The purpose of this study is to find the best dose of the drug and measure the safety and efficacy of LY4152199 in participants with previously treated B-cell malignancies. Participants will have the option to continue taking LY4152199 until the study ends.
Must have a diagnosis of either follicular lymphoma or diffuse large B-cell lymphoma.
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 lymphoma must have at least 1 bi-dimensionally measurable lesion or in the absence of measurable lymphadenopathy, documentation of bone marrow involvement.
Known or suspected peripheral blood involvement by malignant cells with an absolute lymphocyte count of greater than or equal to (≥) 5000 cells per microliter (μL).
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.