Enhancing cocaine metabolism with butyrylcholinesterase as a treatment strategy

Human Butyrylcholinesterase Mutants for (-)-Cocaine Hydrolysis: A Correlation Relationship between Catalytic Efficiency and Total Hydrogen Bonding Energy with an Oxyanion Hole

A combined computational and experimental study has been carried out to explore and test a quantitative correlation relationship between the relative catalytic efficiency (RCE) of human butyrylcholinesrase (BChE) mutant-catalyzed hydrolysis of substrate (-)-cocaine and the total hydrogen bonding energy (tHBE) of the carbonyl oxygen of the substrate with the oxyanion hole of the enzyme in the modeled transition-state structure (TS1), demonstrating a satisfactory linear correlation relationship between ln(RCE) and tHBE. The satisfactory correlation relationship has led us to computationally predict and experimentally confirm new human BChE mutants that have a further improved catalytic activity against (-)-cocaine, including the most active one (the A199S/F227S/S287G/A328W/Y332G mutant) with a 2790-fold improved catalytic efficiency (kcat/KM = 2.5 × 109 min-1 M-1) compared to the wild-type human BChE. Compared to the reference mutant (the A199S/S287G/A328W/Y332G mutant) tested in the reported clinical development of an enzyme therapy for cocaine dependence treatment, this new mutant (with a newly predicted additional F227S mutation) has an improved catalytic efficiency against (-)-cocaine by ∼2.6-fold. The good agreement between the computational and experimental ln(RCE) values suggests that the obtained correlation relationship is robust for computational prediction. A similar correlation relationship could also be explored in studying BChE or other serine hydrolases/esterases with an oxyanion hole stabilizing the carbonyl oxygen in the rate-determining reaction step of the enzymatic hydrolysis of other substrates.

Highly selective butyrylcholinesterase inhibitors related to Amaryllidaceae alkaloids - Design, synthesis, and biological evaluation

Butyrylcholinesterase (BChE) is one of the most frequently implicated enzymes in the advanced stage of Alzheimer's disease (AD). As part of our endeavors to develop new drug candidates for AD, we have focused on natural template structures, namely the Amaryllidaceae alkaloids carltonine A and B endowed with high BChE selectivity. Herein, we report the design, synthesis, and in vitro evaluation of 57 novel highly selective human BChE (hBChE) inhibitors. Most synthesized compounds showed hBChE inhibition potency ranging from micromolar to low nanomolar scale. Compounds that revealed BChE inhibition below 100 nM were selected for detailed biological investigation. The CNS-targeted profile of the presented compounds was confirmed theoretically by calculating the BBB score algorithm, these data were corroborated by determining the permeability in vitro using PAMPA-assay for the most active derivatives. The study highlighted compounds 87 (hBChE IC₅₀ = 3.8 ± 0.2 nM) and 88 (hBChE IC₅₀ = 5.7 ± 1.5 nM) as the top-ranked BChE inhibitors. Compounds revealed negligible cytotoxicity for the human neuroblastoma (SH-SY5Y) and hepatocellular carcinoma (HepG2) cell lines compared to BChE inhibitory potential. A crystallographic study was performed to inspect the binding mode of compound 87, revealing essential interactions between 87 and hBChE active site. In addition, multidimensional QSAR analyses were applied to determine the relationship between chemical structures and biological activity in a dataset of designed agents. Compound 87 is a promising lead compound with potential implications for treating the late stages of AD.

Catalytic activities of a highly efficient cocaine hydrolase for hydrolysis of biologically active cocaine metabolites norcocaine and benzoylecgonine

Cocaine is a widely abused, hepatotoxic drug without an FDA-approved pharmacotherapy specific for cocaine addiction or overdose. It is recognized as a promising therapeutic strategy to accelerate cocaine metabolism which can convert cocaine to pharmacologically inactive metabolite(s) using an efficient cocaine-metabolizing enzyme. Our previous studies have successfully designed and discovered a highly efficient cocaine hydrolase, denoted as CocH5-Fc(M6), capable of rapidly hydrolyzing cocaine at the benzoyl ester moiety. In the present study, we determined the kinetic parameters of CocH5-Fc(M6) against norcocaine (k_cat = 9,210 min^-1, K_M = 20.9 μM, and k_cat/K_M = 1.87 × 10⁵ min^-1 M^-1) and benzoylecgonine (k_cat = 158 min^-1, K_M = 286 μM, and k_cat/K_M = 5.5 × 10⁵ min^-1 M^-1) for the first time. Further in vivo studies have demonstrated that CocH5-Fc(M6) can effectively accelerate clearance of not only cocaine, but also norcocaine (known as a cocaine metabolite which is more toxic than cocaine itself) and benzoylecgonine (known as an unfavorable long-lasting metabolite with some long-term toxicity concerns) in rats. Due to the desired high catalytic activity against norcocaine, CocH5-Fc(M6) is capable of quickly detoxifying both cocaine and its more toxic metabolite norcocaine after intraperitoneally administering lethal dose of 60 or 180 mg/kg cocaine. In addition, the ability of CocH5-Fc(M6) to accelerate clearance of benzoylecgonine should also be valuable for the use of CocH5-Fc(M6) in treatment of cocaine use disorder.

Systematically Modulating Aptamer Affinity and Specificity by Guanosine-to-Inosine Substitution

Aptamers are widely used in small molecule detection applications due to their specificity, stability, and cost effectiveness. One key challenge in utilizing aptamers in sensors is matching the binding affinity of the aptamer to the desired concentration range for analyte detection. The most common methods for modulating affinity have inherent limitations, such as the likelihood of drastic changes in aptamer folding. Here, we propose that substituting guanosine for inosine at specific locations in the aptamer sequence provides a less perturbative approach to modulating affinity. Inosine is a naturally occurring nucleotide that results from hydrolytic deamination of adenosine, and like guanine, it base pairs with cytosine. Using the well-studied cocaine binding aptamer, we systematically replaced guanosine with inosine and were able to generate sequences having a range of binding affinities from 230 nM to 80 μM. Interestingly, we found that these substitutions could also modulate the specificity of the aptamers, leading to a range of binding affinities for structurally related analytes. Analysis of folding stability via melting temperature shows that, as expected, aptamer structure is impacted by guanosine-to-inosine substitutions. The ability to tune binding affinity and specificity through guanosine-to-inosine substitution provides a convenient and reliable approach for rapidly generating aptamers for diverse biosensing applications.

Acetylcholine signaling genes are required for cocaine-stimulated egg laying in Caenorhabditis elegans

The toxicity and addictive liability associated with cocaine abuse are well-known. However, its mode of action is not completely understood, and effective pharmacotherapeutic interventions remain elusive. The cholinergic effects of cocaine on acetylcholine receptors, synthetic enzymes, and degradative enzymes have been the focus of relatively little empirical investigation. Due to its genetic tractability and anatomical simplicity, the egg laying circuit of the hermaphroditic nematode, Caenorhabditis elegans, is a powerful model system to precisely examine the genetic and molecular targets of cocaine in vivo. Here, we report a novel cocaine-induced behavioral phenotype in C. elegans, cocaine-stimulated egg laying. In addition, we present the results of an in vivo candidate suppression screen of synthetic enzymes, receptors, degradative enzymes, and downstream components of the intracellular signaling cascades of the main neurotransmitter systems that control C. elegans egg laying. Our results show that cocaine-stimulated egg laying is dependent on acetylcholine synthesis and synaptic release, functional nicotinic acetylcholine receptors, and the C. elegans acetylcholinesterases.

Anti-tyrosinase, anti-cholinesterase and cytotoxic activities of extracts and phytochemicals from the Tunisian Citharexylum spinosum L.: Molecular docking and SAR analysis

Previously phytochemical investigations carried out on the flowers and trunk bark extracts of Citharexylum spinosum L. tree, allowed the isolation of twenty molecules belonging to several families of natural substances [triterpene acids, iridoid glycosides, phenylethanoid glycosides, 8,3'-neolignan glycosides, together with other phenolic compounds]. In the present work, a biological evaluation (anti-tyrosinase, anticholinesterase and cytotoxic activities) was performed on the prepared extracts and the isolated secondary metabolites. The results showed that the EtOAc extract of the trunk bark displayed the highest anti-tyrosinase effect with a percent inhibition of 55.0 ± 1.8% at a concentration of 100 µg/mL. The highest anticholinesterase activity was presented by the same extract with an IC₅₀ value of 99.97 ± 3.01 µg/mL. The EtOAc extract of flowers and that of the trunk bark displayed the best cytotoxic property with IC₅₀ values of 96.00 ± 2.85 and 88.75 ± 2.00 µg/mL, respectively, against the human cervical cancer cell line (HeLa), and IC₅₀ values of 188.23 ± 3.88 and 197.00 ± 4.25 µg/mL, respectively, against the human lung cancer (A549) cell lines. Biological investigation of the pure compounds showed that the two 8,3'-neolignan glycosides, plucheosides D₁-D₂, generate the highest anti-tyrosinase potency with a percent inhibition of 61.4 ± 2.0 and 79.5 ± 2.3%, respectively, at a concentration of 100 µM. The iridoid glycosides exhibited a significant anticholinesterase activity with IC₅₀ values ranging from 17.19 ± 1.02 to 52.24 ± 2.50 µM. Triterpene pentacyclic acids and iridoid glycosides exerted encouraging cytotoxic effects against HeLa with IC₅₀ values ranging from 9.00 ± 1.10 to 25.00 ± 1.00 µM. The study of the structure-activity relationship (SAR) has been sufficiently and widely discussed. The natural compounds that exhibited the significant bioactivities were docked.

Catalytic activities of cocaine hydrolases against the most toxic cocaine metabolite norcocaethylene

A majority of cocaine users also consume alcohol. The concurrent use of cocaine and alcohol produces the pharmacologically active metabolites cocaethylene and norcocaethylene, in addition to norcocaine. Both cocaethylene and norcocaethylene are more toxic than cocaine itself. Hence, a truly valuable cocaine-metabolizing enzyme for cocaine abuse/overdose treatment should be effective for the hydrolysis of not only cocaine, but also its metabolites norcocaine, cocaethylene, and norcocaethylene. However, there has been no report on enzymes capable of hydrolyzing norcocaethylene (the most toxic metabolite of cocaine). The catalytic efficiency parameters (kcat and KM) of human butyrylcholinesterase (BChE) and two mutants (known as cocaine hydrolases E14-3 and E12-7) against norcocaethylene have been characterized in the present study for the first time, and they are compared with those against cocaine. According to the obtained kinetic data, wild-type human BChE showed a similar catalytic efficiency against norcocaethylene (kcat = 9.5 min-1, KM = 11.7 μM, and kcat/KM = 8.12 × 105 M-1 min-1) to that against (-)-cocaine (kcat = 4.1 min-1, KM = 4.5 μM, and kcat/KM = 9.1 × 105 M-1 min-1). E14-3 and E12-7 showed an improved catalytic activity against norcocaethylene compared to wild-type BChE. E12-7 showed a 39-fold improved catalytic efficiency against norcocaethylene (kcat = 210 min-1, KM = 6.6 μM, and kcat/KM = 3.18 × 107 M-1 min-1). It has been demonstrated that E12-7 as an exogenous enzyme can efficiently metabolize norcocaethylene in rats.

Systemic Safety of a Recombinant AAV8 Vector for Human Cocaine Hydrolase Gene Therapy: A Good Laboratory Practice Preclinical Study in Mice

Cocaine addiction continues to impose major burdens on affected individuals and broader society but is highly resistant to medical treatment or psychotherapy. This study was undertaken with the goal of Food and Drug Administration (FDA) permission for a first-in-human clinical trial of a gene therapy for treatment-seeking cocaine users to become and remain abstinent. The approach was based on intravenous administration of AAV8-hCocH, an adeno-associated viral vector encoding a modified plasma enzyme that metabolizes cocaine into harmless by-products. To assess systemic safety, we conducted "Good Laboratory Practice" (GLP) studies in cocaine-experienced and cocaine-naive mice at doses of 5E12 and 5E13 vector genomes/kg. Results showed total lack of viral vector-related adverse effects in all tests performed. Instead, mice given one injection of AAV8-hCocH and regular daily injections of cocaine had far less tissue pathology than cocaine-injected mice with no vector treatment. Biodistribution analysis showed the vector located almost exclusively in the liver. These results indicate that a liver-directed AAV8-hCocH gene transfer at reasonable dosage is safe, well tolerated, and effective. Thus, gene transfer therapy emerges as a radically new approach to treat compulsive cocaine abuse. In fact, based on these positive findings, the FDA recently accepted our latest request for investigational new drug application (IND 18579).

Dimerization of human butyrylcholinesterase expressed in bacterium for development of a thermally stable bioscavenger of organophosphorus compounds

Human butyrylcholinesterase (BChE) is a widely distributed plasma enzyme. For decades, numerous research efforts have been directed at engineering BChE as a bioscavenger of organophosphorus insecticides and chemical warfare nerve agents. However, it has been a grand challenge to cost-efficiently produce BChE in large-scale. Recently reported studies have successfully designed a truncated BChE mutant (with amino-acid substitutions on 47 residues that are far away from the catalytic site), denoted as BChE-M47 for convenience, which can be expressed in E. coli without loss of its catalytic activity. In this study, we aimed to dimerize the truncated BChE mutant protein expressed in a prokaryotic system (E. coli) in order to further improve its thermal stability by introducing a pair of cross-subunit disulfide bonds to the BChE-M47 structure. Specifically, the E377C/A516C mutations were designed and introduced to BChE-M47, and the obtained new protein entity, denoted as BChE-M48, with a pair of cross-subunit disulfide bonds indeed exists as a dimer with significantly improved thermostability and unaltered catalytic activity and reactivity compared to BChE-M47. These results provide a new strategy for optimizing protein stability for production in a cost-efficient prokaryotic system. Our enzyme, BChE-M48, has a half-life of almost one week at a 37°C, suggesting that it could be utilized as a highly stable bioscavenger of OP insecticides and chemical warfare nerve agents.

Treating Cocaine Addiction, Obesity, and Emotional Disorders by Viral Gene Transfer of Butyrylcholinesterase

Butyrylcholinesterase (BChE), a plasma enzyme that hydrolyses the neurotransmitter, acetylcholine relatively well, with far lower efficiency than acetylcholinesterase (AChE) but with the capability to degrade a broad range of bioactive esters. AChE is universally understood as essential to cholinergic neurotransmission, voluntary muscle performance, and cognition, among other roles, and its catalytic impact is essential for life. A total absence of BChE activity, whether by enzyme inhibition or simple lack of enzyme protein is not only compatible with life, but does not lead to obvious physiologic disturbance. However, very recent studies at Mayo Clinic have amassed support for the concept that BChE does have a true physiological role as a "ghrelin hydrolase" and, pharmacologically, as a cocaine hydrolase. Human subjects and animal mutations that lack functional BChE show higher than normal levels of ghrelin, an acylated peptide that drives hunger and feeding, along with certain emotional behaviors. Mice treated by viral gene transfer of BChE show higher plasma levels of enzyme and lower levels of ghrelin. Ghrelin is acknowledged as a driver of food-seeking and stress. This brief review examines some key phenomena and considers means of modulating BChE as treatments for cocaine addiction, anxiety, aggression, and obesity.

A quantitative LC-MS/MS method for simultaneous determination of cocaine and its metabolites in whole blood

As new metabolic pathways of cocaine were recently identified, a high performance liquid chromatography tandem mass spectrometry (LC-MS/MS) method was developed to simultaneously determine cocaine and nine cocaine-related metabolites in whole blood samples. One-step solid phase extraction was used to extract all of the ten compounds and corresponding internal standards from blood samples. All compounds and internal standards extracted were separated on an Atlantis T3 (100Å, 3μm, 2.1mm×150mm I.D) column and detected in positive ion and high sensitivity mode with multiple reaction monitoring. This method was validated for its sensitivity, linearity, specificity, accuracy, precision, recovery, and stability. All of the ten compounds were quantifiable ranging from the lower limit of quantification (LLOQs) of ∼10nM (1.9-3.2ng/ml) to ∼1000nM (190-320ng/ml) without any interfering substance. Accuracy and precision were determined, and both of them were within the acceptance criteria of the United States (US) Food and Drug Administration (FDA) and European Medicines Agency (EMA) guidelines. The recovery was above 66.7% for all compounds. Stability tests demonstrated the stability of compounds under different storage conditions in whole blood samples. The method was successfully applied to a pharmacokinetic study with co-administration of cocaine and alcohol in rats.

Effects of a cocaine hydrolase engineered from human butyrylcholinesterase on metabolic profile of cocaine in rats

Accelerating cocaine metabolism through enzymatic hydrolysis at cocaine benzoyl ester is recognized as a promising therapeutic approach for cocaine abuse treatment. Our more recently designed A199S/F227A/S287G/A328W/Y332G mutant of human BChE, denoted as cocaine hydrolase-3 (CocH3), has a considerably improved catalytic efficiency against cocaine and has been proven active in blocking cocaine-induced toxicity and physiological effects. In the present study, we have further characterized the effects of CocH3 on the detailed metabolic profile of cocaine in rats administrated intravenously (IV) with 5 mg/kg cocaine, demonstrating that IV administration of 0.15 mg/kg CocH3 dramatically changed the metabolic profile of cocaine. Without CocH3 administration, the dominant cocaine-metabolizing pathway in rats was cocaine methyl ester hydrolysis to benzoylecgonine (BZE). With the CocH3 administration, the dominant cocaine-metabolizing pathway in rats became cocaine benzoyl ester hydrolysis to ecgonine methyl ester (EME), and the other two metabolic pathways (i.e. cocaine methyl ester hydrolysis to BZE and cocaine oxidation to norcocaine) became insignificant. The CocH3-catalyzed cocaine benzoyl ester hydrolysis to EME was so efficient such that the measured maximum blood cocaine concentration (∼38 ng/ml) was significantly lower than the threshold blood cocaine concentration (∼72 ng/ml) required to produce any measurable physiological effects.

Is it possible to reverse aged acetylcholinesterase inhibited by organophosphorus compounds? Insight from the theoretical study

The main treatment for organophosphorus (OP) compound poisoning in clinics is to restore the activity of acetylcholinesterase (AChE) through oxime-induced reactivation of the phosphorylated OP-AChE adduct. It suffers from a competitive and irreversible aging reaction of the phosphorylated OP-AChE adduct, resulting in permanent inactivity of AChE. However, it was recently reported that N-methyl-2-methoxypyridinium species can act as methylating agents to methylate the methyl methane-phosphonate monoanion, in which the reaction mimics the reverse of the aging reaction of the phosphorylated OP-AChE adduct. If the aging reaction could be really reversed, the efficiency for the OP detoxification should be significantly improved, bringing up the possibility to develop an agent to reverse the aging process of the phosphorylated OP-AChE adduct. However, such a reaction with the N-methyl-2-methoxypyridinium species in the enzyme is still not reported so far. It is of great interest to know whether or not this reaction is observable in the enzyme, and more importantly, if it turns out to be not observable in the enzyme, why such a reaction proceeds quickly in aqueous solution but not in the enzyme. In the present study, we performed DFT calculations and quantum mechanical/molecular mechanical (QM/MM) calculations to reveal the fundamental mechanism for the methylation of both the methyl methane-phosphonate monoanion and the aged sarin-AChE adduct by N-methyl-2-methoxypyridinium species, respectively. The obtained results support the SN2 reaction mechanism, not the stepwise mechanism, for the methylation of the methyl methane-phosphonate monoanion by 9 reported N-methyl-2-methoxypyridinium compounds. The calculated free energy barriers are in good agreement with the experimental data. The methylation of the aged sarin-AChE adduct by one N-methyl-2-methoxypyridinium compound (labeled as compound 2) also employs the SN2 reaction mechanism with an extremely high free energy barrier of 30.4 ± 3.5 (or 26.6) kcal mol(-1), implying that this reaction in the enzyme hardly occurs. Our results clearly show that compound 2 forms a strong π-π stacking interaction with the aromatic ring of the W86 residue of AChE, making itself unable to approach sarin for the reverse of the aging process. On the basis of the structure and mechanism, several possible strategies have been suggested for designing methylating agents with higher activity against the aged sarin-AChE adduct.

Long-Term Blockade of Cocaine Self-Administration and Locomotor Activation in Rats by an Adenoviral Vector-Delivered Cocaine Hydrolase

A promising approach in treating cocaine abuse is to metabolize cocaine in the blood using a mutated butyrylcholinesterase (BChE) that functions as a cocaine hydrolase (CocH). In rats, a helper-dependent adenoviral (hdAD) vector-mediated delivery of CocH abolished ongoing cocaine use and cocaine-primed reinstatement of drug-seeking for several months. This enzyme also metabolizes ghrelin, an effect that may be beneficial in maintaining healthy weights. The effect of a single hdAD-CocH vector injection was examined in rats on measures of anxiety, body weight, cocaine self-administration, and cocaine-induced locomotor activity. To examine anxiety, periadolescent rats were tested in an elevated-plus maze. Weight gain was then examined under four rodent diets. Ten months after CocH-injection, adult rats were trained to self-administer cocaine intravenously and, subsequently, cocaine-induced locomotion was tested. Viral gene transfer produced sustained plasma levels of CocH for over 13 months of testing. CocH-treated rats did not differ from controls in measures of anxiety, and only showed a transient reduction in weight gain during the first 3 weeks postinjection. However, CocH-treated rats were insensitive to cocaine. At 10 months postinjection, none of the CocH-treated rats initiated cocaine self-administration, unlike 90% of the control rats. At 13 months postinjection, CocH-treated rats showed no cocaine-induced locomotion, whereas control rats showed a dose-dependent enhancement of locomotion. CocH vector produced a long-term blockade of the rewarding and behavioral effects of cocaine in rats, emphasizing its role as a promising therapeutic intervention in cocaine abuse.

Reaction Pathway for Cocaine Hydrolase-Catalyzed Hydrolysis of (+)-Cocaine

A recently designed and discovered cocaine hydrolase (CocH), engineered from human butyrylcholinesterase (BChE), has been proven promising as a novel enzyme therapy for treatment of cocaine overdose and addiction because it is highly efficient in catalyzing hydrolysis of naturally occurring (-)-cocaine. It has been known that the CocH also has a high catalytic efficiency against (+)-cocaine, a synthetic enantiomer of cocaine. Reaction pathway and the corresponding free energy profile for the CocH-catalyzed hydrolysis of (+)-cocaine have been determined, in the present study, by performing first-principles pseudobond quantum mechanical/molecular mechanical (QM/MM)-free energy (FE) calculations. Acordingt to the QM/MM-FE results, the catalytic hydrolysis process is initiated by the nucleophilic attack on carbonyl carbon of (-)-cocaine benzoyl ester via hydroxyl oxygen of S198 side chain, and the second reaction step (i.e. dissociation of benzoyl ester) is rate-determining. This finding for CocH-catalyzed hydrolysis of (+)-cocaine is remarkably different from that for the (+)-cocaine hydrolysis catalyzed by bacterial cocaine esterase in which the first reaction step of the deacylation is associated with the highest free energy barrier (~17.9 kcal/mol). The overall free energy barrier (~16.0 kcal/mol) calculated for the acylation stage of CocH-catalyzed hydrolysis of (+)-cocaine is in good agreement with the experimental free energy barrier of ~14.5 kcal/mol derivated from the experimental kinetic data.

Fate of systemically administered cocaine in nonhuman primates treated with the dAd5GNE anticocaine vaccine

Cocaine use disorders are mediated by the cocaine blockade of the dopamine transporter in the central nervous system (CNS). On the basis of the concept that these effects could be obviated if cocaine were prevented from reaching its cognate receptors in the CNS, we have developed an anticocaine vaccine, dAd5GNE, based on a cocaine analog covalently linked to capsid proteins of an E1(-)E3(-) serotype 5 adenovirus. While the vaccine effectively blocks systemically administered cocaine from reaching the brain by mediating sequestration of the cocaine in the blood, the fact that cocaine also has significant peripheral effects raises concerns that vaccination-mediated redistribution could lead to adverse effects in the visceral organs. The distribution of systemically administered cocaine at a weight-adjusted typical human dose was evaluated along with cocaine metabolites in both dAd5GNE-vaccinated and control nonhuman primates. dAd5GNE sequestration of cocaine to the blood not only prevented cocaine access to the CNS, but also limited access of both the drug and its metabolites to other cocaine-sensitive organs. The levels of cocaine in the blood of vaccinated animals rapidly decreased, suggesting that while the antibody limits access of the drug and its active metabolites to the brain and sensitive organs of the periphery, it does not prolong drug levels in the blood compartment. Gross and histopathology of major organs found no vaccine-mediated untoward effects. These results build on our earlier measures of efficacy and demonstrate that the dAd5GNE vaccine-mediated redistribution of administered cocaine is not likely to impact the vaccine safety profile.

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.

Future pharmacological treatments for substance use disorders

Substance use disorders represent a serious public health and social issue worldwide. Recent advances in our understanding of the neurobiological basis of the addictive processes have led to the development of a growing number of pharmacological agents to treat addictions. Despite this progress, there are no approved pharmacological treatments for cocaine, methamphetamine and cannabis addiction. Moving treatment development to the next stage will require novel ways of approaching substance use disorders. One such novel approach is to target individual vulnerabilities, such as cognitive function, sex differences and psychiatric comorbidities. This review provides a summary of promising pharmacotherapies for alcohol, opiate, stimulant and nicotine addictions. Many medications that target positive and negative reinforcement of drugs, as well as individual vulnerabilities to addiction, are in different phases of development. Clinical trials testing the efficacy of these medications for substance use disorder are warranted.

The interaction of quaternary reversible acetylcholinesterase inhibitors with the nicotinic receptor

Acetylcholinesterase inhibitors (AChEIs) are used in the treatment of myasthenia gravis (MG). We investigated the effects of AChEIs on peripheral nicotinic receptors (nAChR), which play a crucial role in the treatment of MG symptoms. The positive modulation of those receptors by AChE inhibitors could have an added value to the anti-AChE activity and might be useful in the therapy of MG. Furthermore, to estimate the potential drawbacks of the compounds, cytotoxicity has been assessed on various cell lines. The whole-cell mode of the patch-clamp method was employed. The experiments were performed on medulloblastoma/rhabdomyosarcoma cell line TE671 expressing human embryonic muscle-like receptor with subunits alpha2betagammadelta. The effect of the compounds on cell viability was measured by standard MTT assay (Sigma Aldrich) on ACHN (renal cell adenocarcinoma), HeLa (immortal cell line derived from a cervical carcinoma), HEPG2 (hepatocellular carcinoma) and BJ (skin fibroblasts) cell lines. No positive modulation by the tested AChE inhibitors was observed. Moreover, the compounds exhibited antagonistic activity on the peripheral nAChR. Standard drugs used in MG treatment were shown to be less potent inhibitors of muscle-type nAChR than the newly synthesized compounds. The new compounds showed very little effect on cell viability, and toxicities were comparable to standards. Newly synthesized AChEIs inhibited peripheral nAChR. Furthermore, the inhibition was higher than that of standards used for the treatment of MG. They could be used for the study of nAChR function, thanks to their high antagonizing potency and fast recovery of receptor activity after their removal. However, since no positive modulation was observed, the new compounds do not seem to be promising candidates for MG treatment, even though their cytotoxic effect was relatively low.

Kinetic characterization of human butyrylcholinesterase mutants for the hydrolysis of cocaethylene

It is known that the majority of cocaine users also consume alcohol. Alcohol can react with cocaine to produce a significantly more cytotoxic compound, cocaethylene. Hence a truly valuable cocaine-metabolizing enzyme as treatment for cocaine abuse/overdose should be efficient for not only cocaine itself, but also cocaethylene. The catalytic parameters (kcat and KM) of human BChE (butyrylcholinesterase) and two mutants (known as cocaine hydrolases E14-3 and E12-7) for cocaethylene are characterized in the present study, for the first time, in comparison with those for cocaine. On the basis of the obtained kinetic data, wild-type human BChE has a lower catalytic activity for cocaethylene (kcat=3.3 min(-1), KM=7.5 μM and kcat/KM=4.40 × 10(5) M(-1)·min(-1)) compared with its catalytic activity for (-)-cocaine. E14-3 and E12-7 have a considerably improved catalytic activity against cocaethylene compared with the wild-type BChE. E12-7 is identified as the most efficient enzyme for hydrolysing cocaethylene in addition to its high activity for (-)-cocaine. E12-7 has an 861-fold improved catalytic efficiency for cocaethylene (kcat=3600 min(-1), KM=9.5 μM and kcat/KM=3.79 × 10(8) M(-1)·min(-1)). It has been demonstrated that E12-7 as an exogenous enzyme can indeed rapidly metabolize cocaethylene in rats. Further kinetic modelling has suggested that E12-7 with an identical concentration as that of the endogenous BChE in human plasma can effectively eliminate (-)-cocaine, cocaethylene and norcocaine in simplified kinetic models of cocaine abuse and overdose associated with the concurrent use of cocaine and alcohol.

Kinetic characterization of high-activity mutants of human butyrylcholinesterase for the cocaine metabolite norcocaine

It has been known that cocaine produces its toxic and physiological effects through not only cocaine itself, but also norcocaine formed from cocaine oxidation catalysed by microsomal CYP (cytochrome P450) 3A4 in the human liver. The catalytic parameters (kcat and Km) of human BChE (butyrylcholinesterase) and its three mutants (i.e. A199S/S287G/A328W/Y332G, A199S/F227A/S287G/A328W/E441D and A199S/F227A/S287G/A328W/Y332G) for norcocaine have been characterized in the present study for the first time and compared with those for cocaine. On the basis of the obtained kinetic data, wild-type human BChE has a significantly lower catalytic activity for norcocaine (kcat=2.8 min(-1), Km=15 μM and kcat/Km=1.87 × 10(5) M(-1)·min(-1)) compared with its catalytic activity for (-)-cocaine. The BChE mutants examined in the present study have considerably improved catalytic activities against both cocaine and norcocaine compared with the wild-type enzyme. Within the enzymes examined in the present study, the A199S/F227A/S287G/A328W/Y332G mutant (CocH3) is identified as the most efficient enzyme for hydrolysing both cocaine and norcocaine. CocH3 has a 1080-fold improved catalytic efficiency for norcocaine (kcat=2610 min(-1), Km=13 μM and kcat/Km=2.01 × 10(8) M(-1)·min(-1)) and a 2020-fold improved catalytic efficiency for cocaine. It has been demonstrated that CocH3 as an exogenous enzyme can rapidly metabolize norcocaine, in addition to cocaine, in rats. Further kinetic modelling has suggested that CocH3 with an identical concentration with that of the endogenous BChE in human plasma can effectively eliminate both cocaine and norcocaine in a simplified kinetic model of cocaine abuse.

Pharmacokinetic approaches to treatment of drug addiction

The pharmacokinetic approach to treatment targets the drug molecules themselves, aiming to reduce their concentration at the site of action, thereby reducing or preventing any pharmacodynamic effect. This approach might be useful in the treatment of acute drug toxicity/overdose and in the long-term treatment of addiction. Early clinical trials with anticocaine and antinicotine vaccines have shown reduced drug use and good tolerability. Also showing promise in animal studies are monoclonal antibodies against cocaine, methamphetamine and phencyclidine, as well as the enhancment of cocaine metabolism with genetic variants of human butyrylcholinesterase, using a bacterial esterase or catalytic monoclonal antibodies. Pharmacokinetic treatments offer potential advantages in terms of patient compliance, absence of medication interactions and benefit for patients who cannot take standard medications.

Modeling in vitro inhibition of butyrylcholinesterase using molecular docking, multi-linear regression and artificial neural network approaches

Butyrylcholinesterase (BChE) has been an important protein used for development of anti-cocaine medication. Through computational design, BChE mutants with ∼2000-fold improved catalytic efficiency against cocaine have been discovered in our lab. To study drug-enzyme interaction it is important to build mathematical model to predict molecular inhibitory activity against BChE. This report presents a neural network (NN) QSAR study, compared with multi-linear regression (MLR) and molecular docking, on a set of 93 small molecules that act as inhibitors of BChE by use of the inhibitory activities (pIC₅₀ values) of the molecules as target values. The statistical results for the linear model built from docking generated energy descriptors were: r(2)=0.67, rmsd=0.87, q(2)=0.65 and loormsd=0.90; the statistical results for the ligand-based MLR model were: r(2)=0.89, rmsd=0.51, q(2)=0.85 and loormsd=0.58; the statistical results for the ligand-based NN model were the best: r(2)=0.95, rmsd=0.33, q(2)=0.90 and loormsd=0.48, demonstrating that the NN is powerful in analysis of a set of complicated data. As BChE is also an established drug target to develop new treatment for Alzheimer's disease (AD). The developed QSAR models provide tools for rationalizing identification of potential BChE inhibitors or selection of compounds for synthesis in the discovery of novel effective inhibitors of BChE in the future.

Prospects, promise and problems on the road to effective vaccines and related therapies for substance abuse

This review addresses potential new treatments for stimulant drugs of abuse, especially cocaine. Clinical trials of vaccines against cocaine and nicotine have been completed with the generally encouraging result that subjects showing high titers of antidrug antibody experience a reduction in drug reward, which may aid in cessation. New vaccine technologies, including gene transfer of highly optimized monoclonal antibodies, are likely to improve such outcomes further. In the special case of cocaine abuse, a metabolic enzyme is emerging as an alternative or added therapeutic intervention, which would also involve gene transfer. Such approaches still require extensive studies of safety and efficacy, but they may eventually contribute to a robust form of in vivo drug interception that greatly reduces the risks of addiction relapse.

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.

Modification of pharmacokinetic and abuse-related effects of cocaine by human-derived cocaine hydrolase in monkeys

Although substantial research effort has focused on developing pharmacological treatments for cocaine abuse, no effective medications have been developed. Recent studies show that enzymes that metabolize cocaine in the periphery, forestalling its entry into the brain, can prevent cocaine toxicity and its behavioral effects in rodents. Here we report on effects of one such enzyme (Albu-CocH) on the pharmacokinetic and behavioral effects of cocaine in squirrel monkeys. Albu-CocH was developed from successive mutations of human butyrylcholinesterase (BChE) and has 1000-fold greater catalytic activity against cocaine than naturally occurring BChE. Pharmacokinetic studies showed that Albu-CocH (5 mg/kg) had a half-life of 56.6 hours in squirrel monkeys. In these studies, plasma levels of cocaine following i.v. 1 mg/kg cocaine were reduced 2 hours after administration of Albu-CocH, whereas plasma levels of the cocaine metabolite ecgonine methyl ester were increased. These effects were still evident 72 hours following Albu-CocH administration. In behavioral experiments in monkeys, pre-treatment with 5 mg/kg Albu-CocH dramatically decreased self-administration of a reinforcing dose of i.v. cocaine (30 µg/kg/injection) for over 24 hours. Pre-treatment with 5 mg/kg Albu-CocH also attenuated the reinstatement of extinguished cocaine self-administration by an i.v. priming injection of cocaine (0.1 or 0.3 mg/kg) and, in separate studies, attenuated the discriminative-stimulus effects of cocaine. The ability of Albu-CocH to attenuate the abuse-related effects of cocaine in squirrel monkeys indicates that further investigation of BChE mutants as potential treatment for cocaine abuse and toxicity is warranted.

Interception of cocaine by enzyme or antibody delivered with viral gene transfer: a novel strategy for preventing relapse in recovering drug users

Recent progress in enzyme engineering has led to versions of human butyrylcholinesterase (BChE) that hydrolyze cocaine efficiently in plasma, reduce concentrations reaching reward neurocircuity in the brain, and weaken behavioral responses to this drug. Along with enzyme advances, increasingly avid anti-cocaine antibodies and potent anti-cocaine vaccines have also been developed. Here we review these developments and consider the potential advantages along with the risks of delivering drug-intercepting proteins via gene transfer approaches to treat cocaine addiction.

Why does the G117H mutation considerably improve the activity of human butyrylcholinesterase against sarin? Insights from quantum mechanical/molecular mechanical free energy calculations

Human butyrylcholinesterase (BChE) is recognized as the most promising bioscavenger for organophosphorus (OP) warfare nerve agents. The G117H mutant of human BChE has been identified as a potential catalytic bioscavenger with a remarkably improved activity against OP nerve agents such as sarin, but it still does not satisfy the clinical use. For further design of the higher-activity mutants against OP nerve agents, it is essential to understand how the G117H mutation improves the activity. The reaction mechanisms and the free energy profiles for spontaneous reactivation of wild-type BChE and its G117H mutant phosphorylated by sarin have been explored, in this study, by performing first-principles quantum mechanical/molecular mechanical free energy calculations, and the remarkable role of the G117H mutation on the activity has been elucidated. For both the wild-type and G117H mutant enzymes, H438 acts as a general base to initiate the spontaneous reactivation that consists of two reaction steps: the nucleophilic attack at the phosphorus by a water molecule and decomposition of the pentacoordinated phosphorus intermediate. The calculated overall free energy barriers, i.e., 30.2 and 23.9 kcal/mol for the wild type and G117H mutant, respectively, are in good agreement with available experimental kinetic data. On the basis of the calculated results, the mutated residue (H117 in the G117H mutant) cannot initiate the spontaneous reactivation as a general base. Instead, it skews the oxyanion hole and makes the phosphorus more open to the nucleophilic water molecule, resulting in a remarkable change in the rate-determining step and significantly improved catalytic activity of human BChE.

Antiandrogen pretreatment alters cocaine pharmacokinetics in men

Among cocaine users, men experience more adverse brain and vascular effects than their female counterparts. This could be caused by testosterone, which may potentiate some of cocaine's effects. We examined whether antiandrogen (flutamide, FL) pretreatment alters cocaine's acute behavioral, physiologic, and pharmacokinetic effects in men with histories of occasional cocaine use. Participants (N = 8) were pretreated with oral FL (250 mg) and placebo on separate study days followed by intravenous (IV) cocaine (0.4 mg/kg). Vital signs, subjective ratings, and blood samples for cocaine and metabolites were obtained at baseline and for 90 minutes after cocaine administration. FL, itself, had no effects on physiologic or subjective responses; however, after cocaine, heart rate recovered faster with FL pretreatment. Flutamide reduced peak plasma cocaine levels (Wilcoxon signed-rank z = 2.1, P < 0.04) and area under the curve (AUC; z = 1.96, P < 0.05). Additionally, FL reduced EME levels (z = 1.96, P < 0.05) and AUC for BE and EME (z = 2.38, P < 0.02 and z = 1.96, P < 0.05, respectively). These results suggest that FL may alter cocaine pharmacokinetics in men. Because cocaine and BE are vasoconstrictive, the data imply that FL might reduce some of cocaine's cardiovascular effects.

Combined cocaine hydrolase gene transfer and anti-cocaine vaccine synergistically block cocaine-induced locomotion

Mice and rats were tested for reduced sensitivity to cocaine-induced hyper-locomotion after pretreatment with anti-cocaine antibody or cocaine hydrolase (CocH) derived from human butyrylcholinesterase (BChE). In Balb/c mice, direct i.p. injection of CocH protein (1 mg/kg) had no effect on spontaneous locomotion, but it suppressed responses to i.p. cocaine up to 80 mg/kg. When CocH was injected i.p. along with a murine cocaine antiserum that also did not affect spontaneous locomotion, there was no response to any cocaine dose. This suppression of locomotor activity required active enzyme, as it was lost after pretreatment with iso-OMPA, a selective BChE inhibitor. Comparable results were obtained in rats that developed high levels of CocH by gene transfer with helper-dependent adenoviral vector, and/or high levels of anti-cocaine antibody by vaccination with norcocaine hapten conjugated to keyhole limpet hemocyanin (KLH). After these treatments, rats were subjected to a locomotor sensitization paradigm involving a “training phase" with an initial i.p. saline injection on day 1 followed by 8 days of repeated cocaine injections (10 mg/kg, i.p.). A 15-day rest period then ensued, followed by a final “challenge" cocaine injection. As in mice, the individual treatment interventions reduced cocaine-stimulated hyperactivity to a modest extent, while combined treatment produced a greater reduction during all phases of testing compared to control rats (with only saline pretreatment). Overall, the present results strongly support the view that anti-cocaine vaccine and cocaine hydrolase vector treatments together provide enhanced protection against the stimulatory actions of cocaine in rodents. A similar combination therapy in human cocaine users might provide a robust therapy to help maintain abstinence.

Laser captured hepatocytes show association of butyrylcholinesterase gene loss and fibrosis progression in hepatitis C-infected drug users

Chronic hepatitis C virus (HCV) infection is complicated by hepatic fibrosis. Hypothesizing that early fibrogenic signals may originate in cells susceptible to HCV infection, hepatocyte gene expression was analyzed from persons with chronic HCV at different stages of liver fibrosis. Four HCV-infected subjects with precirrhosis liver fibrosis (Ishak fibrosis 3-5) were matched for age, race, and gender to five HCV-infected subjects with no evidence of fibrosis (Ishak fibrosis 0). Hepatocytes from each subject were isolated from liver biopsies using laser capture microdissection. Transcriptome profiling was performed on hepatocyte RNA using hybridization arrays. We found that hepatocytes in precirrhosis fibrosis were depleted for genes involved in small molecule and drug metabolism, especially butyrylcholinesterase (BCHE), a gene involved in the metabolism of drugs of abuse. Differential expression of BCHE was validated in the same tissues and cross-sectionally in an expanded cohort of 143 HCV-infected individuals. In a longitudinal study, serum BCHE activity was already decreased at study inception in 19 fibrosis progressors compared with 20 fibrosis nonprogressors (P < 0.05). Nonprogressors also had decreased BCHE activity over time compared with initial values, but these evolved a median (range) 8.6 (7.8-11.4) years after the study period inception (P < 0.05). Laser captured portal tracts were enriched for immune related genes when compared with hepatocytes but precirrhosis livers lost this enrichment.

CONCLUSION

Chronic HCV is associated with hepatocyte BCHE loss years before hepatic synthetic function is impaired. These results indicate that BCHE may be involved in the pathogenesis of HCV-related fibrosis among injection drug users.

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.

Pharmacokinetic strategies for treatment of drug overdose and addiction

The pharmacokinetic treatment strategy targets the drug molecule itself, aiming to reduce drug concentration at the site of action, thereby minimizing any pharmacodynamic effect. This approach might be useful in the treatment of acute drug toxicity/overdose and in the long-term treatment of addiction. Phase IIa controlled clinical trials with anticocaine and antinicotine vaccines have shown good tolerability and some efficacy, but Phase IIb and III trials have been disappointing because of the failure to generate adequate antibody titers in most participants. Monoclonal antibodies against cocaine, methamphetamine and phencyclidine have shown promise in animal studies, as has enhancing cocaine metabolism with genetic variants of human butyrylcholinesterase, with a bacterial esterase, and with catalytic monoclonal antibodies. Pharmacokinetic treatments offer potential advantages in terms of patient adherence, absence of medication interactions and benefit for patients who cannot take standard medications.

Accelerating cocaine metabolism as an approach to the treatment of cocaine abuse and toxicity

One pharmacokinetic approach to the treatment of cocaine abuse and toxicity involves the development of compounds that can be safely administered to humans and that accelerate the metabolism of cocaine to inactive components. Catalytic antibodies have been developed and shown to accelerate cocaine metabolism, but their catalytic efficiency for cocaine is relatively low. Mutations of human butyrylcholinesterase and a bacterial cocaine esterase found in the soil of coca plants have also been developed. These compounds accelerate cocaine metabolism and antagonize the behavioral and toxic effects of cocaine in animal models. Of these two approaches, the human butyrylcholinesterase mutants show the most immediate promise as they would not be expected to evoke an immune response in humans.

Cocaine hydrolase gene therapy for cocaine abuse

Rapid progress in the past decade with re-engineering of human plasma butyrylcholinesterase has led to enzymes that destroy cocaine so efficiently that they prevent or interrupt drug actions in the CNS even though confined to the blood stream. Over the same time window, improved gene-transfer technology has made it possible to deliver such enzymes by endogenous gene transduction at high levels for periods of a year or longer after a single treatment. This article reviews recent advances in this field and considers prospects for development of a robust therapy aimed at aiding recovering drug users avoid addiction relapse.

Bacterial cocaine esterase: a protein-based therapy for cocaine overdose and addiction

Cocaine is highly addictive and there are no pharmacotherapeutic drugs available to treat acute cocaine toxicity or chronic abuse. Antagonizing an inhibitor such as cocaine using a small molecule has proven difficult. The alternative approach is to modify cocaine's pharmacokinetic properties by sequestering or hydrolyzing it in serum and limiting access to its sites of action. We took advantage of a bacterial esterase (CocE) that has evolved to hydrolyze cocaine and have developed it as a therapeutic that rapidly and specifically clears cocaine from the subject. Native enzyme was unstable at 37°C, thus limiting CocE's potential. Innovative computational methods based on the protein's structure helped elucidate its mechanism of destabilization. Novel protein engineering methodologies were applied to substantially improve its stability in vitro and in vivo. These improvements rendered CocE as a powerful and efficacious therapeutic to treat cocaine intoxication and lead the way towards developing a therapy for addiction.

Cocaine hydrolase encoded in viral vector blocks the reinstatement of cocaine seeking in rats for 6 months

BACKGROUND

Cocaine dependence is a pervasive disorder with high rates of relapse. In a previous study, direct administration of a quadruple mutant albumin-fused butyrylcholinesterase that efficiently catalyzes hydrolysis of cocaine to benzoic acid and ecgonine methyl ester acutely blocked cocaine seeking in an animal model of relapse. In the present experiments, these results were extended to achieve a long-duration blockade of cocaine seeking with a gene transfer paradigm using a related butyrylcholinesterase-based cocaine hydrolase (CocH).

METHODS

Male and female rats were allowed to self-administer cocaine under a fixed-ratio 1 schedule of reinforcement for approximately 14 days. Following the final self-administration session, rats were injected with CocH vector or a control injection (empty vector or saline), and their cocaine solutions were replaced with saline for 14 days to allow for extinction of lever pressing. Subsequently, they were tested for drug-primed reinstatement by administering intraperitoneal injections of saline (S), cocaine (C) (5, 10, and 15 mg/kg), and d-amphetamine according to the following sequence: S, C, S, C, S, C, S, d-amphetamine. Rats then received cocaine-priming injections once weekly for 4 weeks and, subsequently, once monthly for up to 6 months.

RESULTS

Administration of CocH vector produced substantial and sustained CocH activity in plasma that corresponded with diminished cocaine-induced (but not amphetamine-induced) reinstatement responding for up to 6 months following treatment (compared with high-responding control animals).

CONCLUSIONS

These results demonstrate that viral transfer of CocH may be useful in promoting long-term resistance to relapse to cocaine addiction.

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.

Cocaine esterase-cocaine binding process and the free energy profiles by molecular dynamics and potential of mean force simulations

The combined molecular dynamics (MD) and potential of mean force (PMF) simulations have been performed to determine the free energy profiles for the binding process of (-)-cocaine interacting with wild-type cocaine esterase (CocE) and its mutants (T172R/G173Q and L119A/L169K/G173Q). According to the MD simulations, the general protein-(-)-cocaine binding mode is not affected by the mutations; e.g.. the benzoyl group of (-)-cocaine is always bound in a subsite composed of aromatic residues W151, W166, F261, and F408 and hydrophobic residue L407, while the carbonyl oxygen on the benzoyl group of (-)-cocaine is hydrogen-bonded with the oxyanion-hole residues Y44 and Y118. According to the PMF-calculated free energy profiles for the binding process, the binding free energies for (-)-cocaine with the wild-type, T172R/G173Q, and L119A/L169K/G173Q CocEs are predicted to be -6.4, -6.2, and -5.0 kcal/mol, respectively. The computational predictions are supported by experimental kinetic data, as the calculated binding free energies are in good agreement with the experimentally derived binding free energies, i.e., -7.2, -6.7, and -4.8 kcal/mol for the wild-type, T172R/G173Q, and L119A/L169K/G173Q, respectively. The reasonable agreement between the computational and experimental data suggests that the PMF simulations may be used as a valuable tool in new CocE mutant design that aims to decrease the Michaelis-Menten constant of the enzyme for (-)-cocaine.

Human butyrylcholinesterase-cocaine binding pathway and free energy profiles by molecular dynamics and potential of mean force simulations

In the present study, we have performed combined molecular dynamics and potential of mean force (PMF) simulations to determine the enzyme-substrate (ES) binding pathway and the corresponding free energy profiles for wild-type butyrylcholinesterase (BChE) binding with (-)/(+)-cocaine and for the A328W/Y332G mutant binding with (-)-cocaine. According to the PMF simulations, for each ES binding system, the substrate first binds with the enzyme at a peripheral anionic site around the entrance of the active-site gorge to form the first ES complex (ES1-like) during the binding process. Further evolution from the ES1-like complex to the nonprereactive ES complex is nearly barrierless, with a free energy barrier lower than 1.0 kcal/mol. So, the nonprereactive ES binding process should be very fast. The rate-determining step of the entire ES binding process is the subsequent evolution from the nonprereactive ES complex to the prereactive ES complex. Further accounting for the entire ES binding process, the PMF-based simulations qualitatively reproduced the relative order of the experimentally derived binding free energies (ΔG(bind)), although the simulations systematically overestimated the magnitude of the binding affinity and systematically underestimated the differences between the ΔG(bind) values. The obtained structural and energetic insights into the entire ES binding process provide a valuable base for future rational design of high-activity mutants of BChE as candidates for an enzyme therapy for cocaine overdose and abuse.

Active site gating and substrate specificity of butyrylcholinesterase and acetylcholinesterase: insights from molecular dynamics simulations

Butyrylcholinesterase (BChE) and acetylcholinesterase (AChE) are highly homologous proteins with distinct substrate preferences. In this study we compared the active sites of monomers and tetramers of human BChE and human AChE after performing molecular dynamics (MD) simulations in water-solvated systems. By comparing the conformational dynamics of gating residues of AChE and BChE, we found that the gating mechanisms of the main door of AChE and BChE are responsible for their different substrate specificities. Our simulation of the tetramers of AChE and BChE indicates that both enzymes could have two dysfunctional active sites due to their restricted accessibility to substrates. The further study on catalytic mechanisms of multiple forms of AChE and BChE would benefit from our comparison of the active sites of the monomers and tetramers of both enzymes.

Recent progress in protein drug design and discovery with a focus on novel approaches to the development of anti-cocaine medications

Cocaine is highly addictive and no anti-cocaine medication is currently available. Accelerating cocaine metabolism, producing biologically inactive metabolites, is recognized as an ideal anti-cocaine medication strategy, especially for the treatment of acute cocaine toxicity. However, currently known wild-type enzymes have either too low a catalytic efficiency against the abused cocaine, in other words (-)-cocaine, or the in vivo half-life is too short. Novel computational strategies and design approaches have been developed recently to design and discover thermostable or high-activity mutants of enzymes based on detailed structures and catalytic/inactivation mechanisms. The structure- and mechanism-based computational design efforts have led to the discovery of high-activity mutants of butyrylcholinesterase and thermostable mutants of cocaine esterase as promising anti-cocaine therapeutics. The structure- and mechanism-based computational strategies and design approaches may be used to design high-activity and/or thermostable mutants of many other proteins that have clear therapeutic potentials and to design completely new therapeutic enzymes.

Computational design of a thermostable mutant of cocaine esterase via molecular dynamics simulations

Cocaine esterase (CocE) has been known as the most efficient native enzyme for metabolizing naturally occurring cocaine. A major obstacle to the clinical application of CocE is the thermoinstability of native CocE with a half-life of only ∼11 min at physiological temperature (37 °C). It is highly desirable to develop a thermostable mutant of CocE for therapeutic treatment of cocaine overdose and addiction. To establish a structure-thermostability relationship, we carried out molecular dynamics (MD) simulations at 400 K on wild-type CocE and previously known thermostable mutants, demonstrating that the thermostability of the active form of the enzyme correlates with the fluctuation (characterized as the root-mean square deviation and root-mean square fluctuation of atomic positions) of the catalytic residues (Y44, S117, Y118, H287, and D259) in the simulated enzyme. In light of the structure-thermostability correlation, further computational modelling including MD simulations at 400 K predicted that the active site structure of the L169K mutant should be more thermostable. The prediction has been confirmed by wet experimental tests showing that the active form of the L169K mutant had a half-life of 570 min at 37 °C, which is significantly longer than those of the wild-type and previously known thermostable mutants. The encouraging outcome suggests that the high-temperature MD simulations and the structure-thermostability relationship may be considered as a valuable tool for the computational design of thermostable mutants of an enzyme.

Reaction mechanism for cocaine esterase-catalyzed hydrolyses of (+)- and (-)-cocaine: unexpected common rate-determining step

First-principles quantum mechanical/molecular mechanical free energy calculations have been performed to examine the catalytic mechanism for cocaine esterase (CocE)-catalyzed hydrolysis of (+)-cocaine in comparison with CocE-catalyzed hydrolysis of (-)-cocaine. It has been shown that the acylation of (+)-cocaine consists of nucleophilic attack of the hydroxyl group of Ser117 on the carbonyl carbon of (+)-cocaine benzoyl ester and the dissociation of (+)-cocaine benzoyl ester. The first reaction step of deacylation of (+)-cocaine, which is identical to that of (-)-cocaine, is rate-determining, indicating that CocE-catalyzed hydrolyses of (+)- and (-)-cocaine have a common rate-determining step. The computational results predict that the catalytic rate constant of CocE against (+)-cocaine should be the same as that of CocE against (-)-cocaine, in contrast with the remarkable difference between human butyrylcholinesterase-catalyzed hydrolyses of (+)- and (-)-cocaine. The prediction has been confirmed by experimental kinetic analysis on CocE-catalyzed hydrolysis of (+)-cocaine in comparison with CocE-catalyzed hydrolysis of (-)-cocaine. The determined common rate-determining step indicates that rational design of a high-activity mutant of CocE should be focused on the first reaction step of the deacylation. Furthermore, the obtained mechanistic insights into the detailed differences in the acylation between the (+)- and (-)-cocaine hydrolyses provide indirect clues for rational design of amino acid mutations that could more favorably stabilize the rate-determining transition state in the deacylation and, thus, improve the catalytic activity of CocE. This study provides a valuable mechanistic base for rational design of an improved esterase for therapeutic treatment of cocaine abuse.