Purpose: While CD33 directed immunotherapies have caught significant interest in recent years, the only approved antibody-drug conjugate targeting this antigen for AML is gemtuzumab ozogamicin, which targets the IgV-domain of CD33. Unfortunately, in its current form, these are not effective in a significant proportion of patients due to the presence of a splicing SNP resulting in the loss of IgV-domain. This, however, can be mitigated by targeting the IgC2-domain of CD33; thus, this study aimed to characterize CD33-D2 isoform using the recently developed CD33-D2-targeting antibody HL2541.
Methods: Genetically engineered AML cell lines expressing CD33 isoforms were tested for antibody-bound internalization and response to GO in vitro. AML-bearing NSG-SGM3 mice were used to evaluate CD33-D2 localization and targeting by the HL2541 antibody in vivo.
Results: HL2541-bound-CD33-D2 is internalized similar to CD33-FL upon binding the antibody component of GO. Co-existence of both isoforms compromises the internalization by >2.5-3-fold for each isoform in the AML cell lines, further resulting in 7-9.5-fold higher IC50 values compared to cells expressing only CD33-FL. Finally, we demonstrate that AML cells expressing CD33-D2 localize to bones in mice and are targeted by HL2541antibody in vivo.
Conclusion: The results establish the relevance of targeting IgC domain as an alternative immunotarget to supplement AML chemotherapy.
Keywords: Acute myeloid leukemia; CD33; antibodies; immunotarget; immunotherapy.
CD33 is a protein found on most cancer cells in people with a type of blood cancer called acute myeloid leukemia (AML). A current treatment called gemtuzumab ozogamicin (GO) targets this protein. GO is made of an antibody that carries a drug to kill cancer cells. When GO sticks to CD33, the protein pulls it inside the cell, which leads to the cell being killed. But for many patients, this does not work well. That is because they have a change in their genes that removes the part of CD33 that GO targets. To solve this problem, we tested a new antibody called HL2541. This new antibody sticks to a different part of CD33. We wanted to see if HL2541 could still attach to and enter AML cells that do not have the part of CD33 targeted by GO. In lab tests, we saw that HL2541 could get inside these cancer cells, just like GO. We also tested it in mice and found that HL2541 could find the cancer cells in the body. These results show that HL2541 can find and enter AML cells with the changed CD33 protein. This means HL2541 might help make a new treatment for patients who do not respond to GO.