一种新型 IgG 型 FLT3xCD3 双特异性抗体，用于治疗 AML 和 B-ALL。

BACKGROUND: In lymphoid malignancies, the introduction of chimeric antigen receptor T (CAR-T) cells and bispecific antibodies (bsAbs) has achieved remarkable clinical success. However, such immunotherapeutic strategies are not yet established for acute myeloid leukemia (AML), the most common form of acute leukemia in adults. Common targets in AML such as CD33, CD123, and CLEC12A are highly expressed on both AML blasts and on normal myeloid cells and hematopoietic stem cells (HSCs), thereby raising toxicity concerns. In B-cell acute lymphoblastic leukemia (B-ALL), bsAbs and CAR-T therapy targeting CD19 and CD22 have demonstrated clinical success, but resistance via antigen loss is common, motivating the development of agents focused on alternative targets. An attractive emerging target is FLT3, a proto-oncogene expressed in both AML and B-ALL, with low and limited expression on myeloid dendritic cells and HSCs. METHODS: We developed and characterized CLN-049, a T cell-activating bsAb targeting CD3 and FLT3, constructed as an IgG heavy chain/scFv fusion. CLN-049 binds the membrane proximal extracellular domain of the FLT3 protein tyrosine kinase, which facilitates the targeting of leukemic blasts regardless of FLT3 mutational status. CLN-049 was evaluated for preclinical safety and efficacy in vitro and in vivo. RESULTS: CLN-049 induced target-restricted activation of CD4+ and CD8+ T cells. AML cell lines expressing a broad range of surface levels of FLT3 were efficiently lysed on treatment with subnanomolar concentrations of CLN-049, whereas FLT3-expressing hematopoietic progenitor cells and dendritic cells were not sensitive to CLN-049 killing. Treatment with CLN-049 also induced lysis of AML and B-ALL patient blasts by autologous T cells at the low effector-to-target ratios typically observed in patients with overt disease. Lysis of leukemic cells was not affected by supraphysiological levels of soluble FLT3 or FLT3 ligand. In mouse xenograft models, CLN-049 was highly active against human leukemic cell lines and patient-derived AML and B-ALL blasts. CONCLUSIONS: CLN-049 has a favorable efficacy and safety profile in preclinical models, warranting evaluation of its antileukemic activity in the clinic.

背景：在淋巴恶性肿瘤中，嵌合抗原受体 T 细胞（CAR-T）和双特异性抗体（bsAb）的引入取得了显著的临床成功。然而，这种免疫治疗策略尚未在急性髓系白血病（AML）中确立，AML 是成人中最常见的急性白血病形式。AML 中的常见靶点，如 CD33、CD123 和 CLEC12A，在 AML blasts 和正常髓系细胞和造血干细胞（HSCs）上均高度表达，从而引发了毒性问题。在 B 细胞急性淋巴细胞白血病（B-ALL）中，针对 CD19 和 CD22 的 bsAb 和 CAR-T 治疗已显示出临床成功，但通过抗原丢失产生耐药性很常见，这促使人们开发针对替代靶点的药物。一个有吸引力的新兴靶点是 FLT3，它是 AML 和 B-ALL 中表达的原癌基因，在髓系树突状细胞和 HSCs 上的表达水平较低且有限。

方法：我们开发并表征了 CLN-049，这是一种针对 CD3 和 FLT3 的 T 细胞激活 bsAb，构建为 IgG 重链/scFv 融合物。CLN-049 结合 FLT3 蛋白酪氨酸激酶的膜近端细胞外结构域，从而促进了白血病blasts 的靶向，而与 FLT3 突变状态无关。在体外和体内评估了 CLN-049 的临床前安全性和疗效。

结果：CLN-049 诱导了 CD4+和 CD8+T 细胞的靶向激活。在用亚纳摩尔浓度的 CLN-049 处理时，表达广泛表面水平 FLT3 的 AML 细胞系被有效地裂解，而表达 FLT3 的造血祖细胞和树突状细胞对 CLN-049 的杀伤不敏感。CLN-049 处理还诱导了自体 T 细胞对 AML 和 B-ALL 患者blasts 的裂解，其效应物与靶细胞的比例在临床上明显疾病患者中观察到的低水平。白血病细胞的裂解不受生理性可溶性 FLT3 或 FLT3 配体水平的影响。在小鼠异种移植模型中，CLN-049 对人白血病细胞系和患者来源的 AML 和 B-ALL blasts 具有高度活性。

结论：CLN-049 在临床前模型中具有良好的疗效和安全性，值得在临床上评估其抗白血病活性。