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

The former sedative-hypnotic and recreational drug methaqualone (Quaalude) is a moderately potent, non-selective positive allosteric modulator (PAM) at GABA receptors (GABARs) (Hammer et al., 2015). In the present study, we have identified a novel methaqualone analog, 2-phenyl-3-(p-tolyl)quinazolin-4(3H)-one (PPTQ), in a screening of 67 analogs at five αβγ GABAR subtypes and delineated its functional properties and mechanism of action at wild-type and mutant GABARs expressed in Xenopus laevis oocytes by two-electrode voltage clamp electrophysiology. PPTQ was found to be an allosteric agonist and a PAM (ago-PAM) at human αβγ and αβδ GABARs, exhibiting intrinsic activity at micromolar concentrations and potentiating the GABA-evoked signaling through the receptors at concentrations down to the low-nanomolar range. Whereas PPTQ exclusively increased the potency of GABA at the αβγ receptor, it increased both GABA potency and efficacy at αβδ and displayed modest potency-based preference for αβδ over αβγ. In elaborate mutagenesis and competition experiments PPTQ was found to act through the same or an overlapping site as etomidate in the transmembrane β/α subunit interfaces, whereas it did not seem to target the other three transmembrane interfaces in the GABAR. Finally, the PPTQ site was shown to be allosterically linked with sites targeted by neurosteroids and barbiturates but not with the high-affinity benzodiazepine site in the αβγ receptor. In conclusion, the development of a highly potent, bioavailable GABAR ago-PAM by subtle modifications to the methaqualone scaffold demonstrates that derivatization of this infamous drug from the past can lead to modulators with distinct functional characteristics at the receptors.