Dissociation of increased sulfation from sulfate replenishment and hepatoprotection in acetaminophen-poisoned mice by N-acetylcysteine stereoisomers

Drugs and pharmaceuticals: management of intoxication and antidotes

The treatment of patients poisoned with drugs and pharmaceuticals can be quite challenging. Diverse exposure circumstances, varied clinical presentations, unique patient-specific factors, and inconsistent diagnostic and therapeutic infrastructure support, coupled with relatively few definitive antidotes, may complicate evaluation and management. The historical approach to poisoned patients (patient arousal, toxin elimination, and toxin identification) has given way to rigorous attention to the fundamental aspects of basic life support--airway management, oxygenation and ventilation, circulatory competence, thermoregulation, and substrate availability. Selected patients may benefit from methods to alter toxin pharmacokinetics to minimize systemic, target organ, or tissue compartment exposure (either by decreasing absorption or increasing elimination). These may include syrup of ipecac, orogastric lavage, activated single- or multi-dose charcoal, whole bowel irrigation, endoscopy and surgery, urinary alkalinization, saline diuresis, or extracorporeal methods (hemodialysis, charcoal hemoperfusion, continuous venovenous hemofiltration, and exchange transfusion). Pharmaceutical adjuncts and antidotes may be useful in toxicant-induced hyperthermias. In the context of analgesic, anti-inflammatory, anticholinergic, anticonvulsant, antihyperglycemic, antimicrobial, antineoplastic, cardiovascular, opioid, or sedative-hypnotic agents overdose, N-acetylcysteine, physostigmine, L-carnitine, dextrose, octreotide, pyridoxine, dexrazoxane, leucovorin, glucarpidase, atropine, calcium, digoxin-specific antibody fragments, glucagon, high-dose insulin euglycemia therapy, lipid emulsion, magnesium, sodium bicarbonate, naloxone, and flumazenil are specifically reviewed. In summary, patients generally benefit from aggressive support of vital functions, careful history and physical examination, specific laboratory analyses, a thoughtful consideration of the risks and benefits of decontamination and enhanced elimination, and the use of specific antidotes where warranted. Data supporting antidotes effectiveness vary considerably. Clinicians are encouraged to utilize consultation with regional poison centers or those with toxicology training to assist with diagnosis, management, and administration of antidotes, particularly in unfamiliar cases.

Chiral toxicology: it's the same thing...only different

Chiral substances possess a unique architecture such that, despite sharing identical molecular formulas, atom-to-atom linkages, and bonding distances, they cannot be superimposed. Thus, in the environment of living systems, where specific structure-activity relationships may be required for effect (e.g., enzymes, receptors, transporters, and DNA), the physiochemical and biochemical properties of racemic mixtures and individual stereoisomers can differ significantly. In drug development, enantiomeric selection to maximize clinical effects or mitigate drug toxicity has yielded both success and failure. Further complicating genetic polymorphisms in drug disposition, stereoselective metabolism of chiral compounds can additionally influence pharmacokinetics, pharmacodynamics, and toxicity. Optically pure pharmaceuticals may undergo racemization in vivo, negating single enantiomer benefits or inducing unexpected effects. Appropriate chiral antidotes must be selected for therapeutic benefit and to minimize adverse events. Enantiomers may possess different carcinogenicity and teratogenicity. Environmental toxicology provides several examples in which compound bioaccumulation, persistence, and toxicity show chiral dependence. In forensic toxicology, chiral analysis has been applied to illicit drug preparations and biological specimens, with the potential to assist in determination of cause of death and aid in the correct interpretation of substance abuse and "doping" screens. Adrenergic agonists and antagonist, nonsteroidal anti-inflammatory agents, SSRIs, opioids, warfarin, valproate, thalidomide, retinoic acid, N-acetylcysteine, carnitine, penicillamine, leucovorin, glucarpidase, pesticides, polychlorinated biphenyls, phenylethylamines, and additional compounds will be discussed to illustrate important concepts in "chiral toxicology."

Synthesis, physical properties, toxicological studies and bioavailability of L-pyroglutamic and L-glutamic acid esters of paracetamol as potentially useful prodrugs

Paracetamol ester prodrugs with L-pyroglutamic and L-glutamic acid, biosynthetic precursors of glutathione, have been synthesized to reduce paracetamol hepatotoxicity and improve bioavailability. The toxicological studies of paracetamol esters show that only L-5-oxo-pyrrolidine-2-paracetamol carboxylate reduces toxicity after administration of an overdose. The glutathione hepatic values in mice obtained by intraperitoneal injection of the ester are superimposable on controls and the oral LD50 was found to be greater than 2000 mg kg-1 and the intraperitoneal LD50 was 1900 mg kg-1. These results taken together with hydrolysis and bioavailability data show that ester is a potential candidate as a prodrug of paracetamol.

Effects of N-acetylcysteine stereoisomers on oxygen-induced lung injury in rats

The effects of the stereoisomers of N-acetylcysteine (L-NAC and D-NAC) on oxygen-induced lung oedema have been studied in rats. The NAC-isomers were given by an osmotic minipump in order to attain continuous administration, either intravenously or intragastrically. In some experiments, plasma concentrations of NAC, cysteine and glutathione (total concentrations, i.e., concentrations obtained after reduction of the samples with dithiothreitol) were recorded. Exposure to oxygen induced an almost two-fold increase of the lung wet weight. When L-NAC or D-NAC were given intravenously, in dose of 1.1 mmol/day/kg body weight, the increase of lung wet weight was prevented by 40-50%. The plasma concentrations were approximately 40 microM (L-NAC) and approximately 90 microM (D-NAC). Following intragastrical administration of the same doses, plasma concentrations of L-NAC and D-NAC reached approximately 3 and approximately 60 microM, respectively. Using this method of administration, only D-NAC significantly diminished the increase of the lung wet weight. The difference in plasma concentrations of the NAC isomers, particularly after intragastric administration, most likely reflects the fact that L-NAC is effectively hydrolysed in most tissues, while D-NAC is resistant to enzymatic hydrolysis, thus penetrating largely intact into the systemic circulation. The data presented shows that NAC, regardless of stereoconfiguration, will protect the lung against oxygen toxicity, provided sufficient systemic levels are obtained. Since D-NAC is not a precursor of L-cysteine, formation of glutathione cannot explain the protective effects of this isomer. L- and D-NAC may therefore act via direct antioxidant/radical scavenging mechanisms and not necessarily as precursors of glutathione in this model.

Effect of D- or L-methionine and cysteine on the growth inhibitory effects of feeding 1% paracetamol to rats

Rats fed 1% paracetamol in the diet failed to grow and a dose-dependent inhibition of growth was observed and found to be independent of hepatoxicity. Addition of 0.5% D- or L-methionine, or L-cysteine to a diet containing 1% paracetamol restored growth. Addition of L-methionine to the drinking water was equally effective. Feeding D-cysteine or sodium sulphate were ineffective. Acute paracetamol toxicity was also prevented by D- as well as by L-methionine. It is concluded that the inhibition of growth was due to depletion of sulphur amino acids in the course of paracetamol metabolism. This was sometimes followed by episodes of liver cell injury. Since the normal human dosage of paracetamol is up to 4 g/day, which is equivalent to 1% of the diet, the possibility of induction of amino acid deficiency by chronic use of paracetamol in normal dosage is raised.

Selective effects of N-acetylcysteine stereoisomers on hepatic glutathione and plasma sulfate in mice

Administration of 1200 mg/kg N-acetyl-L-acetyl-L-cysteine to mice apparently increased the rate of glutathione synthesis resulting in prolonged elevation of the hepatic glutathione pool. At 3 hr, peak glutathione concentrations nearly doubled. The fraction of glutathione in the oxidized state remained at normally low values over this period. The L isomer also significantly increased plasma and urinary concentrations of inorganic sulfate. Plasma concentrations nearly doubled at their peak and urinary excretion over 24 hr rose some threefold above control. Consistent with the known stereoselectivity of many biological processes, the unnatural D isomer of N-acetylcysteine failed to increase hepatic glutathione. Liver concentrations remained similar to control suggesting that the D isomer was unable to increase the rate of glutathione synthesis. The D isomer further differed from its L enantiomer in failing to increase the plasma concentration and the urinary excretion of inorganic sulfate. Congruent with these observations, much more N-acetyl-D-cysteine (47% of dose) was recovered unchanged in 24 hr urine than N-acetyl-L-cysteine (6.1% of dose). These findings are of toxicologic interest because they identify the N-acetylcysteine stereoisomers as a pair of agents that may be useful in separating biologic effects of sulfhydryls that are related primarily to their physical properties (i.e. reduction, radical scavenging) from those that accrue from their ability to increase glutathione and sulfate availability in vivo.