A Study to Assess FLX-787 in Subjects With Motor Neuron Disease Experiencing Muscle Cramps.

Inclusion Criteria: * Documented diagnosis of Motor Neuron Disease (MND) \[including Amyotrophic Lateral Sclerosis (ALS), Primary Lateral Sclerosis (PLS) or Progressive Muscular Atrophy (PMA)\] * Expected survival \> 6 months * Weekly muscle cramping (defined as: a sustained muscle contraction that's most often painful and lasts seconds to minutes) Exclusion Criteria: * Presence of major gastrointestinal disorders, such as inflammatory bowel disease, diverticulitis, active peptic ulcer disease, or significant gastroesophageal reflux disease (i.e., not well-controlled on antacids or proton pump inhibitors), or oral or esophageal lesions/ulcers * Presence of laryngospasm or significant swallowing problems * Presence of percutaneous endoscopic gastrostomy, esophagogastroduodenoscopy, or G-tube * Unable or unwilling to discontinue medications for cramps and/or opiates * Inability to tolerate a spicy sensation in the mouth or stomach * Actively using illicit drugs or history of chronic substance abuse within the past year prior to screening, including abuse of alcohol * Intention to change the current level of tobacco use or use of nicotine-containing products (i.e., new smokers or those actively trying to quit may not enrolled) * Participated in a clinical study (except natural history studies without administration of an investigational product) within 30 days prior to screening