Selective inhibition of acetaminophen oxidation and toxicity by cimetidine and other histamine H2-receptor antagonists in vivo and in vitro in the rat and in man.

Acetaminophen-induced hepatotoxicity results from hepatic enzymatic oxidation of acetaminophen to a toxic, electrophilic intermediate. Acetaminophen is ordinarily eliminated after conjugation with glucuronic acid and sulfate to nontoxic derivatives. Cimetidine has been shown to inhibit the hepatic oxidation of a number of drugs and to protect rats from acetaminophen-induced hepatic necrosis. The aim of this study was to define the mechanism by which cimetidine reduced acetaminophen-induced hepatic necrosis and to determine whether inhibition of formation of the reactive metabolite(s) of acetaminophen occurred also in man. In vivo cimetidine pretreatment decreased covalent binding of [3H]acetaminophen to the liver from 552 +/- 23.8 to 170 +/- 31.6 nmol/g protein 2 h after a toxic dose of acetaminophen in 3-methylcholanthrene pretreated rats (P less than 0.05). Cimetidine pretreatment also significantly reduced the rate of hepatic glutathione depletion. Both cimetidine and metiamide produced dose-dependent inhibition of acetaminophen oxidation in vitro, whereas inhibition by ranitidine and cimetidine sulfoxide was quantitatively less. Inhibition of acetaminophen oxidation by cimetidine and metiamide was primarily competitive with an inhibition constant (Ki) of 130 +/- 16 and 200 +/- 50 microM, respectively. By contrast, cimetidine inhibited acetaminophen glucuronidation minimally with a Ki of 1.39 +/- 0.23 mM. Similar results were obtained using human liver microsomes as a source of enzymes. In a dose-related fashion, cimetidine also reduced acetaminophen-induced toxicity to human lymphocytes when incubated with microsomes and NADPH. Pharmacokinetics of acetaminophen elimination were studied in normal volunteers with and without co-administration of cimetidine 300 mg every 6 h. In normal volunteers, cimetidine decreased the fractional clearance of the oxidized (potentially toxic) metabolites of acetaminophen more than the conjugated metabolites. This finding confirmed the hypothesis that cimetidine is a relatively selective inhibitor of the oxidation of acetaminophen to reactive metabolites in man as well as in animals. When considered together with the results of previous studies showing improved survival and decreased hepatoxicity in acetaminophen-poisoned animals, the present results provide a rational basis for assessing possible benefits of cimetidine treatment of acetaminophen overdoses in man.

Uptake of the noncytotoxic transport probe procainamide in the Chinese hamster ovary model of multidrug resistance.

Many of the cytotoxic substrates of the multidrug transporter are organic cations. Cimetidine, procainamide, and tetraethylammonium bromide were used in a Chinese hamster ovary model of multidrug resistance, to study handling of noncytotoxic cationic transport probes. Cimetidine and procainamide, but not tetraethylammonium, accumulated to a greater extent (5-fold) in the sensitive CHOAUXB1 (AB) cell line than in the resistant CHRC5 (C5) cell line. Accumulation of both cimetidine and procainamide was significantly increased by verapamil in C5 but not AB. Procainamide accumulation in both AB and C5 was temperature dependent and occurred by passive diffusion. Diltiazem, nifedipine, rifampin, tamoxifen, rhodamine, and ethidium also increased procainamide accumulation in C5 but not AB. Azide in glucose-free medium increased procainamide accumulation in C5, and this was reversed when glucose, but not 3-O-methylglucose, was added. Procainamide efflux rates were similar in AB and C5 and not affected by verapamil or azide. The initial rate of procainamide uptake was higher in AB than in C5, and both verapamil and azide increased the initial rate of procainamide uptake in C5. Thus, differences in accumulation of the noncytotoxic transport probe procainamide in the colchicine-sensitive and colchicine-resistant components of the Chinese hamster ovary cell line mimic the accumulation of known cytotoxic substrates for the multidrug transporter, such as colchicine, vinblastine, and doxorubicin. The differential accumulation of procainamide is due to differences in rates of drug influx, rather than efflux. Since procainamide influx is passive and decreased accumulation in the resistant line appears to parallel M(r) 170,000 glycoprotein presence and activity, we would speculate that decreased procainamide accumulation may be due to an indirect effect of the M(r) 170,000 glycoprotein, such as its effect on intracellular pH.

The stimulation by methotrexate of human chorionic gonadotropin and placental alkaline phosphatase in cultured choriocarcinoma cells.

Treatment of the BeWo line of choriocarcinoma cells with methotrexate in doses that inhibit DNA synthesis causes a tenfold increase in synthesis of human chorionic gonadotropin and a threefold increase in activity of placental alkaline phosphatase. No concomitant increase in lactic dehydrogenase activity occurs under these conditions. This effect of methotrexate can be blocked by simultaneous addition of thymidine or folinic acid, neither of which alone increases human chorionic gonadotropin synthesis or placental alkaline phosphatase activity in BeWo cells.

Stimulation of human chorionic gonadotropin by JAr line choriocarcinoma after inhibition of DNA synthesis.

Treatment of choriocarcinoma cells (JAr line) for 24 hr with methotrexate, fluorodeoxyuridine, 1-beta-D-arabinofuranosylcytosine, or hydroxyurea, in doses that inhibit DNA synthesis, results in a 2.5- to 12-fold increase in human chorionic gonadotropin (HCG) synthesis. Under the conditions of use, these agents do not significantly depress RNA or protein synthesis. Simultaneous addition of thymidine with methotrexate or fluorodeoxyuridine and of deoxycytidine with 1-beta-D-arabinofuranosylcytosine blocks the HCG stimulation. Despite a rapid inhibition of DNA synthesis, HCG stimulation is gradual and reaches a peak after drug removal. Decline in HCG synthesis is coincident with recovery of DNA synthesis, and there is a positive correlation between the duration of inhibition of DNA synthesis and the amount of HCG increase. Although most of the methotrexate-induced stimulation of HCG synthesis occurs after methotrexate removal, preventing protein synthesis during the exposure to methotrexate abrogates the increase. The stimulation of HCG synthesis by methotrexate is more sensitive to protein synthesis inhibition during the first 12 hr of a 24-hr exposure than it is during the second 12 hr. Thus, in accord with the kinetics, specificity, dose dependence, and protein synthesis requirement, HCG synthesis seems to be stimulated as a direct consequence of inhibition of DNA synthesis, with a requirement for some other protein synthesis-dependent event(s) preceding the stimulation.

The ontogony of the human placental glucocorticoid receptor and inducibility of heat-stable alkaline phosphatase.

The human placenta was found to contain a cytosol receptor for glucocorticoids. The concentration of this receptor in term placenta was 27-fold higher than that found in cytosol from first trimester placenta. The levels of cytosol glucocorticoid receptor in three trophoblastic cell lines (JAr, BeWo, and JEG) were also determined and all were found to be low. The ability of prednisolone, a potent glucocorticoid, to stimulate heat-stable alkaline phosphatase activity found in these cells was tested. Although control experiments demonstrated that the conditions were adequate to stimulate HeLa cell alkaline phosphatase, none of the trophoblastic lines responded to prednisolone administration. This result may be explained by the observation that the JAr cells lacked any detectable glucocorticoid receptor and the receptor levels in cytosol prepared from JEG and BeWo cells were 12% and 2%, respectively, of those measured in HeLa cytosol. Our studies also suggest that the increase in serum levels of heat-stable alkaline phosphatase observed during pregnancy may reflect increasing placental sensitivity to glucocorticoids as a result of increased receptor levels.

Epidermal growth factor stimulates production of progesterone in cultured human choriocarcinoma cells.

Epidermal growth factor (EGF) stimulates the production of progesterone by JEG-3, a clonal strain of human choriocarcinoma cells. Stimulation occurs in a time and dose-dependent manner. In addition, EGF increases [14C]-acetate incorporation into [14C]-cholesterol in JEG-3 cells, and this may constitute its mechanism of action in enhancing progesterone synthesis.

Characterization of steroid production in cultured human choriocarcinoma cells.

Progesterone is the major steroid synthesized by the JEG-3, BeWo, and JAR cell lines of choriocarcinoma. A lesser amount of pregnenolone is produced. The 17alpha-hydroxy derivatives of these steroids are only minimally present in three lines. The addition of fetal calf serum to the culture medium modestly increases the synthesis of these steroids, but increases the quantity of 17beta-estradiol produced by 30- to 90-fold. The addition of dehydroepiandrosterone, dehydroepiandrosterone sulfate, androstenedione, androstenediol, and testosterone was shown to stimulate 17beta-estradiol synthesis. There is a clear dose-response relationship between the amount of testosterone added and the quantity of 17beta-estradiol produced. These results indicate that 3beta-hydroxysteroid dehydrogenase-isomerase, 17beta-ol dehydrogenase, and aromatase are active in cultured choriocarcinoma cells, whereas 17beta-hydroxylase and 17-20 desmolase do not appear to be functional in these cells. It is concluded that the stereoidogenic capabilities of choriocarcinoma cells in culture are similar to those of the in vivo placenta and support their use as an experimental model of placental steroidogenesis.

Epidermal growth factor stimulates secretion of human chorionic gonadotropin by cultured human choriocarcinoma cells.

Epidermal growth factor (EGF) binds to JEG-3 cells, a tissue culture line of human choriocarcinoma. EGF also stimulates secretion of human chorionic gonadotropin (hCG) and to a lesser extent the secretion of free hCG-alpha.

Cimetidine inhibits renal procainamide clearance.

Procainamide and cimetidine are eliminated in large part by the kidneys. Both are secreted by an active transport mechanism in the proximal tubule and each inhibits secretion of the other in the isolated, perfused rabbit tubule. In this study in man, cimetidine inhibited renal clearance of oral procainamide by 36%. This was associated with a 28% decrease in the ratio of systemic clearance of procainamide to bioavailability, an 18% decrease in the elimination rate constant, and a 24% prolongation of elimination t1/2. These results suggest that cimetidine increases plasma t1/2 and decreases systemic clearance of procainamide in part by inhibiting its active secretion by the kidneys.

Effect of ketoconazole on hepatic oxidative drug metabolism.

Several clinical reports have suggested (but not demonstrated) that ketoconazole, a broad-spectrum antifungal drug, may inhibit hepatic oxidative drug metabolism in man. We recently demonstrated that ketoconazole inhibits caffeine and aminopyrine oxidation in the rat; we now study the influence of ketoconazole on theophylline and chlordiazepoxide kinetics in man. These studies were performed before and after varying doses of ketoconazole within the currently accepted therapeutic range. Ketoconazole had no effect on theophylline clearance, whereas the drug impaired chlordiazepoxide clearance from plasma. After a single dose of ketoconazole, there was a 20% decrease in clearance and a 26% decrease in volume of distribution without evidence of inhibition of drug metabolism. These changes apparently were not related to ketoconazole dose. After repetitive dosing with ketoconazole, chlordiazepoxide clearance decreased by 38% and was associated with reduced concentrations of its first oxidative metabolite, N-desmethylchlordiazepoxide. We conclude that ketoconazole inhibits at least one subset of the hepatic mixed-function oxidase system, but is not as general an inhibitor of hepatic oxidative drug metabolism as cimetidine appears to be. For some coadministered drugs, ketoconazole may also have an effect on other kinetic parameters such as volume of distribution. Therefore, we caution that clinically important drug interactions may occur with the concurrent use of ketoconazole.

Transforming growth factor alpha distribution in rectal crypts as a biomarker of decreased colon cancer risk in patients consuming cellulose.

Data from rat experimental carcinogenesis studies indicate that supplemental dietary cellulose reduces the incidence of colon cancer. Epidemiology studies also indicate that high dietary fiber reduces the risk of colorectal cancer in humans. Patients diagnosed with sporadic adenomas were entered into a randomized clinical trial to determine if supplemental dietary cellulose would reduce the patients' risk for colon cancer. Immunohistochemical staining for transforming growth factor alpha (TGF-alpha) was done on biopsies of rectal mucosa taken from patients at the time of initial polypectomy and 1 year later. Results were evaluated for utility as a surrogate end point biomarker for reduction in colon cancer risk. There was a significant decrease in the fraction of the rectal crypt cells that stained for TGF-alpha in six of seven of the patients given the cellulose supplements but in only one of six of the patients not given cellulose. Thus, whether evaluated as a group or in individual patients, there was a significant decrease in TGF-alpha in rectal crypts due to cellulose intervention, which correlated with the expected ability of supplemental dietary cellulose to decrease the risk for colon cancer. Long-term testing of the ability of dietary cellulose to reduce adenoma recurrence is under way to validate the use of TGF-alpha as a surrogate end point biomarker.

Effect of aspirin on prostaglandin E2 formation and transforming growth factor alpha expression in human rectal mucosa from individuals with a history of adenomatous polyps of the colon.

Colorectal cancer is the second-most frequent cause of cancer mortality in the United States. Human epidemiology and laboratory studies indicate that aspirin may be an effective colorectal cancer chemopreventive agent. This study was designed to determine whether treatment with 81 mg of aspirin per day for 3 months would alter two putative surrogate end point biomarkers of chemoprevention of colorectal cancer [i.e., mucosal prostaglandin E2 (PGE2) formation and transforming growth factor alpha (TGF-alpha) expression] in normal-appearing rectal mucosa from individuals with a history of adenomatous polyps. Rectal biopsies were obtained by flexible sigmoidoscopy at three sequential time points: (a) after a 1-month placebo run-in period (baseline), (b) after 3 months of ingesting 81 mg of aspirin (as a single tablet) once per day, and (c) after 3 months of ingesting a placebo tablet once per day (washout period). Daily aspirin significantly suppressed PGE2 formation, but this significant suppression was completely reversed when aspirin was withdrawn. The extent of TGF-alpha staining in rectal crypts was also reduced significantly (P = 0.039) by daily aspirin. After a 3-month placebo-washout period, however, the mean extent of TGF-alpha staining was not significantly different from either baseline or the aspirin time point. Thus, 81 mg of aspirin daily significantly reduced rectal mucosal PGE2 formation and TGF-alpha expression in patients with a history of adenomatous polyps. These putative surrogate end point biomarkers may be useful intermediate end points in future colorectal cancer chemoprevention trials.

Steroid elimination 24 hours after liver transplantation using daclizumab, tacrolimus, and mycophenolate mofetil.

BACKGROUND: Corticosteroids have long been a cornerstone of orthotopic liver transplant (OLTx) immunosuppression. Newer, more potent, agents have successfully allowed for more rapid tapering and discontinuation of corticosteroids in OLTx recipients. We hypothesize that corticosteroids can be safely avoided after the first postoperative day (POD) using these newer agents. METHODS: Thirty adult OLTx recipients were prospectively enrolled in a randomized open-label, institutional review board-approved protocol. Fifteen patients (group A) received our standard regimen of tacrolimus, mycophenolate mofetil, and corticosteroids, and 15 patients (group B) received daclizumab, 2 mg/kg on POD 0 and 14, with tacrolimus, mycophenolate mofetil, and corticosteroids on POD 0 and 1 and then discontinuation. In both groups, mycophenolate mofetil was tapered off between 3 and 4 months after OLTx. Bone mineral densitometry was performed at 1, 3, and 6 months after OLTx. Quantitative hepatitis C virus (HCV) polymerase chain reaction was obtained at days 0, 7, 14, 21, and 28. Retransplant recipients, patients with autoimmune hepatitis, or status 1 or 2A patients were excluded. RESULTS: Patient and graft survival rates were 93% (group A) and 100% (group B) with mean follow-up of 18 months. Patients in group B had more rejection diagnosed (25%) compared with group A (6.7%). Yet, the incidence of biopsy-proven acute rejection requiring steroid therapy was 6.7% in both groups. Hispanic race was common in groups A and B (87% and 74%). A total of six biopsies were performed in group B, with three patients having mild rejection responding to an increase in tacrolimus without the need for corticosteroids. One patient in group B was switched to cyclosporine for severe neurotoxicity and remains on monotherapy with normal graft function. No patient in either group developed a requirement for additional antihypertensive medication. Likewise, there were no patients with new-onset diabetes. The bone mineral densitometry was higher in group B at every time point but did not reach statistical significance. Serum cholesterol level was significantly (P=0.03) lower in group B at 6 months after OLTx. Serum triglycerides were also lower, but the difference was not significant. Quantitative polymerase chain reaction for HCV-positive patients (group A, n=7; group B, n=8) frequently increased after OLTx. There was no correlative decrease associated with daclizumab. At present, two patients in group A have documented HCV recurrence. CONCLUSION: Corticosteroids can be safely avoided after POD 1 with the current regimen. With early follow-up, there is no difference in hypertension or diabetes or bone density. Lipid panels tended to be lower in patients who were not on corticosteroids. Longer term follow-up will be needed to demonstrate the potential advantage of corticosteroid avoidance in regard to hypertension, diabetes, and possibly HCV recurrence.

Effect of aging on hepatic elimination of cimetidine and subsequent interaction of aging and cimetidine on aminopyrine metabolism.

Aging and cimetidine may each impair hepatic microsomal drug metabolism. To test if and by what mechanisms advanced age may increase sensitivity to the inhibitory effects of cimetidine, the interaction of these two factors with aminopyrine metabolism in the rat was studied using a correlative approach. Initial studies using the aminopyrine breath test indicated that a 40 mg/kg dose of cimetidine, i.p., impaired the 14CO2 exhaled by up to 76% more in aged (26-month) than in young (3- to 4-month-old) rats. Using an isolated liver perfusion to dissect out hepatic components of this phenomenon, it was found that various doses of cimetidine impaired aminopyrine clearance to a greater degree (P less than 0.05) in aged than in young livers. However, cimetidine metabolism in this system ranged from 36 to 78% less in aged versus young livers (P less than 0.05). Subsequent in vitro studies indicated that microsomes isolated from aged livers also averaged a 76% lower rate of cimetidine metabolism (P less than 0.05). A fixed cimetidine concentration, however, inhibited aminopyrine demethylation to the same degree in aged versus young rats (P less than 0.05). In vivo pharmacokinetics showed an age-related decrease in both aminopyrine and cimetidine systemic clearance. In the young rat the liver contributed about 30% to total systemic clearance of cimetidine. In the aged rat, all clearance was renal. Despite a decrease in glomerular filtration rate, net tubular cimetidine secretion was well-maintained. Despite this, absence of the hepatic component resulted in decreased overall systemic clearance of the drug in aged rats. It is concluded that (1) the aged rat liver exhibits impaired cimetidine metabolism, resulting in decreased overall systemic clearance of the drug despite normal net renal tubular secretion, (2) there is no age-related enhanced sensitivity to cimetidine of the hepatic microsomal oxidizing system using aminopyrine as the probe drug, and (3) the larger inhibition of aminopyrine metabolism in aged rats following various doses of cimetidine is due to decreased overall cimetidine clearance, resulting in higher concentrations of the inhibitor in the liver of aged rats.

Hepatic oxidative drug metabolism and the microsomal milieu in a rat model of congenital hyperbilirubinemia.

The aims of this study were to evaluate the hypothesis that impaired glucuronidation of bilirubin and possibly of drug oxidation in the liver of homozygous (jj) Gunn rats may be due to an altered microsomal milieu. Accordingly, we investigated and compared in vivo and in vitro demethylation of aminopyrine, hepatic cytochrome P-450 levels, microsomal lipid composition, and microsomal membrane fluidity in icteric, homozygous (jj) Gunn rats and in their anicteric heterozygous (jJ) littermates. In both males and females, [14C]aminopyrine demethylation in vivo, using the 14CO2 breath test, was unimpaired in the icteric animals. Likewise, cytochrome P-450 levels in the icteric and nonicteric groups were similar, and aminopyrine kinetics in vitro in the females were comparable in icteric and nonicteric littermates. The main lipid classes were also similar in the homozygous and heterozygous female Gunn rats, whereas only minor changes were seen in the phospholipid fatty acyl composition with a small, but significant, increase in the unsaturated index in the icteric group. Despite this, there was no apparent effect on hepatic microsomal membrane fluidity as measured by the order parameter of I[12,3] and the rotational correlation time of I[1,14] in either female or male sets of homozygous and heterozygous Gunn rats. Our data, therefore, do not support an alteration of composition or fluidity of the microsomal milieu as a mechanism of impaired bilirubin glucuronidation and possibly of oxidation in these animals. They also absolve long-term unconjugated hyperbilirubinemia as a mechanism of hepatic microsomal dysfunction. Our study, therefore, indirectly suggests that abnormal glucuronidation of bilirubin and some other aglycones in homozygous Gunn rats is due to genetic abnormalities involving the enzyme(s) itself.

The effect of route of administration on the cimetidine-theophylline drug interaction.

The effect of route of cimetidine administration on cimetidine-mediated inhibition of theophylline oxidation was examined in healthy individuals. Based on the evidence that cimetidine-mediated inhibition of drug oxidation is competitive and, therefore, dependent on cimetidine concentration in the liver, oral cimetidine was tested to determine whether it would cause greater inhibition of drug oxidation than intravenous (IV) cimetidine. Both oral and IV cimetidine decreased theophylline clearance to the same extent. However, when clearance was corrected for cimetidine AUC, oral cimetidine resulted in a greater inhibition than IV cimetidine. Thus, the potential for increased inhibitory effect of oral cimetidine was balanced by decreased absorption after oral administration. Degree of inhibition (absolute change in theophylline clearance) and percent of inhibition after cimetidine correlated with the basal theophylline clearance. Individuals with higher basal theophylline clearances had greater degree and percent of inhibition than individuals with lower basal theophylline clearances.

Effect of cyclosporine on colchicine partitioning in the rat liver.

Colchicine is secreted into bile as a major pathway of elimination. Cyclosporine (CsA) inhibits colchicine biliary secretion. In the present study, the effects of cyclosporine and its vehicle (cremophor) on the partitioning of colchicine across the liver were studied. CsA decreased the colchicine bile/plasma ratio from 484 +/- 39 to 53 +/- 3 (P < 0.001). This effect was due to both a decrease in bile/liver partitioning (control, 35.1 +/- 1.2, vs CsA treatment, 9.2 +/- 0.5; p < 0.001) as well as a decrease in liver/plasma partitioning (control, 13.7 +/- 0.8, vs CsA treatment, 5.7 +/- 0.4; P < 0.001). Cremophor also decreased the colchicine bile/plasma ratio (317 +/- 19, P < 0.02 vs control), but this effect was due to a decrease in the liver/plasma ratio (9.99 +/- 0.7, P < 0.02 vs control) rather than the bile/liver ratio (31.9 +/- 2.1, P > 0.2 vs control). Inhibition at the canalicular membrane is consistent with the location of gp-170, the presumed transporter of colchicine.

Effect of the nonimmunosuppressive cyclosporin analog SDZ PSC-833 on colchicine and doxorubicin biliary secretion by the rat in vivo.

Colchicine and doxorubicin are secreted into bile as a major pathway of their elimination. Colchicine and doxorubicin are also substrates for P-glycoprotein, and P-glycoprotein has been demonstrated to be present at the liver canalicular membrane. Cyclosporin (CsA) inhibits colchicine biliary secretion in vivo. In the present study, the effects of SDZ PSC-833, a nonimmunosuppressive cyclosporin D analog, on the biliary secretion of colchicine and doxorubicin were investigated. SDZ PSC-833 given at a bolus dose of 2 mg/kg promptly decreased colchicine biliary clearance from 9.05 +/- 0.2 to 2.41 +/- 0.43 ml min-1 kg-1 (P < 0.001) and the colchicine bile/plasma ratio from 146 +/- 8 to 35 +/- 5 (P < 0.001). SDZ PSC-833 also inhibited doxorubicin biliary clearance (basal: 10.5 +/- 3 vs post-SDZ PSC-833: 2.48 +/- 0.94 ml min-1 kg-1; P = 0.06) and the doxorubicin bile/plasma ratio (basal: 228 +/- 64 vs post-SDZ PSC-833: 48 +/- 22; P < 0.01). Colchicine renal secretion was completely inhibited by SDZ PSC-833. Thus, SDZ PSC-833 inhibits the constitutive transport of the multi-drug-resistance substrates colchicine and doxorubicin and is more potent than cyclosporin in this regard. The possibility of increased toxicity to normal tissues because of impaired elimination of cytotoxic agents will need to be considered if SDZ PSC-833 is used to chemosensitize cancer cells.

Potential use of cimetidine for treatment of acetaminophen overdose.

Acetaminophen, a drug frequently taken in intentional and accidental overdose, causes liver toxicity when concentration of the cytochrome P-450-derived metabolite exceeds the metabolic capacity of available glutathione. Present treatment of acetaminophen overdose involves oral N-acetylcysteine (NAC), which enhances liver glutathione synthesis. An alternative or additive approach to therapy would be to inhibit the formation of the toxic metabolite by inhibiting the cytochrome P-450 system. The H2-receptor antagonist cimetidine inhibits the cytochrome P-450 system, does not interfere with the administration or function of NAC, and therefore affords additive protection. Also, it has little effect on the nontoxic routes of elimination of acetaminophen and is itself quite nontoxic. That cimetidine protects against acetaminophen toxicity in animal models has been demonstrated on the basis of improved survival, as well as decreases in several critical elements used to monitor acetaminophen toxicity: classic histologic changes, aminotransferase activity, metabolite covalent binding, and liver glutathione depletion. Administration of cimetidine well after the overdose is also protective. In contrast, animal models of acetaminophen toxicity demonstrate that ranitidine does not afford protection from acetaminophen hepatotoxicity. Clinical data in well-done trials in humans will be needed to support the experimental animal data.

Aging and benzodiazepine binding in the rat cerebral cortex.

The specific binding of 3H-diazepam in the cerebral cortex was investigated in membrane preparations from 6 and 30 month old Fisher-344 rats. No age-related differences in the association, equilibrium, or dissociation, binding characteristics were observed. The increased sensitivity of the elderly to the central sedative effects of the benzodiazepines does not, therefore, appear to involve changes in binding to the receptor site located in the cortex.