Novel pharmacological approaches to treatment of drug overdose and addiction

Long-lasting blocking of interoceptive effects of cocaine by a highly efficient cocaine hydrolase in rats

Cocaine dependence is a serious world-wide public health problem without an FDA-approved pharmacotherapy. We recently designed and discovered a highly efficient long-acting cocaine hydrolase CocH5-Fc(M6). The present study examined the effectiveness and duration of CocH5-Fc(M6) in blocking interoceptive effects of cocaine by performing cocaine discrimination tests in rats, demonstrating that the duration of CocH5-Fc(M6) in blocking cocaine discrimination was dependent on cocaine dose and CocH5-Fc(M6) plasma concentration. Particularly, a dose of 3 mg/kg CocH5-Fc(M6) effectively attenuated discriminative stimulus effects of 10 mg/kg cocaine, cumulative doses of 10 and 32 mg/kg cocaine, and cumulative doses of 10, 32 and 56 mg/kg cocaine by ≥ 20% for 41, 19, and 10 days, and completely blocked the discriminative stimulus effects for 30, 13, and 5 days with corresponding threshold plasma CocH5-Fc(M6) concentrations of 15.9, 72.2, and 221 nM, respectively, under which blood cocaine concentration was negligible. Additionally, based on the data obtained, cocaine discrimination model is more sensitive than the locomotor activity to reveal cocaine effects and that CocH5-Fc(M6) itself has no long-term toxicity regarding behavioral activities such as lever pressing and food consumption in rats, further demonstrating that CocH5-Fc(M6) has the desired properties as a promising therapeutic candidate for prevenance of cocaine dependence.

Epigenetics: a link between addiction and social environment

The detrimental effects of drug abuse are apparently not limited to individuals but may also impact the vulnerability of their progenies to develop addictive behaviours. Epigenetic signatures, early life experience and environmental factors, converge to influence gene expression patterns in addiction phenotypes and consequently may serve as mediators of behavioural trait transmission between generations. The majority of studies investigating the role of epigenetics in addiction do not consider the influence of social interactions. This shortcoming in current experimental approaches necessitates developing social models that reflect the addictive behaviour in a free-living social environment. Furthermore, this review also reports on the advancement of interventions for drug addiction and takes into account the emerging roles of histone deacetylase (HDAC) inhibitors in the etiology of drug addiction and that HDAC may be a potential therapeutic target at nucleosomal level to improve treatment outcomes.

Butyrylcholinesterase levels and subjective effects of smoked cocaine in healthy cocaine users

BACKGROUND

Butyrylcholinesterase (BChE) is beginning to attract attention as a possible target for cocaine abuse treatment because of its role in metabolizing cocaine.

OBJECTIVE

The purpose of this analysis was to assess whether endogenous BChE levels are associated with the subjective effects of cocaine.

METHODS

Data from 28 participants in five inpatient cocaine self-administration studies were included in the present analysis. Four minutes after each smoked cocaine dose, participants rated their drug-related effects from 0-100 using a computerized self-report Visual Analogue Scale (VAS). The main outcome measures were nine change-in-VAS ratings between a baseline placebo dose and a 25-mg smoked cocaine dose.

RESULTS

After controlling for age, sex, total years of cocaine use, total milligrams of cocaine administered before the 25-mg dose being analyzed, and baseline diastolic blood pressure, endogenous BChE was not significantly associated with any of the nine change-in-VAS ratings.

CONCLUSION

Though BChE appears to be a possible target for cocaine abuse treatment, these data suggest that endogenous levels of BChE may not play a role in modifying the subjective effects of cocaine. Future larger studies of BChE in respect to the subjective effects produced by cocaine are needed to confirm or refute these findings.

Catalytic activities of a cocaine hydrolase engineered from human butyrylcholinesterase against (+)- and (-)-cocaine

It can be argued that an ideal anti-cocaine medication would be one that accelerates cocaine metabolism producing biologically inactive metabolites via a route similar to the primary cocaine-metabolizing pathway, i.e., hydrolysis catalyzed by butyrylcholinesterase (BChE) in plasma. However, wild-type BChE has a low catalytic efficiency against naturally occurring (-)-cocaine. Interestingly, wild-type BChE has a much higher catalytic activity against unnatural (+)-cocaine. According to available positron emission tomography (PET) imaging analysis using [(11)C](-)-cocaine and [(11)C](+)-cocaine tracers in human subjects, only [(11)C](-)-cocaine was observed in the brain, whereas no significant [(11)C](+)-cocaine signal was observed in the brain. The available PET data imply that an effective therapeutic enzyme for treatment of cocaine abuse could be an exogenous cocaine-metabolizing enzyme with a catalytic activity against (-)-cocaine comparable to that of wild-type BChE against (+)-cocaine. Our recently designed A199S/F227A/S287G/A328 W/Y332G mutant of human BChE has a considerably improved catalytic efficiency against (-)-cocaine and has been proven active in vivo. In the present study, we have characterized the catalytic activities of wild-type BChE and the A199S/F227A/S287G/A328 W/Y332G mutant against both (+)- and (-)-cocaine at the same time under the same experimental conditions. Based on the obtained kinetic data, the A199S/F227A/S287G/A328 W/Y332G mutant has a similarly high catalytic efficiency (kcat/KM) against (+)- and (-)-cocaine, and indeed has a catalytic efficiency (k(cat/)K(M) = 1.84 × 10(9) M(-1) min(-1)) against (-)-cocaine comparable to that (k(cat)/K(M) = 1.37 × 10(9) M(-1) min(-1)) of wild-type BChE against (+)-cocaine. Thus, the mutant may be used to effectively prevent (-)-cocaine from entering brain and producing physiological effects in the enzyme-based treatment of cocaine abuse.

Modeling of pharmacokinetics of cocaine in human reveals the feasibility for development of enzyme therapies for drugs of abuse

A promising strategy for drug abuse treatment is to accelerate the drug metabolism by administration of a drug-metabolizing enzyme. The question is how effectively an enzyme can actually prevent the drug from entering brain and producing physiological effects. In the present study, we have developed a pharmacokinetic model through a combined use of in vitro kinetic parameters and positron emission tomography data in human to examine the effects of a cocaine-metabolizing enzyme in plasma on the time course of cocaine in plasma and brain of human. Without an exogenous enzyme, cocaine half-lives in both brain and plasma are almost linearly dependent on the initial cocaine concentration in plasma. The threshold concentration of cocaine in brain required to produce physiological effects has been estimated to be 0.22±0.07 µM, and the threshold area under the cocaine concentration versus time curve (AUC) value in brain (denoted by AUC2_∞) required to produce physiological effects has been estimated to be 7.9±2.7 µM·min. It has been demonstrated that administration of a cocaine hydrolase/esterase (CocH/CocE) can considerably decrease the cocaine half-lives in both brain and plasma, the peak cocaine concentration in brain, and the AUC2_∞. The estimated maximum cocaine plasma concentration which a given concentration of drug-metabolizing enzyme can effectively prevent from entering brain and producing physiological effects can be used to guide future preclinical/clinical studies on cocaine-metabolizing enzymes. Understanding of drug-metabolizing enzymes is key to the science of pharmacokinetics. The general insights into the effects of a drug-metabolizing enzyme on drug kinetics in human should be valuable also in future development of enzyme therapies for other drugs of abuse.

In this computational study, we have examined, for the first time, the potential effects of a drug-metabolizing enzyme on drug pharmacokinetics in human, showing that a high-activity drug-metabolizing enzyme can completely/effectively prevent the drug of abuse from entering brain to produce physiological effects. Based on this encouraging insight, it is feasible to develop enzyme therapies for drugs of abuse. Through pharmacokinetic modeling, we have demonstrated that, without an exogenous enzyme, the drug half-lives in both brain and plasma are almost linearly dependent on the initial drug concentration in plasma. This finding indicates that one may not simply say the half-life of a drug without clearly indicating the actual dose condition. We have also demonstrated for the first time how a high-activity drug-metabolizing enzyme can considerably decrease the peak concentration of drug in brain and drug half-lives in both brain and plasma. In addition, we have calculated the minimum (threshold) concentration of cocaine in brain required to produce physiological effects. The predicted threshold concentration, along with all of the general insights obtained in this study, will provide a rational base for future design of further experimental studies required for the enzyme therapy development.

Reaction Pathway and Free Energy Profile for Cocaine Hydrolase-Catalyzed Hydrolysis of (-)-Cocaine

Reaction pathway of (-)-cocaine hydrolysis catalyzed by our recently discovered most efficient cocaine hydrolase, which is the A199S/F227A/S287G/A328W/Y332G mutant of human butyrylcholinesterase (BChE), and the corresponding free energy profile have been studied by performing first-principles pseudobond quantum mechanical/molecular mechanical (QM/MM)-free energy (FE) calculations. Based on the QM/MM-FE results, the catalytic hydrolysis process consists of four major reaction steps, including the nucleophilic attack on carbonyl carbon of (-)-cocaine benzoyl ester by hydroxyl group of S198, dissociation of (-)-cocaine benzoyl ester, nucleophilic attack on carbonyl carbon of (-)-cocaine benzoyl ester by water, and finally the dissociation between (-)-cocaine benzoyl group and S198 of the enzyme. The second reaction step is rate-determining. The calculated free energy barrier associated with the transition state for the rate-determining step is ~15.0 kcal/mol, which is in excellent agreement with the experimentally-derived activation free energy of ~14.7 kcal/mol. The mechanistic insights obtained from the present study will be valuable for rational design of more active cocaine hydrolase against (-)-cocaine. In particular, future efforts aiming at further increasing the catalytic activity of the enzyme against (-)-cocaine should focus on stabilization of the transition state for the second reaction step in which the benzoyl ester of (-)-cocaine dissociates.

Reaction pathway and free energy profiles for butyrylcholinesterase-catalyzed hydrolysis of acetylthiocholine

The catalytic mechanism for butyrylcholineserase (BChE)-catalyzed hydrolysis of acetylthiocholine (ATCh) has been studied by performing pseudobond first-principles quantum mechanical/molecular mechanical-free energy (QM/MM-FE) calculations on both acylation and deacylation of BChE. Additional quantum mechanical (QM) calculations have been carried out, along with the QM/MM-FE calculations, to understand the known substrate activation effect on the enzymatic hydrolysis of ATCh. It has been shown that the acylation of BChE with ATCh consists of two reaction steps including the nucleophilic attack on the carbonyl carbon of ATCh and the dissociation of thiocholine ester. The deacylation stage includes nucleophilic attack of a water molecule on the carboxyl carbon of substrate and dissociation between the carboxyl carbon of substrate and hydroxyl oxygen of Ser198 side chain. QM/MM-FE calculation results reveal that the acylation of BChE is rate-determining. It has also been demonstrated that an additional substrate molecule binding to the peripheral anionic site (PAS) of BChE is responsible for the substrate activation effect. In the presence of this additional substrate molecule at PAS, the calculated free energy barrier for the acylation stage (rate-determining step) is decreased by ~1.7 kcal/mol. All of our computational predictions are consistent with available experimental kinetic data. The overall free energy barriers calculated for BChE-catalyzed hydrolysis of ATCh at regular hydrolysis phase and substrate activation phase are ~13.6 and ~11.9 kcal/mol, respectively, which are in reasonable agreement with the corresponding experimentally derived activation free energies of 14.0 kcal/mol (for regular hydrolysis phase) and 13.5 kcal/mol (for substrate activation phase).

Reaction pathway and free energy profile for butyrylcholinesterase-catalyzed hydrolysis of acetylcholine

A catalytic mechanism for the butyrylcholinesterase (BChE)-catalyzed hydrolysis of acetylcholine (ACh) has been studied by performing pseudobond first-principles quantum mechanical/molecular mechanical-free energy calculations on both acylation and deacylation of BChE. It has been shown that the acylation with ACh includes two reaction steps, including nucleophilic attack on the carbonyl carbon of ACh and dissociation of choline ester. The deacylation stage includes nucleophilic attack of a water molecule on the carboxyl carbon of the substrate and dissociation between the carboxyl carbon of the substrate and the hydroxyl oxygen of the Ser198 side chain. Notably, despite the fact that acetylcholinesterase (AChE) and BChE are very similar enzymes, the acylation of BChE with ACh is rate-determining, which is remarkably different from the AChE-catalyzed hydrolysis of ACh, in which the deacylation is rate-determining. The computational prediction is consistent with available experimental kinetic data. The overall free energy barrier calculated for BChE-catalyzed hydrolysis of ACh is 13.8 kcal/mol, which is in good agreement with the experimentally derived activation free energy of 13.3 kcal/mol.

Design of high-activity mutants of human butyrylcholinesterase against (-)-cocaine: structural and energetic factors affecting the catalytic efficiency

The present study was aimed to explore the correlation between the protein structure and catalytic efficiency of butyrylcholinesterase (BChE) mutants against (-)-cocaine by modeling the rate-determining transition state (TS1), i.e., the transition state for the first step of chemical reaction process, of (-)-cocaine hydrolysis catalyzed by various mutants of human BChE in comparison with the wild type. Molecular modeling of the TS1 structures revealed that mutations on certain nonactive site residues can indirectly affect the catalytic efficiency of the enzyme against (-)-cocaine through enhancing or weakening the overall hydrogen bonding between the carbonyl oxygen of (-)-cocaine benzoyl ester and the oxyanion hole of the enzyme. Computational insights and predictions were supported by the catalytic activity data obtained from wet experimental tests on the mutants of human BChE, including five new mutants reported for the first time. The BChE mutants with at least ∼1000-fold improved catalytic efficiency against (-)-cocaine compared to the wild-type BChE are all associated with the TS1 structures having stronger overall hydrogen bonding between the carbonyl oxygen of (-)-cocaine benzoyl ester and the oxyanion hole of the enzyme. The combined computational and experimental data demonstrate a reasonable correlation relationship between the hydrogen-bonding distances in the TS1 structure and the catalytic efficiency of the enzyme against (-)-cocaine.

Fundamental reaction mechanism and free energy profile for (-)-cocaine hydrolysis catalyzed by cocaine esterase

The fundamental reaction mechanism of cocaine esterase (CocE)-catalyzed hydrolysis of (-)-cocaine and the corresponding free energy profile have been studied by performing pseudobond first-principles quantum mechanical/molecular mechanical free energy (QM/MM-FE) calculations. On the basis of the QM/MM-FE results, the entire hydrolysis reaction consists of four reaction steps, including the nucleophilic attack on the carbonyl carbon of (-)-cocaine benzoyl ester by the hydroxyl group of Ser117, dissociation of (-)-cocaine benzoyl ester, nucleophilic attack on the carbonyl carbon of (-)-cocaine benzoyl ester by water, and finally dissociation between the (-)-cocaine benzoyl group and Ser117 of CocE. The third reaction step involving the nucleophilic attack of a water molecule was found to be rate-determining, which is remarkably different from (-)-cocaine hydrolysis catalyzed by wild-type butyrylcholinesterase (BChE; where the formation of the prereactive BChE-(-)-cocaine complex is rate-determining) or its mutants containing Tyr332Gly or Tyr332Ala mutation (where the first chemical reaction step is rate-determining). Besides, the role of Asp259 in the catalytic triad of CocE does not follow the general concept of the "charge-relay system" for all serine esterases. The free energy barrier calculated for the rate-determining step of CocE-catalyzed hydrolysis of (-)-cocaine is 17.9 kcal/mol, which is in good agreement with the experimentally derived activation free energy of 16.2 kcal/mol. In the present study, where many sodium ions are present, the effects of counterions are found to be significant in determining the free energy barrier. The finding of the significant effects of counterions on the free energy barrier may also be valuable in guiding future mechanistic studies on other charged enzymes.