Irradiated NK-92 Targets AML Leukemic Stem Cells in Vivo and Gene-Modified CD16+NK-92 Mediates Antibody Dependent Cell Mediated Cytotoxicity (ADCC) Against CD123+ Cells

Abstract Abstract 1909 Introduction: Patients with acute myeloid leukemia (AML) in remission relapse frequently from residual leukemic stem cells (LSCs) that are CD34+CD38-CD123+. NK-92 is an infusible CD16- malignant NK cell line cytotoxic against AML and after irradiation for prevention of NK cell malignancy has minimal adverse events in Phase I trials. Here, we tested irradiated NK-92 (iNK-92) against primary AML in a NOD/SCID gamma null (NSG) xenograft model. To optimize killing of LSCs in vitro we utilized a gene modified CD16+NK-92 against the CD123+ leukemia cell line OCI/AML5 treated with and without an anti-CD123 monoclonal antibody (7G3) to facilitate antibody dependent cell mediated cytotoxicity (ADCC). Methods: NSG mice were irradiated with 225 or 325 cGy before infusion with primary AML blasts (1 or 3×10e6 cells). iNK-92 was started 1 or 10 days post AML infusion and given 1–3 times weekly (15 or 20×10e6 cells/dose) to a total dose of 60–75 ×10e6 cells/mouse. In vitro cytotoxicity was measured by the chromium release assay. OCI/AML5 or primary AML was pretreated with 7G3 or anti-Class I antibody respectively at 10 ug/ml 2 hours before ADCC assays. Results: We developed an NSG AML xenograft model using a CD34+CD38+ primary AML sample containing a small fraction of CD34+CD38- cells that were predominantly CD123+. 3×10e6 primary cells induced leukemia in NSG mice by 6 weeks and maintained comparable potency up to quaternary transplantations with a stable immunophenotype. In vitro cytotoxicity against LSCs was assessed by treating 1×10e6 first passage BM derived primary AML +/− 5×10e6 iNK-92 ×4 hours and injecting into 2 cohorts of 5 mice. At 6 weeks mice were sacrificed and bone marrow harvested. Average leukemic engraftment in the iNK-92 group (79.8, 3.48, 92.1, 81.3, 86.1, Av= 68.6%) was less than untreated AML inoculated group (95.0, 93.4, 19.4, 95, 97.3, Av=80.0%), but not statistically significant (p=0.62). Removing one poorly engrafted outlier mouse from each group yielded a higher engraftment in the control (Av=95.1%) versus the treatment group (Av=84.8%) and was significant (p=0.011). To test iNK-92 to treat engrafted leukemia, NSG mice were infused with 3×10e6 AML cells starting day 10 with 20×10e6 iNK-92 weekly ×6 doses. Survival was improved in the treatment group to near statistical significance (p=0.055), but all mice ultimately succumbed to disease. Addition of IL-2 to iNK-92 did not improve outcomes (p=0.13). 3×10e6 primary AML cells were also infused into 2 cohorts of 4 mice and treated +/− iNK-92 from day 2 and given 15×10e6 cells twice weekly to 75×10e6 total dose. BM (1×10e6 cells) from each of 4 primary recipients in control and treatment was serially transplanted 1:1 into four new NSG mice. BM engraftment occurred in all AML only cohort secondary mice (80.8, 93.3, 80.4, 96.4 Av=87.7%) while one mouse from iNK-92 group was leukemia free with engraftment at background levels of non-injected mice (96.4, 94.7, 1.8, 95.7 Av=72.2%). Proportion of LSCs in secondary transplanted mice was: AML (8.01, 9.48, 8.66, 5.25, Av=7.85%) and AML + iNK-92 (7.13, 3.46, 0.03, 4.0 Av=3.66%) which was statistically significant (p=0.05). CD16+NK-92 killed CD123+ OCI/AML5 cells at effector:target (E:T) ratios of 25:1, 10:1, 5:1 and 1:1 (% lysis +/−SD: 35.0 +/−4.0, 9.0+/−6.6, -2.0 +/−0.1, 1.7 +/−3.3) and was significantly enhanced (2–6x) when targets were coated with anti-CD123 mAb (% lysis +/−SD: 64.3 +/−3.1, 48.5 +/−4.1, 20.9 +/−0.1, 10.1+/−3.3). Further, CD16+NK-92 also killed primary AML at E:T ratios of 25:1, 10:1, 5:1 and 1:1 (% lysis +/−SD: 34.7 +/−4.6, 15.6 +/− 4.7, 11.5 +/− 2.0, 7.7 +/−1.6) which was significantly enhanced (2–3x) by coating with anti-Class I monoclonal antibodies (% lysis +/−SD: 64.8 +/−10.4, 31.1 +/− 8.9, 30.2 +/− 9.4, 23.9+/−2.8). Conclusion: iNK-92 reduces engraftment of AML and improves survival in a primary AML xenograft model. CD16+NK-92 cytotoxicity against primary AML is enhanced with anti-class I antibodies via ADCC, demonstrating that CD16+NK-92 can be redirected against bulk primary leukemia using a highly expressed cell surface marker. Finally, CD16+NK-92 cytotoxicity can be improved against OCI/AML5 with anti-CD123 antibodies via ADCC, providing the first proof-of-principle for the targeting of leukemic stem cells by combining humoral and cellular approaches. Disclosures: No relevant conflicts of interest to declare.