NPPA and Alzheimer disease: Wu et al. (2013) showed significantly elevated levels of TDO in the cerebellum, but not cerebrum, of 3xTg-AD mice and hippocampi of humans with AD. Because 3-HK is increased in the serum (Schwarz et al., 2013) and QA is increased in the hippocampus of AD patients (Guillemin et al., 2005a) as well as 3xTg-AD mice (Fertan et al., 2019a), the KP may be a worthy target for AD treatment (Figure 1). There is evidence that reducing KP activity can ameliorate some of the symptoms of AD in animal models (Vamos et al., 2009; Zwilling et al., 2011; Yu et al., 2014; Deora et al., 2017).