ALBERT

Description: 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.

Description: TLK199 is a novel glutathione analog inhibitor of the enzyme GST P1-1. GST P1-1 has been shown to be a negative regulator of c-Jun N-terminal kinase, which has been implicated in the control of cellular growth and differentiation. Studies in rodents have shown that TLK199 treatment increases the levels of circulating white blood cells and stimulates the production of CFU-GM in bone marrow. In a rodent model of 5-FU-induced neutropenia, treatment with TLK199 accelerated the recovery of neutrophil levels at a rate similar to that observed with G-CSF. In a recently reported Phase 2 clinical trial in patients with refractory MDS involving multilineage blood cell dysfunction, treatment with TLK199 resulted in a 65% Hematological Improvement of neutrophils, erythrocytes and/or platelets, confirming the myelorestorative effect of TLK199 in humans. The present study was designed to determine if the myelostimulative effects of TLK199 on neutrophils and their precursors are due in part to increased levels of endogenous G-CSF. TLK199 was tested in normal and neutropenic rodents for its effects on G-CSF production. A single administration of TLK199 (p.o. or i.p.) to normal mice resulted in up to 500% increase in G-CSF serum levels. In a rat model of 5-FU-induced neutropenia, TLK199 administration accelerated the recovery of circulating neutrophils, which corresponded to maximal G-CSF serum levels being observed earlier than vehicle control. The ability of TLK199 to enhance G-CSF production in primary human bone marrow stromal cells was also examined. Treatment of these cells with TLK199 potentiated IL-1b-induced production of G-CSF, GM-CSF and IL-6 from 200 to 400%. These data suggest that the enhancement of G-CSF production may contribute to the accelerated recovery of neutrophils observed with TLK199 treatment under neutropenic conditions. Together with the results from the Phase 2 clinical trial in MDS, these data support further clinical assessment of TLK199 treatment for chemotherapy-induced cytopenia.