SCD Stem Cell Mobilization and Apheresis Using Motixafortide

This study is being done to see if the study drug, motixafortide, is safe in participants with sickle cell disease (SCD). Investigators also want to see if the drug will help the body increase the number of stem cells that can be collected for possible future transplant use. PRIMARY OBJECTIVE * To characterize the safety and tolerability of motixafortide in participants with SCD as determined by the incidence of adverse events (AEs). SECONDARY OBJECTIVES * To characterize the efficacy of a single dose (Part A) or two doses (Part B) of motixafortide for hematopoietic stem cell (HSC) mobilization and apheresis collection in participants with SCD as determined by the yield of CD34+ cells (CD34+ cells/kg). * To measure the mobilization effects of single-day (Part A) or daily dosing (Part B) dosing with motixafortide in the peripheral blood in participants with SCD as determined by peak peripheral blood CD34+ counts * To recommend a phase 2 dosing strategy based on safety, efficacy, and mobilization effects