GABAergic inhibition is increasingly considered one of the most important factors in controlling the mode of operation of thalamo-cortical and re-entrant cortico-thalamic pathways as well as of the afferent septal-hippocampus and of the re-entrant hippocampal-septal glutamatergic pathways that are substrates for the regulation of complex brain functions, including normal sensory, gating, cognition, vigilance, spatial orientation, volition, and consciousness. These functions are altered in schizophrenia and bipolar disorder patients with mania. Thus, one may attribute the appearance of psychotic symptoms in these mental disorders to a deficit in GABAergic modulation in critical cortico-thalamic and cortico-hippocampal brain circuits.
Indeed, schizophrenia and bipolar disorder patients with mania who manifest an auditory gating deficit and who suffer from other associated problems, such as auditory hallucinations, cognitive impairment, delusions, and defects of volition (lack of motivation, flat affect, and social withdrawal), express downregulation of GABAergic synthesis and function in their cortical, thalamic, septal, and hippocampal circuits (Guidotti et al., 2000). The neuroanatomical and neurochemical abnormalities of the GABAergic system in postmortem brain of patients with schizophrenia and bipolar disorder with mania can be summarized as follows: 1) decreased GABA release and uptake (Reynolds et al., 1990; Sherman et al., 1991; Simpson et al., 1998); 2) increased denervation-dependent GABAA receptor density measured by 3H-muscimol binding, particularly in layer II of the prefrontal cortex (PFC) and in superficial layers of other adjacent cortical areas (Benes et al., 1992; Benes, 2000); 3) increased expression of the αl GABAA receptor subunit mRNA in the PFC (Impagnatiello et al., 1998); 4) a dorsolateral PFC decrease of presynaptic GABA transporter-1-positive cartridges in GABAergic axon terminals of chandelier cells impinging on the initial segments of pyramidal axons (Woo et al., 1998), and decreased GAD67 expression (this enzyme is important in the synthesis of GABA and is one of the two molecular forms of glutamic acid decarboxylase expressed in GABAergic neurons, the other being GAD65) (Akbarian et al., 1995; Impagnatiello et al., 1998; Benes 2000; Bunney and Bunney, 2000; Guidotti et al., 2000; Volk et al., 2000), which taken together suggest that GABAergic function may be downregulated; and 5) an altered cortical distribution of nicotinamide adenine dinucleotide phosphate-positive GABAergic cells (Akbarian et al., 1996).
Collectively, these data support the hypothesis that GABAergic downregulation in cortical and hippocampal networks involved in sensory information processing act as a vulnerability factor in the etiopathogenesis of psychosis and may contribute to sensory gating deficits and to the presence of visual and auditory hallucinations, cognitive impairment, and social withdrawal. As a general strategy, our studies are directed at testing whether drugs that tend to normalize a deficient GABAergic system may also be beneficial in correcting the deficit of auditory gating in schizophrenia. This deficit, commonly known as a defective prepulse inhibition of startle (PPI) is considered the most important among the few objective measurements of brain neuronal network functional alterations that can be measured in patients with psychotic disorders and in their relatives (Swerdlow & Geyer, 1998).
New therapeutic opportunities arise today due to increasing insights into the molecular architecture and diversity of components involved in signal transduction at GABAA receptors (FIG. 2). Based on the brain expression of seven subunit families comprising at least 18 different subunits (α1-6, β1-3, δ, ε, θ, p1-3) that are assembled in heteropentameric GABAA receptors, an extraordinary structural heterogeneity of the target for GABA is operative in the CNS (Barnard, 2001). Most GABAA receptor subtypes in brain are believed to be composed of αβγ subunits associated to form the above-mentioned heteropentameric structures termed GABAA receptors (FIG. 2). Although the physiological significance of this heterogeneity is only partially understood, the pharmacological significance of the various subunits with respect to, their association with various actions of anxiolytic benzodiazepines and congeners is beginning to be quite well understood. For example, GABAA receptors containing α and γ subunits are substrates for a wide spectrum of actions elicited by clinically available agonists acting at the modulatory benzodiazepine binding sites. These sites are located at the interface of the postsynaptic ectodomains of contiguous α and γ subunits on GABAA receptors composed of αβγ subunits (FIG. 2). Benzodiazepine—sensitive GABAA receptors are characterized by the expression of α1, α2 α3 or α5 subunits in their heteropentameric structure and by the contiguity of these α subunits with γ2 subunits. The GABA-gated ion channel opening frequency of these receptors is promptly enhanced to maximal efficacy by agonists of benzodiazepine sites currently available on the US market and this is the basis for their therapeutic effectiveness in the treatment of anxiety disorders, sleep disturbances, muscle spasms, and epilepsy. However, maximal amplification of GABA-gated channels by benzodiazepines is also the basis of undesirable side effects, such as sedation, amnesia, tolerance, and dependence (Costa and Guidotti, 1996).
In present clinical practice the consensus is that in schizophrenia protracted benzodiazepine monotherapy is not effective, despite anecdotical reports of antipsychotic efficacy in the first few days of treatment. The lack of antipsychotic action after protracted benzodiazepine treatment is presumably attributable to the well known phenomenon of tolerance that inevitably and rapidly occurs after administration of clinically-available benzodiazepine. Moreover their use is generally contraindicated because dlinically-available benzodiazepine drugs also elicit sedation and amnesia, and if used repeatedly in addition to tolerance produce dependence (Costa et al., 2001).
Recent studies (Möhler, 2001) have established that the sedative and amnestic action of benzodiazepines is related to the amplification of GABA action at GABAA α1 receptors including α1 subunits contiguous to the γ2 subunits, whereas their anxiolytic and anticonvulsant and antipsychotic actions are mediated presumably by similar actions at GABAA receptors having α2, α3 or α5 receptor subunits (Table 1).
Thus, opportunities for the design of a new generation of benzodiazepine binding site ligands that are partial agonists and act specifically at GABAA receptor subtypes, are now emerging. These new drugs have reduced tolerance and dependence liability and selectively act on GABAA receptor subtypes to control psychotic symptoms in schizophrenia patients without sedative or amnestic action.
U.S. Pat. No. 5,317,018 of May 31, 1994 describes the use of imidazo[1,5-a]benzodiazepine-3carboxamide derivatives and compositions containing such compounds for treating anxiety and panic disorders and idiopathic and psychomotor epilepsy. One such compound is 6-(2 bromophenyl)-8-fluoro-4H-imidazo[1,5a][1,4]benzodiazepine-3-carboxamide which is often referred to as imidazenil.
Unlike other partial allosteric modulators, imidazenil has a high affinity for the receptor, does not act on GABAA α1 subunit and is slowly metabolized in compounds that may act on GABAA α1 subunit. Thus, imidazenil does not produce sedation and does not produce tolerance or dependence as indeed occurs with other partial allosteric modulators of the action of GABA at GABAA receptors (Costa et al. 2001).
Auta et al PNAS 97 (No.5) 2314-2319 (Feb. 29, 2000) describe the use of imidazenil to prevent alprazolam-induced aquisition in patas monkeys and note that during a seventeen day treatment no tolerance of the effects of imidazenil were noted. Costa et al TiPS May 1996, 192-200 suggest that imidazenil may be a prototype of a new generation of anxiolytic and anticonvulsant drugs that have minimal disruptive effects on learning and memory, and are virtually devoid of the tolerance liability and other unwanted side-effects of classic benzodiazepines. As a reference compound, we used the clinically available benzodiazepine alprazolam, an 8chloro-1-methyl-6-phenyl-4-[1,2,4]triazolo[4,3a][1-4]benzodiazepine whose formula is also indicated in FIG. 1.