Study of Oral Navitoclax Tablet In Combination With Oral Ruxolitinib Tablet When Compared With Oral Ruxolitinib Tablet To Assess Change In Spleen Volume In Adult Participants With Myelofibrosis

Myelofibrosis is a type of bone marrow cancer that usually develops slowly and disrupts body's normal production of blood cells. It causes bone marrow scarring, leading to severe anemia that can cause weakness and fatigue. It can also cause a low number of blood-clotting cells called platelets, which increases risk of bleeding. Myelofibrosis often causes an enlarged spleen. The purpose of this study is to see if a combination of navitoclax and ruxolitinib is more effective and safe in assessment of change in spleen volume when compared to ruxolitinib in participants with myelofibrosis. Navitoclax is an investigational drug for the treatment of myelofibrosis. Participants in this study are divided into two groups, called treatment arms. Each group receives a different treatment. Adult participants with a diagnosis of myelofibrosis will be enrolled. Around 230 participants will be enrolled in approximately 190 sites worldwide. Participants will receive oral navitoclax tablet with oral ruxolitinib tablet or oral ruxolitinib tablet with oral placebo (no active drug) tablet and treatment may continue untill the participant cannot tolerate the study drug, or benefit is not achieved, or other reasons which qualify for discontinuation of the study drug. There may be a higher treatment burden for participants in this trial compared to their standard of care. Participants will attend regular visits during the course of the study at a hospital or clinic. The effect of the treatment will be checked by medical assessments, blood tests, magnetic resonance imaging (MRI) or computed tomography (CT) scan, bone marrow tests, checking for side effects, and completing questionnaires.