NKTR-255 enhances in vitro ADCC of NK cells and synergizes with daratumumab to reduce MM growth in a humanized MM mouse model. (A) NK cells isolated from peripheral blood mononuclear cells of patients with MM treated for 14 days with NKTR-255 at indicated doses (or rhIL-15 as positive control) were employed to test ADCC. MM cell lines were labeled with CTV stain, pre-treated with or without daratumumab, and co-incubated with the effector cells in a 4-hour in vitro cytotoxicity assay. The percentage of target cell killing over background (spontaneous lysis of target cells not exposed to NK cells) is shown and results are depicted as mean ± standard deviation. (B-C) A fully humanized mouse model engrafted with NCI-H929 MM tumor cell line was used to study the efficacy of NKTR-255, daratumumab, or their combination on tumor growth and NK cell proliferation. Humanized mice were boosted with the human recombinant cytokines IL-15, IL-3, IL-4 and GM-CSF. One week after boost, mice were engrafted with 5 × 10⁶ NCI-H929 cells subcutaneously. When tumors reached an average volume of 50 mm³ these mice were randomized into vehicle, NKTR-255 (0.3 mg/kg weekly), daratumumab (5 mg/kg weekly), or NKTR-255 (0.3 mg/kg weekly) + daratumumab (5 mg/kg weekly). Tumor volume (B) was monitored 3 times a week using a caliper. Blood samples were collected at the end of the experiment for immunophenotyping, and the absolute count of NK cells was assessed per cohort of treatment (C). N = 5 mice/group. ∗P < .05, ∗∗P < .01, ∗∗∗P < .001, ^nsP ≥ .05. CNT, control; MM, multiple myeloma; ns, not significant; NK, natural killer; SD, standard deviation.