Synergism Between Chemotherapy with 6-Mercaptopurine and DNA Vaccination Against Ph⁺ Acute Lymphoblastic Leukemia In Syngeneic Mice

Abstract 3265

Long term remission and cure in 70–80 % of children and 40–50% of adults with acute lymphoblastic leukemia (ALL) is achieved with polychemotherapy. The BFM study protocol includes maintenance chemotherapy with 6-mercaptopurine (6-MP) and methotrexate. However, prognosis of patients with Philadelphia chromosome positive (Ph⁺) ALL remains poor and most patients with Ph⁺ ALL eventually relapse unless they undergo allogeneic stem cell transplantation. Thus, new relapse prevention strategies are needed for patients with Ph⁺ ALL.

We compared the efficacy of 6-MP with a DNA-based vaccine in a syngeneic Balb/c mouse model. We also evaluated whether 6-MP inhibits the immunization by the DNA-vaccine or alternatively, acts synergistically in combination therapy. 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), exhibiting an unique structure and containing 6 CpG-motifs, as non-specific immune adjuvant. One day after the application of a lethal challenge dose of the syngeneic BCR-ABL^p185 expressing leukemia cell line BM185, mice received either daily oral applications of 6-MP (50mg/m²/d) or were immunized twice with the DNA vaccine BCR-ABL/GM-CSF/IL-12/dSLIM intracutaneously or received both 6-MP and the DNA-vaccine.

While chemotherapy with 6-MP lead to a significant longer mean tumor-free and overall survival compared to the control and a survival rate of 10%, immunization with the DNA vaccine BCR-ABL/GM-CSF/IL-12/dSLIM also showed a significant longer mean tumor-free and overall survival compared to the control and a survival rate of 56%. More intriguingly, the combination therapy with 6-MP and DNA vaccination further improved the outcome and lead to a survival rate of 100%. Immunization experiments and CTL assays showed that BCR-ABL specific sequences are essential to prevent Ph⁺ ALL. In cell depletion studies we show that both CD4⁺ T cells and natural killer cells and to a lower degree CD8⁺ cells contribute to the anti-leukemia effect of our DNA-based vaccine. In conclusion, we provide survival and functional data that 6-MP combined with a leukemia-specific DNA-vaccine acts synergistically in the treatment of Ph⁺ ALL. This approach might provide an option to optimize maintenance therapy in order to prevent a relapse in patients with Ph⁺ ALL.