Countermeasures against organophosphorus (OP) anticholinesterases (organophosphates) have been extensively investigated. One means of safeguarding against exposure to OP compounds is use of a prophylactic agent, which can be either enzyme- or inhibitor-based. Butyrylcholinesterase (BChE) has been shown to completely protect against OP intoxication by binding to toxicant molecules before reaching the target enzyme (acetylcholinesterase) in tissues. Its use, however, is limited due to the large amount of enzyme needed for effective protection. Reduction of the enzyme dose necessary for long-term use may be achieved by pharmacokinetic or pharmacodynamic means. The effects of 1) poly-l-lysine grafted poly(ethylene glycol) copolymer complexation and 2) re-engineering of enzyme surface loops on BChE related to prophylactic capacity (e.g. substrate and inhibitor kinetics, in vitro enzyme inactivation) were investigated. Complexation with PLL-g-PEG resulted in a decreased turnover number (kcat) with butyrylthiocholine (BTChI), but relatively similar bimolecular rate constants (ki) with paraoxon (7-12%). More extensive reductions in ki were noted with other inhibitors (up to 60%). Copolymer-complexed enzymes were also less sensitive to heat and protease-mediated enzyme inactivation. While PLL-g-PEG complexation may improve the in vivo pharmacokinetics of BChE, we also investigated a pharmacodynamic approach. A catalytic variant of BChE, G117H, has been previously developed, though its interaction with OP compounds is too slow to be toxicologically relevant. Five BChEG117H mutants were produced with a 3-residue insertion into an identified hypermobile surface loop on BChEG117H (278-285). While loop mutants displayed decreased catalytic activity against BTChI, one loop mutant (ENA variant) had a slight but significant increase in the enzyme's reactivation rate (k3) with paraoxon (a 50% increase). In addition to enzyme-based strategies, we also investigated the inhibitor-based prophylactic drug pyridostigmine bromide (PB). PB is effective when used in conjunction with standard antidotes, though its pharmacokinetics are suboptimal. We 3) studied the ability of nanocrystalline cellulose hydrogel formulations (NCC-PB) as an oral PB delivery vehicle for enhanced dosing logistics. In vitro and in vivo evaluations of four NCC-PB formulations suggested that the time course of cholinesterase inhibition may be extended in comparison to free PB. Overall, these studies highlight a range of approaches aimed at improving both available and developing prophylactic agents against organophosphate toxicity.