A Study to Investigate Safety and Efficacy With SAR445514 in Participants With Relapsed/Refractory Multiple Myeloma (RRMM) and Relapsed/Refractory Light-chain Amyloidosis (RRLCA)

Inclusion Criteria: Participants must have a documented diagnosis of multiple myeloma (Part 1a, 2, and 3a) or light chain amyloidosis (Part 1b and 3b). Participants with RRMM (Part 1, 2, and 3a) * Participants with measurable disease for RRMM * Participants with MM must have received at least 2 prior lines of therapy which must include at least 2 consecutive cycles of a second or third generation immunomodulator, steroid, proteasome inhibitor and anti-CD38 monoclonal antibody (MoAb). * Participants must have documented evidence of progressive disease (PD), as per IMWG 2016 criteria. Participants with RR LCA (Part 1b and 3b) must have received at least 1 prior line of treatment comprising at least 1 proteasome inhibitor. * Participants with measurable disease according to ISA 2012. * Participants must have documented evidence of progressive disease (PD), as per ISA 2012 criteria. * One or more organ impacted by amyloidosis as per National comprehensive cancer network (NCCN) guidelines. For dose escalation, body weight within 40 to 120 kg Capable of giving signed informed consent Exclusion Criteria: Participants are excluded from the study if any of the following criteria apply: Medical conditions * Primary refractory MM defined as participants who never achieved at least a minimal response with any treatment during the disease course * Second primary malignancy Participants with RRMM (Part 1a, 2, and 3a) * For MM participants, primary systemic LCA and plasma cell leukemia * For MM participants, congestive heart failure (New York Heart Association \[NYHA\]) Grade ≥II; cardiomyopathy, active ischemia, or any other uncontrolled cardiac condition Participants with RR LCA (Part 1b and 3b) * For LCA participants, evidence of clinically significant cardiovascular condition, defined as one or more of the following: 1. N-terminal prohormone of brain natriuretic peptide (NT-proBNP) \>8500 ng/mL 2. New York Heart Association (NYHA) classification III or IV heart failure 3. Heart failure that, in the opinion of the Investigator, is not primarily related to LCA cardiomyopathy (including, but not limited to, ischemic heart disease, uncorrected valvular disease, infections) 4. Prior event (history) in the last 6 months of acute coronary syndrome, myocardial infarction or unstable angina as well as participants who during the last 6 months experienced a percutaneous cardiac intervention with stent and/or a coronary artery bypass 5. Hospitalization in the last 4 weeks prior to treatment related to a cardiovascular event 6. Participants with prior history of arrhythmia and/or cardiac conduction disorders for which a pacemaker or an implantable cardioverter defibrillator (ICD) is required but has not been placed. This includes, but may not be limited to, sustained ventricular tachycardia, association of an atrioventricular, or sinoatrial nodal dysfunction * For LCA participants, a systolic blood pressure \<100 mmHg or a diastolic blood pressure \<55 mmHg. * For LCA participants: previous or current diagnosis of symptomatic MM, including the presence of lytic bone disease, plasmacytomas, ≥60% plasma cells in the BM, or hypercalcemia. All participants * Uncontrolled infection within 14 days prior to study treatment. * Known acquired immunodeficiency syndrome-related illness or known human immunodeficiency virus (HIV) disease requiring antiviral treatment or active hepatitis A (defined as positive hepatitis A antigen or positive IgM); HIV serology at screening will be tested for participants in countries where it is required by local regulations. * Uncontrolled or active hepatitis B virus (HBV) infection: participants with positive B surface antigen (HBsAg) and/or HBV deoxyribonucleic acid (DNA) * Active hepatitis C virus (HCV) infection: positive HCV ribonucleic acid (RNA) and negative anti-HCV. Prior/concomitant therapy * Any anti-MM drug treatment within 14 days before study treatment * Prior allogenic hematopoietic stem cell (HSC) transplant with active graft-versus-host disease (GvHD) (GvHD any grade and/or being under immunosuppressive treatment within the last 2 months prior to first IMP). * Any major procedure within 14 days before the initiation of the study treatment * Administration of an anti-CD38 monoclonal antibody (isatuximab or daratumumab) less than 90 days prior to the first administration of study treatment. * Administration of an anti-BCMA agent (including, but not limited to, CAR T-cells, TCEs, antibody drug conjugate) less than 21 days prior to the administration of study treatment. * Unresolved toxicities from prior anticancer therapy, defined as not having resolved to CTCAE Version 5.0 Grade 1, at the exception of residual Grade 2 peripheral neurotoxicity related to bortezomib and/or thalidomide and considered as stable. * Participants with a contraindication to dexamethasone. Prior/concurrent clinical study experience * Received any other investigational drugs or prohibited therapy for this study within 28 days or 5 half-lives from study treatment, whichever is shorter Diagnostic assessments * Hemoglobin \<8 g/dL (5.0 mmol/L) * Platelets \<50 × 10\^9/L (not permissible to transfuse a participant within 1 week prior to the screening platelet count to reach this level). * Absolute neutrophil count (ANC) \<1000 μL (1 × 10\^9/L). * Creatinine clearance \<30 mL/min (Modification of Diet in Renal Disease Formula). * Total bilirubin \>1.5 × upper limit of normal (ULN) (unless the subject has documented Gilbert syndrome in which case direct bilirubin should not be \>2.5 × ULN). * Aspartate aminotransferase (AST/SGOT) or Alanine aminotransferase (ALT/SGPT) \>2.5 × ULN. * Participants with Grade 3 or 4 hypercalcemia (corrected serum calcium of \>12.5 mg/dL; \>3.1 mmol/L; ionized calcium \>1.6 mmol/L; or requiring hospitalization) will not be eligible unless participants recover to Grade 2 or less under anti-hypercalcemia treatment. Other exclusions * Individuals accommodated in an institution because of regulatory or legal order; prisoners or participants who are legally institutionalized * Participant not suitable for participation, whatever the reason, as judged by the Investigator * Sensitivity to any of the study interventions, or components thereof, or drug or other allergy that, in the opinion of the Investigator, contraindicates participation in the study.