PROTEXIA^® - RECOMBINANT HUMAN BUTYRYLCHOLINESTERASE FOR THE PREVENTION AND TREATMENT OF NERVE AGENT TOXICITY

Background
Protexia^® is a recombinant version of human butyrylcholinesterase (BChE), a naturally occurring protein found in minute quantities in blood (2 mg/liter). BChE functions as a natural bioscavenger, like a sponge, to absorb and degrade organophosphate poisons (e.g. nerve agents) before they cause neurological damage. Protexia^® is being developed as a pre- and post-exposure therapy for casualties on the battlefield or civilian victims of nerve agent attacks. Nerve agents belong to a class of compounds known as organophosphate (OP) agents. OP nerve agents, such as sarin gas, soman, tabun or VX, enter the blood stream via inhalation or absorption through the skin. The nerve agents travel in the circulatory system to the brain and muscles causing the nerves to become over-stimulated which lead to massive convulsions and death in severe cases.

Pyridostigmine bromide (PB) is the only FDA approved product for use as a "pre-treatment adjunct" only for poisoning by the nerve agent, soman. It confers no protection on its own but enhances the protection conferred by post-exposure treatment. The current standard of care for post-exposure treatment involves repeated doses of a cocktail of drugs including atropine, oxime reactivators (2PAM) and anti-convulsants. Available pre-and post-treatment options are inadequate and there is a clear need for more efficacious countermeasures.

The Nerve Agent Threat
The potency of OP agents was recognized during World War II, when they were developed as nerve agents for use in chemical weapons. In recent history, terrorists have deployed nerve agents as weapons of mass destruction. The sarin nerve gas attack in the Tokyo subway system in 1995 exposed the vulnerability of North American and European cities to chemical weapons. Following 9/11, the U.S. government embarked upon an intensive anti-terrorism campaign and has allocated unprecedented financial resources through Project BioShield to develop new technologies and products to address these threats.
 

Development Status
PharmAthene plans to commence a Phase I human safety trial of Protexia^® in September 2008. Results from the Phase I trial will be available by the second quarter of 2009. In September 2006 PharmAthene was awarded a multi-year contract valued at up to $219 million from the Department of Defense (DoD) U.S. Army Space and Missile Command, for advanced development of Protexia^®. Under the contract, PharmAthene will be responsible for the conduct and oversight of all product development activities. The initial stage of development, for which $40.5 million has been allocated, includes manufacturing process development, preclinical safety and toxicity testing, submission of an Investigational New Drug (IND) Application with the United States Food and Drug Administration (FDA), and initiation of a Phase I clinical trial. Following the successful completion of the Phase I clinical trial, the government may exercise its option to fund additional development activities beyond the initial $40.5 million, leading to FDA licensure. The contract also provides the Department of Defense with the option to procure an initial 90,000 doses of Protexia^®. The DoD contract was awarded through a full and open competitive solicitation seeking novel second generation prophylactic products for use in humans to prevent and treat poisoning from organophosphorus (OP) nerve agents such as sarin gas, soman, tabun and VX.
 

Protexia^®: Mechanism of Action
The mechanism of action of hBChE is reversal of the acute toxicity associated with OP agents used in chemical warfare (cholinergic crisis). Protexia^® rescue therapy removes nerve agents directly from the bloodstream by breaking them down into inactive components, rather than just treating the neurotoxic symptoms, as is the case with existing therapies. In-vitro and in-vivo studies demonstrate that hBChE serves as a safe and potent scavenger for nerve agents. Of the cholinesterases evaluated so far, human serum BChE (purified from plasma) has advanced the furthest in terms of preclinical development. However, human serum BChE is not a viable commercial option because it is only expressed in minute quantities in human plasma and production of large quantities needed for civilian and military stockpiles is not possible.

Protexia^®'s capability as a medical countermeasure has been demonstrated in-vivo to protect animals from multiple lethal doses of a broad spectrum of nerve agent chemical weapons, including sarin, soman, tabun and VX. Protexia^® has several likely advantages, including providing protection both pre- and post-exposure, detoxification of OP nerve agents with full spectrum protection and a very acceptable safety profile.

Protexia^® is being developed for two indications-pre-exposure prophylaxis and as a post-exposure therapy.

 

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