RWJ-800088 is a novel, potent polyethylene glycol (PEG)-conjugated thrombopoietin (TPO) mimetic that increases platelet levels and protects against thrombocytopenia. A nonclinical safety program was customized for this peptide that takes into account its protein-like structure, synthetic chemical nature, agonist pharmacologic activity, and mode of administration. In repeat-dose toxicity studies, the salient findings were dose-related increases in circulating platelet counts, mean platelet volume, and megakaryocytes in the bone marrow with no antibody formation. Reversible myelofibrosis and hyperostosis were observed in rats, but not dogs, when the circulating platelet levels exceeded 3× those of vehicle controls. The bone effects were due to the exaggerated pharmacologic effect and excessive stimulation and elevation of megakaryocytes by TPO, which results in intramedullary proliferation of fibroblasts and mesenchymal cells followed by osseous metaplasia. These findings support the use of platelet elevations of >3× as a stopping criterion to prevent potential adverse bone-related effects in humans.