Synergism Between Imatinib Mesylate and a BCR-ABL-Specific DNA Vaccine against Pre-Established Ph ⁺ Acute Lymphoblastic Leukemia in Syngeneic Mice.

Abstract 3095

Poster Board III-32

Polychemotherapy leads to long term remission and cure in 40-50% of adults and 70-80 % of children with acute lymphoblastic leukemia (ALL). In contrast, prognosis of patients with Philadelphia chromosome positive (Ph⁺) ALL remains poor. The tyrosine kinase inhibitor imatinib mesylate has become an integral part of front-line therapy for Ph⁺ ALL, with remission rates exceeding 90% but most patients who do not undergo allogeneic stem cell transplantation eventually relapse. Thus, new relapse prevention strategies are needed for patients with Ph⁺ ALL.

We evaluated the synergism between low dose imatinib and a DNA-based vaccine after application of a lethal challenge dose of the syngeneic BCR-ABL^p185 expressing leukemia cell line BM185 in Balb/c mice. Non-viral minimalistic immunogenically defined gene expression vectors (MIDGE) encoding a BCR-ABL^p185 fusion specific peptide, GM-CSF and IL12 were used for in vivo transfection of murine skin. In addition, we used natural DNA-based double stem-loop immunomodulators (dSLIM), containing three CpG-motifs as non-specific immune adjuvant.

Subsequently following the leukemia challenge, mice received daily intraperitoneal injections of either 150, 100, 50 or 25 mg/kg imatinib. While doses of 150 or 100mg/kg lead to 100% and the dose of 50 mg/kg to 40.0% survival, respectively, the dose of 25mg/kg did not result in longer tumor-free and overall survival compared to non-treated controls (survival rate of 0%). Mice immunized with the DNA vaccine BCR-ABL/GM-CSF/IL-12/dSLIM intracutaneously on days 2 and 16 after leukemia challenge without imatinib therapy showed a significant longer mean tumor-free and overall survival (p<0.05) compared to the control and a survival rate of 58,3%. The combination of low dose imatimib therapy (25mg/kg) and DNA vaccination on days 2 and 16 further improved the outcome and lead to a significant longer mean tumor-free and overall survival (p<0.0001) compared to the control and a survival rate of 66,7%. No further enhancement of survival could be achieved when the vaccine was given on days 2 and 9 or on days 2, 9 and 16. Immunization experiments and CTL assays showed that BCR-ABL specific sequences are essential to prevent Ph⁺ acute lymphoblastic leukemia. In cell depletion studies we showed that both CD8+ T cells and natural killer cells contribute to the anti-leukemia effect of our DNA-based vaccine. In conclusion, we provide survival and functional data that low dose imatimib therapy combined with a leukemia-specific DNA-vaccine act synergistically in the treatment of Ph⁺ ALL. This approach might provide a treatment option for minimal residual disease to prevent a relapse of Ph⁺ ALL in patients.