Inclusion Criteria:  Classified as WHO functional class II/III/IV despite treatment with maximally tolerated doses of 2 or more treatment modalities (exp. PDE5 inhibitors, guanylate cyclase stimulators, endothelin receptor antagonists, and prostanoids) Baseline 6MWT: greater than 100 meters and less than 550 meters Established diagnosis of Group 3 pulmonary hypertension as a result of interstitial lung disease OR established diagnosis of Group 1 pulmonary hypertension as a result of connective tissue disease, idiopathic, hereditary, drugs, or toxins.  Right heart catheterization at randomization showing pre-capillary pulmonary hypertension (mPAP ≥ 25 mmHg and PVR > 400 dynes * sec * cm^ -5) and:  PCWP <20 mmHg for Group 3 PH patients and a PCWP <15 for Group 1 PAH patients Participants on chronic medication for PAH, PH, or underlying lung disease must be on a stable and optimized dose for at least 90 days prior to the first dose of the study drug. Female participants who are heterosexually active must use an acceptable method of contraception: condoms (male or female) with or without a spermicidal agent, diaphragm or cervical cap with spermicide, IUD, or Hormone-based contraceptive Be able to provide written informed consent and comply with study requirements Be able to read, speak and understand English  Exclusion Criteria:  Participants with a diagnosis of PAH or PH for reasons due to any of the following:  Group 2, 4, or 5 Group 1 due to HIV, veno-occlusive disease, porto-pulmonary hypertension, congenital heart disease  Group 3 due to severe chronic obstructive pulmonary disease (COPD) or obstructive sleep apnea (OSA)  Note: participants with overlapping syndromes will be evaluated on a case-by-case basis by the recruiting physician* Total Lung Capacity (TLC) less than 60% predicted FEV1/FVC less than 50% predicted or FEV1 less than 55% predicted Inability to safely attempt completion of the 6MWD test Use of experimental PAH treatments within the past 3 months Current systemic treatment with hymecromone  Left sided heart disease as defined by either a PCWP greater than 20 mmHg and/or left ventricular ejection fraction less than 40%  Note: participants with abnormal left ventricular function attributable entirely to impaired left ventricular filling due to the effects of right ventricular overload (ie right ventricular hypertrophy and/or dilatation) are not excluded  Participants must not have 3 or more of the following left ventricular disease / dysfunction risk factors:  Body mass index (BMI) greater than 30kg/m2 History of essential hypertension requiring medication Diabetes mellitus  Historical evidence of significant coronary disease established by any of the following:  History of myocardial infarction History of percutaneous coronary intervention or coronary artery bypass graft Angiographic evidence of greater than 50% stenosis in at least one coronary artery Positive stress test with imaging Stable angina Significant valvular heart disease as determined by more than moderate findings on echocardiogram or history of valve replacement Pregnant or actively breastfeeding Female participants with childbearing potential not willing to use a form of birth control (including abstinence) during the study Inability to undergo right heart catheterization Acute pulmonary embolism within 90 days of randomization Exacerbation of underlying lung disease or active pulmonary or upper respiratory infection within 30 days of randomizations Use of any inhaled tobacco products or significant history of drug abuse within 3 months prior to randomization Subject is receiving greater than 10L/min of oxygen supplementation by any mode of delivery at rest at baseline Body mass index greater than 40kg/m2 Participants with history of dysphagia, achalasia, or difficulty swallowing capsules, tablets or pills Participants with liver failure or AST or ALT greater than 2 times the upper limit of normal Participants with total bilirubin levels greater than 2 times the upper limit of normal Participants with CrCl less than 45 Use of any investigational drug/device, or participation in any investigational study with therapeutic intent within 30 days prior to randomization Known allergy to hymecromone or any component thereof Known allergy to any component of placebo (including wheat allergy, celiac disease, rare hereditary problems of galactose intolerance, Lapp lactase deficiency, or glucose-galactose malabsorption) Physician concern that participant may not adhere to the study protocol Significant psychiatric, addictive, or other disorder that compromises the subject's ability to provide informed consent