Pawpaw extract was prepared by pulp extraction with methanol and liquid-liquid partitioning with EtOAc. Annonacin was isolated from the crude EtOAc extract via column chromatography using a gradient solvent system of increasing polarity. Mass spectroscopy, nuclear magnetic resonance and infrared spectroscopy were used to compare isolated material with synthetic annonacin and a natural annonacin sample. Toxicity of isolated annonacin and the total EtOAc extract were determined in primary rat cortical neurons using the MTT (3- (4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) assay. The effects of annonacin on tau phosphorylation were determined by immunocytochemistry and western blot analysis.