MBG453 in Lower Risk MDS

Inclusion Criteria: * Lower risk MDS patients (IPSS-R score ≤ 3.5 at diagnosis) who have progressed or are refractory to/intolerant of prior therapy and meet one of the following categories: * RBC transfusion dependent according to IWG criteria must either be unresponsive to prior ESA therapy or have an EPO level \> 500 * Prior HMA therapy * Patients with the following cytopenias who otherwise are felt to require treatment per the treating physician: * Platelets \< 50k/uL * ANC \< 500 cells/uL * Patients with MDS with isolated del(5q) ("5q- syndrome") must have progressed on or not tolerated lenalidomide * Patients who are not felt to be candidates for or lack other standard treatment options. Patients with prior luspatercept exposure are eligible. * Patients with dysplastic type CMML (WBC \< 13,000 cells/uL) meeting the above criteria are eligible; responses will be assessed using MDS criteria * Age ≥18 years. Because no dosing or adverse event data are currently available on the use of MGB453 in participants \<18 years of age, children are excluded from this study, but will be eligible for future pediatric trials. * ECOG performance status ≤2 (see Appendix A). * Participants must have adequate organ and marrow function as defined below within 21 days of treatment: * Total bilirubin ≤ 2 mg/dL (unless due to Gilbert's in which case it must be \<3 mg/dL) * AST(SGOT)/ALT(SGPT) ≤3 × institutional ULN * Creatinine clearance ≥30 mL/min/1.73 m2 (by MDR…