Reduce Intensity Conditioning Donor Stem Cell Transplant for the Treatment of Relapsed Multiple Myeloma

This phase II trial studies how well a reduced intensity conditioning regimen after donor stem cell transplant works in treating patients with multiple myeloma that has come back (relapsed). Drugs used in chemotherapy, such as cyclophosphamide, tacrolimus, and mycophenolate mofetil, work in different ways to stop the growth of cancer cells, either by killing the cells, by stopping them from dividing, or by stopping them from spreading. Immunotherapy with monoclonal antibodies, such as daratumumab, may help the body's immune system attack the cancer, and may interfere with the ability of tumor cells to grow and spread. Giving a reduce intensity conditioning regimen consisting of cyclophosphamide, tacrolimus, mycophenolate mofetil, and daratumumab after donor stem cell transplant may improve survival and reduce the risk of multiple myeloma coming back.