CD123 binding agents and uses thereof

An isolated bispecific antibody or antigen-binding fragment comprising a first heavy chain (HC1), a second heavy chain (HC2), first light chain (LC1) and a second light chain (LC2), such that the HC1 and the LC1 pair to form a first antigen-binding site that immunospecifically binds IL3-Rα, and the HC2 and the LC2 pair to form a second antigen-binding site that immunospecifically binds CD3, or an IL3-Rα and CD3-bispecific binding fragment thereof, wherein:

• i) HC1 and LC1 comprise either of the following pairs:
• a. SEQ ID NO:
• 203 and SEQ ID NO:
• 204, or
• b. SEQ ID NO:
• 205 and SEQ ID NO:
• 206, respectively;
• and
• ii) HC2 and LC2 comprise any one of the following pairs:
• a. SEQ ID NO:
• 193 and SEQ ID NO:
• 194,
• b. SEQ ID NO:
• 195 and SEQ ID NO:
• 196,
• c. SEQ ID NO:
• 197 and SEQ ID NO:
• 198,
• d. SEQ ID NO:
• 199 and SEQ ID NO:
• 200, or
• e. SEQ ID NO:
• 201 and SEQ ID NO:
• 202, respectively.

A bispecific antibody or antigen-binding fragment comprising:

• a. a paired heavy and light chain that immunospecifically binds CD3, wherein said heavy chain comprises SEQ ID NO:
• 184 and said light chain comprises SEQ ID NO:
• 190, and
• b. a paired heavy and light chain that immunospecifically binds IL3-Rα, wherein
• i. said heavy chain comprises SEQ ID NO:
• 120 and said light chain comprises SEQ ID NO:
• 165, or
• ii. said heavy chain comprises SEQ ID NO:
• 136 and said light chain comprises SEQ ID NO:
• 168.