Magenta Therapeutics, Inc. announced that it has received a clinical hold letter from the U.S. Food and Drug Administration (FDA) related to its Investigational New Drug Application (IND) filed in June 2021 to initiate a Phase 1/2 clinical trial of MGTA-117 in patients with acute myeloid leukemia (AML) and myelodysplastic syndrome (MDS). The FDA is requiring that Magenta develop an additional bioassay to be used in conjunction with the PK/PD model to inform dose escalation decisions in addition to safety monitoring. This was the only clinical hold item identified by the FDA and does not relate to the toxicology or manufacturing of MGTA-117. Magenta has initiated the development of the bioassay and intends to work closely with the FDA to determine the application of the bioassay for dose escalation. The FDA has indicated its willingness to continue its engagement on this issue through its “Type A” meeting structure, which Magenta anticipates will help facilitate communication and resolution of the clinical hold. The Phase 1/2 clinical trial of MGTA-117 is intended as an initial evaluation of its safety, pharmacokinetics (PK) and pharmacodynamics (PD) as a single agent in the relapsed/refractory AML and MDS patient population. Magenta anticipates transitioning the study to transplant-eligible patients after adequate data related to the safety, PK and PD of MGTA-117 have been collected in the relapsed/refractory AML and MDS patient population. Magenta also expects to develop MGTA-117 as a targeted conditioning agent for patients with genetic diseases prior to delivery of ex vivo gene therapies.