Study to Evaluate Adverse Events and Movement of Intravenously (IV) Infused ABBV-787 in Adult Participants With Relapsed/Refractory (R/R) Acute Myeloid Leukemia (AML)

Inclusion Criteria:

* Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1.
* Laboratory Criteria matching those outlined in the protocol.
* QT interval corrected for heart rate (QTc) \<= 470 msec using Fridericia's correction, and no other clinically significant cardiac abnormalities.
* Documented diagnosis of non-promyelocytic acute myeloid leukemia (AML), per 2022 European Leukemia Net (ELN) criteria.
* Participants with relapsed/refractory (R/R) acute myeloid leukemia (AML) who have been treated with up to 3 prior lines of therapy and are refractory to or intolerant of all established AML therapies that are known to clearly provide clinical benefit at the judgement of the investigator.
* Must have a white blood cell (WBC) count \< 25 × 10\^9 /L prior to initiation of study drug (Note: Hydroxyurea or leukapheresis is permitted to meet this criterion and for use through Cycle 3 to control for hyperleukocytosis.).

Exclusion Criteria:

* Have received a CD33-targeting therapy within 3 months prior to the first dose of ABBV-787.
* Stem cell transplant within 3 months prior to first dose of study drug.
* Have received anticancer therapy including chemotherapy, radiation therapy, immunotherapy, biologic, or any investigational therapy within 14 days or 5 half-lives of the drug (whichever is shorter) prior to the first dose of ABBV-787.
* History of documented pneumonitis that required treatment with systemic steroids within the last 6 months, nor any evidence of active pneumonitis.
* Unresolved toxicity of Grade \>= 2 from prior anticancer therapy, or to levels dictated in the eligibility criteria, with the exception of alopecia.
* Known active severe or poorly controlled acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection.