Inclusion Criteria:

• Subject aged ≥ 18 years.
• Histologically confirmed multiple mylemona 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. 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:

• Prior use of budesonide for any medical condition
• 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 or chronic diarrhea at baseline.
• Prior history of receiving an allogenic stem cell transplant
• Current evidence or diagnosis of microscopic colitis or chronic diarrhea at time of screening
• History of other malignancies with the exception of malignancies for which all treatment was completed ≥ 2 years before study enrollment and are considered to be adequately treated with no evidence of disease or symptoms and/or will not require therapy during the study duration.
• Known active brain metastases or cranial epidural disease.
  • Note: Brain metastases or cranial epidural disease adequately treated with radiotherapy and/or surgery and stable for at least 4 weeks before the first dose of study treatment will be allowed on trial. Subjects must be neurologically asymptomatic and without corticosteroid treatment at the time of the first dose of study treatment.
• 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.