excretion of glutathione degradation products (an estimate of the toxic pathway) with toxicity. A direct correlation could not always be demonstrated, and it was concluded that factors additional to toxic metabolite formation modified acetaminophen-induced hepatotoxicity in the mouse.

N-acetylcysteine-induced inhibition of gastric emptying: A mechanism affording protection to mice from the hepatotoxicity of concomitantly administered acetaminophen☆

Swiss Webster male mice, 22 +/- 3 g, killed 17-18 h following the concomitant oral administration of acetaminophen (350 mg/kg) and N-acetyl-cysteine (NAC, 100-500 mg/kg, treated) had statistically significant lower plasma transaminases (GOT and GPT) than control mice (acetaminophen + water). Possible mechanisms underlying this protective effect of NAC were examined. NAC (500 mg/kg) reduced [14C]acetaminophen-derived radioactivity in the blood and tissues but increased the percentage of the dose in the gastrointestinal tract. Depletion of hepatic sulphydryl compounds below 75% of the control value was prevented by NAC treatment, whereas urinary excretion of mercapturate and sulfate, metabolites derived from sulphydryls, were proportionally increased and excretion of unchanged drug was decreased by NAC. Absorption of acetaminophen from the small intestine was prevented by NAC and this was attributed to an inhibition in gastric emptying. Since all changes observed following NAC treatment could be attributed to inhibition of gastric emptying, it was considered the major mechanism responsible for affording in mice protection from acetaminophen-induced hepatocellular damage following concomitant oral administration.

Metabolism of [14C]Acetylisoniazid and [14C]Acetylhydrazine by the Rat and Rabbit

Male rats and rabbits were singly dosed with either 1-[14C]acetyl isoniazid (acetylisonicotinoylhydrazine, acetyl-INH, 200 mg/kg po) or 1-[14C]acetylhydrazine (50 or 100 mg/kg ip). Urine and expired 14CO2 were collected, and after 6 hr the animals were killed for the analysis of tissue 14C concentrations and covalent binding of 14C to hepatic protein. Rats excreted proportionately more 14C in urine and had lower 14C levels in their tissues compared to rabbits. When acetyl-INH was administered, covalent hepatic protein binding of the acetyl moiety was greater in the rabbit than the rat, but the opposite was observed when acetylhydrazine was administered. Analysis of blood and urine by TLC revealed that the rabbit more rapidly metabolized both acetyl-INH to acetylhydrazine, and acetylhydrazine to diacetylhydrazine than did the rat. These observations suggest that higher amidase activity in the rabbit compared to the rat leads to faster conversion of acetyl-INH to acetylhydrazine which in turn leads to greater covalent binding and hepatotoxicity.

Effect of ethanol on the pharmacokinetics of 2-14C-methaqualone in the rat.

Abstract Blood concentrations of radioactivity at 2, 3 and 4 hr and tissue concentrations at 4 hr following 2- 14 C-methaqualone (25 mg/kg po) administration, were statistically higher in rats simultaneously dosed with ethanol (3 gm/kg po) than in controls receiving only methaqualone. The major route of elimination was biliary excretion and ethanol inhibited the biliary clearance of carbon-14. An inhibition of metabolism by ethanol could not be demonstrated and reduced clearance was attributed to a depression of active secretory processes by ethanol. Consequently, an elevation of plasma concentrations of free drug and an increased uptake of methaqualone into lipoid tissues such as the brain occurred offering an explanation for reports of potentiation following administration of the ethanolmethaqualone combination.

Use of a discrete diode array spectrophotometer for the quantitative and qualitative analysis of cytochrome P-450

Cytochrome P-450, the hemoprotein located in the endoplasmic reticulum of mammalian cells and responsible for the metabolism of xenobiotics, was qualitatively and quantitatively analyzed using the HP-8450A diode array spectrophotometer. The diode array instrument was compared to a conventional spectrophotometer and the advantages of the diode array instrument over conventional spectrophotometry with respect to the analysis of cytrochrome P-450 were discussed.

Effects of rifampin pretreatment on hepatic parameters in the rabbit

Rifampin pretreatment in the rabbit caused a selective induction of hepatic parameters resembling in some respects phenobarbital induction. Concomitant with induction, a transient selective inhibition of hepatic parameters was also observed. This two-fold effect of rifampin offers an explanation for the discrepancy surrounding the dosage and species differences in hepatic induction reported in the literature.