Leukemia-Specific Ligands Against Childhood Acute Lymphoblastic leukemia

Abstract 3269

The one-bead-one-compound (OBOC) combinatorial library method was invented by Kit Lam to identify cancer cell surface targeting ligands (Peng L et al. Nat. Chem. Biol. 2006, Aina OH et al., Mol. Pharm. 2007). Using this method, LLP2A, a ligand against activated a4b1 integrin, was discovered that targets malignant lymphoma with high affinity and specificity (Peng L et al., Mol Cancer Ther. 2008). We tested whether childhood acute lymphoblastic leukemia (ALL) cells expressed activated a4b1 integrin and bound to LLP2A using primary leukemia cells and leukemia cells engrafted in NOD/SCID/IL2Rg null (NSG) mice. Expression of activated a4b1 integrin was observed in three primary leukemia samples (1 T-ALL, 1 precursor B (preB) ALL, and 1 relapsed preB ALL) by the rainbow beads (color-coded polystyrene bead mixture displaying unique ligands against known receptors such as a4b1, a3b1, avb3, a5b1 integrins (Luo J et al., J. Comb. Chem. 2008). To quantify activated a4b1 integrin expression, primary leukemia cells were analyzed by flow cytometry using biotinylated LLP2A/streptavidin-PE. Nine samples (7 preB ALL and 2 relapsed preB ALL) were analyzed. All samples, except for one relapsed preB ALL sample, showed expression of activated a4b1 integrin at different levels. In order to determine the specificity of activated a4b1 integrin expression, hematopoietic stem cells from three normal bone marrow (BM) samples were analyzed and showed very low levels of activated a4b1 integrin expression. Furthermore, we have identified 32 analogues of LLP2A using three fresh preB ALL samples to screen two LLP2A-focused libraries. Peptide microarrays using a polystyrene slide coated with neutravidin and biotinylated ligands will be prepared with these analogues. We will determine the binding affinity and specificity of these analogues to a series of primary ALL cells. Binding specificity to leukemia cells will be determined by excluding ligands which bind to normal hematopoietic progenitor (CD34) cells. The property of the selected ligands will be tested in vivo using NSG mice engrafted with primary ALL cells. Updated results with these new ligands will be presented at the meeting.