6557 Background: There is a great interest to develop targeting drugs for CLL, both MAbs and small molecules, to improve the outcome for the disease. ROR1, a receptor tyrosine kinase, is overexpressed on CLL cells but not on normal cells. ROR1 is constitutively phosphorylated and siRNA transfection induces leukemic cell death. The aim of the study is to produce small molecules inhibiting the cytoplasmic tyrosine kinase activity of ROR1, which could induce specific killing of leukemic cells. Methods: A high-throughput screening assay in 384-format has been developed measuring phosphorylation of a substrate peptide by human recombinant intracellular ROR-1 kinase domain. A collection of 80.000 small molecules has been screened generating two chemical series. Novel leads have been synthesized and structure-activity-relationship has been developed using the assay. Results: Three compounds were selected (KAN0173631, KAN0438063, KAN0438175T). Freshly isolated leukemic CLL cells were used as targets in addition to PBMC from healthy donors to test cytotoxicity. The three compounds induced specific apoptosis of CLL cells (Annexin V/PI and MTT) both at 24 and 48h. Efficacy index (EI), i.e. killing of leukemic cells in relation to normal PBMC was favourable. The most promising drug KAN0438063 had an EI of 40 i.e. killed 40 times more CLL cells than PBMC at an IC50 of 10µM. The compounds induced PARP cleavage and cleavage of caspases 8 and 9 as well as down regulation of Mcl-1 and Bcl-xl (Western Blot). ROR1 was dephosphorylated (Western Blot). The selective apoptotic effect was compared to other small molecules targeting non-ROR1 structures in CLL (PCI-32765, CAL-101, R406, R788, STK-156485, STK-156133) and our compounds were significantly more effective (EI) (p