Therapeutic superiority of a TCR-like antibody to an intracellular WT1 oncogene peptide compared with the tyrosine kinase inhibitor imatinib in a mouse model of Philadelphia chromosome positive (Ph+) ALL.
3047 Background: Acute and chronic leukemias demonstrate significantly increased expression of the Wilms tumor gene 1 (WT1) product, including CD34+ CML stem cells, making WT1 an attractive therapeutic target. However, WT1 protein is intracellular and currently un-druggable. ESKM is a fully human IgG1 antibody that targets a 9 amino acid sequence (RMF) of the protein WT1 in the context of HLA-A0201. Methods: BV173 is an HLA-A0201 positive Ph+ ALL cell line. It over-expresses WT1 and binds strongly to ESKM. We evaluated the in vitro and in vivo efficacy of ESKM in combination with TKIs. Antibody-dependent cell-mediated cytotoxicity (ADCC) was evaluated in vitro by chromium release assay, utilizing human PBMC effectors. In vivo tumor growth was assessed in NSG mice bearing disseminated luciferase tagged BV173 with bioluminescence imaging and flow cytometry of the bone marrow after sacrifice. Imatinib was used at maximum tolerated doses for these mice as determined in pilot studies. Results: The addition of imatinib in vitro did not affect the ability of ESKM to perform ADCC. The BV173 engrafted NSG mice treated with ESKM with and without TKIs showed tumor regression one week after beginning therapy, clearing leukemia from the liver and spleen. Mice relapsed primarily in the bone marrow, with increasing luciferase signal after two weeks of therapy. Compared to untreated control animals, after 5 weeks of therapy, imatinib alone only reduced tumor growth by 45%; ESKM alone reduced growth by 81%, and the combination of ESKM and imatinib reduced growth by more than 95%. Flow cytometry of cells remaining after treatment showed binding of ESKM, suggesting escape was not due to down regulation of the epitope. Conclusions: In this mouse model of Ph+ ALL, ESKM antibody therapy is superior to imatinib and the combination of both modalities is additive. This antibody is efficacious in vitro and in vivo against WT1 overexpressing leukemias, in context of HLA-A0201. This combination holds promise as a therapy for leukemias in patients who are HLA-A0201 positive, with the potential of improved cytoreduction in patients with Ph+ leukemias.