Medical Aspects of Chemical Warfare (2008) - The Black Vault

Nerve Agent Bioscavenger: Development of a New Approach to Protect Against Organophosphorus Exposureexposure. 46 For example, Genovese and Doctor evaluatedthe effects of highly purified Eq BChE on learnedand unlearned behavior in rats. 47 Administration of 500to 7500 U of Eq BChE (resulting in circulating BChElevels as high as ~ 55 U/mL) did not affect acquisitionor retention of a passive avoidance task. Additionally,no disruption of performance of a food-maintainedoperant behavior task was observed. To evaluateunlearned performance, 24-hour spontaneous motoractivity was evaluated before and after administrationof Eq BChE. There was no disruption of either the totalnumber of activity counts nor the circadian pattern ofactivity when monitored for 10 days following administration.Finally, the enzyme was shown to providesignificant protection against performance degradationproduced by 7-(methylethoxyphosphinyloxy)-1--methylquinolinium iodide (MEPQ), a peripherallyactive OP compound. The safety and efficacy of FBSAChE and Eq BChE was also evaluated in rhesusmonkeys using a memory-intensive serial proberecognition task, in which subjects were required torecall a list of stimuli. 13,15,19 Repeated administration ofa commercial preparation of Eq BChE that produceda 7- to 18-fold increase in circulating BChE levels didnot systematically affect task performance, 41 and rhesusmonkeys pretreated with 460 to 503 nmol of EqBChE were protected against 2 or 3 times the LD 50s ofsoman or sarin. 15Similar studies were conducted to address the safetyand efficacy of pHu BChE in mice, 17 rats, 48 guineapigs, 35 and rhesus monkeys. 19 In all cases, doses ofpHu BChE sufficient to protect against OP exposurewere devoid of behavioral side effects. Brandeis et aldemonstrated that pHu BChE was protective againstsoman and had no apparent effect on spatial memoryas assessed by a Morris water-maze task. 48 Similarly,Raveh et al evaluated the safety of pHu BChE and itstherapeutic efficacy against VX and soman toxicityusing standardized observations and behavioral performanceon a spatial discrimination task in rhesusmonkeys. 19 For subjects in which the ratio of enzymeto OP was near or greater than 1, no or mild signs oftoxicity were observed, largely with recovery by thenext day. Regarding the safety of pHu BChE, three offour monkeys exposed to either 13 mg (10,400 U) or 34mg (27,200 U) of pHu BChE did not show any observabledeficits resulting from pHu BChE administrationalone. 19 The transient behavioral effect observed in thefourth monkey was attributed to a nonspecific malaiseinduced by this enzyme preparation.More recently, the behavioral safety of large dosesof pHu BChE alone were evaluated in mice and rhesusmonkeys. Clark et al showed that in mice, 2000U of pHu BChE (the equivalent of 30 times the doserequired for protecting humans from 2 times the LD 50of soman) did not significantly alter acoustic startleor prepulse inhibition behavior. 49 Similarly, administrationof 30 mg/kg of pHu BChE was devoid of anyadverse effects in rhesus monkeys when performancewas assessed using a six-item serial probe recognitiontask. 50 Taken together, these studies demonstrate thatpHu BChE pretreatment can provide protection againstOP exposure while being devoid of adverse behavioraland physiological effects.RECOMBINANT STOICHIOMETRIC BIOSCAVENGERSPlasma-derived Hu BChE represents a first-generationbiological scavenger. This material is obtainedfrom outdated human plasma (Cohn Fraction IV-4paste), and the overall availability is related to thequantity of fraction of the processed human plasmaavailable at any given time. Sufficient amounts ofCohn Fraction IV-4 paste are generated in the UnitedStates by blood processing establishments to produceat least 100,000 doses of the bioscavenger productper year. Although this amount of material may beadequate for use by first responders in case of civilianexposure or deliberate, accidental, or limited combatengagement, it is not sufficient to protect the entirepopulation or even the entire military. To identify amore reliable source of Hu BChE, recent research effortsfocused on the development of Hu BChE fromrecombinant expression systems. If successful, suchefforts will allow for a constant supply of materialof reproducible purity and activity without dependenceon the supply of outdated plasma. There are avariety of potential sources of recombinant Hu BChE(rHu BChE), including transgenic plants, 51 transgenicanimals, 52 transfected insect larvae, 53 or algae. 54 Inaddition, rHu BChE can be expressed in cell lines. 55,56The cell-derived rHu BChE was shown to be a mixtureof monomers, dimers, and tetramers and containedincomplete glycan structures. 57 Similarly, goat-milk–derived rHu BChE is primarily a dimer, with some proteinpresent as monomers and tetramers. In contrast,pHu BChE is predominantly tetrameric and possessesmostly biantennary complex and some high mannoseglycan structures. Also, goat-milk–derived rHu BChEhas a different glycosylation pattern than that of pHuBChE and contains a carbohydrate moiety that hasbeen demonstrated to be immunogenic in humans. 58Because of the lack of subunit assembly and completeglycan structures, rHu BChE has a much shortercirculatory half-life than pHu BChE. 57 To enhance its249