Additional Study Information: This is a Phase 2 study for patients with myelofibrosis previously treated with one or more Janus Kinase(JAK) Inhibitor. A JAK inhibitor targets a certain pathway that may be causing cancer to grow, bystopping or inhibiting the cycle of development of the pathway. There is one JAK inhibitor that has beenapproved for the treatment of patients with myelofibrosis, called ruxolitinib. This research study is evaluating PF-04449913 in patients with myelofibrosis. PF-04449913 is in a class of drugs known as hedgehog inhibitors and is designed to stop the growth of certain cancer stem cells which may be allowing the cancer to grow. The hedgehog pathway is different than the JAK pathway. PF-04449913 is investigational, which means it has not been approved by the FDA to treat patients with myelofibrosis.This study will be conducted in two parts: a lead-in phase and a randomized phase. The lead-in phase will be done to confirm that PF-04449913 100 mg daily dose is safe and tolerated in patients with previously treated myelofibrosis. The randomized phase will compare the effects of the study drug, PF-04449913 versus placebo to find out which is better for treating patients with myelofibrosis who have been previously treated with one or more JAK inhibitors.