Transport characteristics of paraquat across rat intestinal brush-border membrane

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.

Modulation of P-glycoprotein efflux pump: induction and activation as a therapeutic strategy

P-glycoprotein (P-gp) is an ATP-dependent efflux pump encoded by the MDR1 gene in humans, known to mediate multidrug resistance of neoplastic cells to cancer therapy. For several decades, P-gp inhibition has drawn many significant research efforts in an attempt to overcome this phenomenon. However, P-gp is also constitutively expressed in normal human epithelial tissues and, due to its broad substrate specificity, to its cellular polarized expression in many excretory and barrier tissues, and to its great efflux capacity, it can play a crucial role in limiting the absorption and distribution of harmful xenobiotics, by decreasing their intracellular accumulation. Such a defense mechanism can be of particular relevance at the intestinal level, by significantly reducing the intestinal absorption of the xenobiotic and, consequently, avoiding its access to the target organs. In this review, the current knowledge on this important efflux pump is summarized, and a new focus is brought on the therapeutic interest of inducing and/or activating P-gp for limiting the toxicity caused by its substrates. Several in vivo and in vitro studies validating the use of such a therapeutic strategy are discussed. An extensive literature search for reported P-gp inducers/activators and for the experimental models used in their characterization was conducted. Those studies demonstrate that effective antidotal pathways can be achieved by efficiently promoting the P-gp-mediated efflux of deleterious xenobiotics, resulting in a significant reduction in their intracellular levels and, consequently, in a significant reduction of their toxicity.

Regulation of expression and activity of multidrug resistance proteins MRP2 and MDR1 by estrogenic compounds in Caco-2 cells. Role in prevention of xenobiotic-induced cytotoxicity

ABC transporters including MRP2, MDR1 and BCRP play a major role in tissue defense. Epidemiological and experimental studies suggest a cytoprotective role of estrogens in intestine, though the mechanism remains poorly understood. We evaluated whether pharmacologic concentrations of ethynylestradiol (EE, 0.05pM to 5nM), or concentrations of genistein (GNT) associated with soy ingestion (0.1-10μM), affect the expression and activity of multidrug resistance proteins MRP2, MDR1 and BCRP using Caco-2 cells, an in vitro model of intestinal epithelium. We found that incubation with 5pM EE and 1μM GNT for 48h increased expression and activity of both MRP2 and MDR1. Estrogens did not affect expression of BCRP protein at any concentration studied. Irrespective of the estrogen tested, up-regulation of MDR1 and MRP2 protein was accompanied by increased levels of MDR1 mRNA, whereas MRP2 mRNA remained unchanged. Cytotoxicity assays demonstrated association of MRP2 and MDR1 up-regulation with increased resistance to cell death induced by 1-chloro-2,4-dinitrobenzene, an MRP2 substrate precursor, and by paraquat, an MDR1 substrate. Experiments using an estrogen receptor (ER) antagonist implicate ER participation in MRP2 and MDR1 regulation. GNT but not EE increased the expression of ERβ, the most abundant form in human intestine and in Caco-2 cells, which could lead in turn to increased sensitivity to estrogens. We conclude that specific concentrations of estrogens can confer resistance against cytotoxicity in Caco-2 cells, due in part to positive modulation of ABC transporters involved in extrusion of their toxic substrates. Although extrapolation of these results to the in vivo situation must be cautiously done, the data could explain tentatively the cytoprotective role of estrogens against chemical injury in intestine.

Paraquat poisonings: mechanisms of lung toxicity, clinical features, and treatment

Paraquat dichloride (methyl viologen; PQ) is an effective and widely used herbicide that has a proven safety record when appropriately applied to eliminate weeds. However, over the last decades, there have been numerous fatalities, mainly caused by accidental or voluntary ingestion. PQ poisoning is an extremely frustrating condition to manage clinically, due to the elevated morbidity and mortality observed so far and due to the lack of effective treatments to be used in humans. PQ mainly accumulates in the lung (pulmonary concentrations can be 6 to 10 times higher than those in the plasma), where it is retained even when blood levels start to decrease. The pulmonary effects can be explained by the participation of the polyamine transport system abundantly expressed in the membrane of alveolar cells type I, II, and Clara cells. Further downstream at the toxicodynamic level, the main molecular mechanism of PQ toxicity is based on redox cycling and intracellular oxidative stress generation. With this review we aimed to collect and describe the most pertinent and significant findings published in established scientific publications since the discovery of PQ, focusing on the most recent developments related to PQ lung toxicity and their relevance to the treatment of human poisonings. Considerable space is also dedicated to techniques for prognosis prediction, since these could allow development of rigorous clinical protocols that may produce comparable data for the evaluation of proposed therapies.

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.

Pharmacokinetics and tissue distribution of the anticholinergics tiotropium and ipratropium in the rat and dog

Ipratropium, a current treatment for chronic obstructive pulmonary disease (COPD) and tiotropium, a longer acting anticholinergic bronchodilator currently being developed for COPD are structurally related to atropine. In this study, the intravenous (i.v.), oral (p.o.) and intratracheal (i.tr.) single dose pharmacokinetics (PK) of tiotropium and ipratropium were determined in rat and dog. In rats, concentration-time profiles of tiotropium and ipratropium after single i.v. bolus administration of 7-8 mg kg(-1) are similar. Both drugs are highly cleared (Cl between 87 and 150 ml min(-1) kg(-1)) and extensively distributed into tissues (volume of distribution V(ss) between 3 and 15 l kg(-1)). In dogs, this holds also true for both drugs (Cl between 34 and 42 ml min(-1) kg(-1), V(ss) between 2 and 10 l kg(-1)), although different dose regimen were applied (i.v. bolus of 0.08 mg kg(-1) vs. infusion of 0.1 mg kg(-1) h(-1) for 3 h). Tiotropium plasma concentrations increased linearly in rats over a wide dose range following single i.v. administration. Both ipratropium and tiotropium showed a comparable terminal elimination half-life in rat urine (21-24 h) after single i.v. administration, which was much longer than the corresponding half-life in plasma (6-8 h). Whole body autoradiography in rats revealed a broad and rapid tissue distribution of [(14)C]tiotropium radioactivity after single i.v. administration. A comparable distribution pattern has also been reported earlier for ipratropium.

Paraquat2+/H+ exchange in isolated renal brush-border membrane vesicles

The mechanism(s) by which paraquat (1,1'-dimethyl-4,4'-bipyridinium), a divalent organic cation (OC) and proximal tubule nephrotoxicant, crosses renal cell membranes is unclear. The structurally-related monovalent OC, 1-methyl-4-phenylpyridinium (MPP+), crosses the renal brush border via OC/H+ exchange using the same pathway by which tetraethylammonium (TEA) is transported. We examined whether paraquat shares the TEA(MPP+)/H+ exchanger by examining 14C-paraquat transport in rabbit renal BBMV. Compared to a pH equilibrium condition (pH 7.5in:7.5o), an H-gradient (pH 6in:7.5o) stimulated the 5 s and 60 s uptakes of 230 microM paraquat by 51% and 108%, respectively, and this stimulation was blocked by both 20 mM unlabeled paraquat and TEA. Pre-loading BBMV with 2 mM unlabeled TEA (under conditions of pH equilibrium) stimulated by 3-fold the 60 s uptake of 120 microM paraquat and by 5 min produced a transient intravesicular accumulation of paraquat that exceeded equilibrium (2 h) uptake by 45%. The presence of 200 microM paraquat in the extravesicular solution competitively inhibited H-gradient-stimulated transport of 14C-TEA in renal BBMV, increasing the apparent Kt for TEA transport from 169 microM to 379 microM, without significantly influencing the Jmax (16.0 vs. 15.4 nmol mg-1 min-1). The calculated Ki for paraquat (presumably equal to its Kt for transport) after transport was between 160 and 220 microM (depending upon the method of estimation). Significantly, the Kt for MPP+/H exchange is 12 microM, suggesting that the affinity of the exchanger is profoundly influenced by the presence on paraquat of a second positive charge. We conclude that renal transport of paraquat involves the OC/H+ exchanger of proximal cell luminal membranes and that this pathway may play a role in the renal secretion of polyvalent organic cations.

Further evidence that the blood/brain barrier impedes paraquat entry into the brain

The distribution of the non-selective herbicide paraquat was examined in the brain following subcutaneous administration of 20 mg kg-1 paraquat ion containing [14C]paraquat to male adult rats in order to determine whether paraquat crosses the blood/brain barrier. Following administration, [14C]paraquat reached a maximal concentration in the brain (0.05% of administered dose) within the first hour and then rapidly disappeared from the brain. However, 24 h after administration of the herbicide, about 13% of the maximal recorded concentration of paraquat remained in the brain (1.6 nmol g-1 wet weight) and could not be removed by intracardiac perfusion. Using measurements of [14C]paraquat in dissected brain regions and using quantitative autoradiography we demonstrated an asymmetrical distribution in and around the brain at 30 min (maximal concentration) and 24 h after administration. Most of the paraquat was associated with five structures, two of which, the pineal gland and linings of the cerebral ventricles lie outside the blood/brain barrier whilst the remaining three brain areas, the anterior portion of the olfactory bulb, hypothalamus and area postrema do not have a blood/brain barrier. Overall, the distribution of [14C]paraquat in the brain 24 h after systemic administration was highly correlated to the blood volume. These data indicate that any remaining paraquat in the brain 24 h after systemic administration is associated with elements of the cerebro-circulatory system, such as the endothelial cells that make up the capillary network and that there is a limited entry of paraquat into brain regions without a blood/brain barrier.(ABSTRACT TRUNCATED AT 250 WORDS)