Preclinical Evaluation of Tysabri as a Novel Adjuvant Therapy against Drug Resistant B-ALL.

Abstract 3089

Poster Board III-26

Novel treatment strategies for pediatric acute lymphoblastic leukemia (ALL) have turned a rapidly deadly diagnosis into a highly treatable entity, but we are still failing 25% of our pediatric ALL patients who die of recurrent ALL. Definitive studies have demonstrated that adhesion of leukemia and lymphoma cells to extracellular matrices or stromal cells protects them against the toxicity of cytoreductive chemotherapy drugs. In this context, a specific role for CD49d, a dominant adhesion molecule for normal lymphocytes, was demonstrated for acute myeloid leukemia (AML) and other malignant hematopoietic cells. The finding that CD49d blockade sensitizes AML cells to chemotoxicity may be of therapeutic potential, as is suggested by recent findings for AML cells engrafted in NOD/SCID mice. CD49d is and is similarly expressed on acute lymphoblastic leukemia (ALL) cells, but our knowledge about CD49d adhesion-mediated chemoprotection of B-ALL is limited. We hypothesized whether similar to primary AML blasts, xenografted ALL cells resistant to chemotherapy can be sensitized to chemotherapy by disrupting their CD49d-mediated adhesive interaction with stroma. To test our hypothesis we used as a CD49d inhibitor the humanized anti-human CD49d antibody natalizumab, or Tysabri®, which is in clinical use for the treatment of relapsing or refractory Multiple Sclerosis. To determine the potential of Tysabri as a single agent to decrease leukemia progression, we engrafted 5-7 weeks old NOD/SCID mice with primary drug resistant B-ALL labeled with lentiviral luciferase to allow monitoring of leukemia using noninvasive bioluminescent imaging. Tysabri administered upon detection of engraftment on Day15 post-injection of leukemia in the dose of either 1 mg (n=3) or 6 mg (n=3) led to remarkably slower leukemia progression regardless of the dose compared to the control group treated with saline only (n=2). Additional administration of Tysabri on day 29 and day 37 did not result in further containment of leukemogenesis but still showed a marked reduction in progression compared to the saline treated control group. In addition, we determined in vivo that a weekly administration of Tysabri in the dose of 5mg/kg/d resulted in prolonged survival compared to the treated control (p<0.05). Next, we assessed the effect of adjuvant anti-CD49d antibody-mediated dislodgement of ALL cells of drug resistant patients in combination with chemotherapy. The group treated for 4 weeks with chemotherapy including Vincristine, Dexamethasone and L-Asparaginase (VDL) in combination with Tysabri (5mg/kg/d) admistered once weekly showed decreased progression of leukemia and significantly prolonged survival (p<0.05) compared to the VDL only treated control group. No toxicity of Tysabri treatment was observed. Taken together, our data indicates the potential of Tysabri as a novel adjuvant therapy for treatment of drug resistant B-ALL. Given the availability of clinical-grade CD49d blocking antibody, clinical studies can follow immediately, should our hypothesis be confirmed in further in vitro an in vivo studies.