N-Desmethyl Pimavanserin -d9 hydrochloride (AC-279-d9 hydrochloride) is the deuterium labeled N-Desmethyl Pimavanserin. N-Desmethyl Pimavanserin is the active metabolite of Pimavanserin. Pimavanserin is a selective inverse agonist of the 5-HT2A receptor with pIC 50 and pK d of 8.73 and 9.3, respectively.