Imatinib is not an antibody at all. It is a small molecule inhibitor of the c-abl tyrosine kinase. Antibodies generally will not cross the cell membrane and thus are ineffective against intracellular proteins.
For MLL fusion proteins, people are working on specific small molecule inhibitors that disrupt the fusions found in leukemia MLL-AF9, MLL-AF4 etc. The trick is to inhibit the fusion proteins, but not the endogenous MLL1 protein, which is important for hematopoesis. This is not an easy task.