A SINGLE-DOSE OF THROMBOPOIETIN SHORTLY AFTER MYELOSUPPRESSIVE TOTAL-BODY IRRADIATION PREVENTS PANCYTOPENIA IN MICE BY PROMOTING SHORT-TERMMULTILINEAGE SPLEEN-REPOPULATING CELLS AT THE TRANSIENT EXPENSE OF BONE MARROW-REPOPULATING CELLS

Thrombopoietin (TPO) has been used in preclinical myelosuppression mod els to evaluate the effect on hematopoietic reconstitution. Here we re port the importance of dose and dose scheduling for multilineage recon stitution after myelosuppressive total body irradiation (TBI) in mice. After 6 Gy TBI, a dose of 0.3 mu g TPO/mouse (12 mu g/kg) intraperito neally (IP), 0 to 4 hours after TBI, prevented the severe thrombopenia observed in control mice, and in addition stimulated red and white bl ood cell regeneration. Time course studies showed a gradual decline in efficacy after an optimum within the first hours after TBI, accompani ed by a replacement of the multilineage effects by lineage dominant th rombopoietic stimulation. Pharmacokinetic data showed that IP injectio n resulted in maximum plasma levels 2 hours after administration. On t he basis of the data, we inferred that a substantial level of TPO was required at a critical time interval after TBI to induce multilineage stimulation of residual bone marrow cells. A more precise estimate of the effect of dose and dose timing was provided by intravenous adminis tration of TPO, which showed an optimum immediately after TBI and a sh arp decline in efficacy between a dose of 0.1 mu g/mouse (4 mu g/kg; p lasma level 60 ng/mL), which was fully effective, and a dose of 0.03 m u g/mouse (1.2 mu g/kg; plasma level 20 ng/mL), which was largely inef fective. This is consistent with a threshold level of TPO required to overcome initial c-mpl-mediated clearance and to reach sufficient plas ma levels for a maximum hematopoietic response. In mice exposed to fra ctionated TBI (3 x 3 Gy, 24 hours apart), IP administration of 0.3 mu g TPO 2 hours after each fraction completely prevented the severe thro mbopenia and anemia that occurred in control mice. Using short-term tr ansplantation assays, ie, colony-forming unit-spleen (CFU-S) day 13 (C FU-S-13) and the more immature cells with marrow repopulating ability (MRA), it could be shown that TPO promoted CFU-S-13 and transiently de pleted MRA. The initial depletion of MRA in response to TPO was replen ished during longterm reconstitution followed for a period of 3 months . Apart from demonstrating again that MRA cells and CFU-S-13 are separ ate functional entities, the data thus showed that TPO promotes short- term multilineage repopulating cells at the expense of more immature a ncestral cells, thereby preventing pancytopenia. The short time interv al available after TBI to exert these effects shows that TPO is able t o intervene in mechanisms that result in functional depletion of its m ultilineage target cells shortly after TBI and emphasizes the requirem ent of dose scheduling of TPO in keeping with these mechanisms to obta in optimal clinical efficacy. (C) 1998 by The American Society of Hema tology.