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799 Activity of CD19/CD3 Bispecific Antibodies in Chronic Lymphocytic LeukemiaClinically Relevant Abstract

Molecular Pharmacology, Drug Resistance—Lymphoid and Other Diseases
Program: Oral and Poster Abstracts
Type: Oral
Session: 605. Molecular Pharmacology, Drug Resistance—Lymphoid and Other Diseases: Chronic Lymphocytic Leukemia and other Lymphoid Diseases
Monday, December 11, 2017: 4:30 PM
Bldg B, Lvl 2, B216-B217 (Georgia World Congress Center)

Hannah R. Robinson, BA1,2, Junpeng Qi, PhD3*, Sivasubramanian Baskar, PhD1*, Erika Cook, BA1*, Inhye E. Ahn, MD4, Sarah E. M. Herman, PhD1*, Christoph Rader, PhD3* and Adrian Wiestner, MD1

1Hematology Branch, National Heart, Lung, and Blood Institute, National Institutes of Health, Bethesda, MD
2Cleveland Clinic Lerner College of Medicine, Cleveland, OH
3Department of Immunology and Microbiology, The Scripps Research Institute, Scripps Florida, Jupiter, FL
4National Heart, Lung, and Blood Institute, National Institutes of Health, Bethesda, MD

Kinase inhibitors such as ibrutinib have advanced treatment of chronic lymphocytic leukemia (CLL). However, there remains a need for adjunct treatments capable of deepening response or overcoming resistance to kinase inhibitors. CD19/CD3 bispecific antibodies (bsAbs) recruit endogenous T cells to form cytolytic synapses with CD19+ tumor cells. Blinatumomab, a CD19/CD3 bsAb designed in the 54 kDa BiTE format, is FDA approved for the treatment of some forms of acute lymphoblastic leukemia, and has potential for use in other B-cell malignancies. However, due to its short half-life of 2.1 hours, blinatumomab requires continuous intravenous dosing for efficacy. We have developed a novel CD19/CD3 bsAb in the 100 kDa single chain-Fv Fc format (19/3-scFv-Fc). With a half-life of approximately 7 days, 19/3-scFv-Fc may be suitable for weekly dosing, providing a significant logistical advantage. Here we investigated the potential use of 19/3-scFv-Fc for treatment of CLL.

We first cultured peripheral blood mononuclear cells (PBMCs) from treatment-naïve CLL patients with bsAbs and measured CLL cell death by flow cytometry. After 12 days of exposure, median specific-killing compared to the non-targeting control HER2/CD3-scFv-Fc was 91.2% for blinatumomab (P = 0.0009) and 92.1% for 19/3-scFv-Fc (P = 0.003)(n = 12). Both CD19/CD3 bsAbs induced over 25-fold expansion of autologous CD8 and CD4 T cells, as demonstrated by increase in absolute cell counts and by CFSE proliferation assays. Activation markers CD69 and CD25, as well as granzyme B, were also increased in both CD8 and CD4 T cell subsets cultured with blinatumomab or 19/3-scFv-Fc.

As blinatumomab and 19/3-scFv-Fc demonstrated comparable activity against CLL ex vivo, we next evaluated efficacy of bsAbs in vivo using the NOD/SCID/IL2Rγnull (NSG) patient-derived xenograft model. Fifty million CLL PBMCs were injected per mouse, with 2-5 mice tested per treatment group and patient combination. bsAbs were then given once-weekly, with blinatumomab dosed at 0.25 mg/kg and 19/3-scFv-Fc at 0.5 mg/kg to achieve equal molar concentrations. Treatment with 19/3-scFv-Fc resulted in elimination of over 98% of CLL cells in the blood (P < 0.0001) and spleen (P = 0.0026) compared to treatment with HER2/CD3-scFv-Fc (n = 4). Blinatumomab failed to induce any response, either with once-weekly dosing or daily dosing for 8 days.

Ibrutinib has been shown to improve T cell dysfunction characteristic of CLL, suggesting immunotherapy may work well with concurrent ibrutinib treatment. Culture of CLL PBMCs from ibrutinib-treated patients (n = 10, 12 ± 1 months on ibrutinib) with 19/3-scFv-Fc induced T cell activation and increased granzyme B expression comparable to that seen in cells obtained from treatment-naïve patients. However, treatment with 19/3-scFv-Fc drove significantly more killing of ibrutinib-treated CLL cells compared to treatment-naïve CLL cells after 3 days of exposure (43.7% versus 2.23% median specific-killing, P = 0.004). By day 7, 95.3% of ibrutinib-treated and 45.3% of treatment-naïve CLL cells were eliminated (P = 0.02).

Resistance to ibrutinib has been commonly linked to mutations in BTK and/or PLCG2 and is associated with early mortality. We assessed activity of 19/3-scFv-Fc in 3 patients with acquired ibrutinib resistance, manifest 34 to 44 months from the start of ibrutinib. Multiple BTK and/or PLCG2 mutations were present in all 3 patients, with cancer cell fractions of the most abundant mutation in each patient ranging from 8-32%. 19/3-scFv-Fc treatment in culture resulted in >90% specific-killing of CLL cells from all 3 ibrutinib-resistant patients (range 91.50 - 97.83%, P = 0.0066). After engraftment of ibrutinib-resistant cells in NSG mice, 1 dose of 19/3-scFv-Fc eliminated >90% of circulating CLL cells, while HER2/CD3-scFv-Fc had no anti-leukemic activity (P < 0.0001), indicating that 19/3-scFv-Fc can effectively target mutant CLL clones resistant to ibrutinib. Taken together, these data support the investigation of 19/3-scFv-Fc as a promising immunotherapy for CLL, either in combination with ibrutinib or as rescue therapy in ibrutinib-resistant disease.

Disclosures: Wiestner: Pharmacyclics: Research Funding; Acerta Pharma: Research Funding.

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