The current treatment of relapsed or refractory AML is associated with low rates of complete response (CR) and considerable complications. As a result, only a minority of patients (pts) proceed to allogeneic hematopoietic stem cell transplantation (alloHSCT) with curative intent. Furthermore, outcomes for AML pts with disease relapse after alloHSCT are dismal. Blastic plasmacytoid dendritic cell neoplasm (BPDCN) is a rare and incurable blood cancer with a median survival of <18 months and no standard of care. Thus, there are clear unmet therapeutic needs in both these conditions. CD123 is overexpressed on AML blasts and leukemic stem cell (LSC)-enriched cell subpopulations compared to normal hematopoietic stem cells and myeloid progenitors. High levels of CD123 expression is also one of the diagnostic hallmarks for BPDCN. All these features make CD123 an attractive target for T cell based adoptive immunotherapy. We have previously demonstrated preclinically the anti-AML activity of CD123-chimeric antigen receptor (CAR) T cell therapy (Mardiros, Blood 2013). The CD123CAR contains an anti-CD123 single-chain variable fragment, an optimized IgG4 CH2CH3 linker, a CD28 co-stimulatory domain, and a CD3 zeta signaling domain and is used to engineer T cells for patients enrolled on a single center, first-in-human phase I dose escalation clinical trial open at our Institution (NCT02159495). The primary objective is to test the safety and activity of escalating doses of CD123CAR T cells for patients with relapsed or refractory AML (cohort 1) and BPDCN (cohort 2). Donor-derived or autologous T cells from leukapheresed peripheral blood mononuclear cells were lentivirally transduced with the CD123CAR vector. Prior to T cell infusion, all patients undergo a lymphodepleting regimen including fludarabine 25-30 mg/m2 daily for 3 days and cyclophosphamide 300 mg/m2 daily for 3 days. Pts receive a single dose of CD123CAR T cells with an option for a second infusion if they continue to meet safety and eligibility criteria and still have CD123+ disease. To date, 14 patients have been enrolled and 7 treated (6 AML, 1 BPDCN). All 6 patients in the AML cohort had refractory AML following alloHSCT, and a median of 4 (range: 4-7) prior lines of therapy. Of the 2 pts treated at dose level (DL) 1 (50M CAR+ T), 1 achieved a morphologic leukemic-free state, which lasted 2 months. She received a second infusion 3 months later with subsequent blast reduction from 77.9% to 0.9% (flow cytometry) after 35 days. Of the 4 pts on DL 2 (200M CAR+ T), 1 achieved CR and became transfusion independent. She proceeded to a second alloHSCT on day 70. Restaging on day +161 post-transplant showed she has remained in MRD-negative CR with good engraftment and 100% donor chimerism. Another pt with CR prior to treatment remained in CR at day 28 and has proceeded to a second alloHSCT. The remaining 2 patients had reductions in blast counts, but did not achieve remission. All toxicities were reversible and manageable: cytokine release syndrome (CRS; 4 grade 1, 1 grade 2); 1 adenoviral pneumonia requiring intubation; and 1 grade 3 rash due to drug hypersensitivity. There were no dose limiting toxicities and no treatment-related cytopenias. One pt with prior alloHSCT had mild recurrent cutaneous GVHD, which occurred after the completion of CAR T treatment. Correlative studies including T cell persistence and CD123 expression are underway and will be reported. In the BPDCN cohort, 1 pt has been treated so far, a 74-year-old man with a persistent bulky subcutaneous mass after clinical trial treatment with a CD123 antibody-drug conjugate. Following lymphodepletion, he received a single dose of 100M autologous CD123CAR T cells and continues to be in CR at 60 days post-infusion. Skin biopsies at the tumor site on days 14 and 28 showed no evidence of disease. Restaging work-up at day 28 revealed disease-free bone marrow, no new masses by CT scan, normalized blood counts, and complete resolution of clinical symptoms. The pt tolerated the treatment well with no CRS or neurologic toxicity. In this first-in-human clinical trial of CD123CAR T cell therapy, we have demonstrated the feasibility and safety of targeting CD123. We have also observed promising anti-leukemic activity in both AML and BPDCN. Importantly, no myeloablative effects have been observed, supporting further testing of this immunotherapeutic strategy in both transplant eligible and ineligible patients. Disclosures Stein: Stemline: Consultancy; Amgen: Consultancy, Speakers Bureau.

Bispecific antibodies are emerging as a promising new immunotherapy modality and are actively being evaluated in clinical trials for patients with lymphoma. As the first BsAb to receive regulatory approval for lymphoma, mosunetuzumab, an antiCD20/anti-CD3 BsAb, is an exciting new option for patients with relapsed or refractory (R/R) follicular lymphoma. The approval was based on results from an international, multicenter, phase 2 trial in patients with relapsed or refractory (R/R) follicular lymphoma following at least 2 prior lines of systemic therapy. Mosunetuzumab demonstrated remarkable efficacy with an overall response rate of 80% and complete response rate of 60%. Here we provided an overview of the latest clinical data on mosunetuzumab in lymphoma presented at the 2022 ASH Annual Meeting.

Introduction: The current treatment of R/R AML and BPDCN is associated with poor outcome. CD123 is overexpressed on AML blasts, leukemic stem cells, and BPDCN cells compared to normal hematopoietic stem cells. This differential expression feature makes CD123 an attractive target for cellular immunotherapy. Here we report results of an ongoing single-center, first-in-human phase 1 dose-escalation study (NCT02159495) evaluating CD123CAR T cells in treating patients with R/R AML (cohort 1) and BPDCN (cohort 2). This study is designed to determine the safety and antileukemic activity of CD123CAR T cells. Methods: Vectors containing the CD123CAR composed of a single-chain variable fragment, an optimized IgG4 linker, a CD28 co-stimulatory domain, and a CD3 zeta domain are used to engineer donor-derived or autologous T cells via lentiviral transduction. Prior to T-cell infusion, all patients undergo lymphodepleting regimen with the majority receiving fludarabine 25-30 mg/m2 and cyclophosphamide 300-500 mg/m2 daily for 3 days. Patients can receive a second infusion if they continue to meet eligibility criteria. To date, 18 patients have been enrolled and 9 treated (7 AML, 2 BPDCN). Results: All 7 patients in the AML cohort had at least one prior allogeneic stem cell transplant and median of 4 (range: 4-10) prior lines of therapy. Of the 2 patients treated at the dose level (DL) 0 (50M CAR+ T), one achieved a morphologic leukemic-free state (MLFS), which lasted 70 days. She received a second infusion 3 months later with blast reduction from 77.9% to 0.9% (flow cytometry) after 35 days. Of the 5 patients on the DL1 (200M CAR+ T), 1 achieved complete remission (CR) with incomplete count recovery at day 28, and 1 MLFS that improved to CR at day 84. The remaining 3 patients had stable disease. All toxicities were reversible and manageable. No patient developed grade 3 or above cytokine release syndrome or neurotoxicity. There were no treatment-related dose-limiting toxicities and no treatment-related cytopenias longer than 12 weeks. One patient with prior cutaneous GVHD developed grade 1 rash 5 weeks after CAR T infusion, which resolved after clinical management. Peak of T-cell expansion occurred within the first 14 days. We did not observe any CD123-loss leukemic variants. In the BPDCN cohort, 2 patients treated at DL0 (100M CAR+ T) tolerated the treatment well with no grade 3 or above treatment-related toxicities. One patient achieved CR with no evidence of disease in the bone marrow and skin at day 28. Conclusions: In this first-in-human clinical trial of CD123CAR T cell therapy, we have demonstrated the feasibility, safety, and promising antileukemic activity of targeting CD123 in patients with AML or BPDCN. Based on the results we have observed and our collaborative work with Mustang Bio at optimizing the manufacturing platform, we will be leading a multicenter phase 1/2 clinical trial to determine the activity of the cells in achieving remission in patients with AML, BPDCN, and myelodysplastic syndrome. This abstract is also being presented as Poster B22. Citation Format: Lihua E. Budde, Joo Song, Marissa Del Real, Young Kim, Candida Toribio, Brent Wood, Jamie Wagner, Emanuela Marcucci, Anthony Stein, Guido Marcucci, Christine E. Brown, Stephen J. Forman. CD123CAR displays clinical activity in relapsed/refractory (r/r) acute myeloid leukemia (AML) and blastic plasmacytoid dendritic cell neoplasm (BPDCN): Safety and efficacy results from a phase 1 study [abstract]. In: Proceedings of the AACR Special Conference on Tumor Immunology and Immunotherapy; 2018 Nov 27-30; Miami Beach, FL. Philadelphia (PA): AACR; Cancer Immunol Res 2020;8(4 Suppl):Abstract nr PR14.

Acute myeloid leukemia (AML) is the most common acute leukemia in adults. The cure rate for primary AML patients is only 35% and decreases with age. Novel and effective immunotherapies for patients with relapsed and/or refractory (r/r) AML remain an urgent unmet need. CD33 is an attractive immunotherapeutic target for myeloid malignancies given its expression on more than 85% of AML patient samples. We therefore set out to design and test CD33 chimeric antigen receptor (CD33CAR) T cells preclinically as a single agent and in combinational therapy. To assess antileukemic responses of CD33CAR T cells in vitro and in vivo, we enriched CD4/CD8 T cells from peripheral blood mononuclear cells (PBMCs) and genetically modified them to express a second-generation CD33CAR. CD33CAR T cells exhibited potent antigen dependent CD107a degranulation, IFN-γ production and killing activities against AML cells in vitro. Using a NOD-SCID-IL2Rgnull (NSG) xenograft model engrafted with MOLM-14-ffluc, a CD33 expressing AML cell line transduced with lentivirus carrying firefly luciferase (ffluc) and enhanced green fluorescent protein (eGFP), 3 million CD33CAR or mock T cells were introduced intravenously. CD33 CAR T cell-treated group displayed 98.2% leukemic regression 4 days post CAR T infusion, and 99.6% reduction on day 31. Bioluminescent imaging (BLI) and Kaplan-Meier analysis demonstrated that CD33CAR T cells significantly decreased leukemic burden and prolonged overall survival compared to mock T cells in vivo. Decitabine, a DNA hypomethylating agent (HMA), is a main therapeutic agent for treating AML. We observed HMA treatment led to increased CD33 expression on MOLM-14 cells in vitro. We hypothesized that decitabine can potentiate CD33CAR T cell-mediated AML killing by increasing CD33 expression. MOLM-14 cells were treated with either decitabine alone, CD33CAR T cells alone, or sequential treatment using various concentrations of decitabine or DMSO followed by CD33CAR or mock T cells in an E:T ratio of 1:100. We determined the target specific killing activities in each group using flow cytometric based analysis 48 and 96 hours later. The decitabine followed by CD33CAR T cells treatment reproducibly resulted in the most robust antileukemic activity with 80.6% MOLM-14 cells killed. In comparison, CD33CAR T cells or decitabine monotherapy resulted in 11.5% and 50.9% killing, respectively. In vivo testing of the combinational effects of decitabine and CD33CAR T cells are underway and will be updated at the meeting. Finally, checkpoint blockade targeting programmed death-1 (PD-1)/programmed death-ligand 1 (PD-L1) has shown survival benefits, particularly in combination with HMA, for patients with r/r AML (Daver et al. 2019). We observed elevated PD-L1 expression on residual AML blasts that survived the treatment with decitabine in combination with CD33CAR T cells. Therefore, we hypothesized that blockade of PD-1/PD-L1 interaction might further improve the antileukemic effect of CD33CAR T cells against AML cells post antigen induction by decitabine. MOLM-14 cells were treated with decitabine for 2 days and CD33CAR T cells were added in an E:T ratio of 1:75. Anti-PD-1 or IgG4 antibody was added to the culture at various concentrations. The most robust CD33 specific killing was seen in the culture with anti-PD-1 antibody added. Further characterization are underway and will be presented. Taken together, our preclinical findings have demonstrated the potency of the CD33CAR T cell therapy and ways to optimize its efficacy. Our results support clinical translation of CD33CAR T cells for patients with AML. Disclosures Budde: F. Hoffmann-La Roche Ltd: Consultancy.