The purpose of this study is to evaluate the long-term safety and the clinical benefit of pelabresib in patients with hematological and/or solid tumor indications or advanced malignancies. Additionally, participants previously enrolled in studies with pelabresib who received placebo or participants who discontinued pelabresib (for any other reason than participating in this extension study), may be enrolled in this extension study to evaluate the survival and leukemia-free survival (for patients with hematological malignancies) or only the Survival Follow-up (for all the other patients).