Overview

The purpose of Part 1 (Dose Escalation) of the study is to assess how safe and tolerable JNJ-95804306 is and to find out the most suitable dose (recommended phase 2 dose \[RP2D\]) of JNJ-95804306. The purpose of Part 2 (Dose Expansion) is to further assess the safety of JNJ-95804306 and determine the anti-tumor activity alone and/or when administered in addition to standard of care (SoC) therapy at the putative RP2D(s) regimens in participants with hematological malignancies (cancer that begins in blood-forming tissue, such as the bone marrow, or in the cells of the immune system). For US sites: The purpose of Part 1 (Dose Escalation) of the study is to assess how safe and tolerable JNJ-95804306 is and to find out the most suitable dose (recommended phase 2 dose \[RP2D\]) of JNJ-95804306. The purpose of Part 2 (Dose Expansion) is to further assess the safety of JNJ-95804306 and determine the anti-tumor activity alone at the putative RP2D(s) regimens in participants with hematological malignancies (cancer that begins in blood-forming tissue, such as the bone marrow, or in the cells of the immune system).

Eligibility

Inclusion criteria:

For Arm A:

• Have a diagnosis of: Acute myeloid leukemia (AML) per International Consensus Classification (ICC) 2022 or myelodysplastic syndromes (MDS) per world health organization (WHO) 2022 classified as moderate high, high, or very high-risk per the molecular international prognostic scoring system (IPSSM). All participants must have relapsed or refractory disease and have exhausted or are ineligible for standard therapeutic options
• Body weight greater than or equal to (\>=) 40 kilograms (kg)
• Eastern cooperative oncology group (ECOG) performance status of 0 to 2

For Arm B:

• Have a diagnosis of chronic lymphocytic leukemia or small lymphocytic lymphoma (CLL/SLL) that meets International workshop on chronic lymphocytic leukemia (iwCLL), National cancer institute (NCI) Working Group Guidelines which is relapsed or refractory and requires treatment with no other approved therapies available that would be more appropriate in the investigator's judgment. a. Participants must have received at least 2 prior lines of therapy; b. Participants who have received at least one prior line of therapy but are not eligible or do not have access to standard second line therapies, will be allowed to enroll
• Body weight \>= 40 kg
• ECOG performance status of 0 to 2
• Have clinically measurable disease
• For US sites: Have a diagnosis of CLL/SLL that meets iwCLL, NCI Working Group Guidelines which is relapsed or refractory and requires treatment with no other approved therapies available that would be more appropriate in the investigator's judgment. a. Participants must have received at least 2 prior lines of therapy

Exclusion criteria:

For Arm A:

• Has acute promyelocytic leukemia according to world health organization (WHO) 2016 criteria or known active central nervous system (CNS) involvement of AML/MDS, unless in specific cohort (s) per study evaluation team (SET) decision
• Need for supplemental oxygen use to maintain adequate oxygenation
• Have evidence of uncontrolled systemic viral, bacterial, or fungal infection. Antimicrobial prophylaxis is permitted
• For US sites: Has acute promyelocytic leukemia according to WHO 2016 criteria or known active CNS involvement of AML/MDS

For Arm B:

• Need for supplemental oxygen use to maintain adequate oxygenation
• Have evidence of uncontrolled systemic viral, bacterial, or fungal infection requiring initiation of parenteral treatment as medical intervention
• Developed Richter's transformation or prolymphocytic leukemia
• Known active CNS or leptomeningeal involvement of CLL/SLL