Schizophrenia is a chronic, severe, and disabling brain disease. Nearly 1 percent of the population develops schizophrenia during their lifetime - more than 2 million Americans suffer from this disease in a given year. The schizophrenia world market generated total revenues of nearly $12 billion in 2004 and is still growing steadily. Atypical antipsychotic drugs, such as clozapine, risperidone, and olanzapine are widely used to treat schizophrenia but are not fully effective in moderating the positive, negative and cognitive symptoms that different patients present. These drugs act primarily via antagonism of dopamine and serotonin receptors. However, a substantial literature supports the involvement of glutamatergic circuits in mediating the symptoms of schizophrenia in humans and animal models of this disorder. As a result, glutamate receptor ligands are under active analysis as potential alternative and adjuvant therapy for the treatment of this disorder. Among these are activators of the glycine site on the NMDA receptor and group II metabotropic glutamate receptor (mGluR) agonists. N-Acetylaspartylglutamate (NAAG) is the third most prevalent transmitter in the mammalian nervous system and a group II mGluR (mGluR3>>mGluR2) selective agonist. It is inactivated by extracellular peptidases (GCPII and III) to N-acetylaspartate (NAA) and glutamate (Glu) following the release of the peptide into the synaptic space. Inhibition of GCPII and III increases NAAG levels with the consequent activation of presynaptic group II mGluRs. Group II mGluR activation inhibits the release of glutamate and reduces the schizophrenia-like behavioral symptoms elicited by phencyclidine (PCP). Our research team demonstrated that NAAG peptidase inhibition and subsequent group II mGluR activation by NAAG also reduces these behaviors and thus potentially represents a new therapeutic approach to schizophrenia and acute PCP intoxication. After extensive SAR studies, our research team has identified a number of urea-based compounds, including ZJ 43, ZJ 11 and ZJ 17, as NAAG peptidase inhibitors with nanomolar potency. Systemic administration of the NAAG peptidase inhibitor ZJ 43 reduces PCP-induced behaviors in a rat model of schizophrenia, and this action of ZJ 43 is blocked by the group II mGluR antagonist LY341495. However, there exists an important concern that these compounds are too polar to readily penetrate the blood-brain barrier (BBB) and to advance to human clinical studies. The development of prodrugs will provide the means for precise dose delivery to the brain to improve their efficacy as well as the use of lower dose ranges that obviate the problem of unwanted systemic side effects. The long-term goal of this research project is to develop these NAAG peptidase inhibitors as novel therapeutics or adjuvant therapies for schizophrenia. In order to accelerate the development and application of these compounds for human clinical trials, the immediate goal of this research proposal is to develop novel prodrugs to improve the BBB penetration capabilities of our candidate NAAG peptidase inhibitors to improve their efficacy. It is noteworthy that our very recent preliminary data have demonstrated that one mono-ester prodrug of ZJ 43 is about 7-fold more active than the parent drug ZJ 43 in our PCP-induced behavioral model of schizophrenia. These findings encouraged us to pursue further studies on the design, synthesis and pharmacological investigation of prodrugs of our NAAG peptidase inhibitors. Through funding from this SBIR Phase I grant, we intend to bring our discovery of the efficacy of NAAG peptidase inhibitors in animal models of schizophrenia to a higher level of preclinical development and ultimately to foster the translation of this concept into clinical trials.
Specific Aim 1 : Synthesis of compounds: Rational design and synthesis of new prodrug forms of lead NAAG peptidase inhibitors. Based upon the structures of our NAAG peptidase inhibitors and the successful results achieved for some commercial prodrugs, thirteen prodrugs including mono-ester and -amide prodrugs of the current best drug candidates ZJ 43, ZJ 11, and ZJ 17 will be synthesized for the testing described in Aim 2. In addition, 1.0 gram each of the NAAG peptidase inhibtors ZJ 43, ZJ 11, and ZJ 17 will be prepared as reference compounds for the purpose of efficacy comparison during the studies of their prodrugs.
Specific Aim 2 : Characterization of the efficacy of the above prodrugs of NAAG peptidase inhibitors in vivo, including PCP-induced behavioral models of schizophrenia (Aim 2.1); For the behaviorally most effective prodrugs: definition of the level of NAAG peptidase inhibition that they produce in vivo (Aim 2.2); Direct measurement of the levels of the prodrugs and active inhibitors in the brain (Aim 2.3); Determination if the prodrugs have any inherent activity as peptidase inhibitors or group II mGluR agonists (Aim 2.4).
Schizophrenia is a chronic, severe, and disabling brain disease. Through funding from this two-year SBIR Phase I grant, we intend to bring our discovery of the efficacy of NAAG peptidase inhibitors and their novel prodrugs in animal models of schizophrenia to a higher level of preclinical development and ultimately to foster the translation of this concept into clinical trials for the treatment of schizophrenia. ? ? ?
