Safety and Efficacy of FSD201 for the Treatment of Chronic Pain Associated With Idiopathic MCAS (MCAD)

Key Inclusion Criteria: * Clinical diagnosis of idiopathic MCAS as per the global consensus diagnostic criteria * Adults with widespread chronic pain (in three or more body regions); * Chronic pains of intensity greater than or equal to 4.0 but less than or equal to 9.0 on a Numeric Pain Rating Scale, symptom duration of more than 6 months; * Subject agrees to use only acetaminophen (up to 1000 mg per dose and not to exceed 3000 mg/day) or diphenhydramine (up to 300 mg/day) as rescue medication for chronic widespread musculoskeletal nociplastic pains throughout the trial; * The subject is willing to maintain current activity and exercise levels throughout the study; * During the study, the subject agrees not to initiate or change any non-pharmacologic interventions (including chiropractic care, physical therapy, psychotherapy, and massage therapy). Any ongoing non-pharmacologic intervention must be stable for at least 4 weeks before screening and should be continued for the duration of the study; Key Exclusion Criteria: * The subject has pain that cannot be clearly differentiated from or that could interfere with the assessment of chronic musculoskeletal nociplastic pain secondary likely to idiopathic MCAS (post-herpetic neuralgia, traumatic injury, prior surgery, complex regional pain syndrome); * Adults with chronic cancer pain; * Adults with inflammatory connective tissue disorder or rheumatological disorder-related pain, for example rheumatoid arthritis; * Adults with focal musculoskeletal pain; * Adults with skin diseases (chronic urticaria, pemphigus, lupus, rosacea etc.); * Adults with endocrinological disorders (acute hypothyroidism, acute adrenal insufficiency etc.); * Adults with systemic gastrointestinal conditions (Inflammatory bowel disorders); * Significant psychological comorbidities: PHQ-9 score greater than 20; and GAD-7 score greater than 15 (indication of severe anxiety that could interfere with accurate logging of pain ratings); * Current or recent (within 12 months of screening) history of a substance use disorder including cannabinoid or alcohol use disorder as defined by the Diagnostic and Statistical Manual of Mental Disorders (5th edition; DSM-5); * Subject has neurologic disorder unrelated to chronic widespread musculoskeletal nociplastic pains (phantom limb from amputation, vitamin B12 deficiency, chronic inflammatory demyelinating polyneuropathy), circulatory disorder (peripheral artery disease), a skin condition in the area of neuropathy that could alter sensation (plantar ulcer), or other painful conditions (arthritis) that could interfere with reporting of pain due to chronic widespread musculoskeletal nociplastic pains; * Current severe or uncontrolled major depressive disorder or anxiety disorders. Mild to moderate major depression or anxiety disorders are permitted provided that the investigator assesses the patient as clinically stable and appropriate for entry into the study. Stable doses of SSRIs are allowed for the treatment of depression if the dose is stable for 60 days before Screening Visit; * Subject has a history of suicide attempt or suicidal behaviour within the last 12 months or has suicidal ideation within the previous 12 months or who is at significant risk to commit suicide, as judged by the Investigator at Screening and/or Randomization Visit; * Patients with active malignancy or history of malignancy, except for basal cell or squamous cell carcinoma and actinic keratosis. Basal cell carcinoma and small squamous cell carcinoma of the skin which have been excised according to guidelines within the last 5 years or in situ cervical carcinoma that has been fully treated and shows no evidence of recurrence are allowed; * Individuals with generalized joint hypermobility secondary to hereditary connective tissue disorders like Ehlers Danlos Syndrome; * Individuals with high baseline serum tryptase levels suggestive of Primary or Secondary MCAS (defined as above the normal range of 2.2 to 13.2 µg/L).