In vitro Reactivating Effects of Standard and Newly Developed Oximes on Malaoxon-Inhibited Mouse Brain Acetylcholinesterase Alessandra Antunes dos Santos 1 , Danfflbia Bonfanti dos Santos 1 , Alcir Luiz Dafre 2 , Andreza Fabro de Bem 1 , Diogo Onofre Souza 4 , Jo¼o Batista Teixeira da Rocha 5 , Kamil Kuca 3 and Marcelo Farina 1 1 Departamento de Bioquímica, 2 Departamento de CiÞncias Fisiológicas, Centro de CiÞncias Biológicas, Universidade Federal de Santa Catarina, Florianópolis, Santa Catarina, Brazil, 3 Centre of Advanced Studies and Department of Toxicology, Faculty of Military Health Sciences, University of Defense, Hradec Kralove, Czech Republic, 4 Departamento de Bioquímica, Instituto de CiÞncias Bµsicas da Safflde, Universidade Federal do Rio Grande do Sul, Porto Alegre, Rio Grande do Sul, Brazil, and 5 Departamento de Química, Centro de CiÞncias Naturais e Exatas, Universidade Federal de Santa Maria, Santa Maria, Rio Grande do Sul, Brazil (Received 19 October 2009; Accepted 9 February 2010) Abstract: Malathion is an organophosphate (OP) pesticide whose toxicity depends on its bioactivation to malaoxon. Human malathion poisoning has been treated with oximes (mainly pralidoxime) in an attempt to reactivate OP-inhibited acetylcholin- esterase (AChE). However, pralidoxime has shown unsatisfactory therapeutic effects in malathion poisoning and its routine use has been questioned. In this study, we evaluated the in vitro potency of standards and newly developed oximes in reactivat- ing malaoxon-inhibited AChE derived from mouse brain supernatants. Malaoxon displayed a concentration-dependent inhibi- tory effect on mouse brain AChE (IC 50 = 2.36 lM), and pralidoxime caused a modest reactivating effect (30% of reactivation at 600 lM). Obidoxime and trimedoxime, as well as K047 and K075, displayed higher reactivating effects (from 55% to 70% of reactivation at 600 lM) when compared with pralidoxime. The results show that obidoxime, trimedoxime, K074 and K075 present higher reactivating effects on malaoxon-inhibited AChE under in vitro conditions when compared with pralidoxime. Taking into account the unsatisfactory effects of pralidoxime as antidotal treatment in malathion poisonings, the present results suggest that obidoxime, trimedoxime, K074 and K075 might be interesting therapeutic strategies to reactivate malao- xon-inhibited AChE in malathion poisonings. Malathion is an organophosphate (OP) compound widely used in agriculture and veterinary practices as well as in sui- cide attempts [1]. Its toxicity requires the bioactivation to malaoxon [2,3] and the subsequent phosphorylation of the serine hydroxyl group at the active site of acetylcholinester- ase (AChE). These events culminate in the inactivation of AChE, causing the accumulation of acetylcholine at the cho- linergic receptors’ sites that, in turn, leads to cholinergic syn- drome [4–7]. Studies have reported the neurotoxic effects of malathion in both humans [8] and animals [9–11]. Malathion has been pointed as the most important contaminant in cases of OP intoxication in Santa Catarina, a state in Southern Brazil, according to unpublished data obtained from a Toxicological Information Center hosted by the Hospital Universitµrio, Florianópolis, SC. The standard antidotal treatment in malathion poisoning includes the administration of a muscarinic cholinergic receptor antagonist (generally atropine) to block the over- stimulation of cholinergic receptors by acetylcholine. The co-administration of oximes also represents an important clinical procedure in malathion poisoning, which is per- formed in an attempt to reactivate the inhibited AChE [12]. From a molecular point of view, oximes are nucleophilic sub- stances that are able to break down the bond between the enzyme and the OP inhibitors, releasing the enzyme to per- form its physiological role [13,14]. Oximes have been investi- gated for many years as compounds with a great potential in the treatment of OP poisoning [15], but, to the present time, few compounds have found clinical application [16–18]. Of particular interest, pralidoxime, the standard antidotal treat- ment in malathion poisoning, has shown unsatisfactory ther- apeutic effects in malathion poisoning [19] and its routine use has been questioned. In this scenario, there is a clear demand for new and ‘broad-spectrum’ AChE reactivators with a higher efficacy than those clinically available [14]. The new oximes from the K-series (K027, K048, K074 and K075), which were recently developed at the Depart- ment of Toxicology, Faculty of Military Health Sciences, Hradec Kralove, Czech Republic, have been reported as excellent reactivators of OP poisoning under in vitro and in vivo conditions [12,20–26]. It is notable that the antidotal effectiveness of some of these oximes in tabun and sarin poi- sonings has been shown to be higher when compared with commercially available oximes [21–23,27]. Considering the high incidence of malathion poisoning, the low effectiveness of the current antidotal treatment in malathion poisoning and the evident beneficial effects of the oximes from the K-series on poisonings with OP com- Author for correspondence: Marcelo Farina, Departamento de Bio- química, Centro de CiÞncias Biológicas, Universidade Federal de Santa Catarina, 88040900, Florianópolis, Santa Catarina, Brazil (fax +554837219672, e-mail farina@ccb.ufsc.br). Basic & Clinical Pharmacology & Toxicology , 107, 768–773 Doi: 10.1111/j.1742-7843.2010.00576.x Ó 2010 The Authors Basic & Clinical Pharmacology & Toxicology Ó 2010 Nordic Pharmacological Society