Affects the Inactivation of Acetylcholinesterase in Blood and Brain by Sarin

The objective of the present study was to develop a kinetics of pharmacodynamics model to properly describe and investigate the in vivo interaction between the selective adenosine A1 agonist N-cyclopentyladenosine (CPA), acetylcholinesterase (AChE) in blood and brain, and the AChE-inhibitor sarin (isopropylmethylphosphonofluoridate). The direct interaction of CPA (2 M) on the inhibition of AChE by sarin was studied in vitro in heparinized rat blood and in 10% (w/v) brain homogenate. CPA did not directly influence the sarin-mediated inactivation of AChE in either system. In sarin-poisoned (144 g/kg s.c.) rats not treated with CPA, AChE was completely inactivated in blood and brain within 7 min. CPA (2 mg/kg i.m.) treatment, 1 min after sarin administration, caused a small delay in the inhibition of AChE in blood. Treatment with CPA, 2 min before sarin, protected the neuronal AChE partially from being inhibited, but not the enzyme localized in blood. With a dose-response-time model the proportion of the dose of sarin reaching the site of action was estimated to be 48 12 or 13 3% after CPA postor pretreatment, respectively. A correlation between the residual AChE activity in the brain and the incidence of cholinergic symptoms could be established with logistic regression analysis: lower inhibition of AChE in the brain precluded the onset of critical symptoms. In conclusion, CPA affects the concentration of sarin reaching the site of action, which contributes to the protection previously observed in sarin-poisoned rats. Sarin (isopropylmethylphosphonofluoridate) is an organophosphorous nerve agent that irreversibly inhibits the essential enzyme acetylcholinesterase (AChE), resulting in excessive amounts of acetylcholine (ACh) in cholinergic synapses. This cholinergic hyperactivity rapidly evolves into a more generalized neuroexcitability, which triggers seizure activity in susceptible brain areas (Kadar et al., 1995; Shih and McDonough, 1997; Van Helden and Bueters, 1999). Novel treatment strategies are currently under development, because the current therapy (atropine, oxime, and diazepam) seemed inadequate with respect to the suppression of the seizure activity and subsequent brain pathology in primates (Hayward et al., 1990; Van Helden et al., 1996; Shih and McDonough, 1997; Lallement et al., 1998). In this respect, a possible role for the adenosine A1 receptor-mediated inhibition of ACh release in the brain was explored recently (Van Helden et al., 1998; Van Helden and Bueters, 1999; Bueters et al., 2002). This approach is aimed at the prevention or attenuation of ACh accumulation, thereby stifling the neurotoxic cascade in its birth. Intracerebral application of a series of adenosine A1 receptor agonists could effectively inhibit the ACh release (Bueters et al., 2000; Materi et al., 2000). Moreover, promising results were obtained with the adenosine A1 receptor agonist N cyclopentyladenosine (CPA) in treating organophosphate-intoxicated rats: cholinergic symptoms were suppressed and mortality was prevented (Van Helden et al., 1998; Bueters et al., 2002). Microdialysis studies confirmed that accumulation of central ACh was attenuated upon CPA administration, which presumably explains the observed protection (Bueters