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Introduction: Spleen Tyrosine Kinase (SYK), a non-receptor type of tyrosine kinase, is a member of Syk/ZAP70 tyrosine kinase family. It plays a pivotal role in the regulation of B-cell receptor (BCR) signal pathway, which regulates proliferation, differentiation and survival of B lymphocytes. The abnormal activation of BCR singling is closely related to transformation and development of B cell lymphoma. Targeting BCR downstream molecules, such as Bruton' tyrosine kinase (BTK) and phosphoinositide-3-kinase δ (PI3Kδ) has emerged as new therapeutic approaches and inhibitors of BTK and PI3Kδ were approved recently by FDA for treatment of some subtypes of B-cell malignancies. Currently, a couple small molecular inhibitors against SYK, another BCR downstream molecule, are under the early clinical development and showed initial efficacy in B cell lymphomas. HMPL-523, discovered and currently being developed in Phase I clinical trial by Hutchison MediPharma, is a novel, highly potent and selective SYK inhibitor (IC50: 0.025 μM). The anti-tumor activity of HMPL-523 was evaluated in this study. Methods: Inhibitory effects of HMPL-523 on cell viability were investigated in a panel of B cell lymphoma cell lines with SYK/BCR dysregulation by CellTiter-Glo luminescent or CCK-8 assay. The effect of HMPL-523 on SYK signaling pathway was detected by western blot. Annexin-V- positive and PI-negative population was recognized as apoptotic cells by FACS. Nude mice bearing B cell lymphoma xenograft tumors with SYK/BCR dysregulation were used to determine anti-tumor activity of HMPL-523 in vivo. Result: HMPL-523 blocked phosphorylation of BLNK, downstream protein of Syk, in human mantle cell line REC-1 and human plasma cell line ARH-7777 with IC50 of 0.105 µM and 0.173 μM, respectively. HMPL-523 also inhibited cell viability of Ba/F3 Tel-Syk with IC50 of 0.033 μM. Furthermore, inhibitory effects of HMPL-523 on cell viability were evaluated in a panel of B -cell lymphoma cell lines with SYK/BCR deregulation. Results showed that HMPL- 523 potently inhibited cell survival with IC50s from 0.4 to 2 μM. Consistent with the effect on cell viability, HMPL-523 increased the apoptotic rate of REC-1 cells. Moreover, HMPL-523 showed the synergistic activities on killing human diffused large B cell lymphoma (DLBCL) in combination with other drugs such as BTK inhibitor, PI3Kδ inhibitors and Bcl2 family inhibitor. The detailed mechanisms underlying the synergism are still under investigation. Anti-tumor activity of HMPL-523 was determined in Syk dependent xenograft models. Daily oral administration of 100 mg/kg HMPL-523 showed potent anti-tumor activity in B cell lymphoma REC-1 (TGI: 59%). Conclusion:HMPL-523 is a highly potent SYK inhibitor with good activity against B-cell lymphoma in pre-clinical in vitro and in vivo models, supporting further clinical research for HMPL-523 as either single agent or combination drug with other agents to treat B-cell malignancies e.g. DLBCL Disclosures Yang: Hutchison MediPharma Ltd: Employment, Research Funding. Deng:Hutchison MediPharma Ltd: Employment, Research Funding. Sun:Hutchison MediPharma Ltd: Employment, Research Funding. Liang:Hutchison MediPharma Ltd: Employment, Research Funding. Wang:Hutchison MediPharma Ltd: Employment, Research Funding. Fan:Hutchison MediPharma Ltd: Employment, Research Funding. Tang:Hutchison MediPharma Ltd: Employment, Research Funding. Yu:Hutchison MediPharma Ltd: Employment, Research Funding. Sun:Hutchison MediPharma Ltd: Employment, Equity Ownership. Zhou:Hutchison MediPharma Ltd: Employment, Research Funding. Dai:Hutchison MediPharma Ltd: Employment, Research Funding. Qing:Hutchison MediPharma Ltd: Employment, Research Funding. Su:Hutchison MediPharma Ltd: Employment, Research Funding. Ren:Hutchison MediPharma Ltd: Employment, Research Funding.