Lack of first-pass metabolism of ethanol at blood concentrations in the social drinking range

PKQuest: measurement of intestinal absorption and first pass metabolism - application to human ethanol pharmacokinetics

BACKGROUND

PKQuest, a new physiologically based pharmacokinetic (PBPK) program, is applied to human ethanol data. The classical definition of first pass metabolism (FPM) based on the differences in the area under the curve (AUC) for identical intravenous and oral doses is invalid if the metabolism is non-linear (e.g. ethanol). Uncertainties in the measurement of FPM have led to controversy about the magnitude of gastric alcohol metabolism. PKQuest implements a new, rigorous definition of FPM based on finding the equivalent intravenous input function that would produce a blood time course identical to that observed for the oral intake. This input function equals the peripheral availability (PA) and the FPM is defined by: FPM = Total oral dose - PA. PKQuest also provides a quantitative measurement of the time course of intestinal absorption.

METHODS

PKQuest was applied to previously published ethanol pharmacokinetic data.

RESULTS

The rate of ethanol absorption is primarily limited by the rate of gastric emptying. For oral ethanol with a meal: absorption is slow (Tilde; 3 hours) and the fractional PKQuest FPM was 36% (0.15 gm/Kg dose) and 7% (0.3 gm/Kg). In contrast, fasting oral ethanol absorption is fast (Tilde; 50 minutes) and FPM is small.

CONCLUSIONS

The standard AUC and one compartment methods significantly overestimate the FPM. Gastric ethanol metabolism is not significant. Ingestion of a coincident meal with the ethanol can reduce the peak blood level by about 4 fold at low doses. PKQuest and all the examples are freely available on the web at www.pkquest.com.

Disposition and first-pass metabolism of ethanol in humans: is it gastric or hepatic and does it depend on gender?

OBJECTIVE

To assess the extent and site of the first-pass metabolism of ethanol and to examine whether first-pass metabolism and disposition of ethanol are dependent on gender.

METHODS

After a standardized lunch, healthy subjects (six women and six men) received on two separate occasions a 60-minute intravenous infusion of ethanol (0.3 gm/kg) and concomitantly an equimolar dose of d3-ethanol/kg either orally (over 20 minutes) or intraduodenally (infused over 30 minutes). Blood levels, urinary excretion of d0- and d3-ethanol, and sedative effects were monitored for 6 hours. Disposition and first-pass metabolism of ethanol were evaluated by applying an open two-compartment model with Michaelis-Menten elimination.

RESULTS

Comparison of the corresponding intravenous/oral versus intravenous/intraduodenal data of each individual revealed that total first-pass metabolism (gastric plus hepatic) was not pronounced in either males (9.1% +/- 4.0%; mean +/- SD) or females (8.4% +/- 3.1%) and that this first-pass metabolism was partly of gastric origin. Dose-corrected values for area under blood concentration-time curve were on average 28% higher (p < 0.0001) in the women than in the men. Mean total blood ethanol disappearance rate was higher (p < 0.001) in women (3.92 +/- 0.40 mmol/L . hr) than in men (3.19 +/- 0.48 mmol/L . hr). Renal clearance was gender-independent and negligible. A linear relationship (p < 0.001) could be found between the blood levels of ethanol and sedation index. Because the slope was steeper in women (1.04) than in men (0.42) a higher central nervous system sensitivity to the sedative effects of ethanol in women can be assumed.

CONCLUSIONS

Under realistic life conditions (social drinking of moderate doses of ethanol after a light lunch) only a minor, gender-independent first-pass metabolism is observed that is partly of gastric origin.

Diet composition, alcohol utilization, and dependence

Experiments were carried out in which a nutritionally balanced liquid diet previously used in this laboratory was modified as to total calorie content and high or low carbohydrate and fat concentration. Ethanol was added at 4.5% and 6.2% of diet weight and provided either 27% or 34-37% of total calories depending upon the changes in nutrient content. Measurements included 8-day food/calorie and ethanol consumption, plasma ethanol level, liver alcohol dehydrogenase (ADH) activity, and rate of audiogenic-induced withdrawal seizures. The original liquid diet with 4.5% ethanol was consumed in significantly lesser amounts than the alcohol-free diet, and essentially no body weight gain occurred, regardless if the major nonalcohol, nonprotein calorie source was fat or carbohydrate. When the calorie content of the diet was boosted through the addition of extra carbohydrate or fat (at the expense of water), appreciable weight gain was noted; in the case of the higher calorie diet boosted with more carbohydrate (maltodextrin) calories, growth was similar to that observed on the alcohol-free control diet. On this latter diet ethanol calories appeared to be utilized close to their theoretical value of 7 kcal/g. Blood alcohol levels were significantly higher on the lower calorie diets and were lowest on the high-calorie, high-carbohydrate, 4.5% ethanol diet. This diet also allowed for the lowest rate of withdrawal seizures despite an ethanol intake that was as high as on the lower calorie diets. Essentially, no differences were noted among ADH activities for the dietary treatments studied and thus, did not explain the differences observed among blood ethanol levels. When the alcohol concentration in the high-carbohydrate, high-calorie diet was raised to 6.2% from 4.5% to provide 34% of total calories, the rats responded by decreasing their food (and alcohol) intake to the same level as did the animals receiving a much lower calorie diet, but with 37% of caloric alcohol content. This suggests that at a diet alcohol concentration of 34-37%, one or more nutrient metabolites become limiting in the utilization of ethanol, resulting in food intake adjustments that maintain similar amounts of alcohol consumption.

Sex- and strain-related differences in first-pass alcohol metabolism in mice

Adult males and females of three strains of mice, C57BL/10J, C57BL/6J and DBA/2J, were intubated or injected intraperitoneally with 0.02 ml/g body weight of a 25% alcohol solution. Thirty minutes later, their blood alcohol levels (BAL) were measured. Another group of mice, including both sexes, representative of the three strains, was fasted for 12 h and sacrificed; their stomachs were removed, homogenized, and assayed for gastric alcohol dehydrogenase (GAD) activity. Higher BALs were found in all intubated females compared to the intubated males. The reverse was observed in the injected group, which showed the males with the highest BAL values. GAD activity was evidenced in both sexes of the three strains and it was highest in the males. Strain-related differences were evident in the intubated groups and not in the injected groups. Intubated DBA animals had the lowest BALs as well as the highest GAD values. The results provide evidence for first-pass alcohol metabolism in mice and show the effects of sex and strain on gastric oxidation of alcohol.

The effect of gastritis on human gastric alcohol dehydrogenase activity and ethanol metabolism

BACKGROUND

Gastric mucosal alcohol dehydrogenase (ADH) may decrease the bioavailability of ingested ethanol. Because this enzyme is found in highest concentrations in the superficial gastric mucosa, diffuse abnormalities of this tissue could lead to a decrease in the first pass metabolism of ethanol.

METHODS

Thirty-three adult subjects undergoing routine upper gastrointestinal endoscopy had gastric biopsies performed for assessment of gastric histology and the measurement of gastric ADH activity. Twenty of these subjects underwent separate oral dosing and intravenous infusion of ethanol (0.15 g/kg body weight) in order to determine the first pass metabolism, and hence bioavailability, of ethanol.

RESULTS

Gastric histology was normal in 10 of the biopsies, showed chronic gastritis alone in 13 and significant glandular atrophy (i.e. atrophic gastritis) in a further 10. Gastric ADH activity in specimens with normal gastric histology was significantly higher than those with chronic gastritis (P = 0.02), and was further decreased in those specimens with significant atrophy (P < 0.00001). However, no correlation was found between gastric ADH activity and the first pass metabolism of ethanol (r = 0.09, P = 0.9).

CONCLUSIONS

These results suggest that although gastric ADH activity was decreased in individuals with abnormal gastric mucosa, ethanol bioavailability was not affected by gastric ADH activity. These data support the view that gastric ADH does not play a significant role in the first pass metabolism of alcohol.

H2-antagonists and alcohol. Do they interact?

There are conflicting data on the existence of significant first-pass metabolism of alcohol (ethanol) in the human stomach and its inhibition by histamine H2-receptor antagonists. Alcohol is predominantly metabolised in the liver by the microsomal alcohol oxidising system, alcohol dehydrogenase (ADH) and a catalase enzyme. Histochemical and kinetic studies have revealed several ADH isoenzymes in the gastric mucosa with different kinetic properties. After small oral doses of alcohol first-pass metabolism in the stomach occurs, as shown by reduced area under the plasma concentration-time curve (AUC) compared with intravenous or intraduodenal administration. The activity of gastric ADH is reduced in women, the elderly, Asian individuals, the fasting state, chronic alcoholism and after gastrectomy. The effect is only present with small (< or = 0.3 g/kg) alcohol doses and with a high alcohol concentration. In a number of studies, cimetidine in therapeutic doses over 7 days produced a significant increase in the AUC and in the peak plasma concentration after administration of alcohol 0.15 and 0.30 g/kg. This was related to an inhibition of gastric ADH activity, as shown by in vitro studies. Ranitidine inhibited gastric ADH to a similar extent on a molar basis, but its effect on alcohol levels in vivo was less constant in various studies. Nizatidine also reduced gastric alcohol first-pass metabolism, but famotidine and roxatidine did not show this effect. In other studies, H2-receptor antagonists did not change AUC and peak alcohol concentration. The controversy is not easy to resolve, since a number of the positive studies did not use a placebo-controlled, randomised, crossover design, while some of the negative studies did not exclude habitual alcohol consumers and included Oriental volunteers, although both groups have been shown to lack significant gastric ADH activity. In this case, when first-pass metabolism of alcohol does not exist, this by definition cannot be abolished by H2-antagonists. The inclusion of oral and intravenous dosage data of alcohol is mandatory to positively identify first-pass metabolism in any individuals. The significance of the effect of H2-antagonists on blood alcohol concentrations is minor. It only occurs in young, male, nonalcoholic, non-Asian individuals, and alcohol must be given in a small (social) dose, in a high concentration, and after meals. An increase in alcohol levels in predisposed patients during treatment with some H2-antagonists cannot be excluded, although the likelihood is small. Furthermore, carefully designed studies are needed to clarify fully the significance of this interaction.

Effect of dihydrotestosterone on the rate of ethanol elimination in healthy men

Testosterone (T) and, more potently, dihydrotestosterone (DHT), are in vitro inhibitors of hepatic alcohol dehydrogenase (ADH). An increase in the rate of ethanol disappearance (RED) in man has been observed after orchidectomy. We thus investigated the influence of DHT administration on RED in 10 healthy male volunteers. RED was estimated after an oral ethanol bolus (0.6 g/kg), before and after a 14-day treatment with DHT (2 x 125 mg per day percutaneously; Andractim, Besins iscovesco, France). A mean 11.5-fold increase in DHT levels and a 2.0-fold decrease in T values were observed after DHT administration, confirming the good compliance with treatment. RED was decreased after DHT (0.168 +/- 0.043 vs. 0.137 +/- 0.043 g/l/hr; mean +/- SD; p less than 0.01 using Wilcoxon's paired comparison test). These data are consistent with a DHT-induced inhibition of hepatic ADH in vivo. This could have a beneficial effect by decreasing acetaldehyde production in alcoholic patients, in whom marked hypoandrogenism frequently occurs.

High blood alcohol levels in women. The role of decreased gastric alcohol dehydrogenase activity and first-pass metabolism

After consuming comparable amounts of ethanol, women have higher blood ethanol concentrations than men, even with allowance for differences in size, and are more susceptible to alcoholic liver disease. Recently, we documented significant "first-pass metabolism" of ethanol due to its oxidation by gastric tissue. We report a study of the possible contribution of this metabolism to the sex-related difference in blood alcohol concentrations in 20 men and 23 women. Six in each group were alcoholics. The first-pass metabolism was determined on the basis of the difference in areas under the curves of blood alcohol concentrations after intravenous and oral administration of ethanol (0.3 g per kilogram of body weight). Alcohol dehydrogenase activity was also measured in endoscopic gastric biopsies. In nonalcoholic subjects, the first-pass metabolism and gastric alcohol dehydrogenase activity of the women were 23 and 59 percent, respectively, of those in the men, and there was a significant correlation (rs = 0.659) between first-pass metabolism and gastric mucosal alcohol dehydrogenase activity. In the alcoholic men, the first-pass metabolism and gastric alcohol dehydrogenase activity were about half those in the nonalcoholic men; in the alcoholic women, the gastric mucosal alcohol dehydrogenase activity was even lower than in the alcoholic men, and first-pass metabolism was virtually abolished. We conclude that the increased bioavailability of ethanol resulting from decreased gastric oxidation of ethanol may contribute to the enhanced vulnerability of women to acute and chronic complications of alcoholism.

Mechanism of ethanol induced hepatic injury

Ethanol is hepatotoxic through redox changes produced by the NADH generated in its oxidation via the alcohol dehydrogenase pathway, which in turn affects the metabolism of lipids, carbohydrates, proteins and purines. Ethanol is also oxidized in liver microsomes by an ethanol-inducible cytochrome P-450 (P-450IIE1) which contributes to ethanol metabolism and tolerance, and activates xenobiotics to toxic radicals thereby explaining increased vulnerability of the heavy drinker to industrial solvents, anesthetic agents, commonly prescribed drugs, over-the-counter analgesics, chemical carcinogens and even nutritional factors such as vitamin A. Induction also results in energy wastage and increased production of acetaldehyde. Acetaldehyde, in turn, causes injury through the formation of protein adducts, resulting in antibody production, enzyme inactivation, decreased DNA repair, and alterations in microtubules, plasma membranes and mitochondria with a striking impairment of oxygen utilization. Acetaldehyde also causes glutathione depletion and lipid peroxidation, and stimulates hepatic collagen synthesis, thereby promoting fibrosis.

Single dose and steady-state pharmacokinetics of adinazolam after oral administration to man

An aqueous solution containing 1 mg of adinazolam mesylate per ml was administered orally as a single dose (40 mg) and with loading doses followed by hourly doses such that final dose rates of 1, 2, and 3 mg h-1 were administered to steady-state. Four subjects exhibited linear steady-state kinetics, while the other four exhibited Michaelis-Menten kinetics, based on measurement by HPLC of both unchanged drug and the major N-demethyl metabolite. The drug is very rapidly absorbed and has an intrinsic clearance of total (bound + free) drug which averaged 2.14 l min-1 based on the steady-state data and 1.17 l min-1 based on the single dose data, but these means do not differ significantly. The apparent metabolite clearance, CLmc/fm (where fm = fraction of adinazolam converted to the N-demethyl metabolite), averaged 0.170 l min-1 based on steady-state data and 0.179 l min-1 based on single dose data and these means do not differ significantly. Pharmacokinetic parameters, such as these clearances, had large intersubject variations. Three types of bioavailabilities were estimated from the data.

The contribution of the stomach to ethanol oxidation in the rat

To estimate the amount of ethanol that can be oxidized in the stomach, steady-state conditions were created in a group of fed rats by giving a loading dose of ethanol (2 g/kg body wt I.V.) followed by continuous infusion either intravenously or intragastrically. The rate of ethanol oxidation was calculated from the rate of infusion required to maintain steady blood levels of approximately 30 mM for at least 3 hours. Gastrointestinal ethanol concentrations and total contents also remained steady. The rate of ethanol oxidation was 19.3% faster during intragastric than during intravenous infusion (p less than 0.01). When measured at the prevailing luminal ethanol concentration (145 mM) and expressed per body weight, the gastric ADH activity represented 14% of the hepatic activity at 30 mM ethanol, suggesting that gastric ADH activity could account for most of the increased rate of oxidation when ethanol is given intragastrically. Thus, gastric ethanol oxidation by a high Km ADH in the rat represents a significant fraction of the total rate of ethanol oxidation and it is therefore one of the factors which determines the bioavailability of orally administered ethanol.