IXAZOMI: This phase III trial is comparing dexamethasone (a steroid) and an oral drug (MLN9708) with another treatment (chosen by the investigator) in patients with relapsed AmyloidosisA Phase 3, Randomized, Controlled, Open-label, Multicenter, Safety and Efficacy Study of Dexamethasone Plus MLN9708 or Physicians Choice of Treatment Administered to Patients With Relapsed or Refractory Systemic Light Chain (AL) Amyloidosis

Inclusion

1. Male or female participants 18 years or older.

2. Biopsy-proven diagnosis of primary systemic light chain amyloidosis (AL amyloidosis) according to the following standard criteria:

   1. Histochemical diagnosis of amyloidosis, as based on tissue specimens with Congo red staining with exhibition of an apple-green birefringence
   2. If clinical and laboratory parameters insufficient to establish AL amyloidosis or in cases of doubt, amyloid typing may be necessary.
3. Measurable disease as defined by serum differential free light chain concentration (dFLC, difference between amyloid forming [involved] and nonamyloid forming [uninvolved] free light chain [FLC]) ≥ 50 mg/L.

4. Objective, measurable major (cardiac or renal) organ amyloid involvement as defined as follows (amyloid involvement of at least 1 required):

   1. Cardiac involvement is defined as the presence of a mean left ventricular wall thickness on echocardiogram greater than 12 mm in the absence of other potential causes of left ventricular hypertrophy (controlled hypertension is allowed) with a noncardiac biopsy showing amyloid, or a positive cardiac biopsy in the presence of clinical or laboratory evidence of involvement. If there is isolated cardiac involvement, then typing of amyloid deposits is recommended.
   2. Renal involvement is defined as proteinuria (predominantly albumin) >0.5 g/day in a 24-hour urine collection.

   Note: Amyloid involvement of other organ systems is allowed, but not required.

5. Must be relapsed or refractory after 1 or 2 prior therapies. For this protocol, relapsed is defined as progressive disease (PD) documented more than 60 days after last dose; refractory is defined as documented absence of hematologic response or hematologic progression on or within 60 days after last dose of prior therapy.

   1. Participant must not have been previously treated with proteasome inhibitors. (The sponsor reserves the right to open the study to proteasome inhibitor-exposed participants in the future, at some time point after the first interim analysis (IA). In that case, the participant may not be refractory to proteasome inhibitor therapy.)
   2. Given that the physician may select from an offered list of regimens to treat a specific participant, the participant may be refractory to an agent/s listed within the list of offered treatment choices
   3. Must have recovered (ie, ≤ Grade 1 toxicity or participant's baseline status) from the reversible effects of prior therapy
   4. If a participant has received a transplant as his/her first-line therapy, he/she must be at least 3 months post transplantation and recovered from the side effects of the stem cell transplant.

6. Must meet criteria for 1 of the following AL Amyloidosis Risk Stages (as defined by N-terminal proBNP [NT-proBNP] cut-off of < 332 pg/mL and troponin T cut-off of 0.035 ng/mL as thresholds):

   1. Stage 1: both NT-proBNP and troponin T under threshold
   2. Stage 2: either NT-proBNP or troponin T (but not both) over threshold;
   3. Stage 3: both NT-proBNP and troponin T over threshold (but NT-proBNP < 8000 pg/mL)
7. Eastern Cooperative Oncology Group (ECOG) Performance Status ≤ 2.

8. Clinical laboratory values:

   1. Absolute neutrophil count ≥ 1000/µL
   2. Platelet count ≥ 75,000/µL
   3. Total bilirubin ≤ 1.5 upper limit of normal (ULN), except for participants with Gilbert's syndrome as defined by > 80% unconjugated bilirubin and total bilirubin ≤ 6 mg/dL
   4. Alkaline phosphatase ≤ 5 x ULN
   5. Alanine aminotransferase (ALT) or aspartate aminotransferase (AST) ≤3 x ULN
   6. Calculated creatinine clearance ≥ 30 mL/min

9. Female participants who:

   1. If they are of childbearing potential, agree to practice 2 effective methods of contraception, at the same time, from the time of signing the informed consent through 90 days after the last dose of study treatment, AND
   2. Must also adhere to the guidelines of any treatment-specific pregnancy prevention program, if applicable, OR
   3. Agree to practice true abstinence when this is line with the preferred and usual lifestyle of the participant. (Periodic abstinence [eg, calendar, ovulation, symptothermal, post-ovulation methods] and withdrawal are not acceptable methods of contraception.).

   Male participants, even if surgically sterilized (ie, status post vasectomy), who:

   1. Agree to practice effective barrier contraception during the entire study treatment period and through 90 days after the last dose of study drug, AND
   2. Must also adhere to the guidelines of any treatment-specific pregnancy prevention program, if applicable, OR
   3. Agree to practice true abstinence when this is line with the preferred and usual lifestyle of the participant. (Periodic abstinence [eg, calendar, ovulation, symptothermal, post-ovulation methods] and withdrawal are not acceptable methods of contraception.)
10. Voluntary written consent must be given before performance of any study-related procedure not part of standard medical care with the understanding that consent may be withdrawn by the participant at any time without prejudice to future medical care.