Gastrointestinal absorption of paraquat in the isolated mucosa of the rat

ADME/T-based strategies for paraquat detoxification: Transporters and enzymes

Paraquat (PQ) is a toxic, organic herbicide for which there is no specific antidote. Although banned in some countries, it is still used as an irreplaceable weed killer in others. The lack of understanding of the precise mechanism of its toxicity has hindered the development of treatments for PQ exposure. While toxicity is thought to be related to PQ-induced oxidative stress, antioxidants are limited in their ability to ameliorate the untoward biological responses to this agent. Summarized in this review are data on the absorption, distribution, metabolism, excretion, and toxicity (ADME/T) of PQ, focusing on the essential roles of individual transporters and enzymes in these processes. Based on these findings, strategies are proposed to design and test specific and effective antidotes for the clinical management of PQ poisoning.

Protective action of Omega-3 on paraquat intoxication in Drosophila melanogaster

Paraquat (PQ) (1,1'-dimethyl-4-4'-bipyridinium dichloride) is the second most widely used herbicide worldwide; however, in countries different sales and distribution remain restricted. Chronic exposure to PQ leads to several diseases related to oxidative stress and mitochondrial dysfunctions including myocardial failure, cancer, and neurodegeneration and subsequently death depending upon the dose level. The aim of this study was to examine if diet supplementation with eicosapentaenoic and docosahexaenoic acids (EPA and DHA, omega-3 long-chain fatty acids) serves a protective mechanism against neuromuscular dysfunctions mediated by PQ using Drosophila melanogaster as a model with focus on mitochondrial metabolism. PQ ingestion (170 mg/kg b.w. for 3 d) resulted in a decreased life span and climbing ability in D. melanogaster. In the brain, PQ increased thioflavin fluorescence and reduced either 4',6-diamidino-2-phenylindole dihydrochloride (DAPI) nuclei staining and neuronal nuclei protein (NeuN) positive neurons, indicating amyloid formation and neurodegenetation, respectively. In the thorax, PQ ingestion lowered citrate synthase activity and respiratory functions indicating a reduction in mitochondrial content. PQ elevated Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) mRNA expression levels, indicative of high calcium influx from cytosol to mitochondrial matrix. In brain and thorax, PQ also increased hydrogen peroxide (H2O2) production and impaired acetylcholinesterase (AChE) activity. Concomitant EPA/DHA ingestion (0.31/0.19 mg/kg b.w.) protected D. melanogaster against PQ-induced toxicity preserving neuromuscular function and slowing down the rate of aging. In brain and thorax, these omega-3 fatty acids inhibited excess H2O2 production and restored AChE activity. EPA/DHA delayed amyloid deposition in the brain, and restored low citrate synthase activity and respiratory functions in the thorax. The effects in the thorax were attributed to stimulated mRNA expression level of genes involved either in mitochondrial dynamics or biogenesis promoted by EPA/DHA: dynamin-related protein (DRP1), mitochondrial assembly regulatory factor (MARF), mitochondrial dynamin like GTPase (OPA1), and peroxisome proliferator-activated receptor-γ coactivator 1α (PGC1α). In conclusion, diet supplementation with EPA/DHA appears to protect D. melanogaster muscular and neuronal tissues against PQ intoxication.

Improvement in survival after paraquat ingestion following introduction of a new formulation in Sri Lanka

BACKGROUND

Pesticide ingestion is a common method of self-harm in the rural developing world. In an attempt to reduce the high case fatality seen with the herbicide paraquat, a novel formulation (INTEON) has been developed containing an increased emetic concentration, a purgative, and an alginate that forms a gel under the acid conditions of the stomach, potentially slowing the absorption of paraquat and giving the emetic more time to be effective. We compared the outcome of paraquat self-poisoning with the standard formulation against the new INTEON formulation following its introduction into Sri Lanka.

METHODS AND FINDINGS

Clinical data were prospectively collected on 586 patients with paraquat ingestion presenting to nine large hospitals across Sri Lanka with survival to 3 mo as the primary outcome. The identity of the formulation ingested after October 2004 was confirmed by assay of blood or urine samples for a marker compound present in INTEON. The proportion of known survivors increased from 76/297 with the standard formulation to 103/289 with INTEON ingestion, and estimated 3-mo survival improved from 27.1% to 36.7% (difference 9.5%; 95% confidence interval [CI] 2.0%-17.1%; p = 0.002, log rank test). Cox proportional hazards regression analyses showed an approximately 2-fold reduction in toxicity for INTEON compared to standard formulation. A higher proportion of patients ingesting INTEON vomited within 15 min (38% with the original formulation to 55% with INTEON, p < 0.001). Median survival time increased from 2.3 d (95% CI 1.2-3.4 d) with the standard formulation to 6.9 d (95% CI 3.3-10.7 d) with INTEON ingestion (p = 0.002, log rank test); however, in patients who did not survive there was a comparatively smaller increase in median time to death from 0.9 d (interquartile range [IQR] 0.5-3.4) to 1.5 d (IQR 0.5-5.5); p = 0.02.

CONCLUSIONS

The survey has shown that INTEON technology significantly reduces the mortality of patients following paraquat ingestion and increases survival time, most likely by reducing absorption.

Identification of an alginate-based formulation of paraquat to reduce the exposure of the herbicide following oral ingestion

The herbicide paraquat has been widely used throughout the world for almost 50 years and is important in sustainable agriculture. When used correctly the chemical poses no known risk to human health. However, it is acutely toxic, and can be fatal, if the concentrated product is ingested orally. Despite many years of research there is no successful treatment for paraquat intoxication. In recent years we have turned our attention to understanding how we can make the product safer, if it is accidentally or intentionally consumed. We present in this paper a novel approach aimed at safening the paraquat product, Gramoxone. Following our previous research on the site and mechanism of paraquat absorption from the gastrointestinal tract we have identified a new formulation of paraquat, Gramoxone INTEON that reduces the absorption of paraquat into the blood. This new formulation contains the polysaccharide, alginate, a natural product extracted from sea-weed. We have designed a preparation of paraquat and alginate with surfactants that is herbicidally active but has the unique property that it gels on contact with gastric acid in the stomach. The resulting mixture slows the dispersion and delivery of the toxic chemical to its site of absorption in the small intestine. Alginates also protect the mucosa against the damaging influence of topical gastric irritants, like paraquat. Our studies have shown that increasing the loading of alginate between 7 and 17 g/L causes a dose-related reduction in paraquat absorption in vitro in isolated rat ileum. This is also observed in vivo, as measured by paraquat plasma kinetics in the rabbit where the Area Under Curve (AUC 0-24h) was reduced from 33.8+/-3 for Gramoxone to 12.5+/-6 (microg/mL)h for a formulation containing 17 g/L alginate. Such a reduction in systemic exposure to paraquat is expected to reduce the acute oral toxicity of the formulation. This should be particularly effective in a vomiting species such as man since we have shown in this investigation that alginates not only reduce the peak plasma paraquat values but also delay the time to peak levels. This provides the opportunity for a more effective emetic response since the highly viscous gelled material should remain in the stomach for longer than the liquid Gramoxone. Further research is required to understand and optimise the safening and herbicidal characteristics of these alginate acid-triggered gel formulations of paraquat. However, we anticipate that this alginate technology in Gramoxone INTEON could have significant benefit in reducing human mortalities associated with the herbicide.

Modulation of bile acids induced by paraquat in rabbits

Rabbit bile was examined for changes in composition induced by paraquat. Paraquat was administered intraperitoneally and changes in bile components were monitored by high performance liquid chromatography. Alterations in the ratios of total glycine/taurine conjugated bile acids (TGC/TTC), cholic acid/deoxycholic acid (CA/DC), cholic acid/chenodeoxycholic acid (CA/CDC) and cholic acid/cholesterol (CA/CH) were measured as an index of paraquat toxicity. A statistically significant increase in the ratio of TGC/TTC was observed, while CA/DC, CA/CDC and CA/CH showed a decrease. Phospholipids, protein, sugar, bilirubin, beta-carotene, vitamin A and vitamin E in the bile and serum of the experimental animals were also monitored. In bile, the levels of cholesterol, phospholipids, protein, sugar, and total bile acids increased while the levels of the antioxidants beta-carotene, vitamin A and vitamin E decreased. A decrease in the bilirubin content of the bile was also observed. These modifications may be useful clinically for assessment of paraquat toxicity.

Prevention of paraquat toxicity in suspensions of alveolar type II cells by paraquat-specific antibodies

1. The herbicide, paraquat, is accumulated by the energy-dependent polyamine uptake pathway of alveolar type II cells. There it undergoes redox cycling that results in an amplified production of toxic reactive oxygen species and depletion of NADPH and other reducing equivalents. These processes account for the lung being the major target organ for paraquat toxicity. 2. We postulated that paraquat-specific antibodies would inhibit the uptake of the herbicide by type II cells and prevent its toxicity. Accordingly, we examined the effects of paraquat-specific monoclonal antibodies and Fab fragments on the uptake, efflux and cytotoxicity of 50 microM paraquat in suspensions of alveolar type II cells isolated from the rat. 3. The uptake of paraquat was linear over 40 min. Over this time, the uptake rate was inhibited significantly (% inhibition, 73-89) by IgG (25 or 50 microM) or Fab fragments (50 or 100 microM). 4. The apparent efflux rate of paraquat, studied over 16 h, was increased significantly from 0.12 h-1 for the control cells in medium to 0.17 h-1 by paraquat-specific Fab fragments but was unaffected by the specific IgG. 5. Cytotoxicity was determined by measuring the release of 51Cr from the cells. The cytotoxicity of 50 microM paraquat was decreased significantly (percent decrease, 56-80%) in the presence of specific antibodies. 6. These studies in vitro suggest some potential for immunotherapy in selected cases of paraquat poisoning.

Transport characteristics of paraquat across rat intestinal brush-border membrane

The mechanism of absorption of paraquat, which is a type of quaternary ammonium compound (QAC), was studied using rat intestinal loops and brush-border membrane vesicles. Approximately 47% and 37% of radioactively labeled paraquat injected into jejunal and ileal loops disappeared, respectively, after 60 min. Since only a small amount of radioactivity was detected in the mucosal fraction, most of the paraquat that disappeared from the intestinal lumen was considered to have been carried away by the bloodstream, indicating that paraquat absorption was greater than expected. In spite of its low lipid solubility, the uptake of paraquat by brush-border membrane vesicles reflected smooth penetration into the intravesicular space rather than binding to the membrane. According to the increase in extravesicular paraquat concentration, paraquat uptake in the early stage was saturable. Moreover, early paraquat uptake was significantly inhibited by structurally-related QACs such as tetramethylammonium and choline, but not by an endogenous dicationic amine (putrescine). On the other hand, inside-negative membrane potential had no significant effect on the time course of paraquat uptake. From these results, it is suggested that paraquat is absorbed through a specialized mechanism associated with the carrier-mediated transport system for choline on the brush-border membrane.

Protective effects of dextran sulfate and polyvinyl sulfate against acute toxicity of paraquat in mice

The protective effects of sodium dextran sulfate (SDS) and potassium polyvinyl sulfate (PPS) against the acute toxicity of paraquat (PQ) in mice were studied. The survival rates of mice treated with SDS (2000 mg/kg) or PPS (2000 mg/kg) immediately after PQ ingestion (200 mg/kg) were 100% or 100%, respectively. When treated with SDS (2000 mg/kg) or PPS (2000 mg/kg) 15 or 30 min after PQ ingestion (200 mg/kg), the survival rates were 83% or 67% for SDS-treated groups and 67% or 33% for PPS-treated groups, respectively. Treatment with SDS (2000 mg/kg) or PPS (2000 mg/kg) immediately after oral administration of PQ (200 mg/kg) increased the fecal excretion of PQ, decreased the urinary excretion of PQ and decreased the contents of PQ in the lung, liver and kidney. Such effects of SDS and PPS were reduced in the treatment with these drugs at 15 min after PQ. The in situ small intestinal absorption of PQ was significantly reduced in the presence of SDS or PPS. The binding of PQ to SDS or PPS was determined by an ultrafiltration method. These results indicate that SDS and PPS inhibit the gastrointestinal absorption of PQ on the basis of the increased intestinal transit of PQ and the binding of PQ to the drugs resulting in the protective effectiveness of SDS and PPS on the acute toxicity of PQ.