Despite advances in the treatment of acute myeloid leukemia (AML), novel therapies are needed to induce deeper and more durable clinical response. Bispecific T-cell Engager (BiTE) molecules, which redirect patient T cells to lyse tumor cells, are a clinically validated modality for hematologic malignancies. Due to broad AML expression and limited normal tissue expression, fms-related tyrosine kinase 3 (FLT3) is proposed to be an optimal BiTE molecule target. Expression profiling of FLT3 was performed in primary AML patient samples and normal hematopoietic cells and nonhematopoietic tissues. Two novel FLT3 BiTE molecules, one with a half-life extending (HLE) Fc moiety and one without, were assessed for T-cell–dependent cellular cytotoxicity (TDCC) of FLT3-positive cell lines in vitro, in vivo, and ex vivo. FLT3 protein was detected on the surface of most primary AML bulk and leukemic stem cells but only a fraction of normal hematopoietic stem and progenitor cells. FLT3 protein detected in nonhematopoietic cells was cytoplasmic. FLT3 BiTE molecules induced TDCC of FLT3-positive cells in vitro, reduced tumor growth and increased survival in AML mouse models in vivo. Both molecules exhibited reproducible pharmacokinetic and pharmacodynamic profiles in cynomolgus monkeys in vivo, including elimination of FLT3-positive cells in blood and bone marrow. In ex vivo cultures of primary AML samples, patient T cells induced TDCC of FLT3-positive target cells. Combination with PD-1 blockade increased BiTE activity. These data support the clinical development of an FLT3 targeting BiTE molecule for the treatment of AML.

中文翻译：

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用于治疗急性髓系白血病的新型 FLT3 BiTE® 抗体构建体的表征

尽管急性髓性白血病 (AML) 的治疗取得了进展，但仍需要新的疗法来诱导更深、更持久的临床反应。双特异性 T 细胞接合剂 (BiTE) 分子可重新引导患者 T 细胞以裂解肿瘤细胞，是一种经过临床验证的治疗血液系统恶性肿瘤的方法。由于广泛的 AML 表达和有限的正常组织表达，fms 相关的酪氨酸激酶 3 (FLT3) 被认为是最佳的 BiTE 分子靶标。FLT3 的表达谱在原发性 AML 患者样本和正常造血细胞和非造血组织中进行。两种新型 FLT3 BiTE 分子，一种具有半衰期延长 (HLE) Fc 部分，另一种没有，评估了 FLT3 阳性细胞系体外、体内和离体的 T 细胞依赖性细胞毒性 (TDCC) . 在大多数原代 AML 和白血病干细胞的表面检测到 FLT3 蛋白，但只有一小部分正常的造血干细胞和祖细胞。在非造血细胞中检测到的 FLT3 蛋白是细胞质的。FLT3 BiTE 分子在体外诱导 FLT3 阳性细胞的 TDCC，在体内 AML 小鼠模型中减少肿瘤生长并增加存活率。这两种分子在食蟹猴体内均表现出可重现的药代动力学和药效学特征，包括消除血液和骨髓中的 FLT3 阳性细胞。在原代 AML 样本的离体培养中，患者 T 细胞诱导 FLT3 阳性靶细胞的 TDCC。结合 PD-1 阻断剂可增加 BiTE 活性。这些数据支持 FLT3 靶向 BiTE 分子用于治疗 AML 的临床开发。