SGN-CD123A, a Pyrrolobenzodiazepine Dimer Linked Anti-CD123 Antibody Drug Conjugate, Demonstrates Effective Anti-Leukemic Activity in Multiple Preclinical Models of AML
Authors: Sutherland May S.K. , Yu Changpu , Walter Roland B. , Westendorf Lori , Valliere-Douglass John , Pan Lucy , Sussman Django , Anderson Martha , Zeng Weiping , Stone Ivan , Klussman Kerry , Ulrich Michelle , Jonas Mechthild , Senter Peter , Drachman Jonathan G. , Benjamin Dennis 
Published: October 2019

Abstract
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.