A Phase III, 52-week, prospective, randomized, double-blind, placebo-controlled, parallel-group, multi-center study, with a primary efficacy endpoint at 12 weeks, to determine the efficacy, safety, and tolerability of fixed doses of 15 mg bid and 30 mg bid of evenamide as add-on in patients with documented treatment-resistant schizophrenia, which is not adequately controlled by a stable therapeutic dose of the patient s current antipsychotic medication(s)