ALBERT

Description: Survival expectations for acute myeloid leukemia (AML) patients remain poor, highlighting the need for further treatment options. The majority of AML blasts express CD123, the alpha subunit of the IL-3 receptor, which regulates the proliferation, survival and differentiation of hematopoietic cells. CD123 is also robustly expressed on leukemic stem cells and is a marker for minimal residual disease (MRD, Roug et al. 2012). Poor prognosis has previously been associated with elevated expression of CD123 on leukemic stem cells and blasts (Vergez et al. 2011, Testa et al. 2002). These findings identify CD123 as a rational therapeutic target in AML. Here we report the preclinical development of a novel CD123-directed immunoconjugate SGN-CD123A, consisting of a humanized anti-CD123 antibody conjugated to a highly potent DNA binding pyrrolobenzodiazepine (PBD) dimer drug via a protease-cleavable dipeptide linker. An engineered cysteine on each heavy chain attaching the PBD dimer to the antibody allows uniform drug loading of approximately two PBD dimers per antibody. Fluorescence microscopy studies show that SGN-CD123A is rapidly internalized and traffics to lysosomes within hours of binding to CD123-positive AML cells. Uptake of the antibody-drug-conjugate (ADC) induced DNA damage as measured by dose-and time-dependent increases in the phosphorylation of histone 2AX (γH2AX) and cell death associated with G2-M cell cycle arrest, caspase-3 activity, formation of cleaved poly ADP-ribose polymerase, and DNA fragmentation in target cells. The anti-leukemic activity of SGN-CD123A was assessed in cytotoxicity assays in 12 AML cell lines and 23 primary AML patient samples with variable cytogenetic abnormalities (favorable, intermediate and adverse) and multi-drug resistance (MDR) status. SGN-CD123A was highly active in 11 of 12 AML cell lines tested (mean IC50, 6 ng/ml; range of 0.02 to 38 ng/ml), including 4 of 5 MDR-positive cell lines, whereas it was inactive in CD123-negative HEL92.1.7 AML cells. SGN-CD123A was also active against 20 of 23 primary samples isolated from AML patients (mean IC50 of responsive samples, 0.8 ng/mL; range of 0.06 to 2.5 ng/ml). In both AML panels, molecular abnormalities, including the presence of a p53 mutation, FLT3-ITD, as well as MDR positivity, did not affect the in vitro cytotoxic activity of SGN-CD123A. In vivo antitumor activity was evaluated in AML xenograft models established with CD123-positive, MDR-negative Molm-13, HNT-34, and THP-1 cell lines and the MDR-positive KG-1 cell line. In all of the in vivo models, a single dose of SGN-CD123A delivered significant antitumor activity. SGN-CD123A dosed once at 10 mcg/kg yielded complete cures and significant survival advantage in the Molm-13 disseminated model of AML (p 〈 0.0001 compared to untreated or control ADC groups). Durable complete regressions were observed with a single dose of 25 or 75 mcg/kg in the MDR-negative HNT-34 subcutaneous model (p =0.0019 to control ADC group). In the THP-1 model, a single 100 mcg/kg dose of SGN-CD123A yielded durable complete regressions in 2 of 8 mice (p=0.0003 to untreated) whereas a higher dose of 300 mcg/kg gave complete tumor regressions in all mice (p 〈 0.0001 to untreated group). SGN-CD123A was also effective in a MDR-positive model of AML. A single dose of 100 mcg/kg SGN-CD123A significantly decreased tumor growth (p=0.003 to controls) whereas a single dose of 300 mcg/kg yielded durable complete regressions compared to the control groups in the KG-1 subcutaneous model of MDR-positive AML (p =0.008). Early evidence of the antitumor activity of SGN-CD123A was found in tumors harvested from THP-1 mice. Within 48h of dosing with SGN-CD123A, tumor cells showed elevated levels of the DNA damage marker γH2AX and changes in nuclear morphology. These data demonstrate that SGN-CD123A exhibits significant antitumor activity against a broad panel of primary AML samples and in preclinical models of MDR-positive AML that are characteristically resistant to chemotherapy. CD123-directed delivery of PBD may represent a promising new approach for the treatment of AML. Disclosures Sutherland: Seattle Genetics, Inc.: Employment. Yu:Seattle Genetics, Inc: Employment, Equity Ownership. Walter:Seattle Genetics, Inc: Consultancy, Research Funding. Westendorf:Seattle Genetics, Inc: Employment. Valliere-Douglass:Seattle Genetics, Inc: Employment. Pan:Seattle Genetics, Inc: Employment. Sussman:Seattle Genetics, Inc: Employment. Anderson:Seattle Genetics, Inc: Employment. Zeng:Seattle Genetics, Inc: Employment. Stone:Seattle Genetics, Inc: Employment. Klussman:Seattle Genetics, Inc: Employment. Ulrich:Seattle Genetics, Inc: Employment. Jonas:Seattle Genetics, Inc: Employment. Senter:Seattle Genetics, Inc: Employment. Drachman:Seattle Genetics, Inc: Employment. Benjamin:Seattle Genetics, Inc: Employment.

Description: SGN-33, a humanized IgG1 anti-CD33 antibody that targets CD33, a sialoadhesion family member expressed on myeloid precursor cells, macrophages, and monocytes, is currently in Phase I clinical trials for acute myeloid leukemia (AML) and myelodysplastic syndrome (MDS). To further characterize and define the mechanism of action of SGN-33, we undertook a series of experiments in AML cell lines and in cultures of primary normal human monocytes and macrophages. SGN-33 has previously been shown to eliminate tumor cells through antibody-mediated effector functions including Antibody-Dependent Cellular Cytotoxicity (ADCC) and Complement Dependent Cytotoxicity (CDC). We have now demonstrated that SGN-33 binding also initiates Antibody-Dependent Cellular Phagocytosis (ADCP). Because CD33 has been hypothesized to mediate signal transduction, we explored the biochemical effects of SGN-33 binding on AML cell lines and normal monocytes and macrophages. SGN-33 binding stimulated tyrosine phosphorylation of CD33 and recruitment of SHP-1, with maximal effects between 5 to 30 minutes. Initiation of signaling by SGN-33 was not dependent upon antibody crosslinking. Furthermore, SGN-33 exposure significantly reduced the syntheses of pro-inflammatory cytokines (TNF-a, IL-6, IL-1b) and chemokines (RANTES, MCP-1, IL-8) by macrophages cultured in the presence of tumor cell conditioned media. Production of these cytokines by monocytes and macrophages activated in vitro by IFN-g and TGF-b were also blocked by SGN-33 binding (up to 100% reduction in TNF-a levels in vitro). These data demonstrate that the mechanisms of action of anti-CD33 immunotherapy may be broader than previously known. Because pro-inflammatory cytokines have been implicated in the growth and survival of tumor cells in patients, the capacity of SGN-33 to block these factors may yield clinical benefit. This hypothesis is currently being evaluated in clinical studies of SGN-33.

Description: Patients with relapsed or refractory B-cell non-Hodgkin lymphoma (r/r NHL) have a poor prognosis. Despite measurable clinical activity seen with new targeted therapies, most r/r NHL patients do not achieve a complete or durable response suggesting there is room to improve upon existing clinical candidates. Here we describe SGN-CD19B, a new CD19-targeted antibody-drug conjugate (ADC), which demonstrates potent preclinical activity against B-cell malignancies. SGN-CD19B is composed of a humanized antibody (hBU12ec) conjugated to a DNA-cross linking pyrrolobenzodiazepine (PBD) dimer drug (SGD-1882) via a protease-cleavable linker. Fluorescence microscopy studies showed that SGN-CD19B is rapidly internalized and traffics to lysosomes within hours of binding to CD19-positive tumor cell lines. Following uptake, SGN-CD19B induces DNA damage as measured by phosphorylation of histone 2AX. This damage subsequently leads to G2-M cell cycle arrest, caspase-3/7 activation, formation of cleaved poly ADP-ribose polymerase and cell death. The antitumor activity of SGN-CD19B was evaluated in preclinical studies using lymphoma and leukemia cell lines. In vitro cytotoxicity studies showed that SGN-CD19B was highly active on a broad panel of CD19+ tumor cell lines with IC50 values ranging from 0.007 to12 ng/ml. SGN-CD19B displayed compelling anti-tumor activity in 4 separate xenograft models of B-cell malignancies including two models of diffuse large B-cell lymphoma (NHL-DLCL2 and RL), one model of Burkett's lymphoma (Ramos) and one model of B-cell acute lymphoblastic leukemia (B-ALL; Nalm-6). Amongst the NHL xenograft models, SGN-CD19B induced significant, dose-dependent tumor growth delay and survival benefit at ≥100 mcg/kg. At 300 mcg/kg, 100% of treated mice achieved complete and durable tumor regressions in 3 of 3 lymphoma models tested. In a disseminated model of B-ALL, a significant dose-dependent increase in survival was observed in mice treated with ≥10 mcg/kg of SGN-CD19B. At 100 mcg/kg, 10/10 mice survived for 〉115 days while the maximal survival for untreated and control groups was ≤37 days. The ability of SGN-CD19B to deplete normal B-cells was examined in conjunction with single-dose toxicology studies in monkeys. SGN-CD19B was tolerated up to 250 mcg/kg without severe toxicity. Reductions in peripheral CD20+ B-lymphocytes were observed at dose levels as low as 10 mcg/kg and higher. The magnitude and duration of B cell depletion was dose-dependent. CD20+ B-lymphocytes were also depleted from lymphoid tissues, which showed minimal to marked, dose-dependent decreases in the number and size of lymphoid follicle germinal centers at doses ≥30 mcg/kg. Normal lymphoid architecture was restored within 16 weeks and circulating CD20+ B-lymphocytes returned to normal levels within 8-12 weeks of dosing. Together, these data demonstrate that SGN-CD19B exhibits antitumor activity against a broad panel of CD19+ B-cell malignancies and causes durable remissions in preclinical models of NHL and B-ALL. SGN-CD19B is pharmacodynamically active in cynomolgus monkeys, resulting in 100% depletion of CD20+ B-lymphocytes at well-tolerated doses. Clinical trials are planned to further evaluate SGN-CD19B in r/r NHL. Disclosures Ryan: Seattle Genetics, Inc.: Employment. Schimpf:Seattle Genetics, Inc.: Employment. Anderson:Seattle Genetics,Inc: Employment. Zeng:Seattle Genetics, Inc: Employment. Emmerton:Seattle Genetics, Inc.: Employment. Miyamoto:Seattle Genetics, Inc.: Employment. Kostner:Seattle Genetics, Inc.: Employment. Yu:Seattle Genetics, Inc.: Employment. Van Epps:Seattle Genetics, Inc.: Employment. Tatalick:Seattle Genetics, Inc.: Employment. Benjamin:Seattle Genetics, Inc: Employment.

Description: Acute myeloid leukemia (AML) remains a therapeutic challenge. Long-term survival rates, in particular for older AML patients, remain poor, highlighting the need for improved and well-tolerated treatment options. AML patients who are unfit for high dose chemotherapy in the US are often prescribed hypomethylating agents (azacitidine or decitabine) although the efficacy of these agents in this population was modest (Estey, Leukemia 2013). SGN-CD33A is a CD33-directed antibody drug conjugate (ADC) that is comprised of a cysteine-engineered anti-CD33 antibody, a cleavable dipeptide linker that is highly stable in circulation, and a PBD dimer that binds DNA with high intrinsic affinity. The ADC is active as a single agent in preclinical models of AML that are characteristically resistant to chemotherapy (multi-drug-resistant, MDR-positive) (Sutherland et al. Blood 2013). In the present study, we tested the activity of SGN-CD33A in combination with hypomethylating agents (HMA), azacitidine or decitabine. We hypothesized that the combination of SGN-CD33A with an HMA will have greater impact on the DNA repair pathway in leukemic cells, furthering the processes of apoptosis and cell death. MDR-positive AML cells were treated for 96 hours with SGN-CD33A and each of the HMAs alone and in combination, evaluating both simultaneous as well as sequential administration. Enhanced tumor cell killing was observed when AML cells were treated concomitantly with the combination of SGN-CD33A and an HMA or pre-treated with HMAs prior to the addition of SGN-CD33A. Superior anti-leukemic activity of subtherapeutic doses of SGN-CD33A in combination with HMAs was observed in mouse xenograft models generated with MDR-positive AML cell lines. In the difficult to treat HEL9217 model, decitabine or a single dose of 200 mcg/kg SGN-CD33A delayed tumor growth, while significant reductions in tumor growth were observed for the combination treatment (p=0.0001). Improved antitumor activity in this model was also observed for SGN-CD33A in combination with azacitidine. SGN-CD33A in combination with decitabine significantly reduced tumor burden compared to either agent alone in the TF1-α AML model (p=0.0002). Similarly, sequenced dosing of azacitidine followed by a subtherapeutic dose of 30 mcg/kg SGN-CD33A in mice bearing KG-1 xenografts delivered greater antitumor activity compared to the individual agents (p=0.0001). To investigate the mechanism underlying the enhancement in antileukemic activity, we looked at the impact of HMAs alone and in combination with SGN-CD33A on myeloid marker expression, PBD drug release, and impact on the DNA damage and apoptotic pathways. In AML cell lines that did not show improved cytotoxic activity with the combination of SGN-CD33A and HMA, HMA treatment appeared to have no positive effect on the cell surface levels of CD33. However in responsive cell lines such as TF1-α, HMA treatment resulted in time- and dose-dependent increases in the levels of CD33. Significant increases in expression were observed between 2 and 4 days with decitabine and after 4 days with azacitidine. In addition, more PBD dimer drug from SGN-CD33A was incorporated into DNA in HMA treated cells. Concomitant, the treatment of AML cells with the combination of SGN-CD33A and HMA resulted in greater DNA damage and apoptosis as shown by the increased levels of the early DNA damage marker, γH2AX, and the formation of cleaved PARP, an apoptosis marker, compared to either agent alone. Studies are in progress to investigate the effects on other components of the DNA repair and cell cycle pathways. These findings demonstrate that SGN-CD33A can be successfully combined with hypomethylating agents to deliver marked antitumor activity in preclinical drug-resistant models of AML. Disclosures Sutherland: Seattle Genetics, Inc.: Employment. Yu:Seattle Genetics, Inc.: Employment. O'Day:Seattle Genetics,Inc: Employment. Alley:Seattle Genetics,Inc.: Employment. Anderson:Seattle Genetics, Inc.: Employment. Emmerton:Seattle Genetics, Inc.: Employment. Zeng:Seattle Genetics, Inc.: Employment. O'Meara:Seattle Genetics, Inc: Employment, Equity Ownership. Feldman:Seattle Genetics,Inc: Employment. Kennedy:Seattle Genetics,Inc: Employment, Equity Ownership, Honoraria, Speakers Bureau. Ryan:Seattle Genetics, Inc.: Employment. Benjamin:Seattle Genetics, Inc: Employment.

Description: Key PointsSGN-CD33A is a novel antibody-drug conjugate, consisting of an engineered anti-CD33 mAb conjugated to a potent DNA cross-linking cytotoxin. SGN-CD33A is highly active in a broad panel of preclinical AML models and, in contrast to GO, is active despite MDR or poor-risk cytogenetics.

Description: Abstract 3589 Outcomes for patients with acute myeloid leukemia (AML) are poor, highlighting the need for novel treatment options. Most AML cells express the myeloid differentiation antigen CD33, making CD33-targeted therapy a potential treatment strategy. Gemtuzumab ozogamicin (GO), an anti-CD33 monoclonal antibody (mAb) conjugated to the cytotoxic agent calicheamicin, has recently been shown to improve survival in newly diagnosed patients with more favorable-risk AML but has insufficient activity in those with poor prognostic features as well as in relapsed disease. Here we report the preclinical testing of a novel CD33-directed antibody-drug conjugate, SGN-CD33A, consisting of a humanized anti-CD33 mAb with 2 engineered cysteine residues through which pyrrolobenzodiazepine (PBD) dimer drug moieties are conjugated via a maleimidocaproyl valine-alanine dipeptide linker. PBD dimers exert their biological activity by covalent binding and interstrand cross-linking of DNA. Fluorescence microscopy studies showed that SGN-CD33A is rapidly internalized and traffics to lysosomes within hours of binding to CD33-positive AML cell lines. Following uptake, SGN-CD33A induces DNA damage as measured by phosphorylation of histone 2AX, subsequently leading to G2-M cell cycle arrest, disruption of mitochondrial membrane integrity, increased caspase-3 activity, formation of cleaved poly ADP-ribose polymerase, DNA fragmentation and cell death. The anti-leukemic activity of SGN-CD33A was assessed in cytotoxicity assays against 12 AML cell lines and 18 primary AML patient samples of mixed cytogenetic origin (favorable, intermediate and unfavorable) and multidrug resistance (MDR) status. SGN-CD33A was highly active against all AML cell lines tested (mean IC50, 22 ng/ml), including 5 of 5 MDR-positive cell lines (mean IC50, 27 ng/mL). In contrast, GO was moderately active in 1 of 5 MDR-positive cell lines (IC50, 227 ng/mL) but inactive against the other 4 (IC50, 〉1000 ng/mL). SGN-CD33A was also active against 15 of 18 primary samples isolated from untreated AML patients at diagnosis (mean IC50 of responsive samples, 8 ng/mL) and was more potent than GO which was active in 10 of 18 AML samples (mean IC50 of responding samples, 27 ng/mL). The 3 AML specimens that were resistant to SGN-CD33A each had low or absent CD33 expression, as determined by flow cytometry. Cytogenetic abnormalities and MDR activity did not correlate with in vitro SGN-CD33A cytotoxicity. In vivo antitumor activity was evaluated in AML mouse xenograft models established with MDR-negative HL-60 and MDR-positive THP-1 and TF1-α cell lines. SGN-CD33A dosed once at 300 mcg/kg yielded durable complete regressions in THP-1 (Figure A) and TF1-α xenografts. In these models of drug-resistant AML, treatment with a single dose of 100 mcg/kg SGN-CD33A significantly delayed tumor growth compared to untreated and non-binding control ADC-treated mice (p

Description: Denintuzumab Mafodotin (SGN-CD19A) is an antibody-drug conjugate (ADC) composed of an anti-CD19 antibody attached to a synthetic cytotoxic agent, monomethyl auristatin F (MMAF). The ADC binds CD19, internalizes, and releases cys-mcMMAF, which ultimately results in G2-M growth arrest and induction of apoptosis in targeted cells (Law et. al. AACR 2011). SGN-CD19A has shown convincing antitumor activity in relapsed/refractory non-Hodgkin lymphoma patients, resulting in a significant number of objective responses, including complete responses with prolonged durability (Moskowitz et. al. Blood 2015). To further characterize the anti-tumor activity of SGN-CD19A, we evaluated whether immune-mediated cell killing and immunogenic cell death (ICD) act in addition to the previously described apoptotic cell death. We then examined whether these mechanisms of cell death are augmented by clinically approved CD20 antibodies. Our results show that SGN-CD19A supports immune-mediated killing of lymphoma cell lines in vitro through antibody-dependent cellular cytotoxicity (ADCC) and antibody-dependent cellular phagocytosis (ADCP). Assessment of disseminated and subcutaneous tumor xenografts showed that SGN-CD19A activity is reduced when the wild type antibody is replaced with IgG variants that lack effector function. We also found depletion of macrophage or NK cells eliminates the improved survival conferred by the parental antibody of SGN-CD19A. Together these data provide strong evidence that effector function contributes to activity of SGN-CD19A in vivo. Next, we implemented an in vitro system pairing lymphoblastoid cell lines (LCLs) with autologous PBMC to investigate the effect of SGN-CD19A-mediated cell death on immune activation. Treatment of LCL with SGN-CD19A drove hallmarks of ICD as indicated by activation of the ER stress response along with upregulation of surface calreticulin. In vitro co-culture of SGN-CD19A treated LCL with autologous PBMC resulted in heightened monocyte activation and expansion of LCL-reactive cytotoxic T cells supporting a role for SGN-CD19A in augmenting anti-tumor immune responses. Finally, we evaluated the anti-tumor activity of SGN-CD19A in the presence and absence of clinically relevant CD20 antibodies. Results showed that CD20-targeted antibodies impact internalization of CD19 antigen leading to improved drug delivery of SGN-CD19A. Blocking B cell signaling inhibits the effect of CD20 antibodies on CD19 and SGN-CD19A. The improved anti-tumor activity observed with SGN-CD19A plus CD20 therapeutics was confirmed in vivo using rituximab. In conclusion, our results demonstrate SGN-CD19A is a multifunctional ADC capable of eliciting cell killing through immune-mediated mechanisms as well as delivery of the cytotoxic payload, cys-mcMMAF. Importantly, the described mechanisms of SGN-CD19A activity are further augmented by CD20 antibodies. These studies support the ongoing clinical evaluation of SGN-CD19A, which is currently being tested in randomized Phase II clinical trials in combination with R-ICE (NCT02592876) as well as R-CHOP (NCT02855359). Disclosures Van Epps: Seattle Genetics: Employment. Heiser:Seattle Genetics: Employment. Cao:Seattle Genetics: Employment. Klussman:Seattle Genetics: Employment. Yu:Seattle Genetics: Employment. Stone:Seattle Genetics: Employment. Neale:Seattle Genetics: Employment. Gardai:Seattle Genetics: Employment. Law:Seattle Genetics, Inc.: Employment, Equity Ownership. Ryan:Seattle Genetics: Employment.

Description: SGN-33 is a humanized IgG1 antibody with signaling properties that also mediates effector functions including Antibody-Dependent Cellular Cytotoxicity (ADCC). SGN-33 targets CD33, a sialoadhesion family member expressed on acute myeloid leukemia (AML) cells, eliciting complete remissions in some AML patients. In the laboratory, SGN-33 mediates natural killer (NK) cell lysis of both multi-drug resistant (MDR- positive) and MDR-negative AML cell lines. Additionally, SGN-33 significantly increases survival in SCID mouse disseminated models of MDR-negative and MDR- positive AML. Immunomodulatory compounds such as thalidomide and its analog, lenalidomide (Revlimid®), have been shown to enhance NK cell function and anti-cancer activity. In particular, Revlimid in combination with rituximab demonstrated greater tumor cell killing compared to the antibody alone. Given these reports, we investigated whether thalidomide or lenalidomide would enhance the anti-tumor effects of SGN-33. In vitro, lenalidomide augmented NK function in a dose-dependent fashion, manifested as significant increases in perforin expression. Thalidomide and lenalidomide enhanced ADCC activity (% maximal specific lysis) while having no significant effects on tumor cell levels of CD33. In vivo, treatment of SCID mice with lenalidomide resulted in a 2-fold increase in the absolute numbers of lymphocytes (including NK cells). In vivo studies evaluating the efficacy of the combination of SGN-33 and lenalidomide in SCID mouse models of AML are in progress. In summary, our studies demonstrate the anti-leukemic activity of SGN-33 against MDR-positive AML cell lines, suggesting that this unconjugated anti-CD33 antibody may yield benefit in clinical settings where other therapies fail due to the MDR phenotype of the tumor cells. Additionally, agents augmenting immunologic effector function might provide clinical benefit in combination with SGN-33. The combination of SGN-33 and lenalidomide is now being evaluated in a clinical trial.

Description: Long-term survival rates for acute myeloid leukemia (AML) patients remain poor, highlighting the need for further treatment options. AML cells express the myeloid marker CD33, making them amenable to CD33-targeted therapy. SGN-CD33A is a novel anti-CD33 antibody-drug conjugate (ADC) composed of a humanized antibody conjugated to a highly potent DNA-binding pyrrolobenzodiazepine (PBD) dimer drug via a protease-cleavable dipeptide linker. An engineered cysteine on each heavy chain attaching the PBD dimer to the antibody allows uniform drug loading of approximately two PBD dimers per antibody. Upon binding to CD33 on the cell surface, SGN-CD33A is internalized, the linker is cleaved by proteases in the lysosomes, and the released drug forms DNA crosslinks, resulting in cell death. SGN-CD33A is active as a single agent against a broad panel of primary AML samples and in preclinical models of AML that are characteristically resistant to chemotherapy (multi-drug-resistant, MDR-positive) (Sutherland et al. Blood 2013). In the present study, we tested the activity of SGN-CD33A in cytotoxicity assays in combination with therapies commonly used in the treatment of myeloid malignancies including cytarabine (Ara-C) and the hypomethylating agents, 5-azacytidine (vidaza) or 5-aza-2-deoxcytidine (decitabine). Significant synergism in tumor cell killing, as assessed by the Chou-Talalaly Combination Index (CI), was observed when MDR-positive AML cell lines, KG-1 and TF1-a, were treated simultaneously with the combination of SGN-CD33A and Ara-C (CI 〈 0.7) or SGN-CD33A and a hypomethylating agent (CI 〈 0.7). Consistent with these observations, mouse xenograft experiments were conducted with AML cell lines, and demonstrated improved antitumor activity with the combinations compared to either agent alone. In the subcutaneous MDR-positive TF1-a model of AML, a single low dose of SGN-CD33A (30 mcg/kg) in combination with Ara-C significantly reduced tumor burden compared to either agent alone or to the nonbinding control ADC groups (p〈 0.0001). Significant reductions in tumor growth were also observed in subcutaneous MDR-positive TF1-a or KG-1 murine models of AML treated with SGN-CD33A in combination with hypomethylating agents. Whereas a single low dose of SGN-CD33A or the hypomethylating agents decitabine or vidaza delayed tumor growth, the combination delivered greater antitumor activity than the individual agents alone. These findings demonstrate that SGN-CD33A can be combined with therapies that are commonly used in treating myeloid malignancies to deliver significantly improved antitumor activity in preclinical drug-resistant models of AML. Disclosures Sutherland: Seattle Genetics: Employment, Equity Ownership. Yu:Seattle Genetics: Employment, Equity Ownership. Anderson:Seattle Genetics: Employment, Equity Ownership. Emmerton:Seattle Genetics: Employment, Equity Ownership. Zeng:Seattle Genetics: Employment, Equity Ownership. O'Meara:Seattle Genetics, Inc.: Employment, Equity Ownership. Kennedy:Seattle Genetics, Inc.: Employment, Equity Ownership. Ryan:Seattle Genetics: Employment, Equity Ownership. Benjamin:Seattle Genetics: Employment, Equity Ownership.

Description: Key Points SGN-CD19B is broadly active in vitro against malignant B-cell lines, including double-hit and triple-hit lymphoma cell lines. SGN-CD19B shows significant antitumor activity in vivo in preclinical models of B-NHL and B-cell–derived acute lymphoblastic leukemia.