Background-Acute leukemia with 11q23 aberrations is associated with a poor outcome with therapy. The lack of efficacy of conventional therapy has stimulated interest in developing novel strategies. Recent studies have shown that 11q23-positive acute leukemia cells express the high molecular weight-melanoma associated antigen (HMW-MAA). This tumor antigen represents a useful target to control growth of human melanoma tumors in patients and in severe combined immunodeficient (SCID) mice, utilizing antibody-based immunotherapy. This effect appears to be

Acute leukemia with 11q23 aberrations is associated with especially poor outcome to standard treatment. HMW-MAA is a membrane bound proteoglycan that has been targeted in the treatment of malignant melanoma. It is widely distributed on 11q23-positive leukemic cells and is not expressed on normal hematopoietic cells. Finally, HMW-MAA is believed to function by signaling through the focal adhesion kinase (Fak)/proline-rich tyrosine kinase 2 (Pyk2) pathways and by supporting angiogenesis via its expression on pericytes. We hypothesized that inhibition of HMW-MAA with mAb directed against specific HMW-MAA determinants would effectively limit the growth of 11q23-positive acute leukemia cells in vitro and in vivo, by affecting Fak/Pyk2 pathways. We examined an 11q23-positive acute myeloid leukemia (AML) cell line (ML-2) and leukemia cells from two patients (one AML, one acute lymphoblastic leukemia) with 11q23 aberrations. Our results demonstrate that ML-2 cells expressed only the VT68.2 and 225.28 determinants, while primary acute leukemia cells expressed VT68.2, 225.28, and 763.74 determinants. Treatment of ML-2 cells with mAb against VT68.2 and 225.28 in vitro did not affect overall proliferation but downregulated phosphorylated (p) Pyk2 expression as determined by Western blotting. Further, anti-HMW-MAA mAb treatment enhanced the anti-proliferative effect of cytarabine (y=0.7875) in vitro. Based on these results, we then evaluated the dose-response relationships and toxicities of anti-HMW-MAA mAb alone and in combination with cytarabine in subcutaneous ML-2 xenografts grown in SCID mice. A trend towards tumor inhibition was noted following high dose (1 mg/kg 2x/week) anti-225 mAb therapy but was not seen at lower (0.25 and 0.5 mg/kg) anti-225 mAb doses. Mice demonstrated no side effects or weight loss as a result of anti-225 therapy. No downregulation of pPyk2 in tumor cells was observed following anti-225 therapy at any dose. Furthermore, combination anti-225.28 HMW-MAA mAb and cytarabine treatment (at increasing cytarabine doses) did not result in improved tumor inhibition as compared to cytarabine alone (p>0.05). We then evaluated the effects of HMW-MAA anti-225.28 and anti-763.74 mAb alone or together for treatment of NOD/SCID mice engrafted with primary 11q23-positive leukemia patient cells. Combination anti-HMW-MAA mAb therapy resulted in no prolongation in survival of engrafted mice as compared to control treated animals. We were able to confirm the presence of high levels of circulating HMW-MAA mAb in the serum of animals 4 days following mAb injection. In addition, explanted ML-2 cells harvested following xenograft growth still maintained expression of the HMW-MAA determinants. We conclude that naïve anti-HMW-MAA mAb neither caused a statistically significant inhibitory effect nor enhanced the effect of cytarabine on 11q23-positive acute leukemia cells in vivo. Future studies using radiolabelled or toxin-conjugated antibodies may enhance the anti-tumor effects of targeted HMW-MAA immunotherapy for 11q23-positive acute leukemia.