The goal of this 2 step-up, exploratory study is to test safey, tolerability and PK/PD profiles in healthy volunteers and safety, tolerability and efficacy in sudden sensorineural hearing loss patients as an early salvage therapy. The main questions it aims to answer are: 1. whether is it safe and tolerable when healthy volunteers and sudden sensorineural hearing loss patients take multiple doses of NS101 against FAM19A5 2. whether is it effective in reversing hearing capability in sudden sensorineural hearing loss patients who fails to show sufficient recovery despite of oral standard steroid therapy. Patients and heathly volunteers will be given NS101 15mg/kg or 30mg/kg systemically less than 3 months per protocol. This is placebo controlled, double blinded study, which means there will be a group who receives placebo (i.e. fake drug) for study purpose.