Cocaine abuse is a major medical and public health problem that continues to defy treatment. The disastrous medical and social consequences of cocaine addiction, such as violent crime, loss in individual productivity, illness and death, have made the development of an effective pharmacological treatment a high priority. However, cocaine mediates its reinforcing and toxic effects by blocking neurotransmitter reuptake and the classical pharmacodynamic approach has failed to yield small-molecule receptor antagonists due to the difficulties inherent in blocking a blocker. An alternative to receptor-based approaches is to interfere with the delivery of cocaine to its receptors and accelerate its metabolism in the body.
The dominant pathway for cocaine metabolism in primates is butyrylcholinesterase (BChE)-catalyzed hydrolysis at the benzoyl ester group (Scheme 1).

Scheme 1. Schematic representation of BChE-catalyzed hydrolysis at the benzoyl ester group.
Only 5% of the cocaine is deactivated through oxidation by the liver microsomal cytochrome P450 system. Cocaine hydrolysis at benzoyl ester group yields ecgonine methyl ester, whereas the oxidation produces norcocaine. The metabolite ecgonine methyl ester is a biologically inactive metabolite, whereas the metabolite norcocaine is hepatotoxic and a local anesthetic. BChE is synthesized in the liver and widely distributed in the body, including plasma, brain, and lung. Extensive experimental studies in animals and humans demonstrate that enhancement of BChE activity by administration of exogenous enzyme substantially decreases cocaine half-life.
Enhancement of cocaine metabolism by administration of BChE has been recognized to be a promising pharmacokinetic approach for treatment of cocaine abuse and dependence. However, the catalytic activity of this plasma enzyme is three orders-of-magnitude lower against the naturally occurring (−)-cocaine than that against the biologically inactive (+)-cocaine enantiomer. (+)-cocaine can be cleared from plasma in seconds and prior to partitioning into the central nervous system (CNS), whereas (−)-cocaine has a plasma half-life of approximately 45-90 minutes, long enough for manifestation of the CNS effects which peak in minutes. Hence, BChE mutants with high activity against (−)-cocaine are highly desired for use in humans. Although some BChE mutants with increased catalytic activity over wild-type BChE have previously been generated, there exists a need for mutant BChE with even higher catalytic activity. Thus, prior mutants provide limited enhancement in catalytic activity over wild-type BChE.
Previous studies such as (a) Masson, P.; Legrand, P.; Bartels, C. F.; Froment, M-T.; Schopfer, L. M.; Lockridge, O. Biochemistry 1997, 36, 2266 (b) Masson, P.; Xie, W., Froment, M-T.; Levitsky, V.; Fortier, P.-L.; Albaret, C.; Lockridge, O. Biochim. Biophys. Acta 1999, 1433, 281, (c) Xie, W.; Altamirano, C. V.; Bartels, C. F.; Speirs, R. J.; Cashman, J. R.; Lockridge, O. Mol. Pharmacol. 1999, 55, 83, (d) Duysen, E. G.; Bartels, C. F.; Lockridge, O. J. Pharmacol. Exp. Ther. 2002, 302, 751, (e) Nachon, F.; Nicolet, Y.; Viguie, N.; Masson, P.; Fontecilla-Camps, J. C.; Lockridge, O. Eur. J. Biochem. 2002, 269, 630, (f) Zhan, C.-G.; Landry, D. W. J. Phys. Chem. A 2001, 105, 1296; Berkman, C. E.; Underiner, G. E.; Cashman, J. R. Biochem. Pharmcol. 1997, 54, 1261; (g) Sun, H.; Yazal, J. E.; Lockridge, O.; Schopfer, L. M.; Brimijoin, S.; Pang, Y.-P. J. Biol. Chem. 2001, 276, 9330, (h) Sun, H.; Shen, M. L.; Pang, Y. P.; Lockridge, O.; Brimijoin, S. J. Pharmacol. Exp. Ther. 2002, 302, 710, (i) Sun, H.; Pang, Y. P.; Lockridge, O.; Brimijoin, S. Mol. Pharmacol. 2002, 62, 220 (hereinafter “Sun et al”); and (j) Zhan, C.-G.; Zheng, F.; Landry, D. W. J. Am. Chem. Soc. 2003, 125, 2462 (hereinafter “Zhan et al”), herein all incorporated by reference, suggested that, for both (−)-cocaine and (+)-cocaine, the BChE-substrate binding involves two different types of complexes: non-prereactive and prereactive BChE-substrate complexes. Whereas the non-prereactive BChE-cocaine complexes were first reported by Sun et al, Zhan et al were the first reporting the prereactive BChE-cocaine complexes and reaction coordinate calculations, disclosed in Zhan et al.
It was demonstrated that (−)/(+)-cocaine first slides down the substrate-binding gorge to bind to W82 and stands vertically in the gorge between D70 and W82 (non-prereactive complex) and then rotates to a position in the catalytic site within a favorable distance for nucleophilic attack and hydrolysis by S198 (prereactive complex). In the prereactive complex, cocaine lies horizontally at the bottom of the gorge. The main structural difference between the BChE-(−)-cocaine complexes and the corresponding BChE-(+)-cocaine complexes exists in the relative position of the cocaine methyl ester group. Reaction coordinate calculations revealed that the rate-determining step of BChE-catalyzed hydrolysis of (+)-cocaine is the chemical reaction process, whereas for (−)-cocaine the change from the non-prereactive complex to the prereactive complex is rate determining. A further analysis of the structural changes from the non-prereactive complex to the prereactive complex reveals specific amino acid residues hindering the structural changes, providing initial clues for the rational design of BChE mutants with improved catalytic activity for (−)-cocaine.
Previous molecular dynamics (MD) simulations of prereactive BChE-cocaine binding were limited to wild-type BChE. Even for the non-prereactive BChE-cocaine complex, only one mutant (A328W/Y332A) BChE binding with (−)-cocaine was simulated and its catalytic activity for (−)-cocaine was reported by Sun et al. No MD simulation was performed on any prereactive enzyme-substrate complex for (−)- or (+)-cocaine binding with a mutant BChE. In addition, all previous computational studies of Sun et al and Zhan et al of BChE interacting with cocaine were performed based on a homology model of BChE when three-dimensional (3D) X-ray crystal structure was not available for BChE, as taught by Nicolet, Y.; Lockridge, O.; Masson, P.; Fontecilla-Camps, J. C.; Nachon, F. J. Biol. Chem. 2003, 278, 41141 (hereinafter “Nicolet et al”), recently reported 3D X-ray crystal structures of BChE. As expected, the structure of BChE is similar to a previously published theoretical model of this enzyme and to the structure of acetylcholinesterase.
The main difference between the experimentally determined BChE structure and its model was found at the acyl binding pocket (acyl loop) that is significantly bigger than expected. It is unclear whether the structural difference at the acyl binding pocket significantly affect BChE binding with (−)-cocaine and (+)-cocaine. Although previous MD simulations of cocaine binding with wild-type BChE and the reaction coordinate calculations point to some amino acid residues that might need to be mutated for the purpose of improving the catalytic activity for (−)-cocaine hydrolysis, it remained unknown which exact amino acid mutations will result in a BChE with a higher catalytic activity for (−)-cocaine.
Computational studies of wild-type BChE and cocaine from Sun, et al, based on a “homology model,” suggest that the rate-determining step for BChE-catalyzed hydrolysis of cocaine is the rotation of the cocaine in the active site of BChE. By decreasing the hindrance of the rotation, the rate of the hydrolysis may be enhanced. Sun, et al describes creating an A328W/Y332A BChE mutant by: (1) replacing Tyr332 with Ala, “to reduce the steric hindrance and the π-π interaction that impede rotation,” and (2) replacing A1a328 with Trp “to provide a cation-π interaction to restore substrate affinity lost in disabling the π-π interaction.”
Sun et al studied the A328W/Y332A BChE mutant using enzyme assays and kinetics. In vitro studies were conducted using human plasma and in vivo studies were conducted using male Sprague-Dawley rats. The mutant was found to have enhanced catalytic properties. The mutant was further studied using molecular modeling. The three dimensional (3D) structure of A328W/Y332A was generated from the computationally generated 3D model of wild-type BChE and changing the relevant residues using commercially available software. Cocaine was docked to the catalytic gorge of the mutant BChE using other commercially available software. The cocaine-enzyme complex was refined by molecular dynamic simulation. The data generated by the molecular modeling studies were consistent with enzyme assays and kinetic data.
It should be noted that all prior computational techniques (molecular docking and molecular dynamics simulation) used by other researchers are based on an empirical force field which cannot be used to perform any necessary reaction coordinate calculation for the detailed understanding of the complicated catalytic reaction process. As it is well-known, it is particularly challenging to model and simulate the detailed reaction pathway and predict the kinetics of such an enzymatic reaction.
U.S. Patent Application Publication Nos. 2004/0121970; 2004/0120939; and 2003/0153062, describe 20+BChE mutants, or “variants,” from human and other animals, each having from one to six amino acid alterations and increased cocaine hydrolysis activity. For example, mutants include F227A/A328W; F227A/S287G/A328W; A119S/S287G/A328W; A328W/Y332M/S287G/F227A, A199S/F227A/S/287G/A328W and A119S/F227A/S287G/A328W/Y332M. The mutants have varying increases in catalytic activity, up to 100-fold increase relative to wild-type BChE.
There exists a need in the art for determining which proposed mutant BChEs should have ever increasing catalytic activity and for generating those mutants which should have enhanced catalytic activity.