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TLK199 is a novel glutathione analog that is a selective inhibitor of the enzyme glutathione S-transferase (GST) P1-1. TLK199 treatment induces hematopoietic cell proliferation and differentiation through activation of the MAP kinase signaling pathway leading to activation of JNK and ERK2. In rodent models of chemotherapy induced neutropenia, treatment with TLK199 accelerated recovery of white cell counts at rates comparable to treatment with G-CSF. We now report TLK199 treatment of myeloid progenitor cells isolated from normal human blood resulted in the increased formation of CFU-GM (46%) CFU-MK (47%) and BFU-E (142%) lineages over baseline. A corresponding increase in the percentage of cells expressing CD11b, a granulocyte and monocyte marker, was observed in the CFU-GM cells. Since TLK199 is currently being evaluated in a Phase 2a trial in patients with refractory MDS, we examined the effect of treatment on formation of BFU-E, CFU-GM and CFU-GEMM before and after TLK199 treatment at doses of 50 to 400 mg/m². A significant increase in the number of hematopoietic progenitor cell colonies measured from patient peripheral blood and bone marrow was observed as early as Day 4 of Cycle 1 as compared to pretreatment baseline. Ten of 12 patients showed an increase in at least one colony forming type (BFU-E, CFU-GM and CFU-GEMM) and 7 of 12 had an increase in all three colony forming types following TLK199 administration. These results correlate with clinical improvement in hematological parameters in peripheral blood and bone marrow observed in MDS patients treated with TLK199. Studies are underway to define the mechanism of bone marrow and peripheral blood count recovery observed following treatment of MDS patients with TLK199 and the role of GST P1-1 as a regulatory element in myeloid proliferation and differentiation.