Interfering with the excitatory amino acids signalling in the brain for the design of novel neuroprotective agents

Abstract

A complex network of central synaptic transmission is regulated, in vertebrates, by simple amino acids: GABA, with inhibitory effects, and glutamate, the most abundant excitatory amino acid in the CNS. These two fundamental neurotransmitters are responsible for the homeostasis of all the CNS functions, from post-natal developmental age to the entire adult life. Even slight dysregulations of GABA-ergic and glutamat-ergic systems underlie the genesis and the progression of a number of neurological pathologies, both neurodegenerative and psychiatric disorders. GABA-based drugs are widely used in clinic for the treatment of different pathological states, such as anxiety, epilepsy and neuropathic pain. Conversely, the dissociative anaesthetic ketamine and the pro-cognitive agent memantine, are the only glutamatergic drugs which have found clinical use, despite the large volume of evidences that unequivocally link glutamate to CNS diseases. Due to the tight linkage between glutamate transmission and several metabolic cascades, both in periphery and in the CNS, the modulation of enzymatic pathways, leading to the biosynthesis of metabolites able to interfere with the glutamatergic system, offers a viable way to design novel therapeutic approaches for the treatment of a wide spectrum of invalidant neurological conditions. The kynurenic pathway of tryptophan metabolism is the main route of degradation of the essential, proteinogenic amino acid L-tryptophan in mammals and is responsible for the production of several neuroactive compounds, most of which endowed with specific actions towards glutamate receptors: i) kynurenic acid, wide spectrum competitive antagonist of ionotropic glutamate receptors and negative allosteric modulator of α7 nicotinic acetylcholine receptors, with neuroprotective properties, ii) 3-hydroxykynurenine, neurotoxic for its ability to generate free radical species, iii) xanthurenic acid, modulator of Group II metabotropic Glu receptors, iv) cinnabarinic acid, selective mGlu4 orthosteric partial agonist and v) quinolinic acid (QUIN), an endogenous excitotoxin acting as a selective NMDARs agonist. Since the inhibition of the production of QUIN has been proposed as a valid approach for the drug discovery in the field of neurological disorders, this doctoral thesis is aimed at the design and synthesis of novel inhibitors of the enzyme responsible for the biosynthesis of QUIN, namely 3-hydroxyanthranilate 3,4-dioxygenase (3-HAO). Previously described 3-HAO inhibitors are halogenated substrate analogues and are characterized by high chemical instability, due to the common o-aminophenol core. In this work, a series of novel potential 3-HAO inhibitors has been designed and synthesized, based on the chemically stable pyridine N-oxide nucleus. Biological tests have shown interesting activity for three of the synthesized compounds in vitro, with IC50 values ranging between 0.6 μM and 100 μM. A preliminary SAR profile has been drawn for this class of 3-HAO inhibitors, based on the relative activities of test compounds and on docking studies performed by our group of molecular modelling. The most active compound, 2-amino-6-methylnicotinic acid 1-oxide, was also tested in vivo and resulted active, showing modulatory activity on the production of different kyurenine pathway metabolites, in lesioned rat brain. These findings demonstrated the suitability of the 2-aminopyridine-1-oxide scaffold for the design of novel, chemically stable 3-HAO inhibitors which, being active in vivo, open the road to further studies, aimed at the full characterization of the involvement of the kynurenine pathway in the onset and the progression of neurodegenerative and autoimmune diseases, such as Huntington’s disease and multiple sclerosis.