Part 1. Design and Synthesis of Cysteine/Cystine Prodrugs and Bioisosteres Including Symmetrical and Unsymmetrical Disulfides Designed to Increase Cystine Levels in the CNS in Order to Drive the Cystine/Glutamate Antiporter: A Novel Treatment for Schizophrenia and Drug Addiction. Part 2. Design and Synthesis of Subtype Selective Ester Bioisosteres of BZR Ligands for Gabaa/Benzodiazepine Receptors to Enhance Metabolic Stability
Date of Award
Doctor of Philosophy
James M. Cook
Mahmum M. Hossain, Guilherme L. Indig, Andrew A. Pacheco, Alan W. Schwabacher
Antiporter, Bioisosteres, Cysteine, Cystine, Glutamate, Prodrugs
Part 1. Schizophrenia is a debilitating disorder that affects almost 1% of the world's population; pharmacotherapy expenditures for this disorder exceed $10 billion dollars even though existing medications exhibit a poor safety/efficacy profile. It is estimated that 75% of patients discontinue drug treatment, in part due to poor safety/efficacy. The current data set demonstrates that cysteine prodrug NAC reverse the behavioral and neurochemical effects of PCP used to model schizophrenia.
As a result cysteine prodrugs represent a highly novel approach to treating schizophrenia; indeed, these compounds may ultimately be more effective than existing medications because these drugs target the pathology underlying schizophrenia and reverse behaviors used to model negative symptoms and diminished cognition produced by PCP, which are behaviors and symptoms that are not treated with current first line medications. Specifically, therapeutic endpoints produced by cysteine prodrugs include increasing stimulation of group II metabotropic glutamate receptors and restoring levels of glutathione. The latter effect has the potential to reverse several specific abnormalities that have been observed in schizophrenia including increased oxidative stress, decreased NMDA receptor function, altered gene expression, and abnormal cell proliferation / synaptic connectivity.
Throughout this study, multiple series of compounds have been presented and explored, specifically 2 series of cysteine/cystine prodrugs, 2 series of cysteine/cystine bioisosteres and 1 series involving the coupling of two different series of compounds, namely, unsymmetrical disulfides ( mixed dimers). Also in this study, it will be shown through the use of in vivo and in vitro screening methods, diketopiperazine cystine prodrug monomers and dialkylated versions show high promise as novel antipsychotic agents. Furthermore, the diketopiperazine cystine prodrug dimers and dialkylated dimers also have shown promise in becoming novel antipsychotic agents by overcoming the detrimental effects of PCP-induced deficits in sensorimotor gating by restoring pre-pulse inhibition in multiple screenings.
Bioisosteres of cysteine and cystine have shown vast improvements over N-Acetylcysteine by competing with C14 uptake and increasing glutamate levels by driving the cystine/glutamate antiporter. It has also been shown that simple modifications to the cysteine/cystine moiety also improve outcomes far greater then N-Acetylcysteine alone. Once the most effective compounds are determined by screening methods, the research strategy benefits by combining the two such compounds as an unsymmetrical disulfide in order to enhance their effects and help eliminate their disadvantages. As an early example to this approach two mixed dimers were synthesized and have shown extremely positive results in screening methods described here.
Part 2. A series of 1,4-benzodiazepines and imidazobenzodiazepines including bioisosteric ligands was synthesized in search of subtype selective ligands for GABAA/benzodiazepine receptor subtypes. In this study, it was clear that the improved method for synthesizing benzodiazepines was successful. This is based on the number and quantities of numerous compounds synthesized utilizing the improved method. Although the efficacy of XHe-II-053 (4) was decreased in Phase I because of the metabolism of the C-3 ester to the acid, the bioisostere EMJ-I-026 (5) has been shown to exhibit non-sedating anxiolytic activity in mice as well as a binding/oocyte profile in vitro consistent with a non-sedating anxiolytic. Seven bioisosteric analogues were designed in order to circumvent any potential metabolic liability in humans of the previously described ligand. In fact, the bioisosteric analogues were much more stable in human liver microsomes than XHe-II-053 (4) again indicating these bioisosteres are potential nonsedating anxiolytics as well as useful for treatment of anxiety disorders in human populations. These ligands were also stable on human blood, brain and kidney.
Gratifyingly, ligand 5 was clearly an fÑ3 Bz/GABAergic receptor subtype selective ligand at pharmacologically relevant doses (approximately 100 to 200 nM) and, presumably, provides an agent to study physiologically processes mediated by fÑ3 subtypes including anxiety and, in addition, was much more stable on human liver microsomes. In this regard fÑ3 subtype selective ligand, oxadiazole 5 (EMJ-I-026), has been evaluated in the light dark paradigm and clearly is a nonsedating anxiolytic, wherein this ligand was anxiolytic with no sedative properties, in vivo, as compared to diazepam. This study indicated that the ester function in these molecules can be replaced with a metabolically more stable ester bioisostere and still retain anxiolytic activity. The indepth study of these ligands in animal models and other receptor systems are underway by collaborators.
Johnson II, Edward Merle, "Part 1. Design and Synthesis of Cysteine/Cystine Prodrugs and Bioisosteres Including Symmetrical and Unsymmetrical Disulfides Designed to Increase Cystine Levels in the CNS in Order to Drive the Cystine/Glutamate Antiporter: A Novel Treatment for Schizophrenia and Drug Addiction. Part 2. Design and Synthesis of Subtype Selective Ester Bioisosteres of BZR Ligands for Gabaa/Benzodiazepine Receptors to Enhance Metabolic Stability" (2012). Theses and Dissertations. 610.