REGN7257 in Adult Patients With Severe Aplastic Anemia That Is Refractory to or Relapsed on Immunosuppressive Therapy

This study is researching an experimental drug called REGN7257 (called "study drug"). The study is focused on patients who have severe aplastic anemia (SAA), a disease of the bone marrow resulting in an impairment of the production of blood cells. The main purpose of this two-part study (Part A and Part B) is to test how safe and tolerable REGN7257 is in patients with SAA in which other Immunosuppressive therapies (ISTs) have not worked well. The study is looking at several other research questions to better understand the following properties of REGN7257: * Side effects that may be experienced by participants taking REGN7257 * How REGN7257 works in the body * How much REGN7257 is present in blood after dosing * If REGN7257 works to raise levels of certain blood counts after treatment * How quickly REGN7257 works to raise levels of certain blood counts * In patients for whom REGN7257 works to raise levels of certain blood counts after treatment, how many continue to show such a response throughout the study * If REGN7257 works to lower the number of platelet and red blood cell transfusions needed * How REGN7257 changes immune cell counts and composition * How the body reacts to REGN7257 and if it produces proteins that bind to REGN7257 (this would be called the formation of anti-drug antibodies \[ADA\])