In vitro and in vivo pharmacological characterization of SSD114, a novel GABAB positive allosteric modulator

Positive allosteric modulators (PAMs) of the GABAB receptor have emerged as a novel approach to the pharmacological manipulation of the GABAB receptor, enhancing the effects of receptor agonists with few side effects. Here, we identified N-cyclohexyl-4-methoxy-6-(4-(trifluoromethyl)phenyl)pyrimidin-2-amine (SSD114) as a new compound with activity as a GABAB PAM in in vitro and in vivo assays. SSD114 potentiated GABA-stimulated [35S]GTPγS binding to native GABAB receptors, whereas it had no effect when used alone. Its effect on GTPγS stimulation was suppressed when GABA-induced activation was blocked with CGP54626, a competitive antagonist of the GABAB receptor. SSD114 failed to potentiate WIN55,212,2-, morphine- and quinpirole-induced [35S]GTPγS binding to cortical and striatal membranes, respectively, indicating that it is a selective GABAB PAM. Increasing SSD114 fixed concentrations induced a leftward shift of the GABA concentration-response curve, enhancing the potency of GABA rather than its efficacy. SSD114 concentration-response curves in the presence of fixed concentrations of GABA (1, 10, and 20 μM) revealed a potentiating effect on GABA-stimulated binding of [35S]GTPγS to rat cortical membranes, with EC50 values in the low micromolar range. Bioluminescence resonance energy transfer (BRET) experiments in Chinese Hamster Ovary (CHO)-cells expressing GABAB receptors showed that SSD114 potentiates the GABA inhibition of adenylyl-cyclase mediated by GABAB receptors. Our compound is also effective in vivo potentiating baclofen-induced sedation/hypnosis in mice, with no effect when tested alone. These findings indicate that SSD114, a molecule with a different chemical structure compared to known GABAB PAMs, is a novel GABAB PAM with potential usefulness in the GABAB-receptor research field.