A Safety and Efficacy Study to Evaluate Luspatercept in Subjects With Myeloproliferative Neoplasm-associated Myelofibrosis Who Have Anemia With and Without Red Blood Cell-transfusion Dependence

Subjects satisfying the eligibility criteria will be assigned to 1 of the following cohorts
(which are enrolling in parallel) based on their eligibility:

- Cohort 1: will contain up to 20 subjects with anemia only that are not currently
receiving RBC transfusions (these subjects will be referred to as "anemia only"
throughout the protocol, defined as 0 RBC units/84 days immediately up to the C1D1 date)

- Cohort 2: will contain up to 20 subjects that are RBC-transfusion dependent (these
subjects will be referred to as "RBC-transfusion dependent" throughout the protocol,
defined as an average RBC transfusion frequency of 2 to 4 RBC units/28 days over at
least the 84 days immediately up to the C1D1 date)

- Cohort 3A: will contain ≥ 10 subjects (Cohort 3 contains 30 subjects in total) who meet
the eligibility criteria for Cohort 1 (anemia only) while being on a stable dose of
ruxolitinib for at least 112 days immediately prior to the enrollment date

- Cohort 3B: will contain ≥ 10 subjects (Cohort 3 contains 30 subjects in total) who meet
the eligibility criteria for Cohort 2 (RBC-transfusion dependent) while being on a
stable dose of ruxolitinib for at least 112 days immediately prior to the enrollment
date Overall, the study will enroll approximately 70 subjects worldwide.

Inclusion Criteria:

Subjects must satisfy the following criteria to be enrolled in the study (with the
enrollment date defined as the date in which the subject is assigned a cohort in Integrated
Response Technology [IRT]) and receive their first dose of luspatercept:

1. Subject is ≥18 years of age at the time of signing the informed consent form (ICF).

2. Subject has Myeloproliferative neoplasm (MPN)-associated myelofibrosis (PMF, post-
Post-polycythemia vera myelofibrosis (PV MF), and/or Post-essential thrombocythemia
myelofibrosis (post-ET MF)) as confirmed from the most recent local bone marrow biopsy
report according to the World Health Organization 2016 criteria.

3. Subject has anemia defined as:

1. Cohorts 1 and 3A

- Obtain ≥ 3 Hemoglobin (Hgb) levels of ≤ 9.5 g/dL recorded on ≥ 3 different
days, including the day of dosing, in the 84-day period immediately up to
the C1D1 date. There must be ≥ 14 days in between each Hgb measurement. No
subjects with an interval ≥ 42 days between hemoglobin measurements will be
enrolled.

- There must not be any Red blood cell (RBC) transfusions within the 84-day
period immediately up to the C1D1 date.

2. Cohorts 2 and 3B

- Average RBC-transfusion frequency: 2 to 4 RBC units/28 days over at least
the 84 days immediately up to the C1D1 date. There must be no interval > 42
days without ≥ 1 RBC-transfusion.

- Subjects must have a Hgb value of < 13 g/dL on C1D1 prior to luspatercept
administration.

- Only RBC transfusions given when the Hgb ≤ 9.5 g/dL are scored in
determining eligibility.

- RBC transfusions given because of bleeding, infection, or chemotherapy
induced anemia are not scored in determining eligibility.

4. Subject has an Eastern Cooperative Oncology Group (ECOG) performance score of ≤ 2.

5. Subject is not anticipated during the 6 months from the C1D1 date to receive a
hematopoietic cell transplant.

6. A female of childbearing potential (FCBP) for this study is defined as a female who:
1) has achieved menarche at some point, 2) has not undergone a hysterectomy or
bilateral therapy does not rule out childbearing potential) for at least 24
consecutive months (ie, has had menses at any time in the preceding 24 consecutive
months). FCBP participating in the study must:

1. Have 2 negative pregnancy tests as verified by the Investigator prior to starting
study therapy. She must agree to ongoing pregnancy testing during the course of
the study, and after end of study treatment. This applies even if the subject
practices true abstinence* from heterosexual contact.

2. Either commit to true abstinence* from heterosexual contact (which must be
reviewed on a monthly basis and source documented) or agree to use, and be able
to comply with, effective contraception** without interruption, 28 days prior to
starting investigational product, during the study therapy (including dose
interruptions), and for 12 weeks (approximately 5 times the mean terminal
halflife of luspatercept based on multiple-dose pharmacokinetics [PK] data) after
discontinuation of study therapy.

7. Male subjects must:

a. Practice true abstinence (which must be reviewed on a monthly basis) or agree to
use a condom during sexual contact with a pregnant female or a female of childbearing
potential while participating in the study, during dose interruptions and for at least
12 weeks (approximately 5 times the mean terminal half-life of luspatercept based on
multiple-dose PK data) following investigational product discontinuation, even if he
has undergone a successful vasectomy

8. Subject must understand and voluntarily sign an ICF prior to any study-related
assessments/procedures being conducted.

9. Subject is willing and able to adhere to the study visit schedule and other protocol
requirements.

Exclusion Criteria:

The presence of any of the following will exclude a subject from enrollment (with the
enrollment date defined as the date in which the subject is assigned a cohort in Integrated
Response Technology (IRT)):

1. Subject use of hydroxyurea or other drugs with potential effects on hematopoiesis or
ongoing adverse events from previous treatment ≤ 112 days immediately up to the
enrollment date.

a. Systemic corticosteroids are permitted for nonhematological conditions providing
the subject is receiving a stable or decreasing dose for ≥ 84 days immediately up to
enrollment and is receiving a constant dose equivalent to ≤ 10 mg prednisone for the
28 days immediately up to enrollment.

2. Cohort 1 and 2 only: subjects treated with Janus kinase 2 gene (JAK2) inhibitors ≤ 112
days immediately up to the enrollment date or if anticipated/substantial likelihood
for subject to receive ruxolitinib within the first 168 days on the study.

3. Cohort 3 only: subjects not receiving a stable dose of ruxolitinib as part of their
standard-of-care therapy for 112 days immediately up to the enrollment date.

4. Subject use of ESAs or androgenic steroids ≤ 112 days immediately up to the enrollment
date.

5. Initiation of iron chelation therapy (ICT) or change with ICT dose within ≤ 112 days
up to the enrollment date.

6. Subject with anemia from iron deficiency, B12 and folate deficiencies, hemolytic
anemia, infection, or bleeding.

7. Pregnant or breastfeeding females.

8. Subject with blood myeloblasts ≥ 5%.

9. Subject with major surgery within 8 weeks up to the enrollment date. Subject must have
completely recovered from any previous surgery immediately up to the enrollment date.

10. Subject with prior history of malignancies, other than disease under study, unless the
subject has been free of the disease for ≥ 5 years. However, subject with the
following history/concurrent conditions is allowed:

- Basal or squamous cell carcinoma of the skin

- Carcinoma in situ of the cervix

- Carcinoma in situ of the breast

- Incidental histologic finding of prostate cancer (T1a or T1b using the tumor,
nodes, metastasis [TNM] clinical staging system)

11. Subject with prior therapy of luspatercept or sotatercept.

12. Subject participation in any other clinical protocol or investigational trial that
involves administration of experimental therapy and/or therapeutic devices within 30
days immediately up to the enrollment date.

13. Subject with prior hematopoietic cell transplant.

14. Subject with any of the following laboratory abnormalities:

- Neutrophils < 1 x 109/L

- White blood count (WBC) > 100 x 109/L

- Platelets

- Cohorts 1 and 2: < 25 x 109/L

- Cohort 3A and 3B: < 50 x 109/L

- All Cohorts: > 1000 x 109/L

- Estimated glomerular filtration rate < 45 mL/min/1.73 m2 (via the 4-variable
modification of diet in renal disease [Modification of diet in renal disease
(MDRD)] formula)

- Aspartate aminotransferase (AST) or alanine transaminase (ALT) > 3.0 x upper
limit of normal (ULN)

- Direct bilirubin ≥ 2 x ULN

o higher levels are acceptable if these can be attributed to active red blood
cell precursor destruction within the bone marrow (ie, ineffective
erythropoiesis)

- Uncontrolled hyperthyroidism or hypothyroidism

15. Subject with stroke, deep venous thrombosis, pulmonary or arterial embolism within 6
months immediately up to the enrollment date.

16. Subject with diastolic blood pressure ≥ 90 mmHg or systolic blood pressure ≥ 140 mmHg
measured during the Screening Period despite appropriate treatment.

17. Subject with inadequately controlled heart disease and/or have a known left
ventricular ejection fraction <35%.

18. Subject with history of severe allergic or anaphylactic reactions or hypersensitivity
to recombinant proteins or excipients in the investigational product (see luspatercept
Investigator's Brochure (IB)).

19. Subject with uncontrolled systemic fungal, bacterial, or viral infection (defined as
ongoing signs/symptoms related to the infection without improvement despite
appropriate antibiotics, antiviral therapy, and/or other treatment).

20. Subject with human immunodeficiency virus (HIV), evidence of active infectious
Hepatitis B (HepB), and/or evidence of active Hepatitis C (HepC).

21. Subject with any significant medical condition, laboratory abnormality, psychiatric
illness, or is considered vulnerable by local regulations (eg, imprisoned or
institutionalized) that would prevent the subject from participating in the study.

22. Subject with any condition including the presence of laboratory abnormalities, which
places the subject at unacceptable risk if he/she were to participate in the study.

23. Subject with any condition or concomitant medication that confounds the ability to
interpret data from the study.

24. Subject on anticoagulant therapy not under appropriate control or subject not on a
stable dose of anticoagulant therapy for ≥ 8 weeks up to the enrollment date.

25. Subject on anagrelide within 28 days immediately up to the enrollment date.

26. Subject with a major bleeding event (defined as symptomatic bleeding in a critical
area or organ and/or bleeding causing a decrease in hemoglobin of ≥ 2g/dL or leading
to transfusion of ≥ 2 units of packed red cells) in the last 6 months prior to
enrollment.