The oxime pro-2-PAM provides minimal protection against the CNS effects of the nerve agents sarin, cyclosarin, and VX in guinea pigs

Abstract

This study examined whether pro-2-PAM, a pro-drug dihydropyridine derivative of the oxime 2-pralidoxime (2-PAM) that can penetrate the brain, could prevent or reverse the central toxic effects of three nerve agents; sarin, cyclosarin, and VX. The first experiment tested whether pro-2-PAM could reactivate guinea pig cholinesterase (ChE) in vivo in central and peripheral tissues inhibited by these nerve agents. Pro-2-PAM produced a dose-dependent reactivation of sarin- or VX-inhibited ChE in both peripheral and brain tissues, but with substantially greater reactivation in peripheral tissues compared to brain. Pro-2-PAM produced 9–25% reactivation of cyclosarin-inhibited ChE in blood, heart, and spinal cord, but no reactivation in brain or muscle tissues. In a second experiment, the ability of pro-2-PAM to block or terminate nerve agent-induced electroencephalographic seizure activity was evaluated. Pro-2-PAM was able to block sarin- or VX-induced seizures (16–33%) over a dose range of 24–32 mg/kg, but was ineffective against cyclosarin-induced seizures. Animals that were protected from seizures showed significantly less weight loss and greater behavioral function 24 h after exposure than those animals that were not protected. Additionally, brains were free from neuropathology when pro-2-PAM prevented seizures. In summary, pro-2-PAM provided modest reactivation of sarin- and VX-inhibited ChE in the brain and periphery, which was reflected by a limited ability to block or terminate seizures elicited by these agents. Pro-2-PAM was able to reactivate blood, heart, and spinal cord ChE inhibited by cyclosarin, but was not effective against cyclosarin-induced seizures.

Keywords::

• Acetylcholinesterase
• brain
• central nervous system
• cholinesterase inhibitors
• cholinesterase reactivation
• guinea pig
• nerve agents
• organophosphorus compounds
• oximes
• pralidoxime
• pro-2-PAM
• sarin
• cyclosarin
• VX.

Acknowledgements

This project was funded by the Defense Threat Reduction Agency–Joint Service and Technology Office, Medical Science and Technology Division–Proposal No. 1.E0008_08_RC_C. The excellent technical team work of Aviva Kafka, Megan Lyman, John O’Donnell, Jacob Skovira, Anna Smelley, and Kerry Van Shura is acknowledged. A portion of this work was presented at the XII International Congress of Toxicology, Barcelona, Spain, July 2010 (Shih et al. 2010c).

Research was conducted in compliance with the Animal Welfare Act of 1966 (P.L. 89–544), as amended and other Federal statutes and regulations relating to animals and experiments involving animals and adhered to the principles stated in the Guide for the Care and Use of Laboratory Animals (National Research Council 1996). The research environment and protocols for animal experimentation were approved by the Institutional Animal Care and Use Committee (IACUC) of the US Army Medical Research Institute of Chemical Defense. The facility where this research was conducted is fully accredited by the Association for Assessment and Accreditation of Laboratory Animal Care International (AAALAC). The opinions or assertions contained herein are the private views of the authors, and are not to be construed as reflecting the views of the Department of the Army or the Department of Defense.

Declaration of interest

The authors declare no conflicts of interest. The authors alone are responsible for the content and writing of the paper.