Prevention Of Minimal Residual Disease In Ph+ ALL

Aggressive Philadelphia chromosome-positive (Ph+) acute lymphoblastic leukemia (ALL) that genetically and phenotypically mimics the human disease can be induced by the introduction of cultured p185 (BCR-ABL)-expressing Arf-null pre/pro-B cells into healthy, unconditioned syngeneic mice. Only 20 polyclonal donor cells are sufficient to induce lethal ALL within 30 days of their IV administration, indicating that BCR-ABL expression and Arf inactivation are sufficient to guarantee leukemogenesis in healthy recipient animals. Leukemic mice enter transient remission in response to treatment with potent second generation tyrosine kinase inhibitors (TKI) such as dasatinib (Sprycel^TM). However, like human patients with Ph+ ALL, the continuously treated animals ultimately relapse with the emergence of leukemic clones containing clinically relevant BCR-ABL mutations, the nature of which depends upon the intensity of TKI treatment. Premature withdrawal of dasatinib when animals are in remission results in re-emergence of leukemia; surprisingly, leukemic B cells recovered from these animals lack BCR-ABL mutations and remain sensitive to dasatinib ex vivo. Hence, minimal residual disease depends upon salutary signaling within the hematopoietic microenvironment. In agreement, the response to TKI therapy can be significantly improved by abrogating cytokine signaling through a knockdown of the common gamma chain of the cytokine receptor. Administration of the Janus kinase (JAK) inhibitor ruxolitinib (Jakafi™) mimics this response. Although ruxolitinib demonstrated no anti-leukemic activity of its own, the overall survival of leukemic mice inoculated with 200,000 p185+ Arf-/- pre/pro-B cells was significantly extended after administration of a targeted combination therapy of ruxolitinib and dasatinib in comparison with mice treated with dasatinib alone. Addition of dexamethasone further reduced the leukemic burden, prevented CNS relapse, and led to prolonged survival. This implicates prevention of minimal residual disease and relapse by a non-toxic combination of targeted treatments. These studies have provided a rationale for a Phase I/II clinical trial employing these agents, particularly in older patients who are ineligible for bone marrow transplantation or do not tolerate cytotoxic chemotherapy.