Impact of Budesonide on Incidence of ≥ Gr2 Diarrhea in Multiple Myeloma (MM) Patients Undergoing Autologous Stem Cell Transplant

Inclusion Criteria: * Subject aged ≥ 18 years. * History of histologically confirmed multiple myeloma and/or Plasma Cell Leukemia diagnosis undergoing ASCT who are determined to be fit by the investigator to undergo ASCT with melphalan 200 mg/m2 or melphalan 140 mg/m2 as conditioning. * Able to provide informed consent and willing to sign an approved consent form that conforms to federal and institutional guidelines. * Adequate organ function as defined as: --Hepatic: * Total Bilirubin ≤ 2 x institutional upper limit of normal (ULN). * AST(SGOT)/ALT(SGPT) ≤ 5 × institutional ULN * For female subjects who have not undergone surgical sterilization: Negative pregnancy test or evidence of post-menopausal status. The post-menopausal status will be defined as having been amenorrheic for 12 months without an alternative medical cause or having undergone surgical sterilization (bilateral oophorectomy or hysterectomy). The following age-specific requirements apply: * Women \< 50 years of age: * Amenorrheic for ≥ 12 months following cessation of exogenous hormonal treatments; and * Luteinizing hormone and follicle-stimulating hormone levels in the post-menopausal range for the institution; or * Underwent surgical sterilization (bilateral oophorectomy or hysterectomy). * Women ≥ 50 years of age: * Amenorrheic for 12 months or more following cessation of all exogenous hormonal treatments; or * Had radiation-induced menopause with last menses \>1 year ago; or * Had chemotherapy-induced menopause with last menses \>1 year ago; or * Underwent surgical sterilization (bilateral oophorectomy, bilateral salpingectomy, or hysterectomy). Inclusion Criteria required for Patients Enrolling in Safety Run-In Cohort only: -Patient must have at least 2.5 x 106 CD34 cells in reserve for use if engraftment is delayed Exclusion Criteria: * Ongoing or current use of oral budesonide at the time of enrollment. * Receiving other investigational agents, unless deemed acceptable after consultation with the PI * Subjects with moderate or severe pre-existing hepatic impairment as classified according to the Child-Pugh system. * Prior history or current diagnosis of inflammatory bowel disease, microscopic colitis at baseline. * Prior history of receiving an allogenic stem cell transplant * Medical, psychiatric, cognitive, or other conditions that may compromise the subject's ability to understand the subject information, give informed consent, comply with the study protocol or complete the study. * Known prior severe hypersensitivity to melphalan or budesonide or any component in their formulations or compounds of similar composition (NCI CTCAE v5.0 Grade ≥ 3). * Subjects taking prohibited medications as described in Section 6.6.1. A washout period of prohibited medications for a period of at least five half-lives or 14 days (whichever is shorter) should occur before the start of treatment.