The efficacy of recombinant TPO in murine And nonhuman primate models for myelosuppression and stem cell transplantation.

Radiation-induced pancytopenia proved to be a suitable model system in mice and rhesus monkeys to study thrombopoietin (TPO) target cell range and efficacy. TPO was highly effective in rhesus monkeys exposed to the midlethal dose of 5-Gy (300 kV x-rays) TBI, a model in which it alleviated thrombocytopenia, promoted red cell reconstitution, accelerated reconstitution of immature CD34+ bone marrow (BM) cells and potentiated the response to growth factors such as GM-CSF and G-CSF. The accelerated reconstitution of BM CD34+ cells appeared to be reflected by a similar rise in peripheral blood CD34+ cells, both being augmented by concomitant GM-CSF. However, TPO was ineffective following transplantation of limited numbers of autologous BM or highly purified stem cells in monkeys conditioned with 8-Gy TBI. In the 5-Gy model, a single dose of TPO 24 h after TBI was effective in preventing thrombocytopenia and was augmented by GM-CSF. The strong erythropoietic stimulation may result in iron depletion and TPO treatment should be accompanied by monitoring of iron status. In mice, similar observations were made and the importance of dose and dose schedule for stimulation of multilineage repopulating cells versus the lineage-dominant thrombopoietic response studied in detail.