Inclusion Criteria:

        Inclusion Criteria:

          -  Must be able to complete the Myelofibrosis Symptom Assessment Form (MFSAF) v4.0 on at
             least 4 out of 7 days prior to randomization.

             -- Has at least 2 symptoms with a score >= 3 or a total score of >= 12, as measured by
             the MFSAF v4.0.

          -  Documented diagnosis of primary myelofibrosis (MF) as defined by the World Health
             Organization (WHO) classification, post polycythemia vera (PPV)-MF, or post essential
             thrombocytopenia (PET)-MF .

          -  Classified as intermediate-2 or high-risk MF, as defined by the Dynamic International
             Prognostic Scoring System Plus (DIPSS+).

          -  Must currently be on treatment or have received prior treatment with a single Janus
             Kinase 2 (JAK2) inhibitor, ruxolitinib, and meet one of the following criteria (in
             addition to the minimum splenomegaly and symptom burden also required for
             eligibility):

               -  Treatment with ruxolitinib for >= 24 weeks that was stopped due to lack of spleen
                  response (refractory), or loss of spleen response or symptom control after a
                  previous response (relapsed), or was continued despite relapsed/refractory
                  status.

               -  Treatment with ruxolitinib for < 24 weeks with documented disease progression
                  while on therapy as defined by any of the following:

                    -  Appearance of new splenomegaly that is palpable to at least 5 cm below the
                       left costal margin (LCM) in participants with no evidence of splenomegaly
                       prior to the initiation of ruxolitinib.

                    -  A >= 100% increase in the palpable distance below the LCM in participants
                       with measurable spleen distance 5 to 10 centimeters (cm) prior to the
                       initiation of ruxolitinib.

                    -  A >= 50% increase in the palpable distance below the LCM in participants
                       with measurable spleen distance > 10 cm prior to the initiation of
                       ruxolitinib.

                    -  A spleen volume increase of >= 25% (as assessed by Magnetic Resonance
                       Imaging [MRI] or Computed Tomography [CT] scan) in participants with a
                       spleen volume assessment prior to the initiation of ruxolitinib.

          -  Prior or current treatment with ruxolitinib for >= 28 days with intolerance defined as
             new RBC 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.

          -  Eastern Cooperative Oncology Group (ECOG) performance status of 0, 1, or 2.

          -  Splenomegaly defined as palpable spleen measurement >= 5 cm below left costal margin
             or spleen volume >= 450 cm3 as assessed centrally by MRI or CT scan.

          -  Baseline platelet count >= 100 × 10^9/L.

        Exclusion Criteria:

          -  Received prior treatment with a BH3-mimetic compound, bromodomain and extra-terminal
             (BET) inhibitor, or prior use of > 1 JAK2 inhibitor or stem cell transplant.

          -  Eligible for allogeneic stem cell transplantation at the time of study entry.

          -  Receiving medication that interferes with coagulation or platelet function within 3
             days prior to the first dose of study drug or during the study treatment period except
             for low dose aspirin (up to 100 mg daily) and low molecular weight heparin (LMWH).

          -  Receiving anticancer therapy for an active malignancy or MF including chemotherapy,
             radiation therapy, hormonal therapy within 30 days prior to first dose of study drug,
             and during the study treatment period (other than any overlapping therapy as part of
             the selected BAT).