Abemaciclib in Patients With HIV-associated and HIV-negative Kaposi Sarcoma

Background: Kaposi Sarcoma (KS) is common in people with human immunodeficiency virus (HIV) but can also occur in people who do not have HIV. KS tumors usually involve the skin, but may also involve lymph nodes, lungs, bone, and gastrointestinal tract. Researchers want to see if a drug that is currently used to treat a type of breast cancer can help. Objective: To find a safe dose of abemaciclib to treat KS and to see if it can shrink lesions or tumors. Eligibility: People ages 18 and older with KS. Design: Participants will be screened with some or all of the following: Medical history Physical exam Blood and urine tests Chest x-ray and/or computed tomography scans Lung or gastrointestinal tract exam with an endoscope (a flexible instrument to examine the interior of the organ) Medicine review Heart function tests KS lesion assessment Skin sample from a KS lesion Treatment will be given in 28-day cycles. Participants will take the study drug tablets by mouth everyday. They will keep a medicine diary. They will get the study drug until their cancer gets worse or they have unacceptable side effects. Participants will have a study visit at the beginning of each cycle. At these visits, they will repeat some screening tests. They may have medical photographs taken of body surfaces. They may complete questionnaires about their quality of life. They may give skin and saliva samples. For skin samples, an area of skin will be numbed. A small circle of skin over an area affected by KS will be removed. Participants will have follow-up visits for up to 2 years after treatment ends.