Safety and Tolerability Study of Oral ABBV-744 Tablet Alone or in Combination With Oral Ruxolitinib Tablet or Oral Navitoclax Tablet in Adult Participants With Myelofibrosis

Myelofibrosis (MF) is a bone marrow illness that affects blood-forming tissues in the body. MF disturbs the body's normal production of blood cells, causing extensive scarring in the bone marrow. This leads to severe anemia, weakness, fatigue, and an enlarged spleen. The purpose of this study is to see how safe and tolerable ABBV-744 is, when given alone, and in combination with ruxolitinib or navitoclax, for adult participants with MF. ABBV-744 is an investigational drug being developed for the treatment of MF. The study has 4 segments - A, B, C, and D. In Segment A, the safe dosing regimen of ABBV-744 is identified and then, given alone as monotherapy. In Segment B, C, and D, combination therapies of ABBV-744 with either ruxolitinib or navitoclax are given. Adult participants with a diagnosis of MF will be enrolled. Around 130 participants will be enrolled in 60 sites worldwide. In Segment A, participants will receive different doses and schedules of oral ABBV-744 tablet to identify safe dosing regimen. Additional participants will be enrolled at the identified monotherapy dosign regimen. In Segment B, participants will receive oral ruxolitinib and ABBV-744 will be given as "add-on" therapy. In Segment C, participants will receive ABBV-744 and oral navitoclax. In Segment D, participants will receive ABBV-744 and ruxolitinib. Participants will receive treatment until disease progression or the participants are not able to tolerate the study drugs. There may be higher treatment burden for participants in this trial compared to their standard of care. Participants will attend regular visits during the study at a hospital or clinic. The effect of treatment will be checked by medical assessments, blood and bone marrow tests, checking for side effects, and completing questionnaires.