This is a 2-phase study during which patients with select myeloid leukemias or advanced myelodysplastic syndrome (MDS), who have failed, refused or are not eligible for standard treatment, will receive investigational study drug ARRY-520.

The study has 3 parts. The first phase of the study, Phase 1, has 2 parts. In the first part of Phase 1, patients with select myeloid leukemias or advanced MDS will receive increasing doses of study drug on different schedules in order to achieve the highest dose possible that will not cause unacceptable side effects. Approximately 30 patients (per schedule) from the US will be enrolled in Part 1 (Completed). In the second part of Phase 1, patients with advanced MDS will receive the best dose of study drug and schedule determined from the first part of the study. Approximately 10 patients from the US will be enrolled in Part 2 (Completed).

In the third part of the study, Phase 2, patients with acute myeloid leukemia (AML) or advanced MDS will receive the best dose of study drug and schedule determined from the first part of the study and will be followed to see what side effects the study drug causes and to see what effectiveness it has, if any, in treating the cancer. Approximately 40 patients from the US will be enrolled in Part 3 (Withdrawn).

Key Inclusion Criteria (Part 2):

• Patients with either Intermediate-2 or High risk MDS or with AML (>20% bone marrow blasts) with stable low or normal white blood cell count (WBC). Patients should have failed one prior chemotherapy regimen which should have included a hypomethylating agent.
• Eastern Cooperative Oncology Group (ECOG) performance status (PS) of 0, 1 or 2.
• Discontinuation of prior treatment at least 2 weeks prior to the start of the study.
• Adequate hepatic and renal function.
• Additional criteria exist.

Key Exclusion Criteria (Part 2):

• Concurrent cytotoxic therapy, or biological, endocrine and immunological response modifiers.
• Previous radiation to >25% of bone marrow.
• Other active malignancies.
• Known positive serology for the human immunodeficiency virus (HIV).
• Central nervous system involvement as documented by spinal fluid cytology.
• Active, uncontrolled infection.
• Additional criteria exist

Pfizer will provide access to individual de-identified participant data and related study documents (e.g. protocol, Statistical Analysis Plan (SAP), Clinical Study Report (CSR)) upon request from qualified researchers, and subject to certain criteria, conditions, and exceptions. Further details on Pfizer's data sharing criteria and process for requesting access can be found at: https://www.pfizer.com/science/clinical\_trials/trial\_data\_and\_results/data\_requests.