NPPA and amyotrophic lateral sclerosis: Metabolites of the kynurenine pathway show diverse properties that can cause contrasting effects in the nerve system and has become an important research area in neurodegenerative disorders such as Alzheimer’s disease (AD), Huntington’s disease (HD), multiple sclerosis (MS), amyotrophic lateral sclerosis (ALS) and Parkinson’s disease (PD), due to the relationship between abnormal KP metabolite levels and these neurological diseases (Füvesi et al., 2012; Maddison and Giorgini, 2015).