Rapid stereoselective hydrolysis of (+)-cocaine in baboon plasma prevents its uptake in the brain: implications for behavioral studies

Radiosynthesis, Preclinical, and Clinical Positron Emission Tomography Studies of Carbon-11 Labeled Endogenous and Natural Exogenous Compounds

The presence of positron emission tomography (PET) centers at most major hospitals worldwide, along with the improvement of PET scanner sensitivity and the introduction of total body PET systems, has increased the interest in the PET tracer development using the short-lived radionuclides carbon-11. In the last few decades, methodological improvements and fully automated modules have allowed the development of carbon-11 tracers for clinical use. Radiolabeling natural compounds with carbon-11 by substituting one of the backbone carbons with the radionuclide has provided important information on the biochemistry of the authentic compounds and increased the understanding of their in vivo behavior in healthy and diseased states. The number of endogenous and natural compounds essential for human life is staggering, ranging from simple alcohols to vitamins and peptides. This review collates all the carbon-11 radiolabeled endogenous and natural exogenous compounds synthesised to date, including essential information on their radiochemistry methodologies and preclinical and clinical studies in healthy subjects.

Carrier-mediated cocaine transport at the blood-brain barrier as a putative mechanism in addiction liability

BACKGROUND

The rate of entry of cocaine into the brain is a critical factor that influences neuronal plasticity and the development of cocaine addiction. Until now, passive diffusion has been considered the unique mechanism known by which cocaine crosses the blood-brain barrier.

METHODS

We reassessed mechanisms of transport of cocaine at the blood-brain barrier using a human cerebral capillary endothelial cell line (hCMEC/D3) and in situ mouse carotid perfusion.

RESULTS

Both in vivo and in vitro cocaine transport studies demonstrated the coexistence of a carrier-mediated process with passive diffusion. At pharmacological exposure level, passive diffusion of cocaine accounted for only 22.5% of the total cocaine influx in mice and 5.9% in hCMEC/D3 cells, whereas the carrier-mediated influx rate was 3.4 times greater than its passive diffusion rate in vivo. The functional identification of this carrier-mediated transport demonstrated the involvement of a proton antiporter that shared the properties of the previously characterized clonidine and nicotine transporter. The functionnal characterization suggests that the solute carrier (SLC) transporters Oct (Slc22a1-3), Mate (Slc47a1) and Octn (Slc22a4-5) are not involved in the cocaine transport in vivo and in vitro. Diphenhydramine, heroin, tramadol, cocaethylene, and norcocaine all strongly inhibited cocaine transport, unlike benzoylecgonine. Trans-stimulation studies indicated that diphenhydramine, nicotine, 3,4-methylenedioxyamphetamine (ecstasy) and the cathinone compound 3,4-methylenedioxypyrovalerone (MDPV) were also substrates of the cocaine transporter.

CONCLUSIONS

Cocaine transport at the BBB involves a proton-antiporter flux that is quantitatively much more important than its passive diffusion. The molecular identification and characterization of this transporter will provide new tools to understand its role in addictive mechanisms.

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.

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.

Biochemical and molecular analysis of carboxylesterase-mediated hydrolysis of cocaine and heroin

BACKGROUND AND PURPOSE

Carboxylesterases (CEs) metabolize a wide range of xenobiotic substrates including heroin, cocaine, meperidine and the anticancer agent CPT-11. In this study, we have purified to homogeneity human liver and intestinal CEs and compared their ability with hydrolyse heroin, cocaine and CPT-11.

EXPERIMENTAL APPROACH

The hydrolysis of heroin and cocaine by recombinant human CEs was evaluated and the kinetic parameters determined. In addition, microsomal samples prepared from these tissues were subjected to chromatographic separation, and substrate hydrolysis and amounts of different CEs were determined.

KEY RESULTS

In contrast to previous reports, cocaine was not hydrolysed by the human liver CE, hCE1 (CES1), either as highly active recombinant protein or as CEs isolated from human liver or intestinal extracts. These results correlated well with computer-assisted molecular modelling studies that suggested that hydrolysis of cocaine by hCE1 (CES1), would be unlikely to occur. However, cocaine, heroin and CPT-11 were all substrates for the intestinal CE, hiCE (CES2), as determined using both the recombinant protein and the tissue fractions. Again, these data were in agreement with the modelling results.

CONCLUSIONS AND IMPLICATIONS

These results indicate that the human liver CE is unlikely to play a role in the metabolism of cocaine and that hydrolysis of this substrate by this class of enzymes is via the human intestinal protein hiCE (CES2). In addition, because no enzyme inhibition is observed at high cocaine concentrations, potentially this route of hydrolysis is important in individuals who overdose on this agent.

Structure-and-mechanism-based design and discovery of therapeutics for cocaine overdose and addiction

(-)-Cocaine is a widely abused drug and there is currently no available anti-cocaine therapeutic. Promising agents, such as anti-cocaine catalytic antibodies and high-activity mutants of human butyrylcholinesterase (BChE), for therapeutic treatment of cocaine overdose have been developed through structure-and-mechanism-based design and discovery. In particular, a unique computational design strategy based on the modeling and simulation of the rate-determining transition state has been developed and used to design and discover desirable high-activity mutants of BChE. One of the discovered high-activity mutants of BChE has a approximately 456-fold improved catalytic efficiency against (-)-cocaine. The encouraging outcome of the structure-and-mechanism-based design and discovery effort demonstrates that the unique computational design approach based on transition state modeling and simulation is promising for rational enzyme redesign and drug discovery. The general approach of the structure-and-mechanism-based design and discovery may be used to design high-activity mutants of any enzyme or catalytic antibody.

Rational design of an enzyme mutant for anti-cocaine therapeutics

(-)-Cocaine is a widely abused drug and there is no available anti-cocaine therapeutic. The disastrous medical and social consequences of cocaine addiction have made the development of an effective pharmacological treatment a high priority. An ideal anti-cocaine medication would be to accelerate (-)-cocaine metabolism producing biologically inactive metabolites. The main metabolic pathway of cocaine in body is the hydrolysis at its benzoyl ester group. Reviewed in this article is the state-of-the-art computational design of high-activity mutants of human butyrylcholinesterase (BChE) against (-)-cocaine. The computational design of BChE mutants have been based on not only the structure of the enzyme, but also the detailed catalytic mechanisms for BChE-catalyzed hydrolysis of (-)-cocaine and (+)-cocaine. Computational studies of the detailed catalytic mechanisms and the structure-and-mechanism-based computational design have been carried out through the combined use of a variety of state-of-the-art techniques of molecular modeling. By using the computational insights into the catalytic mechanisms, a recently developed unique computational design strategy based on the simulation of the rate-determining transition state has been employed to design high-activity mutants of human BChE for hydrolysis of (-)-cocaine, leading to the exciting discovery of BChE mutants with a considerably improved catalytic efficiency against (-)-cocaine. One of the discovered BChE mutants (i.e., A199S/S287G/A328W/Y332G) has a approximately 456-fold improved catalytic efficiency against (-)-cocaine. The encouraging outcome of the computational design and discovery effort demonstrates that the unique computational design approach based on the transition-state simulation is promising for rational enzyme redesign and drug discovery.

Free energy perturbation (FEP) simulation on the transition states of cocaine hydrolysis catalyzed by human butyrylcholinesterase and its mutants

A novel computational protocol based on free energy perturbation (FEP) simulations on both the free enzyme and transition state structures has been developed and tested to predict the mutation-caused shift of the free energy change from the free enzyme to the rate-determining transition state for human butyrylcholinesterase (BChE)-catalyzed hydrolysis of (-)-cocaine. The calculated shift, denoted by DeltaDeltaG(1 --> 2), of such kind of free energy change determines the catalytic efficiency (kcat/KM) change caused by the simulated mutation transforming enzyme 1 to enzyme 2. By using the FEP-based computational protocol, the DeltaDeltaG(1 --> 2) values for the mutations A328W/Y332A --> A328W/Y332G and A328W/Y332G --> A328W/Y332G/A199S were calculated to be -0.22 and -1.94 kcal/mol, respectively. The calculated DeltaDeltaG(1 --> 2) values predict that the change from the A328W/Y332A mutant to the A328W/Y332G mutant should slightly improve the catalytic efficiency and that the change from the A328W/Y332G mutant to the A328W/Y332G/A199S mutant should significantly improve the catalytic efficiency of the enzyme for the (-)-cocaine hydrolysis. The predicted catalytic efficiency increases are supported by the experimental data showing that kcat/KM = 8.5 x 10(6), 1.4 x 10(7), and 7.2 x 10(7) min(-1) M(-1) for the A328W/Y332A, A328W/Y332G, and A328W/Y332G/A199S mutants, respectively. The qualitative agreement between the computational and experimental data suggests that the FEP simulations may provide a promising protocol for rational design of high-activity mutants of an enzyme. The general computational strategy of the FEP simulation on a transition state can be used to study the effects of a mutation on the activation free energy for any enzymatic reaction.

Modeling effects of oxyanion hole on the ester hydrolysis catalyzed by human cholinesterases

Molecular dynamics (MD) simulations and hydrogen bonding energy (HBE) calculations have been performed on the prereactive enzyme-substrate complexes (ES), transition states (TS1), and intermediates (INT1) for acetylcholinesterase (AChE)-catalyzed hydrolysis of acetylcholine (ACh), butyrylcholinesterase (BChE)-catalyzed hydrolysis of ACh, and BChE-catalyzed hydrolysis of (+)/(-)-cocaine to examine the protein environmental effects on the catalytic reactions. The hydrogen bonding of cocaine with the oxyanion hole of BChE is found to be remarkably different from that of ACh with AChE/BChE. Whereas G121/G116, G122/G117, and A204/A199 of AChE/BChE all can form hydrogen bonds with ACh to stabilize the transition state during the ACh hydrolysis, BChE only uses G117 and A199 to form hydrogen bonds with cocaine. The change of the estimated total HBE from ES to TS1 is ca. -5.4/-4.4 kcal/mol for AChE/BChE-catalyzed hydrolysis of ACh and ca. -1.7/-0.8 kcal/mol for BChE-catalyzed hydrolysis of (+)/(-)-cocaine. The remarkable difference of approximately 3 to 5 kcal/mol reveals that the oxyanion hole of AChE/BChE can lower the energy barrier of the ACh hydrolysis significantly more than that of BChE for the cocaine hydrolysis. These results help to understand why the catalytic activity of AChE against ACh is considerably higher than that of BChE against cocaine and provides valuable clues on how to improve the catalytic activity of BChE against cocaine.

Stereoselective neuroimaging in vivo

Stereoselectivity is a basic property of many neuronal processes due to the spatial features of molecules involved in neurotransmission. Today, neuroimaging procedures are available for studying stereoselectivity in the living brain. Mirror-image radiotracers are the molecular tools that are used, together with single photon emission tomography (SPECT) and positron emission tomography (PET), for studying stereoselective neuronal mechanisms. This review presents the findings obtained in those studies of cholinergic, noradrenergic, dopaminergic, serotonergic, glutamatergic, opioid, cannabinoid, and second messenger neurotransmission.

Molecular dynamics simulation of cocaine binding with human butyrylcholinesterase and its mutants

Molecular dynamics (MD) simulations were carried out to study cocaine binding with wild-type human butyrylcholinesterase (BChE) and its mutants based on a recently reported X-ray crystal structure of human BChE. For each BChE-cocaine system, we simulated both the nonprereactive and prereactive complexes in water. Despite the significant difference found at the acyl binding pocket, the simulated structures confirm the fundamental structural and mechanistic insights obtained from earlier computational studies of wild-type BChE with cocaine based on a homology model, e.g. the rate-determining step for BChE-catalyzed hydrolysis of biologically active (-)-cocaine is the (-)-cocaine rotation in the active site from the nonprereactive BChE-(-)-cocaine complex to the prereactive complex. It has been demonstrated that the MD simulations on both the nonprereactive and prereactive BChE-cocaine complexes can clearly reveal whether specific mutations produce the desired BChE-(-)-cocaine binding structures in which the (-)-cocaine rotation is less hindered while the required prereactive BChE-(-)-cocaine binding is maintained. Based on the MD simulations, both A328W/Y332A and A328W/Y332G BChE's are expected to have catalytic activity for (-)-cocaine hydrolysis higher than that of wild-type BChE and the activity of A328W/Y332G BChE should be slightly higher than that of A328W/Y332A BChE due to the less-hindered (-)-cocaine rotation in the mutant BChE's. However, the less-hindered (-)-cocaine rotation is only a necessary condition for a higher activity mutant BChE. The (-)-cocaine rotation is also less hindered in A328W/Y332A/Y419S BChE, but (-)-cocaine binds with A328W/Y332A/Y419S BChE in a way that is not suitable for the catalysis. Thus, A328W/Y332A/Y419S BChE is expected to lose the catalytic activity. The computational predictions were confirmed by our experimental kinetic data, demonstrating that the MD simulation-based computational protocol used in this study is reliable in prediction of the catalytic activity of BChE mutants for (-)-cocaine hydrolysis.

Modeling evolution of hydrogen bonding and stabilization of transition states in the process of cocaine hydrolysis catalyzed by human butyrylcholinesterase

Molecular dynamics (MD) simulations and quantum mechanical/molecular mechanical (QM/MM) calculations were performed on the prereactive enzyme-substrate complex, transition states, intermediates, and product involved in the process of human butyrylcholinesterase (BChE)-catalyzed hydrolysis of (-)-cocaine. The computational results consistently reveal a unique role of the oxyanion hole (consisting of G116, G117, and A199) in BChE-catalyzed hydrolysis of cocaine, compared to acetylcholinesterase (AChE)-catalyzed hydrolysis of acetylcholine. During BChE-catalyzed hydrolysis of cocaine, only G117 has a hydrogen bond with the carbonyl oxygen (O31) of the cocaine benzoyl ester in the prereactive BChE-cocaine complex, and the NH groups of G117 and A199 are hydrogen-bonded with O31 of cocaine in all of the transition states and intermediates. Surprisingly, the NH hydrogen of G116 forms an unexpected hydrogen bond with the carboxyl group of E197 side chain and, therefore, is not available to form a hydrogen bond with O31 of cocaine in the acylation. The NH hydrogen of G116 is only partially available to form a weak hydrogen bond with O31 of cocaine in some structures involved in the deacylation. The change of the estimated hydrogen-bonding energy between the oxyanion hole and O31 of cocaine during the reaction process demonstrates how the protein environment can affect the energy barrier for each step of the BChE-catalyzed hydrolysis of cocaine. These insights concerning the effects of the oxyanion hole on the energy barriers provide valuable clues on how to rationally design BChE mutants with a higher catalytic activity for the hydrolysis of (-)-cocaine.

Catalytic mechanism and energy barriers for butyrylcholinesterase-catalyzed hydrolysis of cocaine

The geometries of the transition states, intermediates, and prereactive enzyme-substrate complex and the corresponding energy barriers have been determined by performing hybrid quantum mechanical/molecular mechanical (QM/MM) calculations on butyrylcholinesterase (BChE)-catalyzed hydrolysis of (-)- and (+)-cocaine. The energy barriers were evaluated by performing QM/MM calculations with the QM method at the MP2/6-31+G* level and the MM method using the AMBER force field. These calculations allow us to account for the protein environmental effects on the transition states and energy barriers of these enzymatic reactions, showing remarkable effects of the protein environment on intermolecular hydrogen bonding (with an oxyanion hole), which is crucial for the transition state stabilization and, therefore, on the energy barriers. The calculated energy barriers are consistent with available experimental kinetic data. The highest barrier calculated for BChE-catalyzed hydrolysis of (-)- and (+)-cocaine is associated with the third reaction step, but the energy barrier calculated for the first step is close to the highest and is so sensitive to the protein environment that the first reaction step can be rate determining for (-)-cocaine hydrolysis catalyzed by a BChE mutant. The computational results provide valuable insights into future design of BChE mutants with a higher catalytic activity for (-)-cocaine.

Computational redesign of human butyrylcholinesterase for anticocaine medication

Molecular dynamics was used to simulate the transition state for the first chemical reaction step (TS1) of cocaine hydrolysis catalyzed by human butyrylcholinesterase (BChE) and its mutants. The simulated results demonstrate that the overall hydrogen bonding between the carbonyl oxygen of (-)-cocaine benzoyl ester and the oxyanion hole of BChE in the TS1 structure for (-)-cocaine hydrolysis catalyzed by A199S/S287G/A328W/Y332G BChE should be significantly stronger than that in the TS1 structure for (-)-cocaine hydrolysis catalyzed by the WT BChE and other simulated BChE mutants. Thus, the transition-state simulations predict that A199S/S287G/A328W/Y332G mutant of BChE should have a significantly lower energy barrier for the reaction process and, therefore, a significantly higher catalytic efficiency for (-)-cocaine hydrolysis. The theoretical prediction has been confirmed by wet experimental tests showing an approximately (456 +/- 41)-fold improved catalytic efficiency of A199S/S287G/A328W/Y332G BChE against (-)-cocaine. This is a unique study to design an enzyme mutant based on transitionstate simulation. The designed BChE mutant has the highest catalytic efficiency against cocaine of all of the reported BChE mutants, demonstrating that the unique design approach based on transition-state simulation is promising for rational enzyme redesign and drug discovery.

Fundamental reaction mechanism for cocaine hydrolysis in human butyrylcholinesterase

Butyrylcholinesterase (BChE)-cocaine binding and the fundamental pathway for BChE-catalyzed hydrolysis of cocaine have been studied by molecular modeling, molecular dynamics (MD) simulations, and ab initio calculations. Modeling and simulations indicate that the structures of the prereactive BChE/substrate complexes for (-)-cocaine and (+)-cocaine are all similar to that of the corresponding prereactive BChE/butyrylcholine (BCh) complex. The overall binding of BChE with (-)-cocaine and (+)-cocaine is also similar to that proposed with butyrylthiocholine and succinyldithiocholine, i.e., (-)- or (+)-cocaine first slides down the substrate-binding gorge to bind to Trp-82 and stands vertically in the gorge between Asp-70 and Trp-82 (nonprereactive complex) and then rotates to a position in the catalytic site within a favorable distance for nucleophilic attack and hydrolysis by Ser-198 (prereactive complex). In the prereactive complex, cocaine lies horizontally at the bottom of the gorge. The fundamental catalytic hydrolysis pathway, consisting of acylation and deacylation stages similar to those for ester hydrolysis by other serine hydrolases, was proposed on the basis of the simulated prereactive complex and confirmed theoretically by ab initio reaction coordinate calculations. Both the acylation and deacylation follow a double-proton-transfer mechanism. The calculated energetic results show that within the chemical reaction process the highest energy barrier and Gibbs free energy barrier are all associated with the first step of deacylation. The calculated ratio of the rate constant (k(cat)) for the catalytic hydrolysis to that (k(0)) for the spontaneous hydrolysis is approximately 9.0 x 10(7). The estimated k(cat)/k(0) value of approximately 9.0 x 10(7) is in excellent agreement with the experimentally derived k(cat)/k(0) value of approximately 7.2 x 10(7) for (+)-cocaine, whereas it is approximately 2000 times larger than the experimentally derived k(cat)/k(0) value of approximately 4.4 x 10(4) for (-)-cocaine. All of the results suggest that the rate-determining step of the BChE-catalyzed hydrolysis of (+)-cocaine is the first step of deacylation, whereas for (-)-cocaine the change from the nonprereactive complex to the prereactive complex is rate-determining and has a Gibbs free energy barrier higher than that for the first step of deacylation by approximately 4 kcal/mol. A further analysis of the structural changes from the nonprereactive 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.

Fundamental reaction mechanism for cocaine hydrolysis in human butyrylcholinesterase

Butyrylcholinesterase (BChE)-cocaine binding and the fundamental pathway for BChE-catalyzed hydrolysis of cocaine have been studied by molecular modeling, molecular dynamics (MD) simulations, and ab initio calculations. Modeling and simulations indicate that the structures of the prereactive BChE/substrate complexes for (-)-cocaine and (+)-cocaine are all similar to that of the corresponding prereactive BChE/butyrylcholine (BCh) complex. The overall binding of BChE with (-)-cocaine and (+)-cocaine is also similar to that proposed with butyrylthiocholine and succinyldithiocholine, i.e., (-)- or (+)-cocaine first slides down the substrate-binding gorge to bind to Trp-82 and stands vertically in the gorge between Asp-70 and Trp-82 (nonprereactive complex) and then rotates to a position in the catalytic site within a favorable distance for nucleophilic attack and hydrolysis by Ser-198 (prereactive complex). In the prereactive complex, cocaine lies horizontally at the bottom of the gorge. The fundamental catalytic hydrolysis pathway, consisting of acylation and deacylation stages similar to those for ester hydrolysis by other serine hydrolases, was proposed on the basis of the simulated prereactive complex and confirmed theoretically by ab initio reaction coordinate calculations. Both the acylation and deacylation follow a double-proton-transfer mechanism. The calculated energetic results show that within the chemical reaction process the highest energy barrier and Gibbs free energy barrier are all associated with the first step of deacylation. The calculated ratio of the rate constant (k(cat)) for the catalytic hydrolysis to that (k(0)) for the spontaneous hydrolysis is approximately 9.0 x 10(7). The estimated k(cat)/k(0) value of approximately 9.0 x 10(7) is in excellent agreement with the experimentally derived k(cat)/k(0) value of approximately 7.2 x 10(7) for (+)-cocaine, whereas it is approximately 2000 times larger than the experimentally derived k(cat)/k(0) value of approximately 4.4 x 10(4) for (-)-cocaine. All of the results suggest that the rate-determining step of the BChE-catalyzed hydrolysis of (+)-cocaine is the first step of deacylation, whereas for (-)-cocaine the change from the nonprereactive complex to the prereactive complex is rate-determining and has a Gibbs free energy barrier higher than that for the first step of deacylation by approximately 4 kcal/mol. A further analysis of the structural changes from the nonprereactive 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.

Anticocaine catalytic antibodies

Cocaine mediates its reinforcing and toxic actions through a "loss of function" effect at multiple receptors. The difficulties inherent in blocking a pleiotropic blocker pose a great obstacle for the classical receptor-antagonist approach and have contributed to the failure (to date) to devise specific treatments for cocaine overdose and addiction. As an alternative, we have embarked on an investigation of catalytic antibodies, a programmable class of artificial enzyme, as "peripheral blockers" -- agents designed to bind and degrade cocaine in the circulation before it partitions into the central nervous system to exert reinforcing or toxic effects. We synthesized transition-state analogs of cocaine's hydrolysis at its benzoyl ester, immunized mice, prepared hybridomas and developed the first anticocaine catalytic antibodies with the capacity to degrade cocaine to nonreinforcing, nontoxic products. We subsequently identified several families of anticocaine catalytic antibodies and found that the most potent antibody possessed sufficient activity to block cocaine-induced reinforcement, organ dysfunction and sudden death in rodent models of addiction, toxicity and overdose, respectively. With the potential to promote cessation of use, prolong abstinence and provide a treatment for acute overdose, the artificial enzyme approach comprehensively responds to the problem of cocaine.

[(11)]Cocaine: PET studies of cocaine pharmacokinetics, dopamine transporter availability and dopamine transporter occupancy

Cocaine was initially labeled with carbon-11 in order to track the distribution and pharmacokinetics of this powerful stimulant and drug of abuse in the human brain and body. It was soon discovered that [(11)C]cocaine was not only useful for measuring cocaine pharmacokinetics and its relationship to behavior but that it is also a sensitive radiotracer for dopamine transporter (DAT) availability. Measures of DAT availability were facilitated by the development of a graphical analysis method (Logan Plot) for reversible systems which streamlined kinetic analysis. This expanded the applications of [(11)C]cocaine to studies of DAT availability in the human brain and allowed the first comparative measures of the degree of DAT occupancy by cocaine and another stimulant drug methylphenidate. This article will summarize preclinical and clinical research with [(11)C]cocaine.

Two-carbon bridge substituted cocaines: enantioselective synthesis, attribution of the absolute configuration and biological activity of novel 6- and 7-methoxylated cocaines

In an effort to learn more about the general structure-activity relationships of cocaine with the aim to elucidate those structural features that might confer antagonistic properties to such analogues, we describe herein our synthetic efforts to prepare two-carbon bridge functionalized (methoxylated and hydroxylated) analogues. Our approach makes use of a modification of the classical Willstatter synthesis of cocaine: Mannich type cyclization of acetonedicarboxylic acid monomethyl ester with methylamine hydrochloride and 2-methoxysuccindialdehyde in a citrate buffer solution afforded the 6- and 7-substituted 2-carbomethoxy-3-tropinones 3a,b and 4a,b in approximate yields of 64%. Reduction of the (+/-)-tropinone derivatives was performed with sodium amalgam in a sulfuric acid solution to afford a mixture of (+/-)-methoxyecgonine and (+/-)-methoxypseudoecgonine derivatives 5, 11 and 6, 7, 12, 13. Benzoylation of these alcohols yielded the desired cocaine and pseudococaine-like compounds 8, 14 and 9, 10, 15, 16. Additionally, we show that enzymatic hydrolysis of these cocaine analogues using pig liver esterase (PLE) affords a practical means for achieving their chemical resolution. The enantiomers of the methoxycocaine analogues were also prepared starting from chiral (+)- and (-)-6-methoxytropinone. All new analogues were examined for their ability to displace [3H]mazindol binding and to inhibit high-affinity uptake of [3H]dopamine into striatal nerve ending (synaptosomes). It appeared evident that methoxylation of the cocaine two-carbon bridge provides compounds of particular interest: the Ki for the binding of the methoxypseudococaines is about two to four times smaller than the Ki for inhibition of dopamine uptake, thus enabling these compounds capable of countering the effects of cocaine to some extent.

Role of cholinergic receptors and cholinesterase activity in hemodynamic responses to cocaine in conscious rats

It has been suggested that toxicity to cocaine is related to the relative rate of cocaine metabolism by cholinesterases and to activation of cholinergic receptors either directly or by reflex mechanisms. We examined these possibilities by altering cholinesterase activity and blocking cholinergic receptors in rats prone or resistant to cocaine-induced cardiovascular toxicity. Rats were instrumented with a pulsed Doppler flow probe on the ascending aorta for measurement of cardiac output and cannulated for arterial pressure and heart rate determination. In conscious rats, cocaine (5 mg/kg iv) elicited pressor responses and a delayed bradycardia but cardiac output and systemic vascular resistance responses varied greatly between rats. Pretreatment with the nonspecific cholinesterase inhibitors physostigmine (0.1-0.2 mg/kg) or neostigmine (0.1 mg/kg) reduced the pressor response by diminishing the increase in systemic vascular resistance. In contrast, inhibition of cocaine metabolism with the selective plasma cholinesterase inhibitor tetraisopropyl pyrophosphoramide (0.5 mg/kg) or increasing cholinesterase activity with human butyryl cholinesterase (9.9 mg/kg iv) did not alter hemodynamic responses to cocaine. Administration of atropine methyl bromide (0.5-1 mg/kg iv) alone or with physostigmine to prevent the cholinomimetic effects of physostigmine reduced the cocaine-induced decrease in cardiac output noted in some animals. These data suggest that the cocaine-induced decrease in cardiac output observed in some rats is, at least in part, dependent on activation of muscarinic receptors. In addition, the rate of cocaine metabolism is not critical for the initial hemodynamic responses to cocaine in conscious rats.

Stereoselective inhibition of human butyrylcholinesterase by phosphonothiolate analogs of (+)- and (-)-cocaine

The hydrolysis of cocaine (benzoylecgonine methyl ester) to ecgonine methyl ester by human butyrylcholinesterase (BuChE; EC 3.1.1.8) has been shown previously to constitute an important means to detoxicate this material to pharmacologically inactive metabolites. The naturally occurring (-)-cocaine is hydrolyzed to ecgonine methyl ester approximately 2000 times slower than the unnatural (+)-cocaine isomer. In good agreement with previous studies, (-)-cocaine bound to human BuChE with relatively good affinity and competitively inhibited the hydrolysis of the spectrophotometric substrate butyrylthiocholine with a Ki value of 8.0 microM. Similarly, (+)-cocaine also showed relatively high affinity for the human BuChE and competitively inhibited butyrylthiocholine hydrolysis with a Ki value of 5.4 microM. The phosphonothiolates corresponding to the transition state analogs for both (-)- and (+)-cocaine hydrolysis were synthesized and tested as inhibitors of human BuChE-catalyzed hydrolysis of butyrylthiocholine. The phosphonothiolate corresponding to the transition state for (-)-cocaine hydrolysis was a competitive inhibitor with a Ki value of 55.8 microM. The phosphonothiolate corresponding to the transition state for (+)-cocaine hydrolysis gave a Ki value of 25.9 microM, but, in addition, it also showed irreversible inhibition with a ki of inactivation of 68.8 min-1 M-1. It is likely that the mechanism-based inhibitor described herein may find use as a mechanistic probe of butyrylcholinesterase action and also possibly aid in the purification of this class of esterases.

Ecgonine methyl ester, a major cocaine metabolite, causes cerebral vasodilation in neonatal sheep

Maternal cocaine abuse has been associated with fetal and neonatal neurologic abnormalities, including hemorrhagic cerebral infarctions, but the mechanisms for cocaine's cerebral effects are unknown. We previously showed that acute cocaine injection causes cerebral vasodilation in cats and immature sheep; others have shown that cocaine causes cerebral vasoconstriction in piglets and in pressurized neonatal sheep arteries. Although methodologic and species differences may explain these conflicting results, we tested another possibility; that is, that ecgonine methyl ester (EME), a major cocaine metabolite in sheep, causes cerebral vasodilation and may account, in part, for cocaine's vascular effects. We studied the cerebral effects of a single i.v. injection of EME (2.5 mg/kg) in eight chronically catheterized, unanesthetized neonatal sheep (4 +/- 2 d old). We measured cerebral hemisphere blood flow (CBF) using radiolabeled microspheres, mean arterial pressure, heart rate, and arteriovenous oxygen content, and we calculated cerebral oxygen consumption (CMRo2) and cerebrovascular resistance at baseline and 0.5, 2, 5, and 60 min after EME injection. EME injection had no systemic effects, including no changes in mean arterial pressure, heart rate, or arterial blood gases. Within 0.5 min of injection, EME caused a 21% decrease in cerebrovascular resistance, which remained decreased for 60 min. CBF increased by 20% at 0.5, 2, and 5 min. Blood flow to brain regions other than the cerebral hemispheres paralleled changes in CBF, with cerebellar flow remaining increased at 60 min. There was no change in CMRo2. There was a small, but physiologically insignificant, decrease in arterial oxygen content. We conclude that EME causes cerebral vasodilation in neonatal sheep and may account, in part, for cocaine's cerebral vascular effects.

Anti-cocaine catalytic antibodies--a novel approach to the problem of addiction

Cocaine reinforces its self-administration in relation to the magnitude of and rate of rise to the peak serum concentration of the drug. Catalytic antibodies are artificial enzymes which could reduce serum cocaine concentrations, deprive the abuser of cocaine's reinforcing effect and thus favor extinction of the addiction. Catalytic antibodies are elicited by immunization with a stable analog of a transition-state for a chemical reaction. Through our new method for synthesizing phosphonate monoesters, we constructed several phosphonate-based transition-state analogs of cocaine hydrolysis. Using these analogs, monoclonal antibodies were elicited and, thus far, nine anti-analog antibodies with hydrolytic activity against cocaine have been identified, cloned and studied. The activity of one of these antibodies, 15A10, is sufficient to commence preclinical studies.

Sensitivity of striatal [11C]cocaine binding to decreases in synaptic dopamine

We have previously shown that tracer concentrations of [11C]cocaine binding to the dopamine transporter (DAT) in human and baboon striatum can be visualized using positron emission tomography (PET). To determine whether the concentration of dopamine normally present in the synaptic cleft can compete with [11C]cocaine for transporter binding sites, we conducted baboon PET studies with drugs (sodium 4-hydroxybutyrate, four studies, 200 mg/kg; gamma-vinylGABA, three studies, 300 mg/kg; and citalopram, three studies, 2 mg/kg) expected to decrease synaptic dopamine. Each study involved two [11C]cocaine injections and PET scans separated by 2-4 h, with drug administration after the first injection, and without movement of the subject between scans. Time-activity data from striatum and from cerebellum were used with the arterial plasma input function to determine graphically by Logan plotting [11C]cocaine distribution volumes for the brain regions. Specific binding of [11C]cocaine to DAT in striatum was calculated as the distribution volume ratio (DVR) for striatum and cerebellum. In nine of the ten studies drug treatment produced a small increase in DVR (range, 1-11%), and in seven of these studies the increase was > or = 7%. The mean increase was 6.2 +/- 4.1%. The reproducibility of the DVR measure was assessed by comparing [11C]cocaine studies conducted without pharmacological treatments using individual baboons on separate days, and thus involving possible repositioning errors, as well as long-term changes in the state of the striatal dopamine system.(ABSTRACT TRUNCATED AT 250 WORDS)

Binding of bromine-substituted analogs of methylphenidate to monoamine transporters

We synthesized the o-, m- and p-bromo derivatives of dl-threo-methylphenidate from the corresponding bromophenylacetonitriles by modification of the literature synthesis of methylphenidate (Panizzon, Helv. Chim. Acta 1944, 27, 1748). In in vitro binding assays all three dl-threo bromo compounds had higher affinities than methylphenidate for dopamine transporter sites labeled with [3H]2 beta-carbomethoxy-3 beta-(4-fluorophenyl)tropane ([3H]WIN 35,428; IC50 = 13, 4, 20 and 82 nM for o-, m-, and p-bromo compounds, and unsubstituted methylphenidate, respectively). They also bound more strongly than methylphenidate to norepinephrine reuptake sites labeled with [3H]nisoxetine (IC50 = 32, 20, 31 and 440 nM, respectively), but were weak ligands (IC50 > or = 1 microM) at the serotonin transporter labeled with [3H]paroxetine. In addition, the bromine substituted derivatives demonstrated similar activity to methylphenidate in vivo in rodents in terms of inhibition of heart uptake of [3H](-)-norepinephrine, elevation of striatal extracellular dopamine, and stimulation of locomotor activity.

Studies with differentially labeled [11C]cocaine, [11C]norcocaine, [11C]benzoylecgonine, and [11C]- and 4'-[18F]fluorococaine to probe the extent to which [11C]cocaine metabolites contribute to PET images of the baboon brain

The psychostimulant drug of abuse, cocaine (benzoylecgonine methyl ester), is rapidly metabolized by cleavage of its two ester groups, to give benzoylecgonine (BE) and ecgonine methyl ester, and by N-demethylation, to give N-norcocaine (NC). The recent use of [N-methyl-11CH3]cocaine to image brain cocaine binding sites with positron emission tomography (PET) raises the question of whether PET images partially reflect the distribution and kinetics of labeled cocaine metabolites. We prepared [O-methyl-11CH3]cocaine by methylation of the sodium salt of BE with [11C]CH3I, and showed that PET baboon brain scans, as well as regional brain kinetics and plasma time-activity curves corrected for the presence of labeled metabolites, are nearly identical to those seen with [N-methyl-11CH3]cocaine. This strongly suggests that 11C metabolites do not significantly affect PET images, because the metabolite pattern is different for the two labeled forms of cocaine. In particular, nearly half the 11C in blood plasma at 30 min was [11C]CO2 when [N-methyl-11CH3]cocaine was administered, whereas [11C]CO2 was not formed from [O-methyl-11CH3]cocaine. Only a trace of [11C]NC was detected in plasma after [O-methyl-11CH3]cocaine administration. Nearly identical brain PET data were also obtained when 4'-[N-methyl-11CH3]fluorococaine and 4'-[18F]fluorococaine (prepared by nucleophilic aromatic substitution from [18F]fluoride- and 4'-nitrococaine) were compared with [N-methyl-11CH3]cocaine. In vitro assays with rat brain membranes showed that cocaine and 4'-fluorococaine were equipotent at the dopamine reuptake site, but that 4'-fluorococaine was about 100 times more potent at the 5-hydroxytryptamine reuptake site.(ABSTRACT TRUNCATED AT 250 WORDS)

Evaluation of the monoamine uptake site ligand [123I]methyl 3 beta-(4-iodophenyl)-tropane-2 beta-carboxylate ([123I]beta-CIT) in non-human primates: pharmacokinetics, biodistribution and SPECT brain imaging coregistered with MRI

The in vivo properties of a new radioiodinated probe of the dopamine and serotonin transporter, [123I]methyl 3 beta-(4-iodophenyl)tropane-2 beta-carboxylate ([123I]beta-CIT) were evaluated in baboons and vervet monkeys. The labeled product was prepared in 65.2 +/- 2.8% yield (mean +/- SEM; n = 18) by reaction of the tributylstannyl precursor with [123I]NaI in the presence of peracetic acid followed by high pressure liquid chromatography (HPLC) purification to give a product with radiochemical purity of 97.5 +/- 0.5% and specific activity of 500-1200 Ci/mmol. After intravenous administration, whole brain activity peaked at 6-10% injected dose within 1 h post injection (p.i.) and washed out in a biphasic manner with clearance half-lives of 1-2 and 7-35 h for the rapid and slow components, respectively. Excretion occurred primarily through the hepatobiliary route, with about 30% of the injected dose appearing in the GI tract after 5 h. Estimates of radiation absorbed dose gave 0.01, 0.1, 0.2 and 0.03 mGy/MBq to the brain, gall bladder wall, lower large intestine wall and urinary bladder wall, respectively. High resolution SPECT imaging in a baboon demonstrated high uptake of tracer in the region of the striatum (striatum:cerebellum ratio 4.0), in the hypothalamus (ratio 2.6) and in a midbrain region comprising raphe, substantia nigra and superior colliculus (ratio 2.0), with regional brain uptakes measured at 210 min p.i. of [123I]beta-CIT. The anatomical locations of the regions on the SPECT image were confirmed by coregistration with MRI. Plasma metabolites and pharmacokinetics were analyzed in baboons and vervets by ethyl acetate extraction and HPLC. The major metabolite was a polar, non-extractable fraction, which increased to > 50% of the plasma activity by 30-45 min p.i. A minor lipophilic (extractable) metabolite was also observed, increasing to about 4% at 2-3 h p.i. The plasma protein bound fraction, determined by ultrafiltration, was 74.8 +/- 1.4% (n = 6). The arterial input function was characterized by the sum of three exponential terms with half-lives of 0.3-1.7, 9.7-24.9 and 77-166 min, respectively, for the concentration of free parent compound. [123I]beta-CIT promises to be a useful marker for SPECT study of the monoamine uptake system in primate brain.

Differential toxicity of cocaine and its isomers, (+)-cocaine and (-)-psi-cocaine, is associated with stereoselective hydrolysis by hepatic carboxylesterases in cultured rat hepatocytes

Cocaine induces acute lethal cell injury in rat hepatocytes following N-oxidative metabolic activation by cytochrome P450-dependent and flavin-dependent monooxygenases. Beside this oxidative bioactivation pathway, hepatic carboxylesterases may cleave the carboxymethylester or the benzoylester linkage which leads to molecules found to be non-toxic in vivo. To elucidate the structural requirements of the cocaine molecule for its bioactivation and inactivation, the cytotoxic potential of the natural (-)-cocaine relative to two isomeric forms, (+)-cocaine* (the unnatural enantiomer) and (-)-psi-cocaine (the C2 epimer of the unnatural cocaine) were investigated. Primary short-term cultures of rat hepatocytes obtained from phenobarbital (PB)-pretreated rats were exposed to the drugs for up to 24 h. (-)-Cocaine produced marked time- and concentration-dependent release of lactate dehydrogenase (LDH) into the extracellular medium, whereas the other forms were not cytotoxic (0-1 mM). Furthermore, depletion of cellular glutathione (GSH) with diethylmaleate enhanced LDH release in (-)-cocaine-treated cells and caused marginal cytotoxicity in hepatocytes exposed to the other isomers. To investigate the mechanisms that could be responsible for these isomer-specific effects, the time-dependent metabolic degradation was determined both in cultured hepatocytes and in hepatic microsomes in the presence or absence of the serine carboxylesterase inhibitors, phenylmethylsulfonylfluoride (PMSF) or NaF. All three cocaine analogs were enzymatically degraded, but the rates of ester cleavage greatly varied among the stereoisomers. (-)-Cocaine was primarily N-oxidized via SKF-525A-sensitive pathways, whereas (+)-cocaine was predominantly hydrolyzed by PMSF-sensitive carboxylesterases. In contrast, (-)-psi-cocaine, which is very stable in the absence of cells at 37 degrees C and pH 7.4, was subject to extremely fast enzymatic ester cleavage. In conclusion, these results indicate that the isomer-specific differential cytotoxicity of (-)-cocaine, (+)-cocaine and (-)-psi-cocaine in hepatocytes may be related to stereoselective differences in the rates of hydrolytic inactivation by hepatic carboxylesterases and that the N-oxidative pathway, resulting in hepatocyte injury, may thus be relevant only for (-)-cocaine.

Comparative PET studies of the kinetics and distribution of cocaine and cocaethylene in baboon brain

Recent studies have suggested that cocaethylene, an active metabolite of cocaine found in blood and postmortem brain of individuals self-administering cocaine and alcohol, may play a role in the increased toxicity seen when coadministering these 2 drugs. We have used positron emission tomography (PET) and carbon-11 (t1/2:20.4 min) labeled cocaine and cocaethylene to compare the short-term kinetics of cocaine and cocaethylene in baboon brain. The regional uptake of [11C]cocaine cocaethylene in baboon brain. The regional uptake of [11C]cocaine ([11C]COC) and [11C]cocaethylene ([11C]CE), 5-8 mCi and 4-6 micrograms, in baboon brain (n = 7) were similar but clearance from whole brain (global, GL) and from striatum (SR), thalamus (TH), and cerebellum (CB) was slower for cocaethylene. Steady-state distribution volumes (DV) were not significantly different in the striatum but were greater for cocaethylene in the thalamus, cerebellum, and whole brain. Debenzoylation of cocaethylene proceeded at about one-third the rate of cocaine, as determined by in vitro incubation of labeled cocaethylene and labeled cocaine with baboon plasma and with purified horse butyryl-cholinesterase (EC 3.1.1.8). Even though the slower clearance of cocaethylene could lead to longer tissue exposures and potentially accentuated or different physiological effects relative to cocaine, the difference between the 2 drugs is not large. Thus it is more likely that the direct actions of cocaine and alcohol on some organs, rather than cocaethylene, account for this enhanced toxicity.

Stereoselective enzymatic hydrolysis of various ester prodrugs of ibuprofen and flurbiprofen in human plasma

The hydrolysis kinetics of various alkyl, glycolamide, aminoethyl, and 2-(1-imidazolyl)ethyl esters of ibuprofen and flurbiprofen in 80% human plasma were investigated using a direct high-performance liquid chromatographic assay for the enantiomers of these acids. In each case, the R-isomer ester was found to undergo faster plasma-catalyzed hydrolysis than the corresponding S-isomer. The difference in the hydrolysis rates between the enantiomeric forms ranged from a factor of 1.4 for the N,N-diethylglycolamide ester of ibuprofen to a factor of 50 and 25 for the 2-(1-imidazolyl)ethyl ester of ibuprofen and flurbiprofen, respectively. Therefore, enantioselective differences in plasma-catalyzed ester prodrug hydrolysis must be taken into account when evaluating prodrugs of racemic mixtures of chiral drugs.

Determination of the enantiomeric composition of samples of cocaine by normal-phase high-performance liquid chromatography with UV detection

A high-performance liquid chromatographic method has been developed for the quantitation of the enantiomers of cocaine. Mixtures of the naturally occurring (-)-cocaine and synthetically produced (+)-cocaine were hydrolyzed in water to (+) and (-)-benzoyl ecgonine. Esterification with an optically pure 2-octanol resulted in diastereoisomers that could be separated on bare silica gel using an acetonitrile-aqueous ammonium phosphate mobile phase.

Activities of the enantiomers of cocaine and some related compounds as substrates and inhibitors of plasma butyrylcholinesterase

The behaviors of the enantiomers of cocaine (benzoylecgonine methyl ester) and related compounds with butyrylcholinesterase (BChE; EC 3.1.1.8) were investigated spectrophotometrically at 235 nm. The unnatural enantiomer, (+)-cocaine, was hydrolyzed by BChE (extinction coefficient 6.7 L.mmol-1.cm-1) at about half the rate of benzoylcholine, but over 2000 times faster than naturally occurring (-)-cocaine. This rapid hydrolysis of (+)-cocaine may account, in part, for its pharmacological inactivity. (+)-Norcocaine, (+)-benzoylecgonine, (-)-psi-cocaine and tropacocaine were also substrates for BChE. Hydrolysis of (+)-cocaine was sensitive to several standard inhibitors of BChE, including those of competitive, carbamate and organophosphorus classes. Although (-)-cocaine was a poor substrate for debenzoylation, it was a fairly good competitive inhibitor (Ki approximately 10 microM) of the hydrolysis of other substrates. The cocaine metabolites (-)-norcocaine, (-)-benzoylecgonine and (-)-ecgonine methyl ester inhibited BChE with Ki values of 15, 76 and 1300 microM, respectively. (+)-psi-Cocaine had Ki = 3 microM, p-Nitro and p-fluoro derivatives of cocaine and analogs with phenyl and p-fluorophenyl groups in place of the benzoyl ester linkage (WIN 35,065-2 and WIN 35,428) inhibited BChE comparably to (-)-cocaine itself. Both cocaine enantiomers were weak inhibitors of acetylcholinesterase (AChE; EC 3.1.1.7) from human erythrocytes with similar Ki values (160-170 microM). Although it is unlikely that the inhibition of BChE is an important factor in the subjective effects of cocaine, it may have implications for the toxicity of cocaine to the fetus, since BChE appears in the development of the central nervous system before AChE, and has been suggested to function as an embryonic acetylcholinesterase.