Isonipecotic acid, also known as piperidine-4-carboxylic acid (P4C), is a conformationally constrained derivative of ó-aminobutyric acid (GABA) and a moderately potent GABA<sub>A</sub> receptor partial agonist. It consists of a piperidine ring with a carboxylic acid in the iso position. The drug showed moderate-efficacy partial agonism of ñ<sub>1</sub>, ñ<sub>2</sub>, ñ<sub>3</sub>, and ñ<sub>5</sub> subunit-containing GABA<sub>A</sub> receptors ( = 46âÂÂ57%), but showed full or near-full agonism of ñ<sub>4</sub> and ñ<sub>6</sub> subunit-containing GABA<sub>A</sub> receptors ( = 83âÂÂ104%). Isonipecotic acid is unable to cross the bloodâÂÂbrain barrier. It was first described in the scientific literature by at least 1944 and was identified as a GABA<sub>A</sub> receptor agonist by 1978.