Inclusion Criteria:

• Subject aged ≥ 18 years.
• Histologically confirmed multiple mylemona and/or Plasma Cell Leukemia 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:

• 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.