This study is designed to investigate the response of unilateral upper extremity (arm) lymphedema, during pharmacologic treatment of lymphedema with oral placebo and oral acebilustat. Participants will receive "study drug" (Acebilustat or placebo), for 9 months. For 3 of these months, the participant will receive placebo; for 6 of these months, the participant will receive active ingredient, acebilustat. The study is blinded which means that the participant will not be told which study pill they are taking.

To enter this study, participants will have to meet requirements that will be evaluated during screening, Visit 1. If eligible, and if they choose to continue, Visit 2, treatment start, will take place 2-7 days later. Study visits are every (approximately) 90 days. Physical exam, including limb measurements and skin measurements (with tape measure and dermal ultrasound) are performed. Phlebotomy for research samples and clinical labs will be performed at study start and end of study.

Inclusion Criteria:

• Upper arm lymphedema, single arm, stage 2, greater than 6 months duration
• Male or female.
• Ages 18-75.
• Prior imaging by lymphoscintigraphy or magnetic resonance lymphangiography that confirms the presence of lymphedema in the affected limb OR, at screening, an affected:unaffected limb volume ratio of ≥1.1 with significant history of lymphedema
• Consistent use of an appropriately sized compression garment for daytime use.
• Willing to maintain a stable regimen of self-care from screening to end-of-study.
• If a potential participant has undergone prior microvascular (vascular lymph node transfer, lymphaticovenous anastomosis) or debulking surgical intervention, at least one year must have elapsed prior to screening AND, at screening, an affected: unaffected limb volume ratio of ≥1.1.
• Lymphedema therapy must be completed at least 8 weeks prior to screening.
• Has received Covid-19 vaccine (Pfizer, Moderna or Johnson & Johnson)
• Ability to understand and the willingness to sign a written informed consent document.
• . If the possibility of conception exits, agrees to use a medically acceptable method of contraception (both male and female) from the signing of the informed consent form through the entire study period; men or women who are surgically sterile (> 6 months after surgery) or women who have been postmenopausal for at least 1 year are not considered to be of childbearing potential.

Exclusion Criteria:

• Concurrent participation in a clinical trial of any other investigational drug or therapy
• Other medical condition that could lead to acute limb edema (e.g. acute blood clot) or other medical condition that could result in symptoms overlapping those of lymphedema (e.g. frozen shoulder).
• History of clotting disorder.
• Chronic (persistent) infection in the affected limb.
• Active cancer treatment or history of cancer treatment within the past 2 years, except for non-melanoma skin cancer or cervical cancer in-situ.
• Chronic kidney disease
• Liver disease
• Pregnancy or nursing.
• Substance abuse (e.g., alcohol or drug abuse) within 6 months prior to screening.
• Any current use of non-steroidal anti-inflammatory drug (NSAID), e.g. ibuprofen, ketoprofen) or prior therapeutic use of ketoprofen.
• Any current use of immunosuppressive or immunomodulatory drugs (e.g., immunosuppressants, anticancer drugs, interleukins, interleukin antagonists or interleukin receptor blockers) or leukotriene pathway inhibitor (zileuton), leukotriene receptor antagonist (e.g montelukast).
• Personal or family history of prolonged QT syndrome
• Any reason (in addition to those listed above) that, in the opinion of the investigator, precludes full participation in the study.
• Any current use of statin drugs. The use of any statin drug should be discontinued at least 2 weeks prior to the trial enrollment