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A clinically effective blocker of cocaine-induced reinforcement does not exist despite decades of effort. As an alternative to therapeutic approaches based on the pharmacology of the cocaine receptor, the delivery of cocaine to its receptor could be interrupted so that a dose of cocaine no longer had a reinforcing behavioral effect. Since there is no prospect for excluding cocaine from the circulation, this approach would require binding of cocaine by a circulating agent.
In the 1970's Schuster and colleagues investigated an immunologic approach1 to substance abuse based on this possibility of interference with CNS delivery. A rhesus monkey was allowed to self-administer heroin to dependence, and then was immunized to an opiate. Despite access to the heroin, the animal no longer self-administered it. The serum anti-opiate antibody titer greatly exceeded the cerebrospinal fluid titer and this localized the antibody effect to the serum. Thus, the association of heroin and circulating anti-heroin antibody must have been sufficiently rapid to block heroin's effect. However, the limitation of the approach was identified in that continued administration of very high doses of heroin exhausted the pool of circulating antibody and the animal resumed heroin self-administration. An antibody would ideally have the characteristics of an enzyme in order to avoid being stoichiometrically "depleted" itself as it depleted its target. The application of such a degradative enzyme to the problem of chronic cocaine abuse would be to deprive the abuser of the reinforcing effect of the drug and thus promote extinction of the addiction. Between binges cocaine addicts often to seek assistance but the relapse rate is > 50% and the proposed treatment could provide a window for appropriate psychosocial and relapse prevention interventions. With the potential to promote cessation of use, prolong abstinenceand provide a treatment for acute overdose, the artificial enzyme approach comprehensively responds to the problem of cocaine.
Our approach relies on the creation of an artificial enzyme to degrade cocaine that is based on the exciting development of catalytic antibodies2,3.Catalytic antibodies not only bind but also act as artificial enzymes which metabolize their target thus freeing the antibody for further binding. The principles of this startling advance are illustrated by considering the hydrolysis of a carboxylic acid ester by an enzyme:
Hydrolysis of the planar ester commonly proceeds through a tetrahedral intermediate which decomposes to yield alcohol and planar carboxylic acid. The rate of the reaction varies with the magnitude of the activation barrier (DG)between the starting ester and the peak or transition-state structure. An enzyme's active site contains a pocket that complements the structure of the hydrolysis transition-state and through various binding interactions the enzyme stabilizes the transition-state relative to the starting material. This differential stabilization decreases DG and contributes to catalysis. The transition state corresponds to a particular configuration of atoms and is thought to resemble the definable species closest to it in energy, i.e. the tetrahedral intermediate in the case of ester hydrolysis. The transition state is unstable and evanescent but phosphonate monoesters are stable compounds which resemble this species in geometry and distribution of charge and on this basis may serve as transition state analogs. An antibody elicited to such an analog will manifest binding interactions complementary to the hydrolysis transition state being modeled. This antibody, by binding to the modeled substrate, will stabilize the transition state relative to the starting state, lower the activation barrier and catalyze the hydrolysis. By binding and destroying its target the catalytic antibody is then freed to bind additional target.
Of all the commonly abused substances, cocaine is the best candidate for this approach. Attached to the ecgonine nucleus of cocaine is a benzoyl ester group which when hydrolyzed results in a virtually inactive product4 - this is one of the pathways of deactivating metabolism in humans. The transition state of that reaction resembles the tetrahedral intermediate of hydrolysis and can be mimicked by a suitably designed phosphonate ester:
A subpopulation of the antibodies elicited by this cocaine analog will function as esterases highly specific for cocaine. Thus, the principal impediment to the immunologic approach suggested two decades earlier - the exhaustibility of the circulating antibody - could be overcome. The anti-cocaine catalytic antibody generated in this fashion would destroy cocaine and be itself available for continued function. The application of such a reagent antibody to the problem of chronic cocaine abuse would be to deprive the abuser of the reinforcing effect of the drug, thereby providing a window of opportunity for appropriate psychosocial and relapse prevention interventions, and promoting extinction of the addiction.
Based on this analysis, we synthesized a transition-state analog of cocaine hydrolysis, immunized mice and prepared hybridomas. The hybridomas were initially selected based on anti-analog binding and then sub-selected with the capacity to degrade cocaine. In this manner, the first artificial cocaine esterases were identified5. Using several transition-state analogs with varying tether sites that exposed unique epitopes to the mouse immune system, we obtained a total of nine anti-cocaine catalytic antibodies6. We recently found that one of these antibodies (Mab 15A10) was sufficiently active to block cocaine-induced reinforcement and toxicity in animal models of addiction and overdose7. Present efforts are focused on improving the antibodies' activity through second generation analogs and through mutagenesis of our most potent catalytic antibody.
References
1. Bones, K.F., B.H. Wainer, F.W. Fitch, R.M. Rothbert and C.R. Schuster. Changes in heroin self-administration by a rhesus monkey after morphineimmunization. Nature 252:708-718, 1074.
2. Pollack, S.J., J.W. Jacobs and P.G. Schultz. Selective chemical catalysis by an antibody. Science 234:1570-1574,1986.
3. Benkovic, S.J., A.D. Napper and R.A. Lerner. Catalysis of a stereospecific bimolecular amide synthesis by an antibody. Proc. Natl. Acad. Sci. USA 85:5355-5358,1988.
4. Misra, A.L., P.K. Nayak, R. Bloch, and S.J. Mule. Estimation and disposition of [3H]benzoyecgonine and pharmacological activity of some cocaine metabolites.J. Pharm. Pharmacol. 27:784-786,1975.
5. Landry, D.W., M. Akabas, C. Redhead, A. Edelman, E. Cragoe, Q. Al-Awqati. Purification and reconstitution of chloride channels from kidney and trachea. Science 255:1469-1472, 1989
6. Yang, G., J. Chun, H. Arakawa-Uramoto, M.A. Gawinowicz, K. Zhao, and D.W. Landry. Anti-cocaine catalytic antibodies: A synthetic solution to improved diversity. J. Am. Chem. Soc. 118(25):5881-5890,1996.
7. Mets, B., G. Winger, C. Cabrera, S. Seo, S. Jamdar, G. Yang, K. Zhao, R.J. Breiscoe, J.H. Woods and D.W. Landry. A catalytic antibody against cocaine prevents cocaine's reinforcing and toxic effects in rats. Proc. Natl. Acad. Sci. USA 95:10176-10181,1998.