In 1994 the ligand of the c-mpl receptor was cloned and found to be thrombopoietin (TPO, c-mpl ligand) (79-83). TPO acts as a lineage-specific HGF by promoting the proliferation and differentiation of megakaryocytes into platelets (84). TPO has also been found to have effects on hematopoietic stem cells, promoting their survival and stimulating their growth in combination with other cytokines such as IL-3 or stem cell factor (SCF) (85,86). Preclinical studies in animals, and several phase 1-2 studies in humans have shown that rHuTPO can reduce the severity and duration of thrombocytopenia after chemotherapy-induced myelosuppression (87-92). There has been great interest in using rHuTPO to improve the thrombocytopenia and multilineage responses in patients with MDS.

Plasma TPO concentrations are inversely correlated to platelet count and bone marrow megakaryocyte mass (93). Plasma TPO concentrations have generally been found to be higher in MDS patients than in normal individuals (94). RA patients had higher plasma TPO concentrations than RAEB/RAEB-T patients, irrespective of similar platelet counts in these groups. The plasma TPO concentration was upregulated and inversely correlated with the platelet count in RA patients, but not in RAEB/RAEB-T

or CMML patients (95,96). These findings suggest that the physiologic pathways for TPO production and metabolism are intact in patients with low-risk MDS but may be deranged in the advanced stages of MDS, in which the leukemic clone has progressed. One study found that the number of TPO receptors (c-mpl) was decreased in MDS marrow cells compared with normal cells, a similar finding across all MDS FAB subtypes (96). Reduced numbers of TPO receptors, in addition to thrombocytopenia, may contribute to the increased plasma TPO concentrations found in MDS patients.

In vitro studies have assessed the effects of recombinant TPO on the proliferation and differentiation of MDS and AML bone marrow progenitor cells. In short-term liquid cultures and progenitor assays, recombinant human megakaryocyte growth and development factor (rHuMGDF) stimulated DNA synthesis and potentiated leukemic cluster growth of marrow mononuclear cells in approx 25% of patients with MDS (97). rHuMGDF-induced blast cell proliferation correlated with increased expression of c-mpl. rHuMGDF stimulated CFU-MK growth in 45% of these patients. In another study, rHuTPO increased blast number from cultured marrow cells from high-risk MDS patients, but not lower-risk individuals (RA/RARS) (98). High lactic dehydro-genase values were associated with rHuTPO-induced blast proliferation in high-risk patients. In a study of 7-d suspension cultures, rHuMGDF increased CFU-MK growth in 1/10 AML and 6/16 MDS marrows, and CD61+ (a marker of megakaryocytes) cell numbers were increased in 9/13 AML and 12/15 MDS samples (99). In this series, rHuMGDF did not increase CFU-GM colony or cluster growth. Stimulation of in vitro megakaryopoiesis by rHuMGDF in MDS samples could be augmented by rHuIL-3 and rHuSCF (100). When fetal calf serum and rHuEPO were added to MDS marrow samples, rHuTPO induced MDS CFU-GM and BFU-E cell proliferation in a dose-dependent manner by up to 100% (101). These in vitro effects were comparable to previous results obtained with rHuG-CSF, rHuGM-CSF, and rHuIL-3. rHuTPO also augmented the stimulatory effects of these cytokines on MDS marrow cells. These in vitro studies indicate that rHuTPO can induce differentiation of MDS megakaryocyte precursors; however, the potential for fostering leukemic cell growth also exists, a concern that needs to be carefully scrutinized in future clinical trials of recombinant rHuTPO in MDS.

A phase 1-2 open-label, sequential dose-escalation study evaluated the effects of a 14-d course of intravenous pegylated rHuMGDF in patients with MDS and AA who had average baseline platelet counts < 30 x 109/L without transfusion (102). Preliminary data indicated that the drug was well tolerated, but only modest increases in platelet counts were observed in the dose range of 1.25-5.0 ^g/kg/d.

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