Prevention of acetaminophen-induced liver toxicity by 2(R,S)-n-propylthiazolidine-4(R)-carboxylic acid in mice

Protective role of Chlorella vulgaris with Thiamine against Paracetamol induced toxic effects on haematological, biochemical, oxidative stress parameters and histopathological changes in Wistar rats

Paracetamol is extensively consumed as an analgesic and antipyretic drug, but at a high dose level, it leads to deleterious side effects, such as hepatic and nephrotoxicity. This research aimed to estimate the prophylactic efficacy of Chlorella vulgaris and/or thiamine against paracetamol (P) induced hepatorenal and cardiac toxicity. Forty-eight female Wistar rats were randomly divided into eight equal groups (n = 6 rats). Group 1, normal control group. Group 2, Paracetamol group. Groups 3, 4 and 5 were treated with Silymarin drug, Chlorella vulgaris alga, Chlorella vulgaris alga supplemented with thiamine, respectively daily for 7 successive days, then all were administered Paracetamol (2gm/kg. bwt.). While, Groups 6, 7 and 8 were treated by Silymarin, Chlorella vulgaris alga, Chlorella vulgaris supplemented with thiamine, respectively daily for 7 successive days without paracetamol administration. Our results clarified that Paracetamol toxicity caused significant adverse effects on hematological, serum biochemical parameters, and oxidant -antioxidant status as well as histopathological picture of heart, liver, and kidney. However, in the Paracetamol intoxicated groups pretreatment either with Chlorella vulgaris alone or plus thiamine successfully improved the undesirable deleterious effects of paracetamol, and restored almost all variables to near their control levels. This study has finished to that oxidative stress participates in the pathogenesis of paracetamol-induced toxicity in rats and using Chlorella vulgaris alga either alone or plus thiamine alongside their health benefits can protect against oxidative harmful effects induced by paracetamol through their free radical scavenging and powerful antioxidant effects, and they can be used as propylactic agents against paracetamol-induced toxicity.

Antioxidant and hepatoprotective activity of Piper retrofractum against Paracetamol-induced hepatotoxicity in Sprague-Dawley rat

The ethanol extracts of Piper retrofractum were investigated for antioxidant and hepatoprotective activity. Hepatoprotective activity against paracetamol-induced acute hepatotoxicity was estimated in Sprague-Dawley rat. In DPPH free radical assay the root and stem extracts showed IC₅₀ values at 133 and 91 µg/mL, respectively, while ascorbic acid at 14 µg/mL. Extracts also exhibited hydroxyl radical scavenging activity and reducing power. HPLC-DAD analysis indicated the presence of some polyphenolic compounds. Treatment of extracts significantly reduced the elevated serum levels of GPT (P < 0.01), GOT (P < 0.01) and bilirubin (P < 0.001). Both extracts restored the reduced level of total proteins and albumin. A significant increase in HDL-c but decrease in LDL-c level was observed compared to induced control. In histopathological study of liver sections, both extracts showed minimal to mild multifocal and diffuse granular degeneration and mild to moderate lobular disarray compared to control group. Results suggest that both extracts can prevent paracetamol induced hepatotoxicity.

GC-MS analysis and hepatoprotective activity of the n-hexane extract of Acrocarpus fraxinifolius leaves against paracetamol-induced hepatotoxicity in male albino rats

CONTEXT

In Egypt, the burden of liver diseases is exceptionally high.

OBJECTIVE

To investigate the components of the n-hexane extract of Acrocarpus fraxinifolius Arn. (Leguminosae) and its hepatoprotective activity against paracetamol (APAP)-induced hepatotoxicity in rats.

MATERIAL AND METHODS

TRACE GC ultra gas chromatogaphic spectrometry was used for extract analysis. Thirty albino rats were divided into six groups (five rats in each). Group 1 was the healthy control; Groups 2 and 3 were healthy treated groups (250 and 500 mg/kg b.w. of the extract, respectively) for seven days. Group 4 was hepatotoxicity control (APAP intoxicated group). Groups 5 and 6 received APAP + extract 250 and APAP + extract 500, respectively.

RESULTS

Chromatographic analysis revealed the presence of 36 components. Major compounds were α-tocopherol (18.23%), labda-8 (20)-13-dien-15-oic acid (13.15%), lupeol (11.93%), phytol (10.95%) and squalene (7.19%). In the acute oral toxicity study, the mortality rates and behavioural signs of toxicity were zero in all groups (doses from 0 to 5 g/kg b.w. of A. fraxinifolius). LD₅₀ was found to be greater than 5 g/kg of the extract. Only the high dose (500 mg/kg b.w.) of extract significantly alleviated the liver relative weight (4.01 ± 0.06) and biomarkers, as serum aspartate aminotransferase (62.87 ± 1.41), alanine aminotransferase (46.74 ± 1.45), alkaline phosphatase (65.96 ± 0.74), lipid profiles (180.39 ± 3.51), bilirubin profiles (2.30 ± 0.06) and hepatic lipid peroxidation (114.20 ± 2.06), and increased body weight (11.58 ± 0.20), serum protein profile (11.09 ± 0.46) and hepatic total antioxidant capacity (23.78 ± 0.66) in APAP-induced hepatotoxicity in rats.

CONCLUSION

Our study proves the antihepatotoxic/antioxidant efficacies of A. fraxinifolius hexane extract.

The cysteine releasing pattern of some antioxidant thiazolidine-4-carboxylic acids

Oxidative stress that corresponds to a significant increase in free radical concentration in cells can cause considerable damage to crucial biological macromolecules if not prevented by cellular defense mechanisms. The low-molecular-weight thiol glutathione (GSH) constitutes one of the main intracellular antioxidants. It is synthesized via cysteine, an amino acid found only in limited amounts in cells because of its neurotoxicity. Thus, to ensure an efficient GSH synthesis in case of an oxidative stress, cysteine should be provided extracellularly. Yet, given its nucleophilic properties and its rapid conversion into cystine, its corresponding disulfide, cysteine presents some toxicity and therefore is usually supplemented in a prodrug approach. Here, some thiazolidine-4-carboxylic acids were synthesized and evaluated for their antioxidant properties via the DDPH and CUPRAC assays. Then, the cysteine releasing capacity of the obtained compounds was investigated in aqueous and organic medium in order to correlate the relevant antioxidant properties of the molecules with their cysteine releasing pattern. As a result, the structures' antioxidative properties were not only attributed to cysteine release but also to the thiazolidine cycle itself.

Eucalyptus globulus extract protects upon acetaminophen-induced kidney damages in male rat

Plants have historically been used in treating many diseases. Eucalyptus globules, a rich source of bioactive compounds, and have been shown to possess antioxidative properties. The purpose of this study, carried out on male Wistar rats, was to evaluate the beneficial effects of Eucalyptus globulus extract upon acetaminophen-induced damages in kidney. Our study is realized in the Department of Biology, Faculty of Sciences of Sfax (Tunisia). 32 Wistar male rats; were divided into 4 batches: a control group (n=8), a group of rats treated with acetaminophen (900 mg/kg) by intraperitoneal injection during 4 days (n=8), a group receiving Eucalyptus globulus extract (130 mg of dry leaves/kg/day) in drinking water during 42 days after 2 hours of acetaminophen administration (during 4 days) (n=8) and group received only Eucalyptus (n=8) during 42 days. After 6 weeks, animals from each group were rapidly sacrificed by decapitation. Blood serum was obtained by centrifugation. Under our experimental conditions, acetaminophen poisoning resulted in an oxidative stress evidenced by statistically significant losses in the activities of catalase (CAT), superoxide-dismutase (SOD), glutathione-peroxidase (GPX) activities and an increase in lipids peroxidation level in renal tissue of acetaminophen-treated group compared with the control group. Acetaminophen also caused kidney damage as evident by statistically significant (p<0.05) increase in levels of creatinine and urea and decreased levels of uric acid and proteins in blood. Histological analysis demonstrated alteration of proximal tubules, atrophy of the glomerule and dilatation of urinary space. Previous administration of plant extract is found to alleviate this acetaminophen-induced damage.

Protective effect of Et-1 receptor antagonist bosentan on paracetamol induced acute liver toxicity in rats

Paracetamol is one of the first rank drugs which cause hepatic damage during drug intoxications. Endothelin (ET) which is known as one of the most potent vasoactive agent has been shown to contribute in the pathophysiology of various diseases. We hypothesized that bosentan, which is a non-selective ET-1 receptor antagonist, can prevent liver damage. This study included 49 female rats. Groups; I: Healthy group, II: Paracetamol (2 g/kg orally). Groups 3, 4 and 5 received NAC 140 mg/kg (2 doses), BOS 45 mg/kg and BOS 90 mg/kg orally, respectively. 1 h after administration of pretreatment drugs, Groups 3, 4, 5 were given paracetamol. VI: received BOS 90 mg/kg. VII: received 140 mg/kg NAC (2 doses). According to biochemical results, TNF-α, ALT and AST levels were statistically increased in the paracetamol group, these parameters were improved in the bosentan groups. Paracetamol administration decreased SOD activity, GSH level and increased level of MDA in the liver, while bosentan administration significantly improved these parameters. In immunohistochemical staining ET-1 receptor expression was excessively increased in paracetamol group, but not in bosentan groups when compared to healthy control. All these results suggest that bosentan exerted protective effects against experimentally induced paracetamol toxicity in liver.

Antioxidant properties of Taraxacum officinale leaf extract are involved in the protective effect against hepatoxicity induced by acetaminophen in mice

Acetaminophen (APAP) hepatotoxicity has been related to several cases of hepatitis, cirrhosis, and hepatic transplant. As APAP hepatotoxicity is related to reactive oxygen species (ROS) formation and excessive oxidative stress, natural antioxidant compounds have been tested as an alternative therapy to diminish the hepatic dysfunction induced by APAP. Taraxacum officinale Weber (Family Asteraceae), commonly known as dandelion, is used for medicinal purposes because of its choleretic, diuretic, antioxidant, anti-inflammatory, and hepatoprotective properties. This study evaluated the hepatoprotective activity of T. officinale leaf extract against APAP-induced hepatotoxicity. T. officinale was able to decrease thiobarbituric acid-reactive substance levels induced by 200 mg/kg APAP (p.o.), as well as prevent the decrease in sulfhydryl levels caused by APAP treatment. Furthermore, histopathological alterations, as well as the increased levels of serum aspartate and alanine aminotransferases caused by APAP, were prevented by T. officinale (0.1 and 0.5 mg/mL). In addition, T. officinale extract also demonstrated antioxidant activity in vitro, as well as scavenger activity against 2,2-diphenyl-1-picrylhydrazyl and nitric oxide radicals. Our results clearly demonstrate the hepatoprotective effect of T. officinale against the toxicity induced by APAP. The possible mechanisms involved include its scavenger activities against ROS and reactive nitrogen species, which are attributed to the content of phenolic compounds in the extract.

p-Aminophenol-induced cytotoxicity in Jurkat T cells: protective effect of 2(RS)-n-propylthiazolidine-4(R)-carboxylic acid

Acetaminophen (APAP) is hepatotoxic and can cause toxicity in Jurkat T cells. p-Aminophenol (PAP), an industrial chemical and APAP metabolite, is nephrotoxic and hepatotoxic. Its potential toxicity in Jurkat T cells was investigated. PAP (10-250 µM) caused toxicity (decreased survival and increased LDH activity in incubation medium) and GSH depletion. At a concentration of 100 µM but not 250 µM, PAP increased DNA fragmentation. It decreased p-Akt levels (Elisa) and at higher concentrations decreased p-Akt expression (Western blotting). It had no effect on FasL expression. The cysteine precursor 2(RS)-n-propylthiazolidine-4(R)-carboxylic acid (250 µM) attenuated the PAP (100 µM)-induced decrease in viability and prevented GSH depletion and increased DNA fragmentation. It attenuated the PAP-induced decrease in p-Akt levels and protected against the decrease in p-Akt expression. The results demonstrate PAP-induced toxicity and suggest that it is due at least in part to apoptosis and involves GSH depletion and p-Akt inactivation.

Changes in mouse liver protein glutathionylation after acetaminophen exposure

The role of protein glutathionylation in acetaminophen (APAP)-induced liver injury was investigated in this study. A single oral gavage dose of 150 or 300 mg/kg APAP in B6C3F1 mice produced increased serum alanine aminotransferase and aspartate aminotransferase levels and liver necrosis in a dose-dependent manner. The ratio of GSH to GSSG was decreased in a dose-dependent manner, suggesting that APAP produced a more oxidizing environment within the liver. Despite the increased oxidation state, the level of global protein glutathionylation was decreased at 1 h and continued to decline through 24 h. Immunohistochemical localization of glutathionylated proteins showed a complex dynamic change in the lobule zonation of glutathionylated proteins. At 1 h after APAP exposure, the level of glutathionylation decreased in the single layer of hepatocytes around the central veins but increased mildly in the remaining centrilobular hepatocytes. This increase correlated with the immunohistochemical localization of APAP covalently bound to protein. Thereafter, the level of glutathionylation decreased dramatically over time in the centrilobular regions with major decreases observed at 6 and 24 h. Despite the overall decreased glutathionylation, a layer of cells lying between the undamaged periportal region and the damaged centrilobular hepatocytes exhibited high levels of glutathionylation at 3 and 6 h in all samples and in some 24-h samples that had milder injury. These temporal and zonal pattern changes in protein glutathionylation after APAP exposure indicate that protein glutathionylation may play a role in protein homeostasis during APAP-induced hepatocellular injury.

Hepatoprotective activity of a vinylic telluride against acute exposure to acetaminophen

Acetaminophen (APAP) hepatotoxicity has been related with several cases of cirrhosis, hepatitis and suicides attempts. Notably, oxidative stress plays a central role in the hepatic damage caused by APAP and antioxidants have been tested as alternative treatment against APAP toxicity. In the present study, we observed the hepatoprotector activity of the diethyl-2-phenyl-2-tellurophenyl vinylphosphonate (DPTVP), an organotellurium compound with low toxicity and high antioxidant potential. When the dose of 200 mg/kg of APAP was used, we observed that all used doses of DPTVP were able to restore the -SH levels that were depleted by APAP. Furthermore, the increase in thiobarbituric acid reactive substances levels and in the seric alanine aminotransferase (ALT) activity and the histopathological alterations caused by APAP were restored to control levels by DPTVP (30, 50 and 100 μmol/kg). On the other hand, when the 300 mg/kg dose of APAP was used, DPTVP restored the non-proteic -SH levels and repaired the normal liver morphology of the intoxicated mice only at 50 μmol/kg. Our in vitro results point out to a scavenging activity of DPTVP against several reactive species, action that is attributed to its chemical structure. Taken together, our results demonstrate that the pharmacological action of DPTVP as a hepatoprotector is probably due to its scavenging activity related to its chemical structure.

Green-tea polyphenols downregulate cyclooxygenase and Bcl-2 activity in acetaminophen-induced hepatotoxicity

Acetaminophen (APAP) elicits hepatotoxicity via multifactorial pathways, including increased apoptosis, cyclooxygenase (Cox-2) generation, reactive metabolite release, and glutathione (GSH) depletion. We previously showed that mice that consumed different antioxidants in their diets were protected against APAP-induced hepatotoxicity. We therefore further investigated the mechanisms by which green-tea polyphenols (GrTP) protect against APAP-induced hepatic damage. Mice were administered a diet supplemented with GrTP or vehicle for 5 consecutive days followed by intraperitoneal (IP) injection of a toxic dose of APAP or sham. APAP administration upregulated Cox-2 and B-cell lymphoma-2 (Bcl-2) production and had an effect, albeit minor, on Cox-1 and Fas expression in hepatic tissue. GrTP supplementation normalized APAP induced Cox-2 expression and Bcl-2 activation (P < 0.01), as evidenced by immunohistochemistry and Western blot analyses. Similarly, APAP administration elicited marked depletion (99%) in hepatic reduced GSH (rGSH) and endogenous S-adenosylmethionine (SAMe) concentrations (twofold) when compared with sham. APAP also caused severe centrilobular apoptosis and necrosis accompanied by leukocyte infiltration and marked elevations in the hepatic enzyme, alanine aminotransferase (ALT) released from damaged hepatocytes, and cytokine tumor necrosis factor alpha (TNF-alpha). GrTP improved hepatic histopathology (P < 0.01) and attenuated ALT activity (P < 0.05) and the depletion of rGSH (P < 0.05). In conclusion, GrTP supplementation attenuated hepatotoxicity by normalizing Cox-2 and Bcl-2 activation, suggesting a potential use for GrTP in treatng APAP toxicity.

Application of prodrugs to inflammatory diseases of the gut

Oral delivery is the most common and preferred route of drug administration although the digestive tract exhibits several obstacles to drug delivery including motility and intraluminal pH profiles. The gut milieu represents the largest mucosal surface exposed to microorganisms with 10^10-12 colony forming bacteria/g of colonic content. Approximately, one third of fecal dry matter is made of bacteria/ bacterial components. Indeed, the normal gut microbiota is responsible for healthy digestion of dietary fibers (polysaccharides) and fermentation of short chain fatty acids such as acetate and butyrate that provide carbon sources (fuel) for these bacteria. Inflammatory bowel disease (IBD) results in breakage of the mucosal barrier, an altered microbiota and dysregulated gut immunity. Prodrugs that are chemically constructed to target colonic release or are degraded specifically by colonic bacteria, can be useful in the treatment of IBD. This review describes the progress in digestive tract prodrug design and delivery in light of gut metabolic activities.

Comparative efficacies of 2 cysteine prodrugs and a glutathione delivery agent in a colitis model

Oxidant-mediated injury plays an important role in the pathophysiology of inflammatory bowel disease (IBD). Recently, antioxidants were shown to modulate colitis in mice. In this study, the protective effects of L-cysteine and glutathione (GSH) prodrugs are further evaluated against progression of colitis in a murine model. ICR mice were fed compounds incorporated into chow as follows: Group (A) received chow supplemented with vehicle. Group (B) was provided 2-(RS)-n-propylthiazolidine-4(R)-carboxylic-acid (PTCA), a cysteine prodrug. Group (C) received D-ribose-L-cysteine (RibCys), another cysteine prodrug that releases L-cysteine. Group (D) was fed L-cysteine-glutathione mixed sulfide (CySSG), a ubiquitous GSH derivative present in mammalian cells. After 3 days, the animals were further provided with normal drinking water or water supplemented with dextran sodium sulfate (DSS). Mice administered DSS developed severe colitis and suffered weight loss. Colonic lesions significantly improved in animals treated with PTCA and RibCys and, to a lesser extent, with CySSG therapy. Hepatic GSH levels were depleted in colitis animals (control vs DSS, P < 0.001), and normalized with prodrug therapies (control vs treatments, P > 0.05). Protein expressions of serum amyloid A and inflammatory cytokines [interleukin (IL)-6, IL-12, tumor necrosis factor-alpha (TNF-alpha), osteopontin (OPN)] were significantly increased in colitis animals and improved with therapies. Immunohistochemistry and Western blot analyses showed significant upregulation of the macrophage-specific markers, COX-2 and CD68, which suggests macrophage activation and infiltration in the colonic lamina propria in colitis animals. These abnormalities were attenuated in prodrug-treated mice. In conclusion, these data strongly support the novel action of the PTCA against colitis, which further supports a possible therapeutic application for IBD patients.

Antioxidants as novel therapy in a murine model of colitis

Reactive oxygen species (ROS) are increased in inflammatory bowel disease (IBD) and have been implicated as mediators of intestinal inflammation. We investigated the hypothesis that antioxidants with diverse properties attenuate disease progression in a murine dextran sodium sulfate (DSS)-induced colitis model. These antioxidants were (A) S-adenosylmethionine, a glutathione (GSH) precursor; (B) green tea polyphenols, a well-known antioxidant; and (C) 2(R,S)-n-propylthiazolidine-4(R)-carboxylic acid (PTCA), a cysteine prodrug, involved in GSH biosynthesis. BALB/c mice were divided into four groups and provided with the above mentioned antioxidants or the vehicle incorporated into chow. The animals were further divided into two subgroups and given normal drinking water (control) or water supplemented with DSS (to induce colitis), and the progression of the disease was studied. DSS-treated mice developed severe colitis as shown by bloody diarrhea, weight loss and pathological involvement (P<.001). However, all the antioxidants significantly improved diarrhea and colon lesions (P<.01), and increased body weights (P<.05). Hematocrits were significantly less affected in DSS-treated animals receiving antioxidants (P<.01). Colon lengths were significantly decreased due to mucosal inflammation in DSS-treated animals, but antioxidant therapy normalized this pathological finding (P<.001). The blood level of reduced GSH was decreased in DSS-treated mice (P<.05) and returned to normal when treated with antioxidants. Serum amyloid A (acute phase protein; P=.0015) and tumor necrosis factor-alpha (TNF-alpha; pro-inflammatory cytokine; P<.01) were significantly increased in DSS-treated animals (161+/-40 pg/ml) and improved with antioxidant treatment (P<.01). Finally, actin cytoskeleton was distorted and fragmented in the mucosa of DSS-treated mice and improved with antioxidant therapy. In conclusion, three structurally dissimilar antioxidants provided protection against DSS-induced colitis in this murine model, supporting a possible role for antioxidant therapy in IBD patients.

Diverse antioxidants protect against acetaminophen hepatotoxicity

The reactive oxygen species-sensitive transcription nuclear factor-kappaB (NF-kappaB) plays a pivotal role in the development of acetaminophen (APAP) hepatotoxicity. We investigated the efficacy of a diverse series of antioxidants in preventing APAP-induced hepatotoxicity. BALB/c mice were divided into four groups and provided with antioxidants incorporated into chow as follows: (1) control diet; or diet supplemented with (2) S-adenosylmethionine (SAMe); (3) green tea polyphenols (GrTP); or (4) (RS)-n-propylthiazolidine-4(R)-carboxylic acid (PTCA). After 5 days on these diets, the animals were further subdivided into (A) given an IP injection with APAP (750 mg/kg), or (B) kept as untreated controls. The animals were sacrificed at 0, 4 h, and 24 h following APAP administration. PAP/vehicle induced marked decreases in hepatic reduced glutathione (GSH) levels and endogenous SAMe concentrations (46%) when compared to controls. APAP also caused severe centrilobular necrosis and marked increase in serum enzyme ALT activity (38-fold). Oral administration of antioxidants significantly attenuated the APAP-induced liver damage and depletion of hepatic GSH. There were profound increases in serum TNF-alpha levels at 4 h following APAP administration in nonsupplemented compared to antioxidant-treated animals, but no significant differences noted after 24 h. Serum amyloid A increased in APAP-challenged mice irrespective of antioxidant treatment. Finally, hepatic SAMe concentrations were drastically decreased 24 h following APAP administration, and these decreases were attenuated by pretreatment with antioxidants. In conclusion, these orally administered antioxidants with dissimilar properties provided protection against liver damage, supporting the potential use of antioxidant therapy in patients with APAP toxicity. This is the first report that GrTP and oral administration of PTCA and SAMe can provide protection against APAP injury in this model.

p-Aminophenol-induced hepatotoxicity in hamsters: Role of glutathione

p-Aminophenol (PAP) is a widely used industrial chemical and a known nephrotoxin. Recently, it was found to also cause hepatotoxicity and glutathione (GSH) depletion in mice. The exact mechanism of liver toxicity is not known. The aims of this study were to determine whether PAP can cause acute hepatotoxicity in hamsters and to further investigate the role of GSH in PAP-induced toxicity. PAP was administered ip to hamsters in doses of 200-800 mg/kg. Liver damage at 24 h after PAP administration was assessed by elevations in plasma enzyme activities and histopathologic examination. GSH and cysteine (Cys) levels in liver at 4 h were determined by HPLC. PAP decreased hepatic GSH concentration to 8% and Cys to 30% of vehicle control values. It increased plasma glutamic pyruvic transaminase (GPT) activity by 47-fold and sorbitol dehydrogenase (SDH) activity by 113-fold. PAP also caused severe centrilobular hepatocellular necrosis. 2(RS)-n-Propylthiazolidine-4(R)-carboxylic acid (PTCA), a Cys precursor, attenuated the PAP-induced decreases in hepatic sulfhydryl levels; GSH and Cys were 39% and 78% of vehicle controls, respectively. PTCA also attenuated the PAP-induced elevations in plasma enzyme activities and hepatic necrosis. It was concluded that PAP hepatotoxicity is associated with depletion of hepatic GSH and can be prevented by PTCA.

Double-prodrugs of L-cysteine: differential protection against acetaminophen-induced hepatotoxicity in mice

A series of double-prodrugs of L-cysteine, designed to release L-cysteine in vivo and stimulate the biosynthesis of glutathione (GSH), were synthesized. To evaluate the hepatoprotective effectiveness of these double-prodrugs, male Swiss-Webster mice were administered acetaminophen (ACP) (2.45 mmol/kg (360 mg/kg), intraperitoneally (i.p.)). Prodrug (2.50 mmol/kg, i.p. or 1.25 mmol/kg, i.p., depending on the protocol) was administered 1 h before ACP as a priming dose. A supplementary dose of prodrug (2.5 mmol/kg, i.p. or 1.25 mmol/kg, i.p. depending on the protocol) was administered 0.5 h after ACP. The plasma alanine amino transferase (ALT) values, 24 h after ACP administration were transformed to logs and the 95% and 99% confidence intervals of the log values were plotted and compared for each group. Hepatoprotection was assessed by the degree of attenuation of plasma ALT levels. With these multiple dose schedules, the use of 2% carboxymethylcellulose as vehicle for the prodrugs was found to be detrimental; therefore, the prodrugs were dissolved in dilute aqueous base and the pH adjusted for administration. When a priming dose was given 1 h before ACP followed by a supplementary dose 0.5 h after ACP, only N,S-bis-acetyl-L-cysteine, where both the sulfhydryl and amino groups of L-cysteine were functionalized with the acetyl group, was found to be effective in protecting mice against the hepatotoxic effects of ACP. This suggests that these acetyl groups were rapidly hydrolyzed in vivo to liberate L-cysteine. In contrast, N-acetylation of 2(R,S)-methylthiazolidine-4(R)-carboxylic acid (MTCA) and its 2-n-propyl analog (PTCA), or N-acetylation of 2-oxothiazolidine-4-carboxylic acid (OTCA), reduced the hepatoprotective effects relative to the parent MTCA, PTCA, and OTCA, indicating that the release of L-cysteine in vivo from these N-acetylated thiazolidine prodrugs was metabolically unfavorable. The carbethoxy group, whether functionalized on the sulfhydryl or on the amino group of L-cysteine, or on the secondary amino group of MTCA, appears to be a poor "pro-moiety," since these carbethoxylated double-prodrugs of L-cysteine did not protect mice from ACP-induced hepatotoxicity.

Acetaminophen-induced suppression of hepatic AdoMet synthetase activity is attenuated by prodrugs of L-cysteine

Administration of acetaminophen (ACP, 400 mg/kg, i.p.) to fasted, male Swiss-Webster mice caused a rapid 90% decrease in total hepatic glutathione (GSH) and a 58% decrease in mitochondrial GSH by 2 h post ACP. This was followed by a time-dependent decrease (72%) in hepatic AdoMet synthetase activity and rise in plasma ALT levels (>10000 U/l) at 24 h post ACP treatment. AdoMet synthetase activity was maintained at 82, 78 and 60% of controls, respectively, by the cysteine prodrugs PTCA, CySSME and NAC. Total hepatic and mitochondrial GSH levels were also protected from severe ACP-induced depletion by CySSME and MTCA. These results suggest that the maintenance of GSH homeostasis by cysteine prodrugs can protect mouse hepatic AdoMet synthetase, a sulfhydryl enzyme whose integrity is dependent on GSH, as well as the liver itself from the consequences of oxidative stress elicited by toxic metabolites of xenobiotics.

Effects of 2(RS)-n-propylthiazolidine-4(R)-carboxylic acid on extrahepatic sulfhydryl levels in mice treated with acetaminophen

The cysteine (Cys) precursor 2(RS)-n-propylthiazolidine-4(R)-carboxylic acid (PTCA) has been shown to protect against acetaminophen (APAP)-induced hepatic GSH, GSSG, and Cys depletion and hepatic necrosis. The aim of this study was to determine the effects of PTCA on the concentrations of sulfhydryl compounds in extrahepatic tissues, including renal cortex, whole blood, and brain, in C57BL/6 mice treated with hepatotoxic doses of APAP. PTCA (1-5 mmol/kg, i.p.) was administered 30 min after the administration of APAP at a dose (800 mg/kg; 5.29 mmol/kg, i.p.) that depleted hepatic GSH and Cys at 4 hr by 95 and 86%, respectively. Tissue concentrations of GSH and Cys were determined by HPLC. At 4 hr following APAP administration, renal cortical GSH and Cys concentrations were decreased to 64 and 39%, respectively, of vehicle-treated control values, and blood concentrations were decreased to 87 and 30%, respectively, of vehicle controls. Brain GSH and Cys were not depleted by APAP. PTCA at 5 mmol/kg (i) attenuated the APAP-induced depletion of GSH and Cys at 4 hr in renal cortex (78 and 65%, respectively, of vehicle controls), (ii) prevented APAP-induced Cys depletion in blood (670% of vehicle controls) with no effect on GSH concentration (94% of vehicle controls), and (iii) increased GSH and Cys concentrations in brain (119 and 411%, respectively, of vehicle controls). The results demonstrate a high degree of tissue selectivity in the APAP-induced depletion of GSH and Cys, and in the effectiveness of PTCA in maintaining and even elevating sulfhydryl levels in extrahepatic tissues of APAP-treated mice.