This is a Phase 1, two-part, multicenter study to evaluate ELA026 in participants ≥18 years old with relapsed/refractory TCM following any line of prior therapy who are eligible for investigational treatments.

This is the first clinical study of ELA026 as a therapy for R/R TCMs. The study will begin with an initial dose finding component (Part 1, Phase 1a), enrolling approximately 6 to 18 participants (up to maximum of 24 participants) to identify up to 2 dosing regimens with an acceptable safety profile. Upon completion of Part 1, the study will proceed to the cohort expansion phase (Part 2, Phase 1b) to further evaluate these regimens.

Parts 1 and 2 will include the following study periods:

• Screening period: up to 28 days
• Treatment period: up to 6 cycles or 24 weeks (1 cycle = 28 days)
• Optional extension period: On a case-by-case basis, participants experiencing ongoing clinical benefits may continue treatment beyond 6 cycles, up to an additional 6 cycles, with approval by the Sponsor
• Safety follow-up: 28 days after the last dose of study treatment
• Survival follow-up: up to 2 years from the end of treatment

Inclusion Criteria:

• Age ≥18 years Participants with a confirmed histologic diagnosis of a TCM who are R/R following any line of prior therapy (participants with CTCLs should have received at least 2 prior lines of systemic therapy for R/R CTCL) and eligible for investigational therapies
• Presence of measurable disease by clinical examination, radiologic imaging (computed tomography, magnetic resonance imaging, or whole body FDG-PET scans), and/or in bone marrow aspirate/biopsy
• Eastern Cooperative Oncology Group performance score of ≤2
• Anticipated life expectancy >6 months per investigator judgment

Exclusion Criteria:

• Participants who are eligible for standard of care or approved therapeutic options for R/R TCMs with established clinical benefit
• Organ dysfunction as defined in the protocol
• Participants with hemophagocytic lymphohistiocytosis (HLH) based on HLH2004 diagnostic criteria
• Participants receiving or planning to start immunotherapy or immune effector cell therapy (such as chimeric antigen receptor [CAR] T-cell therapy, T-cell engagers, or programmed cell death protein 1 [PD1]/programmed cell death ligand 1 [PD-L1] inhibitors)
• Allogeneic hemopoietic stem cell transplant within 100 days prior to the first dose of ELA026 and currently receiving systemic immunosuppressive therapy
• Women of childbearing potential who are planning to become pregnant or are breastfeeding during the conduct of the study, including 60 days after last dose of study drug
• Male participants whose partners are women of childbearing potential and who are planning to become pregnant during the conduct of this trial by the male partner, including within 60 days after the last dose of study drug