A Study Evaluating Tolerability and Efficacy of Navitoclax Alone or in Combination With Ruxolitinib in Participants With Myelofibrosis

Inclusion Criteria: * Participants with documented diagnosis of intermediate-2 or high-risk primary Myelofibrosis, Post Polycythemia Vera Myelofibrosis or Post-essential Thrombocythemia Myelofibrosis. * Participant must be ineligible due to age, comorbidities, or unfit for unrelated or unmatched donor transplantation or unwilling to undergo stem cell transplantation at time of study entry. * Eastern Cooperative Oncology Group (ECOG) of 0, 1, or 2. * Prior treatment must meet at least one of the following criteria: * Prior or current treatment with ruxolitinib and no prior treatment with a Bromodomain and Extra-Terminal motif (BET) proteins inhibitor or another Janus Kinase 2 (JAK-2) inhibitor, and meet all of the following criteria: * Ruxolitinib treatment must meet at least one of the following criteria: * Ruxolitinib treatment for \>=24 weeks with lack of efficacy defined as a lack of spleen response (refractory) or a loss of spleen or symptom response (relapsed) * Ruxolitinib treatment for \<24 weeks with documented disease progression on spleen measurements while on ruxolitinib as defined in the protocol: * Ruxolitinib treatment for \>=28 days with intolerance defined as new red blood cell transfusion requirement (at least 2 units/month for 2 months) while receiving a total daily ruxolitinib dose of \>=30 mg but unable to reduce dose further due to lack of efficacy. * If receiving ruxolitinib at the time of screening, must currently be on a…