68 G. A. PETROIANU ET AL. Copyright © 2005 John Wiley & Sons, Ltd. J. Appl. Toxicol. 2005; 25: 68–73 JOURNAL OF APPLIED TOXICOLOGY J. Appl. Toxicol. 2005; 25: 68–73 Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/jat.1037 Protective agents in acute high-dose organophosphate exposure: comparison of ranitidine with pralidoxime in rats G. A. Petroianu,* M. Y. Hasan, S. M. Nurulain, K. Arafat, M. Shafiullah and O. Naseer Department of Pharmacology, Faculty of Medicine and Health Sciences, UAE University, Al-Ain, United Arab Emirates Received 25 May 2004; Revised 26 June 2004; Accepted 27 July 2004 ABSTRACT: Weak and reversible inhibitors of cholinesterase, when coadministred in excess with a more potent inhibi- tor such as organophosphates, can act in a protective manner. Ranitidine (RAN) is a clinically widely used histamine type 2 (H 2 ) receptor blocker. Ranitidine is also the most potent inhibitor of acetylcholinesterase among H 2 blockers (inhibitory constant K in the low micromolar range) but roughly three orders of magnitude less potent than paraoxon. This study evaluates RAN-conferred protection in acute high-dose organophosphate (paraoxon, POX) exposure in rats in direct com- parison with the therapeutic gold-standard pralidoxime (PRX). Group 1 received 1 μM POX, group 2 received 50 μM RAN, group 3 received 50 μM PRX, group 4 received 1 μM POX + 50 μM RAN and group 5 received 1 μM POX + 50 μM PRX. All substances were applied intraperitoneally. The ani- mals were monitored for 48 h and mortality was recorded at 30 min and 1, 2, 3, 4, 24 and 48 h. Blood was taken for red blood cell acetylcholinesterase (RBC-AChE) measurements at baseline, 30 min and 24 and 48 h. Mortality occurred mainly in the first 30 min after POX administration, with minimal changes occurring thereafter. Mortality (in %) at 30 min in groups 1, 4 and 5 was 52 ± 18, 37 ± 20 and 17 ± 18, respectively, and mortality at 48 h was 59 ± 12, 39 ± 20 and 28 ± 20, respectively. The RBC-AChE activities (in % of baseline values) at 30 min in groups 1, 4 and 5 were 18 ± 16, 47 ± 23 and 48 ± 20, respectively. At 24 h the values were 46 ± 16, 65 ± 24 and 86 ± 17, respectively, and at 48 h the values were 71 ± 19, 78 ± 21 and 110 ± 27, respectively. Coadministration of PRX significantly decreases mortality in the described model at all points in time. Coadministration of RAN statistically significantly decreases mortality at 24 and 48 h. The extent of protection conferred by RAN is less (but not statistically significantly so) than that conferred by the gold-standard PRX. Coadministration of PRX statistically significantly increases RBC-AChE activities in the described model at all points in time. Ranitidine confers a statistically significant protection for the enzyme at 30 min only. We conclude that RAN is potentially of clinical use in reducing mortality in acute high-dose organophosphate exposure. Further studies involving different organophosphates and dosages, as well as different animal species, will be needed both to confirm these initial findings and to address the issue of the optimal timing for RAN preadministration. Copyright © 2005 John Wiley & Sons, Ltd. KET WORDS: organophosphate; paraoxon; treatment; ranitidine; pralidoxime; cholinesterase Introduction Organophosphorus compounds are widely used sub- stances. Accidental and suicidal exposures are frequent. Organophosphates were also involved in a terrorist attack in the Tokyo subway in 1995. The inhibition of esterases (butyrylcholine: 3.1.1.8; acetylcholine: 3.1.1.7) by these substances results from reacting covalently with the ac- tive centre serine by phosphorylation (Levine, 1991). Paraoxon (POX) is a prototypical non-neuropathic organophosphorus compound. The effects of poisoning with organophosphorus compounds have been described extensively (Petroianu et al., 1998). Oximes are the only enzyme reactivators clinically available (Johnson et al., 2000). Clinical experience with oximes, however, is not entirely satisfactory and therefore the search for comple- ments and/or alternatives is ongoing. Ranitidine (RAN) is a clinically widely used histamine type 2 (H 2 ) receptor blocker. The substance is water soluble with a volume of distribution of ca. 1.4 l kg -1 body wt. Protein binding is low at ca. 15%. Following i.v. injection, ca. 70% of the dose is recovered in the urine. The elimination half-life is 2–3 h. Mean peak levels of 576 ng ml -1 (1.6 nM ml -1 ) occur within 15 min following a 50 mg i.m. dose. The molecular weight is 350. The LD 50 in rats is 83 mg kg -1 body wt. (Louis et al., 1981; Grant et al., 1989). Ranitidine is also a weak inhibitor of cholinesterases (Hansen and Bertl, 1983a,b; Laine-Cessac et al., 1993; Kounenis et al., 1994). Ranitidine and nizatidine are the most potent inhibitors of acetylcholinesterase among H 2 * Correspondence to: G. A. Petroianu, Department of Pharmacology and Therapeutics, Faculty of Medicine and Health Sciences, UAE University, PO Box 17666, Al-Ain, United Arab Emirates. E-mail: georg.petroianu@uaeu.ac.ae Contract/grant sponsor: UAE University; Contract/grant number: 18/2002.