The main purpose of this clinical trial is to examine the feasibility and effects of tamoxifen in subjects with pulmonary arterial hypertension (PAH). The study will evaluate how well the drug is tolerated, and its impact on functional condition and selected biomarkers. Changes in tricuspid annular plane systolic excursion (TAPSE) and other parameters determined by transthoracic echocardiography will be evaluated as well as changes in additional metrics such as six minute walk test distance, quality of life assessments, and hormone levels.

Pulmonary arterial hypertension (PAH) is characterized by progressive loss of function by the pulmonary vascular bed due to a variety of factors including obliterative vascular lesions, vasoconstriction, and thrombotic occlusion of the pulmonary arteries. Ultimately, right-sided heart failure ensues with severe limitation of exercise and eventual progression to death or lung transplantation. While there are multiple FDA-approved therapies for PAH representing 3 major pathways of interest, no treatments are curative, and have additional limitations including high expense, multiple side effects, and dosing inconveniences.

The strongest established risk factor for the progressively fatal disease pulmonary arterial hypertension (PAH) is female sex (~3:1 female:male ratio). We and others have found higher circulating estrogen levels, and enhanced estrogen signaling, in PAH patients. Preclinical work by our group and others supports the concept that anti-estrogen therapy, is effective for both prevention and treatment in PAH. Recent and ongoing clinical studies are underway to assess these approaches in humans, including a recent study demonstrating the safety of estrogen reduction in postmenopausal women.

Tamoxifen is the most commonly used selective estrogen receptor modulator (SERM). Due to its extensive use in humans for over three decades, it has an excellent safety profile and its long-term sequelae are well characterized. Furthermore, it is a generic drug which has been FDA-approved for treatment and prevention of breast cancer, particularly those with estrogen receptor-positive neoplasms.

To help to determine whether tamoxifen may be a safe and effective treatment for PAH in women and men, we will conduct a single-center, randomized, double-blind, placebo-controlled Phase II study of subjects with PAH. All subjects will also be treated with background standard of care therapy at the discretion of their PAH care physician.

Inclusion Criteria:

• Previous documentation of mean pulmonary artery pressure greater than or equal to 25 mm Hg with a pulmonary capillary wedge pressure (or left ventricular end-diastolic pressure) less than or equal to15 mm Hg and PVR greater than or equal to 3 WU at any time before study entry, consistent Group 1 PAH classified by accepted international classification.
• Diagnosis of PAH which is idiopathic, heritable, drug- or toxin-induced, or associated with connective tissue disease.
• Age 18 years and older.
• WHO Functional Class I, II, or III status.
• Ability to perform a six minute walk test without significant limitations in musculoskeletal function or coordination, with distance greater than or equal to 150m and less than or equal to 550m.
• Informed consent.

Exclusion Criteria:

• Current treatment with estrogen, progesterone, or any form of sex hormone therapy.
• Current treatment with anti-sex hormone therapy (e.g., anastrozole, fulvestrant, tamoxifen, leuprolide acetate (luporon) or other centrally-acting hormone agents.
• WHO Functional Class IV status.
• History of, or current, breast, uterine, ovarian, or testicular cancer.
• Current pregnancy, or prior pregnancy within 3 months of enrollment.
• Initiation of PAH therapy (prostacyclin analogues, endothelin-1 receptor antagonists, phosphodiesterase-5 inhibitors, riociguat, selexipag) within three months of enrollment; the dose must be stable for at least three months prior to Baseline Visit. Of note, PAH therapy, including diuretics, which is stopped and then restarted or has dose changes which are not related to initiation and up titration will be allowed within 3 months prior to the Baseline Visit, and during the trial for subjects.
• History of thromboembolic event.
• Hospitalized or acutely ill.
• Renal failure (creatinine over 2.0).
• Hypercalcemia.
• Severe osteoporosis (t score < -2.0 OR t score < -2.5 if on bone modifying treatment).
• Current or recent (< 3 months) chronic heavy alcohol consumption.
• Enrollment in a clinical trial or concurrent use of another investigational drug (non FDA approved) or device therapy within 30 days of screening visit.
• Enrollment in any pharmacologic clinical trial within one month of screening.
• Due to potential drug interactions with tamoxifen, subjects using bosentan (CYP3A4) or selexipag (CYP2C8) will be excluded.
• Due to the concerns of pregnancy during PAH and with tamoxifen use, subjects will be excluded who do not use at least two forms of contraception (e.g., IUD plus the use of a barrier contraceptive method).

Since the data that will be produced involves patients with a disease process that is much in need of new treatment options and the investigators agree that data sharing is essential for expedited translation of research results into patient treatment options. A detailed data sharing plan is in place and available upon request. In brief:

It is our plan to make data available at the time it is accepted for publication of the main findings from the final dataset through the use of a data enclave of our own design that would restrict our Data Analyst from sharing any information that would breach participant confidentiality. Potential researchers will contact the PI to discuss specific needs and how the data will be utilized. A formal approval process is in place to evaluate and complete such requests.

2 years after study completion.

Criteria include:

• detailed written description of the project for which the data would be used.
• acknowledge in any publication resulting from the data, the source of the data, crediting the program support.
• agree to submit all papers or reports submitted for publication to the PI for review prior to submission.