This open-label, entry-into-human (EIH) study will evaluate the safety, tolerability, pharmacokinetics (PK), and pharmacodynamics of RO7283420. Escalating doses of RO7283420 will be administered to participants with Acute Myeloid Leukemia (AML) in order to determine the maximum tolerated dose (MTD) and/or recommended Phase II dose (RP2D).

The study will include AML participants with measurable disease, for whom standard-of-care (SOC) is not available. Two Groups of AML participants will be included in this study:

• Group I participants will have hematologic relapse/refractory disease defined as participants not in complete remission (CR) or complete remission with incomplete hematologic recovery (CRi).
• Group II participants will have molecular relapse/persistent disease (participants with a CR or CRi, and a positive MRD based on local multi-parameter flow cytometry (MFC) or molecular assessment).

The study consists of three parts:

• Part A (single-participant dose escalation cohorts) - single participants from Group I will receive increment-based escalating doses until a Grade >=2 AE related to RO7283420 or a clear pharmacodynamic effect
• Part B (multiple-participant dose escalation cohorts) - multiple-participant cohorts of >=3 participants will be enrolled for dose escalation for Group I and Group II independently.
• Part C (dose expansion) - participants will receive the respective identified RP2D for that group.

The treatment period for each participant will be up to 7 months with a maximum number of cycles depending on the dosing frequency the participant receives. Each participant will receive up to 6, 9, and 18 cycles of treatment with RO7283420, when treated with Q3W, every-2-weeks (Q2W), or once-a-week (QW) dosing regimens, respectively. Additional 3, 5, or 9 cycles may be administered for the Q3W, Q2W, and QW dosing regimens, respectively, in case the participants have achieved at least partial remission (PR).

Inclusion Criteria:

• With confirmed diagnosis of primary or secondary AML according to WHO classification 2016, with measurable disease. Eligible participants need to have received standard-of-care (SOC) and have no other SOC options available Participants who are not willing to receive SOC will be not eligible. Two groups of participants (Group I - hematologic relapsed/refractory and Group II - molecular relapsed/refractory) will be included
• Participants who have received hematopoietic stem cell transplant (HSCT) must have the HSCT performed ≥ 90 days prior to the first dose of RO7283420 on Cycle 1 Day 1, having demonstrated hematological engraftment and do not have an active Graft versus Host Disease, not requiring immunosuppressive treatment (including but not limited to cyclosporine, tacrolimus, sirolimus, and mycophenolate), which must be stopped at least 28 days prior to the first dose of RO7283420 on Cycle 1 Day 1
• Eastern Cooperative Oncology Group (ECOG) performance status 0-2
• Peripheral blast counts =< 20,000/mm3 on Cycle 1 Day 1 prior to the first dosing
• Confirmed genotype of HLA-A*02
• Adequate renal (a creatinine clearance of >=50 mL/min as calculated according to the Cockroft-Gault formula) and adequate liver test results
• Male or female participants agree to use contraception and the abstinence requirements to prevent exposure of an embryo to the study treatment

Exclusion Criteria:

• Acute promyelocytic leukemia (APL)
• Core Binding Factor (CBF)-AML Note: participants with r/r CBF-AML after at least 2 salvage attempts can be enrolled into the study
• Group II only: participants with normal karyotype and a favorable molecular profile according to ELN guideline 2017
• Participants with active bacterial, fungal or viral infection considered by the Investigator to be clinically uncontrolled or of unacceptable risk upon the induction of neutropenia (i.e. participants who are or should be on antimicrobial agents for the treatment of active infection)
• Grade >= 2 glomerular proteinuria at screening or on Cycle 1 Day 1 prior to the first dosing.
• Another primary malignancy (other than AML) that requires active therapy. Adjuvant hormonal therapy is allowed
• Clinical evidence or history of central nervous system (CNS) leukemia
• Presence of extramedullary disease at screening
• Current or past history of CNS disease, such as stroke, CNS inflammation, epilepsy, CNS vasculitis, or neurodegenerative disease
• Participants who have a history of clinically significant liver disease, including liver cirrhosis (e.g. Child-Pugh class B and C) or participants who have a history of active or chronic infectious hepatitis unless serology demonstrates clearance of infection
• Participants who might refuse to receive blood products and/or have known hypersensitivity to any of the components of RO7283420, tocilizumab, or dasatinib

Qualified researchers may request access to individual patient level data through the clinical study data request platform (www.vivli.org). Further details on Roche's criteria for eligible studies are available here (https://vivli.org/ourmember/roche/). For further details on Roche's Global Policy on the Sharing of Clinical Information and how to request access to related clinical study documents, see here (https://www.roche.com/research\_and\_development/who\_we\_are\_how\_we\_work/clinical\_trials/our\_commitment\_to\_data\_sharing.htm).