Effects of A771726, the Active Metabolite of Leflunomide,on Multiple Myeloma Cells Proliferation in Vitro.

Multiple Myeloma (MM) is a clonal B-cell malignancy characterized by the accumulation of terminally differentiated plasma cell in the bone morrow, which accounts for 10% of all hematological cancers. Despite major advance of last decade, MM remains an incurable disease in need of new therapeutic approaches. Leflunomide was originally developed as an immunosuppressive drug capable of alleviating the severity of autoimmune disease and preventing allograft and xenograft rejection and it has been FDA approved to use in the treatment of rheumatoid arthritis (RA) since 1998. It rapidly converted in gastrointestinal tract and plasma to open ring metabolite A771726. In current study, we examine the effects of A771726 to the growth of MM cell lines in vivo and show that A771726 inhibits the proliferation of four different well-characterized MM cell lines (MM.1, U266, RPMI8226 and MDR₁₀V) in a dose- and time- dependent manner, regardless their sensitivity to the conventional chemotherapies. Cell cycle analyses further demonstrate that G1-S phase transition is blocked by A771726 in those cells. Moreover, we find that, at higher concentration, A771726 is able to induce apoptosis in at least two of the cell lines tested as measured by an increase of the cells with subG1 DNA content, an increasing in caspase activity and cleavage of caspase substrate. Previous studies suggested that A771726 exerts its anti-proliferate function mostly by inhibiting de novo pyrimidine synthesis in lymphocyte and supplementation of the cultures with exogenous uridine could prevent A771726-induced cell cycle block by restoring the intracellular UTP and CTP lever. However, we find that additions of exogenous uridine can prevent neither A771726-induced cell cycle block nor apoptosis in the MM cell lines tested, suggesting a novel mechanism. It shall be noted that pharmacologically, the highest concentration of A771726 (200uM) used in our study is readily achievable in the plasma through current drug regiment of Leflunomide in RA patients. Since its approval, Leflunomide has been proved to be an effective antirheumatic drug with mild side effects. Given the results from current study, we are tempting to suggest that Leflunomide might also provide a safe alternative in the treatment of MM. The vast knowledge gained from clinical use of Leflunomide in last six year will greatly help us greatly in testing this hypothesis in a clinical setting in a near future.