A Novel Asymmetrical Anti-CLL-1×CD3 Bispecific Antibody, ABL602, Induces Potent CLL1-Specific Antitumor Activity with Minimized Sensitization of Pro-Inflammatory Cytokines

Acute myeloid leukemia (AML) is a disease with high incidence of relapse that is originated and maintained from leukemia stem cells (LSCs). Expression of C-type lectin-like molecule-1 (CLL-1; also known as CLEC12A, c-type lectin domain family 12 member A) is mainly restricted to LSCs but absent in normal hematopoietic stem cells (HSCs), suggesting CLL-1 as an excellent therapeutic target for AML. This unique expression pattern paves the way to develop therapies that potentially eliminate CLL1-positive LSC while preserving CLL1-negative HSC.

To re-direct T cells to AML cells, we generated IgG-based asymmetric (2+1, ABL602) bispecific antibody (BsAb) targeting CLL-1 and CD3. As a 2+1 format BsAb, ABL602 has bivalent binding to CLL-1 for target arm and monovalent binding to CD3. ABL602 exhibited higher binding activity to CLL-1-expressing AML cell lines and greater tumor-killing efficacy than 1+1 format BsAb and benchmark antibody MCLA-117 (Merus; CLEC12AxCD3 bispecific antibody). ABL602 induced potent cytotoxic activities on CLL1-expressing AML cell lines (EC ₅₀ of 0.04~3.05pM and 0.97~16.64pM for U937 and HL-60, respectively) with concomitant T cell activation (EC ₅₀ of 0.10~3.54pM and 0.94~4.92pM for U937 and HL-60, respectively) and cytokine/granzyme B release. Despite strong tumor-killing activity, ABL602 did not kill CLL1-negative cancer cell lines, suggesting that ABL602 induces CLL-1-dependent cytotoxicity. Moreover, ABL602 did not or minimally induce TNF-α and IL-6 in PBMC in the absence of AML cell lines, while MCLA-117 triggered high level of expression of those cytokines. In established orthotopic AML mouse model using HL-60 Luc, ABL602 demonstrated statistically significant anti-tumor activity in a dose-dependent manner. Proportions of bone marrow CD33 ⁺ AML blasts diminished in a dose-dependent manner, while CD3 ⁺ T cells more infiltrated to the bone marrow.

Overall, our results indicate that ABL602, appropriately engineered 2+1 asymmetric BsAb, promotes T-cell activity specifically against CLL1-expressing AML cells and is a promising treatment strategy for AML patients by achieving the desired balance between antitumor activity and safety.