Pre- and post-treatment effect of physostigmine on soman-inhibited human erythrocyte and muscle acetylcholinesterase in vitro

Brain delivery of antidotes by polymeric nanoparticles

Accidental intoxications from environmental pollutants, as well as intentional self- and chemical warfare-related poisonings affect millions of people worldwide each year. While many toxic agents can readily enter the central nervous system (CNS), the blood-brain barrier (BBB) prevents the brain uptake of most pharmaceuticals. Consequently, poisoning antidotes usually cannot reach their site of action in the CNS in therapeutically relevant concentrations, and thus only provide effective protection to the peripheral nervous system. This limitation can be overcome by encapsulating the antidotes in nanoparticles (NP), which can enhance their CNS accumulation without damaging the integrity of the BBB. Among nanocarriers, polymer-based drug delivery systems exhibit remarkable benefits, such as bioavailability, cell uptake and tissue retention. Furthermore, due to their capacity to mask unfavorable physicochemical properties of cargo drugs, polymeric NPs were able to improve BBB transport of various pharmaceuticals. However, while polymer NP-mediated treatment of various pathological brain conditions, such as glioma and Alzheimer's disease were exhaustively studied, the application of polymeric nanocarriers for brain-targeted delivery of antidote molecules has not been adequately examined. To display its therapeutic potential, we review the state of the art of polymer NP-assisted CNS delivery of antidotes for various poisonings, including heavy metal and organophosphorus intoxications.

The oximes HI-6 and MMB-4 fail to reactivate soman-inhibited human and guinea pig AChE: A kinetic in vitro study

Acetylcholinesterase (AChE) inhibited by the organophosphorus nerve (OP) agent soman underlies a spontaneous and extremely rapid dealkylation ("aging") reaction which prevents reactivation by oximes. However, in vivo studies in various, soman poisoned animal species showed a therapeutic effect of oximes, with the exact mechanism of this effect remaining still unclear. In order to get more insight and a basis for the extrapolation of animal data to humans, we applied a dynamic in vitro model with continuous online determination of AChE activity. This model allows to simulate the in vivo toxico- and pharmacokinetics between human and guinea pig AChE with soman and the oximes HI-6 and MMB-4 in order to unravel the species dependent kinetic interactions. It turned out that only HI-6 was able to slow down the ongoing inhibition of human AChE by soman without preventing final complete inhibition of the enzyme. Continuous perfusion of AChE with soman and simultaneous or delayed (8, 15 or 40min) oxime perfusion did not result in a relevant reactivation of AChE (less than 2%). In conclusion, the results of the present study indicate a negligible reactivation of soman-inhibited AChE by oximes at conditions simulating the in vivo poisoning by soman. The observed therapeutic effect of oximes in soman poisoned animals in vivo must be attributed to alternative mechanisms which may not be relevant in humans.

Pharmacological management of anticholinergic delirium - theory, evidence and practice

The spectrum of anticholinergic delirium is a common complication following drug overdose. Patients with severe toxicity can have significant distress and behavioural problems that often require pharmacological management. Cholinesterase inhibitors, such as physostigmine, are effective but widespread use has been limited by concerns about safety, optimal dosing and variable supply. Case series support efficacy in reversal of anticholinergic delirium. However doses vary widely and higher doses commonly lead to cholinergic toxicity. Seizures are reported in up to 2.5% of patients and occasional cardiotoxic effects are also recorded. This article reviews the serendipitous path whereby physostigmine evolved into the preferred anticholinesterase antidote largely without any research to indicate the optimal dosing strategy. Adverse events observed in case series should be considered in the context of pharmacokinetic/pharmacodynamic studies of physostigmine which suggest a much longer latency before the maximal increase in brain acetylcholine than had been previously assumed. This would favour protocols that use lower doses and longer re-dosing intervals. We propose based on the evidence reviewed that the use of cholinesterase inhibitors should be considered in anticholinergic delirium that has not responded to non-pharmacological delirium management. The optimal risk/benefit would be with a titrated dose of 0.5 to 1 mg physostigmine (0.01-0.02 mg kg(-1) in children) with a minimum delay of 10-15 min before re-dosing. Slower onset and longer acting agents such as rivastigmine would also be logical but more research is needed to guide the appropriate dose in this setting.

Pretreatment with Huperzine A-Loaded Poly(lactide-co-glycolide) Nanoparticles Protects against Lethal Effects of Soman-Induced in Mice

Huperzine A (HupA), an alkaloid isolated from theChinese club moss, is a reversible inhibitor of cholinesterases which cross the blood-brain barrier and show high specificity for acetylcholinesterase (AChE). However, HupA induces unwanted side effects in an effective dose against nerve agent poisoning. In the present study, HupA–loaded poly (lactide-co-glycolide) nanoparticles (HupA-PLGA-NP) were prepared using the O/W emulsion solvent evaporation method. The results of SEM demonstrated that HupA-PLGA-NP had an spherical shape and a smooth surface without pores. It’s mean diameter and PDI were 208.5±3.6nm and 0.09±0.01 respectively. The Zeta potential was-35.3±1.8mV and the drug loading was 2.86±0.6%.In vitrodrug release studies showed that HupA-PLGA-NP had a sustained-release behavior in phosphate buffer solution, The accumulated amount of HupA was about 72.1% at 48h with a low burst release within 30min. The LD50values of HupA and HupA-PLGA-NP were 1.40 and 4.85mg/kg respectively, showing that the toxicity of HupA was reduced by 3.5 times. We evaluated the protective efficacy for different doses of HupA or HupA-PLGA-NP against 1.0×LD95(143.0μg/kg) soman toxicity. The results confirmed that HupA (0.3~0.5mg/kg) or HupA-PLGA-NP (0.5~1.5mg/kg) could ensure animals survive. However, about 10% of the animals injected with HupA (0.8mg/kg) died, while no animals died when injected with HupA-PLGA-NP (1.5mg/kg). Aim to 100% survival rate, the effective protective time (12h) of HupA-PLGA-NP (0.5mg/kg,iv) against 1.0×LD95soman toxicity in mice was significantly prolonged compared with that of HupA (4h). The study of AChE activity showed that whole-blood and supernatant of brain diluted by 80-fold and 10-fold respectively were optimum in this study. AChE inhibition after administration of HupA and HupA-PLGA-NP (0.5mg/kg,iv) was recorded and analyzed, The peak values of AChE inhibition in whole-blood and brain by HupA-PLGA-NP (17.6% and 21.8%) were lower than those by HupA (33.7% and 31.9%) and AChE inhibition time by HupA-PLGA-NP was longer than that by HupA. These data confirmed that HupA-PLGA-NP had less toxic and more longer time than HupA against 1.0×LD95soman poisoning and warrant further development as a potent medical countermeasure against chemical warfare nerve agents (CWNAs) poisoning.

Comparative kinetics of organophosphates and oximes with erythrocyte, muscle and brain acetylcholinesterase

There is an ongoing debate whether oximes can effectively counteract the effects of organophosphorus compounds (OP) on brain acetylcholinesterase (AChE) activity and whether there are differences in the kinetic properties of brain and erythrocyte AChE. In order to investigate the kinetics of AChE from different tissues and species the well established dynamically working in vitro model with real-time determination of membrane-bound AChE activity was adapted for use with brain AChE. The enzyme reactor, that was loaded with brain, erythrocyte or muscle AChE, was continuously perfused with substrate and chromogen while AChE activity was on-line analyzed in a flow-through detector. It was possible to determine the Michaelis-Menten constants of human erythrocyte, muscle and brain AChE which were almost identical. In addition, the inhibition kinetics of sarin and paraoxon as well as the reactivation kinetics of obidoxime and HI 6 were determined with human, swine and guinea pig brain and erythrocyte AChE. It was found that the inhibition and reactivation kinetics of brain and erythrocyte AChE were highly comparable in all tested species. These data support the view that AChE from different tissue has similar kinetic properties and that brain AChE is comparably susceptible toward reactivation by oximes.

Determination of acetylcholinesterase activity by the Ellman assay: a versatile tool for in vitro research on medical countermeasures against organophosphate poisoning

Inhibition of acetylcholinesterase (AChE) is the main mechanism of action of organophosphorus compounds (OP), and AChE reactivators (oximes) are at present the only causal therapeutic approach. Being the key target of OP toxicity, AChE may serve as a valuable tool for diagnosis of OP exposure as well as for the investigation of the kinetics of interactions between OP and oximes. At present, the rapid, simple, and cheap spectrophotometric Ellman assay is widely used for diagnosis, therapeutic monitoring and in vitro kinetic investigations. Application of the assay for investigation of the interactions between AChE, inhibitors, and oximes requires the consideration of potential matrix effects (e.g. hemoglobin), side reactions (e.g. oximolysis of substrate) and other determinants (e.g. pH, temperature). By taking these factors into account, the Ellman assay allows the precise and reproducible determination of kinetic constants as a basis for the understanding of toxic OP effects and for the development of improved therapies against poisoning by OP. In addition, advanced applications of the Ellman assay, for example, in a dynamic in vitro model for the real-time activity determination of membrane-bound AChE, enables the proper investigation of relevant tissue, primarily respiratory muscle, and extends the applicability of this method.