Vol. 304, Issue 3, 1307-1313, March 2003

Adenosine A₁ Receptor Agonist N⁶-Cyclopentyladenosine Affects the Inactivation of Acetylcholinesterase in Blood and Brain by Sarin

Tjerk J. H. Bueters , Marloes J. A. Joosen, Herman P. M. van Helden, Ad P. IJzerman and Meindert Danhof

Research Group Medical Countermeasures, TNO Prins Maurits Laboratory, Rijswijk, The Netherlands (T.J.H.B., M.J.A.J., H.P.M.v.H.); Division of Medicinal Chemistry, Leiden/Amsterdam Center for Drug Research, Leiden University, Leiden, The Netherlands (T.J.H.B., A.P.IJ.); and Division of Pharmacology, Leiden/Amsterdam Center for Drug Research, Leiden University, Leiden, The Netherlands (T.J.H.B., M.D.)

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 N⁶-cyclopentyladenosine (CPA), acetylcholinesterase (AChE) in blood and brain, and the AChE-inhibitor sarin (isopropylmethylphosphonofluoridate). The direct interaction of CPA (2 µM) 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 µg/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 post- or 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.