Tenapanor in Synucleinopathy-Related Constipation

Inclusion Criteria 1. Age 50-89 years. 2. Diagnosis of PD within Hoehn and Yahr stages 1-3, confirmed by a neurologist per International Parkinson and Movement Disorder Society criteria. 3. Average weekly stool frequency of ≤5 spontaneous bowel movements (SBMs) and ≤2 complete spontaneous bowel movements (CSBMs) over the past 6 months. These criteria will be verified during a 2-week screening period. 4. Stool consistency ≤3 on the Bristol Stool Form Scale (BSFS). This criterion will be verified during a 2-week screening period. 5. Agreement to use contraception, if applicable. Exclusion Criteria 1. Functional diarrhea or IBS-D/M based on Rome IV Criteria. 2. Symptomatic structural GI abnormalities or inflammatory bowel disease. 3. Significant hepatic (ALT or AST ≥ 2.5x the upper limit of normal) or renal (serum creatinine \>2mg/dl) dysfunction. 4. Pregnancy or lactation. 5. Diagnosis of primary dyssynergic defecation by anorectal manometry.