A Study of LY4152199 in Participants With Previously Treated B-cell Cancers (BAF_FRontier-1 )

• Must have an Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1.
• Estimated life expectancy of greater than or equal to (≥)12 weeks as judged by the Investigator.
• Participants with select tumor types must have measurable or assessable disease as defined below:
• Must have at least 1 bi-dimensionally measurable lesion or in the absence of measurable lymphadenopathy, documentation of bone marrow involvement.
• Must be able to comply with inpatient/outpatient treatment, laboratory monitoring, and required clinic visits for the duration of trial participation.
• Must have adequate organ function.
• Must have failed or were intolerant to at least 2 prior lines of therapy appropriate for available treatment for the specific B-cell malignancy

All Participants

• Known or suspected central nervous system (CNS) involvement by systemic lymphoma.
• Current or history of CNS disease, such as stroke, epilepsy, CNS vasculitis, or neurodegenerative disease
• Any unresolved toxicities from prior therapy greater than National Cancer Institute (NCI) Common Terminology Criteria for Adverse Events (CTCAE) version 5.0 Grade 2 at the time of starting trial treatment except for alopecia.
• Autologous stem cell transplantation within 100 days of this study for post autologous transplant individuals.
• Residual symptoms of neurotoxicity or cytopenias from prior chimeric antigen receptor T-cell therapy (CAR-T) or bispecifics. Exception: Cytopenia related to prior CAR-T or bispecifics allowed if they meet the adequate organ function criteria.
• Known or suspected history of macrophage activation syndrome or hemophagocytic lymphohistiocytosis (HLH).
• Active second malignancies, unless in remission, with life expectancy greater than 2 years with Sponsor approval.
• History of autoimmune disease
• Significant cardiovascular disease
• Active uncontrolled systemic bacterial, viral, fungal, or parasitic infection (except for fungal nail infection), or other clinically significant active disease process
• Vaccination with a live vaccine within 4 weeks prior to signing informed consent form (ICF).
• Have current or had a history of severe allergic or anaphylactic reactions to monoclonal antibody therapy (or recombinant antibody-related fusion proteins).
• Prior treatment with B-cell activating factor receptor (BAFF-R) directed therapies (e.g., monoclonal antibody, CAR-T or bispecific antibody). This exclusion criterion does not apply to participants seeking retreatment.
• Pregnant and/or planning to breastfeed during the trial or within 90 days of the last dose of study intervention.
• Known hypersensitivity to any component or excipient of LY4152199.