Abstract
Acute leukemia is the most common childhood cancer, accounting for approximately 3500 new cases per year in the United States. While a majority of these patients are cured with modern therapy, a proportion will do poorly and have little chance for a cure; the development of novel therapeutic agents is critical for these patients. Vascular endothelial growth factor (VEGF) is known to be a strong promoter of angiogenesis (Shweiki et al., Nature1992. 359:843–5) and elevated levels of VEGF are correlated with poorer prognosis in patients with leukemia (Aguayo et al., Blood.1999. 94:3717–21; Avramis et al., Clin Cancer Res.2006. 12:6978–84). Vandetanib (ZACTIMA™; ZD6474) is an orally active small molecule tyrosine kinase inhibitor with activity against VEGF receptor 2 (VEGFR2), VEGF receptor 1 (VEGFR1), VEGF receptor 3 (VEGFR3), epidermal growth factor receptor (EGFR), platelet derived growth factorβ (PDGFRβ), and Rearranged during transfection (RET). We have used a panel of 18 acute leukemia cell lines derived from patients with acute lymphoid leukemias (ALL) or acute myeloid leukemias (AML) of different lineages and developmental stages, to investigate the potential anti-leukemic effects of vandetanib. Proliferation and/or survival of 4 of the cell lines are inhibited at clinically achievable concentrations of vandetanib, with IC50 values ranging from 85nM to 2.3μM. This anti-tumor activity is dose dependent and results in accumulation of cells in G1 phase. At higher concentrations, treatment with vandetanib induces apoptosis. The vandetanib-sensitive cell lines express VEGFR1 and/or VEGFR3 and do not express VEGFR2 or EGFR. Stimulation of a vandetanib-sensitive AML cell line (Molm-13) with VEGF-C or a mutant form of VEGF-C that specifically stimulates VEGFR-3 results in increased proliferation and/or survival. In this cell line, this effect is specific to VEGFR-3 signaling, as stimulation of VEGFR-1 with VEGF-B does not affect proliferation or survival. Thus, our data suggest that vandetanib-mediated anti-leukemia activity is due to inhibition of VEGFR3 and/or VEGFR1. However, vandetanib-resistant cell lines also express VEGFR1 and/or VEGFR3 and thus, expression of VEGFR1 and/or VEGFR3 does not predict vandetanib sensitivity. In order to determine whether VEGFR signaling may play a role in promoting survival in other contexts, we investigated the interactions between vandetanib and standard pediatric re-induction chemotherapy agents. For these studies, a vandetanib-resistant B-lineage ALL cell line was treated concurrently with vandetanib and either adriamycin, aramycin-C (cytarabine), etoposide, or methotrexate. Anti-tumor activity was measured and Bliss independence was evaluated. Vandetanib exhibits synergistic anti-tumor activity in combination with all 4 of the chemotherapy agents investigated. Thus, vandetanib can mediate direct anti-tumor effects against specific acute leukemia cell lines and can also augment the effects of chemotherapy. Taken together, our data suggest that vandetanib may be an effective agent for treatment of pediatric acute leukemias.
Inhibition of multiplication of Toxoplasma gondii by human monocytes exposed to T-lymphocyte products.
