This clinical trial is evaluating whether addition of navtemadlin to ruxolitinib treatment will provide more clinical benefit than ruxolitinib alone for patients with Myelofibrosis who have a suboptimal response to ruxolitinib treatment alone.

Subjects will start by receiving ruxolitinib alone in the run-in period. Those who demostrate a suboptimal response from ruxolitinib alone will then be randomized 2:1 to receive navtemadlin or navtemadlin placebo as add-on treatment to their ongoing ruxolitinib. Randomized means that subjects will be assigned to a group by chance, like a flip of a coin. The study is blinded, meaning the subjects, doctors, central endpoint assessors and sponsor will not know which add on treatment (navtemadlin or navtemadlin placebo) the subject is receiving.

The main aim of this study is to find out how well elritercept works in lowering the need for RBC transfusions. Other aims are to learn how well elritercept works in reducing the need for RBC transfusions over longer periods of time or in adults with high transfusion needs. The study will also check on how safe elritercept is and how well it is tolerated.