Functional properties and mechanism of action of PPTQ, an allosteric agonist and low nanomolar positive allosteric modulator at GABAA receptors

Structural insights into GABAA receptor potentiation by Quaalude

Methaqualone, a quinazolinone marketed commercially as Quaalude, is a central nervous system depressant that was used clinically as a sedative-hypnotic, then became a notorious recreational drug in the 1960s-80s. Due to its high abuse potential, medical use of methaqualone was eventually prohibited, yet it persists as a globally abused substance. Methaqualone principally targets GABAA receptors, which are the major inhibitory neurotransmitter-gated ion channels in the brain. The restricted status and limited accessibility of methaqualone have contributed to its pharmacology being understudied. Here, we use cryo-EM to localize the GABAA receptor binding sites of methaqualone and its more potent derivative, PPTQ, to the same intersubunit transmembrane sites targeted by the general anesthetics propofol and etomidate. Both methaqualone and PPTQ insert more deeply into subunit interfaces than the previously-characterized modulators. Binding of quinazolinones to this site results in widening of the extracellular half of the ion-conducting pore, following a trend among positive allosteric modulators in destabilizing the hydrophobic activation gate in the pore as a mechanism for receptor potentiation. These insights shed light on the underexplored pharmacology of quinazolinones and further elucidate the molecular mechanisms of allosteric GABAA receptor modulation through transmembrane binding sites.

Pharmacological modulation of GABAA receptors

Ligand-gated ion channels are integral membrane proteins that activate through a change in conformation upon transmitter binding and were identified as key players of brain function. GABA_A receptors are major inhibitory ligand-gated ion channels of this protein family. They are the target of many therapeutic compounds widely used in the clinic and continue to attract the attention of academic and pharmaceutical laboratories. Advances in the knowledge of the structure of GABA_A receptors at the molecular level with unprecedented resolution enabled the determination of the binding sites of many allosteric modulators revealing the nature of their interactions with the receptors. Herein, we review the latest findings on allosteric modulation of GABA_A receptors and their relevance to drug discovery.

Structural Determinants for the Mode of Action of Imidazopyridine DS2 at δ-Containing γ-Aminobutyric Acid Type A Receptors

Despite the therapeutic relevance of δ-containing γ-aminobutyric acid type A receptors (GABA_ARs) and the need for δ-selective compounds, the structural determinants for the mode and molecular site of action of δ-selective positive allosteric modulator imidazo[1,2-a]pyridine DS2 remain elusive. To guide the quest for insight, we synthesized a series of DS2 analogues guided by a structural receptor model. Using a fluorescence-based fluorometric imaging plate reader membrane potential assay, we found that the δ-selectivity and the pharmacological profile are severely affected by substituents in the 5-position of the imidazopyridine core scaffold. Interestingly, the 5-methyl, 5-bromo, and 5-chloro DS2 analogues, 30, 35, and 36, were shown to be superior to DS2 at α4β1δ as mid-high nanomolar potency δ-selective allosteric modulators, displaying 6-16 times higher potency than DS2. Of these, 30 also displayed at least 60-fold selectivity for α4β1δ over α4β1γ2 receptor subtypes representing a potential tool for the selective characterization of δ-containing GABA_ARs in general.

From Methaqualone and Beyond: Structure-Activity Relationship of 6-, 7-, and 8-Substituted 2,3-Diphenyl-quinazolin-4(3H)-ones and in Silico Prediction of Putative Binding Modes of Quinazolin-4(3H)-ones as Positive Allosteric Modulators of GABAA Receptors

Methaqualone (2-methyl-3-(o-tolyl)-quinazolin-4(3H)-one, MTQ) is a moderately potent positive allosteric modulator (PAM) of GABA_A receptors (GABA_ARs). In a previous structure-activity relationship (SAR) study probing the importance of 2- and 3-substituents in the quinazolin-4(3H)-one scaffold, several potent GABA_AR PAMs were identified, including 2,3-diphenylquinazolin-4(3H)-one (PPQ) and 3-(2-chlorophenyl)-2-phenylquinazolin-4(3H)-one (Cl-PPQ). Here, PPQ was applied as lead in a SAR study of 6-, 7-, and 8-substituents in the quinazolin-4(3H)-one by synthesis and functional characterization of 36 PPQ analogs at various GABA_AR subtypes. While none of the new analogs were significantly more potent than PPQ or displayed pronounced subtype selectivity across the GABA_ARs tested, several interesting SAR observations were extracted from the study. In an in silico study, the putative binding modes of MTQ, PPQ, and Cl-PPQ in the transmembrane β₂⁽⁺⁾/α₁^(-) interface of the α₁β₂γ_2S GABA_AR were predicted. Several plausible binding modes were identified for the three PAMs, and rationalization of the molecular basis for their different modulatory potencies was attempted.