J Appl Toxicol. 2015 May;35(5):493-9.

Reversible cholinesterase inhibitors as pre-treatment for exposure to organophosphates: assessment using azinphos-methyl.

Petroianu GA, Nurulain SM, Hasan MY, Kuča K, Lorke DE.

Department of Cellular Biology & Pharmacology, Herbert Wertheim College of Medicine, Florida International University, University Park GL 495 D, 11200 SW 8th St, Miami, FL 33199, Florida, USA

 

Abstract

Pre-treatment with reversible acetylcholinesterase (AChE) inhibitors before organophosphorous compound (OPC) exposure can reduce OPC-induced mortality. However, pyridostigmine, the only substance employed for such prophylaxis, is merely efficacious against a limited number of OPCs. In search of more efficacious and broad-range alternatives, we have compared in vivo the ability of five reversible AChE inhibitors (pyridostigmine, physostigmine, ranitidine, tacrine and K-27) to reduce mortality induced by the OPC azinphos-methyl. Protection was quantified using Cox (1) analysis by determining the relative risk (RR) of death in rats that were administered these AChE inhibitors in equitoxic dosage (25% of LD01) 30 min before azinphos-methyl exposure. Azinphos-methyl-induced mortality was significantly reduced by all five tested compounds as compared with the reference group that was only exposed to azinphos-methyl without prior pre-treatment (RR = 1). The most efficacious prophylactic agents were K-27 (RR = 0.15) and physostigmine (RR = 0.21), being significantly more efficacious than ranitidine (RR = 0.62) and pyridostigmine (RR = 0.37). Pre-treatment with tacrine (RR = 0.29) was significantly more efficacious than pre-treatment with ranitidine, but the difference between tacrine and pyridostigmine was not significant. Our results indicate that prophylactic administration of the oxime K-27 may be a promising alternative in cases of imminent OPC exposure.

PMID: 25186309

 

Supplement:

OPCs are used to treat Alzheimer’s disease, glaucoma and myasthenia gravis; but highly toxic OPC-based nerve agents have also been misused in terrorist attacks, malicious poisonings and as chemical warfare agents. OPCs inhibit AChE, the enzyme which terminates the action of the neurotransmitter acetylcholine at cholinergic synapses. When AChE is inhibited, acetylcholine accumulates in the central and the peripheral nervous systems and subsequently stimulates both nicotinic and muscarinic cholinergic receptors. OPC intoxication can be fatal due to generalized seizures and/or respiratory failure. Standard treatment of OPC poisoning includes the muscarinic receptor antagonist atropine and an oxime-type AChE reactivator. When possible, pre-treatment with reversible AChE inhibitors before exposure to OPCs can produce superior therapeutic results. Since the substances available for such pre-treatment are only effective in exposure to a limited number of OPCs, we have performed a series of experiments evaluating the protective efficacy of several alternative compounds with known AChE-inhibitory activity (pyridostigmine, physostigmine, ranitidine, tacrine and K-27), hoping to find a broad-range compound. These AChE inhibitors were given as pre-treatment before exposure to a group of chemically diverse OPCs: methyl-paraoxon (2), ethyl-paraoxon (3), diisopropylfluorophosphate [DFP] (4) and terbufos sulfone (5).

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Figure 1. Relative risk of death (RR) estimated by Cox (1) analysis in animals exposed to intraperitoneal (i.p.) injections of azinphos-methyl and adjusted for azinphos-methyl dose (high/low) for each of the time points examined (10 min, 30 min, 1 hr, 2 hrs, 3 hrs, 4 hrs, 24 hrs, and 48 hrs). The protective effect of different cholinesterase inhibitors (ranitidine, pyridostigmine, tacrine, physostigmine, K-27), administered prophylactically 30 mins before azinphos-methyl exposure, was compared with no pre-treatment (reference group: azinphos-methyl alone, RR = 1). The injected dose for pre-treatment was approximately ¼ of the LD01. All tested compounds significantly reduced azinphos-methyl-induced mortality. The best protection was given by K-27 and physostigmine, which reduced the cumulative RR to about 15%-21% (RR = 0.15-0.21). This was significantly superior to the two least efficacious compounds, ranitidine (RR=0.62) and pyridostigmine (RR=0.37).

The present study evaluates the prophylactic efficacy of these same cholinesterase inhibitors, when administered before the OPC azinphos-methyl in the same experimental setting, and compares the results with those previously obtained for the other four OPCs. The aim of the study was to determine in vivo to what extent pre-treatment with pyridostigmine, physostigmine, ranitidine, tacrine or K-27 reduces the mortality of animals exposed to the organophosphorus compound azinphos-methyl.

Figure 1 shows the relative risk of death (RR) of the pre-treated experimental animals 10 min through 48 hours after exposure to azinphos-methyl. Pre-treatment with all five tested reversible AChE inhibitors at the tested dosage of 25% of LD01 significantly (p ≤ 0.05) reduced azinphos-methyl-induced mortality as compared to the reference group (azinphos-methyl only, but no pre-treatment, RR=1). Best protection was observed for K-27 (RR=0.15) and physostigmine (RR=0.21), which were significantly more efficacious than ranitidine (RR=0.62) and pyridostigmine (RR=0.37). Taken into consideration our previously published data on methyl-paraoxon (2), ethyl-paraoxon (3), DFP (4) and terbufos sulfone (5) exposure, the experimental oxime K-27 is the most promising compound, since it hardly penetrates the blood brain barrier (6) and since it is at least as effective as physostigmine and significantly more efficacious than the three other tested AChE inhibitors when administered prophylactically before a wide range of chemically diverse OPCs.

 

 

References

  1. Cox DR. 1972. Regression models and life tables. J Roy Statistical Society 34: 189-220.
  2. Lorke DE, Hasan MY, Nurulain SM, Shafiullah M, Kuca K, Petroianu GA. Acetylcholinesterase inhibitors as pretreatment before acute exposure to organophosphates: assessment using methyl-paraoxon. CNS Neurol Disord Drug Targets 11: 1052-1060.
  3. Petroianu GA, Nurulain SM, Shafiullah M, Hasan MY, Kuca K, Lorke DE. 2013. Usefulness of administration of non-organophosphate cholinesterase inhibitors before acute exposure to organophosphates: assessment using paraoxon. J Appl Toxicol 33: 894-900.
  4. Lorke DE, Hasan MY, Nurulain SM, Shafiullah M, Kuca K, Petroianu GA. 2011. Pretreatment for acute exposure to diisopropylfluorophosphate: in vivo efficacy of various acetylcholinesterase inhibitors. J Appl Toxicol 31: 515-523.
  5. Lorke DE, Nurulain SM, Hasan MY, Kuca K, Petroianu GA. 2014. Prophylactic administration of non-organophosphate cholinesterase inhibitors before acute exposure to organophosphates: assessment using terbufos sulfone. J Appl Toxicol 34: 1096-1103.
  6. Lorke DE, Hasan MY, Nurulain SM, Sheen R, Kuca K, Petroianu GA. 2007. Entry of two new asymmetric bispyridinium oximes (K-27 and K-48) into the rat brain: comparison with obidoxime. J Appl Toxicol 27: 482-490.

Acknowledgements:  This study was supported by the Sheikh Hamdan Bin Rashid Al Maktoum Award for Medical Science awarded to Georg Petroianu and grant MH CZ-DRO (UHHK, 00179906) awarded to Kamil Kuča.

 
fig2Contact: Georg Petroianu, MD, PhD, FCP Professor and Chair Department of Cellular Biology and Pharmacology Herbert Wertheim College of Medicine

11200 SW 8th Street, GL 495

Miami, FL 33199

georg.petroianu@fiu.edu

 

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