Patients will receive intravenous (IV) NKTR-255 in 21 or 28 day treatment cycles. During the Part 1 dose escalation portion of the trial, patients will either receive NKTR-255 as monotherapy, NKTR-255 administered as a doublet with daratumumab subcutaneous (DARZALEX FASPRO TM), or NKTR-255 administered as a doublet with rituximab. After determination of the recommended Phase 2 dose (RP2D) of NKTR-255, NKTR-255 will be evaluated in Part 2. During the Part 2 dose expansion portion of the trial, patients will either receive NKTR-255 as monotherapy, NKTR-255 administered as a doublet with daratumumab subcutaneous (DARZALEX FASPRO TM), or NKTR-255 administered as a doublet with rituximab.

This is a Phase 1 study to evaluate safety and tolerability of NKTR-255 alone and in combination with daratumumab or rituximab.

NKTR-255 is a cytokine that is designed to regulate T and natural killer cell activation, proliferation and promote their anti-tumor effects.

This is a Phase 1, open-label, multi-center, dose escalation, dose expansion, safety follow-up, and survival follow-up of NKTR-255 as a single agent and NKTR-255 in combination with DARZALEX FASPRO TM or rituximab. Study treatment is defined as any investigational treatment(s) or marketed product(s), intended to be administered to a study patient according to the study enrollment.

Part 1 will enroll relapsed/refractory multiple myeloma (MM) and Non-Hodgkin's Lymphoma (NHL) patients. In Part 2, Cohort A will enroll NHL patients who have progressed on a chimeric antigen receptor T-cell (CAR-T) product, Cohort B will enroll MM patients who previously received daratumumab and other anti-CD38 therapies to receive NKTR-255 alone and/or in combination with daratumumab, and Cohort C will enroll indolent Non-Hodgkin's Lymphoma (iNHL) patients who previously received rituximab and other therapies to receive NKTR-255 alone and/or in combination with rituximab.

Key Inclusion Criteria:

• Patients must have relapsed or refractory MM or NHL with no available therapies that would confer clinical benefit for their primary disease.
• For MM patients, measurable relapsed or refractory MM as defined by the IMWG Criteria (Kumar, 2016) following treatment with at least 3 lines of therapy with no other available treatment that would confer benefit.
• For NHL patients, measurable or detectable disease according to International Myeloma Working Group (IMWG) and the Lugano Classification. Extranodal NHL disease that is measurable by fluorodeoxyglucose-positron emission tomography (FDG-PET) imaging only is allowed.
• Estimated glomerular filtration rate (eGFR) ≥ 40 mL/min/1.73 m2.
• Eastern Cooperative Oncology Group (ECOG) performance status score ≤ 2

Patient has the following laboratory test results during Screening:

1. Absolute neutrophil count (ANC) or absolute granulocyte count (AGC) ≥ 1000/µL
2. Platelets ≥ 30,000/µL
3. Hemoglobin ≥ 8g/dL
4. Absolute lymphocytes ≥ 500/µL
5. Leukocytes ≥ 3000/µL

Patients are eligible who also meet all the following criteria in these cohorts of Part 2:

NKTR-255 Monotherapy NHL Group Only:

• Patients with NHL who received a commercially approved CD19 CAR-T product and had PD. The first dose of NKTR-255 will be administered within 30 days of the PD.

NKTR-255 with Daratumumab MM Group Only :

• Patients with MM must have had previous exposure to proteasome inhibitor, immunomodulatory agent (IMiD), and anti-CD38 therapy.
• Patients who previously received daratumumab or other anti-CD38 therapies must have at least 3 months washout.

NKTR-255 with Rituximab Group iNHL Group Only:

• Patients with relapsed or refractory iNHL who previously progressed during or following 1 or more prior systemic rituximab-containing (or another treatment with an anti-CD20 antibody-containing) regimens for lymphoma.

Key Exclusion Criteria:

• Patients who have an active, known, or suspected autoimmune disease.
• Any treatment-related neurotoxicity or cytokine release syndrome (CRS) prior to enrollment into the study should return to baseline before NKTR-255 treatment.
• Active central nervous system (CNS) involvement with NHL.
• Patients who have been previously treated with prior interleukin-2 or interleukin-15.
• Patients who received daratumumab or other anti-CD38 therapies previously must have 3 months washout.

NOTE: Other protocol defined Inclusion/Exclusion criteria may apply