NTF3 and hearing loss disorder: Neurotrophic factors, such as brain-derived neurotrophic factor (BDNF) [34], neurotrophin-3 (NT-3) [21, 35], and growth factors (insulin-like growth factor 1 (IGF1) [36] have been studied for a relatively long time and shown to be effective for the treatment of hearing loss [37]; however, they are difficult to use because they show insufficient permeability at the blood–brain and blood-nerve barriers [38].