Additional Study Information: This is a clinical research study for subjects with Myelofibrosis (Primary, Post-Polycythemia Vera, and Post-Essential Thrombocytopenia) and thrombocytopenia. The purpose of this study is to determine if pacritinib is better at reducing spleen size and a reduction in symptoms than the best available therapy. This is a randomized, controlled study. Randomized means individuals will be selected by chance to enter one of the three treatment groups - either the experimental drug Pacritinib, available in two different doses, or best available therapy. Individuals will be taking active study medication, either Pacritinib or best available therapy regardless of the treatment group.