• General

‣ Must be ≥ 65 years of age and capable of giving signed informed consent, which includes compliance with the requirements and restrictions listed in the ICF and in the protocol, at the time of signing the ICF

⁃ Must have a life expectancy ≥ 12 weeks

⁃ Must have an ECOG performance status of 0-2

⁃ Must have available MT-401-OTS product with a ≥ 2/8 HLA match Disease Characteristics

⁃ For participants with AML:

• Must have a confirmed diagnosis of AML or MDS/AML per 2022 WHO Classification of Haematolymphoid Tumours: Myeloid and Histiocytic/Dendritic Neoplasms or 2022 International Consensus Criteria

∙ Must have intermediate or high-risk disease based on ELN 2022 criteria.

∙ If no targetable mutation is present, must have received 1 prior standard regimen with at least 4 cycles of standard therapy containing an HMA or a standard cytarabine-containing induction therapy

∙ If targetable mutation is present, must have received a regimen that includes commercially available targeted therapy unless unable to tolerate or the participant declines (must be documented in the informed consent). If targeted therapy was not administered as part of first-line of therapy, a second regimen is allowed.

∙ Must have either: ≤ 10% bone marrow blasts and ≤ 5% peripheral blasts during screening and not be considered to have hyperproliferating disease at diagnosis or after treatment OR Evidence of MRD based on evaluation at a local laboratory

⁃ For participants with MDS:

• Must have confirmed diagnosis of MDS based on 2022 WHO Classification of Haematolymphoid Tumours: Myeloid and Histiocytic/Dendritic Neoplasms or 2022 ICC criteria

∙ Must have high-risk or very-high-risk disease based on IPSS-M (ie, not evolved to AML)

∙ Must have received standard treatment with at least 4 cycles of an HMA and have evidence of continued disease, including morphologic disease or MRD-positive

∙ Must have bone marrow blasts ≤ 10% at screening Health Status

⁃ Must have adequate coagulation, hepatic, renal, and cardiac function:

• PT/INR and PTT/aPTT \< 1.3 × ULN

∙ AST and ALT \< 3 × ULN; for participants with leukemic infiltration of the liver (documented by biopsy or imaging), AST and ALT \< 5 × ULN is permitted.

∙ Total bilirubin ≤ 1.5 × ULN unless bilirubin rise is due to Gilbert's syndrome or of nonhepatic origin (2 × ULN is permitted)

∙ eGFR ≥ 40 mL/min by the MDRD formula

∙ LVEF ≥ 45% (prior to apheresis and lymphodepletion) Sex

⁃ Women of childbearing potential are eligible to participate if they agree to the following during the intervention period and for at least 1 year after the last infusion of MT-401-OTS:

• Must use a contraceptive method that is highly effective (ie, with a failure rate of \< 1% per year; see Section 10.3), preferably with low user dependency PLUS

∙ Must agree not to donate eggs (ie, ova and oocytes) for the purpose of reproduction

⁃ Male participants are eligible to participate if they agree to the following during the intervention period and for at least 6 months after the last infusion of MT-401-OTS:

• Must refrain from donating sperm

• PLUS either:

∙ Must be abstinent from intercourse as their preferred and usual lifestyle (abstinent on a long-term and persistent basis) and agree to remain abstinent OR

∙ Must agree to use a male condom AND should also be advised of the benefit for a nonpregnant female partner to use a highly effective method of contraception (see Section 10.3) as a condom may break or leak