Study of the Safety and Efficacy of STI-6129 in Patients With Relapsed or Refractory Systemic AL Amyloidosis

Inclusion Criteria: * Confirmed diagnosis of AL amyloidosis by tissue biopsy of an involved organ, or a surrogate site such as abdominal fat demonstrating amyloid deposition by mass spectrometry * The presence of a monoclonal light chain protein in serum * Relapsed or refractory AL amyloidosis is defined as patients who have received ≥ 2 lines of treatment. Patients must have received at least one proteasome inhibitor during their prior therapy. Patients who have received prior daratumumab treatment or prior stem cell transplantation remain eligible. Patients may have relapsed with disease progression or have been refractory to their last prior line of treatment. Progression of disease that develops \> 60 days after the last dose of a treatment regimen in a patient who achieved at least a partial response (PR) defines a relapse. Refractory systemic AL amyloidosis is defined as the development of disease progression during therapy with an anti-AL amyloidosis treatment regimen or within 60 days of the last dose of an anti-AL amyloidosis treatment regimen or the achievement of less than a PR after ≥ 2 cycles * Measurable disease defined as the finding by serum-free light chain (FLC) assay that the difference between the involved and uninvolved FLC (dFLC) is ≥ 50 mg/L * Pulse oximetry ≥ 92% on room air * ECOG performance status of 0, 1, or 2 * Willing to comply with the study schedule and all other protocol requirements * Females of childbearing potential (FCBP) must have 2 negative pregnancy tests prior to treatment. All heterosexually active FCBP and all heterosexually active male patients must agree to use effective methods of birth control throughout the study Exclusion Criteria: * Isolated vascular amyloid in a bone marrow biopsy or a plasmacytoma specimen or isolated soft tissue involvement (localized AL amyloidosis) * Presence of non-AL amyloidosis * A diagnosis of multiple myeloma * A diagnosis of other malignancies if the malignancy has required therapy within the last 3 years or is not in complete remission. Exceptions are non-metastatic basal cell or squamous cell carcinomas of the skin or prostate cancer that does not require treatment * Treatment with an allogeneic hematopoietic stem cell transplantation (HSCT) within 6 months prior to the planned infusion of STI-6129, or active graft-versus-host disease (GVHD) following the allogeneic transplant, or a requirement for currently receiving immunosuppressive therapy following the allogeneic transplant * Revised Mayo Clinic AL amyloidosis stage \> 3 * New York Heart Association (NYHA) class \> 2 * Left ventricular ejection fraction (LVEF) \< 40% * Patients with mean left ventricular wall thickness ≥ 12 mm and/or intraventricular septal thickness \> 25 mm by echocardiogram in the absence of hypertension or valvular heart disease * Patients with NT-proBNP ≥ 1800 ng/L or BNP ≥ 400 ng/L, or cTNT ≥ 0.025 μg/L will be excluded in the dose-escalation stage of the study and can only be included in the PK and expansion stages after evaluation by cardiology and discussion with the principle investigator regarding the risk associated with the treatment * Abnormal baseline hematological laboratory results at Screening: * Hemoglobin \< 8.0 g/dL * Platelet count \< 50,000/μL * Absolute neutrophil count (ANC) \< 1000/μL * Abnormal baseline chemistry laboratory results at Screening: * Serum creatinine ≥ 2.0 mg/dL or estimated creatinine clearance \< 60 mL/min (using the Cockcroft-Gault equation) * Serum aspartate aminotransferase (AST) or alanine aminotransferase (ALT) \> 3x the upper limit of normal (ULN) or serum total bilirubin \> 1.5x ULN (except for patients in whom hyperbilirubinemia is attributed to Gilbert's Syndrome) * INR or aPTT \> 1.5x ULN within 1 week prior to the infusion of STI-6129, unless on a stable dose of an anticoagulant * Pregnant or breastfeeding * Active bacterial, viral, or fungal infection within 72 hours of the infusion of STI-6129; patients with ongoing use of prophylactic antibiotics, antifungal agents, or antiviral agents remain eligible as long as there is no evidence of active infection * Have human immunodeficiency virus (HIV) infection, human T-cell leukemia virus type 1 (HTLV1) infection, or hepatitis B virus (HBV) or hepatitis C virus (HCV) viremia or are at risk for HBV reactivation (at risk for HBV reactivation is defined as being HBs antigen positive, or anti-HBc-antibody positive), or are positive for HBV deoxyribonucleic acid (DNA). HCV ribonucleic acid (RNA) must be undetectable by laboratory test * QTcF \> 470 msec on a baseline ECG * Any condition including the presence of laboratory abnormalities that places the patient at unacceptable risk if the patient was to participate in the study