The potential for effective therapeutic treatment of chemotherapy-induced neutrope-nia and thrombocytopenia was high. In preclinical models, a single injection of either mpl ligand (PEG-rHuMGDF or rHuTPO) was effective in ameliorating radiation-induced thrombocytopenia. The new-generation pegylated forms of filgrastim and leridistim showed that a single injection has therapeutic efficacy equivalent to respective, conventional daily dosing. The potential for single injections of a mpl ligand and a pegylated form of G-CSF to control the thrombocytopenia and neutropenia observed in the oncology setting was evident. However, PEG-rHuMGDF, promegapoietin, leridis-tim, and progenipoietin have been removed from human trials, and development of Peg-leridistim has been stopped. At this writing, TPO remains in clinical trials within the United States, although the optimum schedule appears to require prophylactic administration in dose-intensive chemotherapy regimens. PEG-rHuMGDF remains in clinical trials but only in Japan, China, Korea, and Taiwan. rHuIL-11, although approved for treatment of thrombocytopenia, is not without side effects that may ultimately limit its use in the clinical community. Pegfilgrastim has been approved by the FDA. The ability to administer a single injection that elicits an response equivalent to multiple daily injections will be a welcome addition to the clinical management of neu-tropenia. To conclude, although a number of new hematopoietic growth factors have been developed within the last decade, only two, rHuIL-11 and pegfilgrastim, have been approved by the FDA.

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