Sotatercept in Treating Patients With Myeloproliferative Neoplasm-Associated Myelofibrosis or Anemia

Inclusion Criteria: * MPN-associated myelofibrosis * Anemic patient OR red blood cell (RBC)-transfusion-dependent patient * Alanine aminotransferase (ALT) (serum glutamate pyruvate transaminase \[SGPT\]) and aspartate aminotransferase (AST) (serum glutamic oxaloacetic transaminase \[SGOT\]) equal to or less than 2.5 x upper limit of normal (ULN), or equal to or less than 4 x ULN (if upon judgment of the treating physician, it is believed to be due to extramedullary hematopoiesis \[EMH\] related to myelofibrosis \[MF\]) * Direct bilirubin equal to or less than 1.5 x ULN; or equal to or less than 2 x ULN (if upon judgment of the treating physician, it is believed to be due to extramedullary hematopoiesis related to MF) * Creatinine clearance equal to or more than 50 mL/min * Treatment-related toxicities from prior therapies must have resolved to grade equal to or less than 1 * Women of childbearing potential and men must agree to using medically approved (i.e., mechanical or pharmacological) contraceptive measure for at least 112 days following the last dose of sotatercept (ACE-011); males must agree to use a latex condom or non-latex condom NOT made of natural (animal) membrane during any sexual contact with females of childbearing potential or a pregnant female while participating in the study and for at least 112 days following the last dose of sotatercept (ACE-011), even if he has a vasectomy * For cohort of patients that are already on ruxolitinib therapy: on therapy with ruxolitinib for at least for 6 months, and on stable dose for last 2 months, before starting therapy with sotatercept Exclusion Criteria: * Serious medical condition or psychiatric illness that would prevent, (as judged by the treating physician) the subject from signing the informed consent form or any condition including the presence of laboratory abnormalities, which places the subject at unacceptable risk if he/she were to participate in the study or confounds the ability to interpret data from the study * Pregnant or lactating female * Known positive for human immunodeficiency virus-1 (HIV-1), or active infection with hepatitis-B or -C * Use of any MPN-associated myelofibrosis-directed therapy within 2 weeks prior to study day 1 (other than ruxolitinib at a stable dose for patients in the combination cohort as stated in inclusion criteria) * Symptomatic congestive heart failure, unstable angina, or unstable cardiac arrhythmia * Prior sotatercept * Major surgery within 4 weeks prior to day 1 * Severe allergic or anaphylactic reactions or hypersensitivity to recombinant proteins or excipients in the investigational product * Uncontrolled hypertension (systolic blood pressure \[SBP\] equal to or more than 140 or diastolic blood pressure \[DBP\] equal to or more than 90)