A Phase 2 Study to Evaluate the Efficacy and Safety of GS-6624 in Adult Subjects With Primary, Post Polycythemia Vera or Post Essential Thrombocythemia Myelofibrosis

Inclusion Criteria:

- Must be diagnosed with PMF or post ET/PV MF with intermediate-1, intermediate-2 or
high risk disease according to the IWG prognostic scoring system, or if with low risk
disease then with symptomatic splenomegaly that is ≥ 10 cm below left costal margin
by physical exam.

- Must have adequate organ function as demonstrated by the following:

- ALT (SGPT) and/or AST (SGOT) ≤ 2.5x upper limit of normal (ULN), or ≤ 4x ULN (if
upon judgment of the treating physician, it is believed to be due to extramedullary
hematopoiesis [EMH] related to MF);

- Direct bilirubin ≤ 1.5 x ULN; or ≤ 2x ULN (if upon judgment of the treating
physician, it is believed to be due to extramedullary hematopoiesis [EMH] related to
MF);

- Serum creatinine ≤ 2.5 mg/dL. 2.5 mg/dL.

- In Stage 2, subjects must be on ruxolitinib for at least 8 weeks and on a stable dose
for at least 4 weeks.

- ECOG performance status (PS) ≤ 2

- Treatment-related toxicities from prior therapies must have resolved to Grade ≤ 1

- Women of childbearing potential and men must agree to using one medically approved
(ie, mechanical or pharmacological) contraceptive measure and have their partners
agree to an additional barrier method of contraception for the duration of the study
and for 90 days after the last administration of study drug. Please refer to Section
11 for a definition of female of child bearing potential and a list of acceptable
contraceptive methods for this study.

Exclusion Criteria:

- Any 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.

- Known history of human immunodeficiency virus (HIV), hepatitis C, or hepatitis B.

- History or presence of any form of cancer within the 3 years prior to enrollment,
with the exception of excised basal cell or squamous cell carcinoma of the skin, or
cervical carcinoma in situ or breast carcinoma in situ that has been excised or
resected completely and is without evidence of local recurrence or metastasis.

- Participation in an investigational drug or device trial within 2 weeks prior to
study Day 1 or within 5 times the half-life of the investigational agent in the other
clinical study, if known.

- Use of any cytotoxic chemotherapeutic agents (eg, hydroxyurea), corticosteroids
(prednisone ≤ 10 mg/day or corticosteroid equivalent is allowed), or immunodulators
(eg, thalidomide) within 2 weeks and interferon use within 4 weeks prior to study Day
1.

- Symptomatic congestive heart failure (New York Heart Association Classification >
Class II), unstable angina, or unstable cardiac arrhythmia requiring medication.

- History of surgery within 2 weeks prior to enrollment or anticipated surgery during
the study period.

- Any other condition that might reduce the chance of obtaining data required by the
protocol or that might compromise the ability to give truly informed consent.