Myelopoietin, a chimeric agonist of human interleukin 3 and granulocyte colony-stimulating factor receptors, mobilizes CD34⁺ cells that rapidly engraft lethally X-irradiated nonhuman primates

Myelopoietin (MPO), a multifunctional agonist of interleukin 3 and granulocyte colony-stimulating factor (G-CSF) receptors, was evaluated for its ability to mobilize hematopoietic colony-forming cells (CFC) and CD34⁺ cells relative to control cytokines in normal nonhuman primates. Additionally, the engraftment potential of MPO-mobilized CD34⁺ cells was assessed in lethally irradiated rhesus monkeys. Normal rhesus monkeys were administered either MPO (200 μg/kg/day), daniplestim (a high-affinity interleukin 3 receptor agonist) (100 μg/kg/day), G-CSF (100 μg/kg/day), or daniplestim coadministered with G-CSF (100 μg/kg/day each), subcutaneously for 10 consecutive days. The mobilization kinetics were characterized by peripheral blood (PB) complete blood counts, hematopoietic CFC [granulocyte- macrophage CFC (GM-CFC), megakaryocyte CFC (MK-CFC)], and the immunophenotype (CD34⁺ cells) of PB nucleated cells prior to and on day 3 to days 7, 10, 12, and 14, and at intervals up to day 28 following initiation of cytokine administration. A single large-volume leukapheresis was conducted on day 5 in an additional cohort (n = 10) of MPO-mobilized animals. Eight of these animals were transplanted with two doses of CD34⁺ cells/kg. A maximum 10- fold increase in PB leukocytes (white blood cells) (from baseline 7.8-12.3 x 10³/μL to approximately 90 x 10³/μL) was observed over day 7 to day 10 in the MPO, G-CSF, or daniplestim+G-CSF cohorts, whereas daniplestim alone stimulated a less than onefold increase. A sustained, maximal rise in PB- derived GM-CFC/mL was observed over day 4 to day 10 for the MPO-treated cohort, whereas the daniplestim+ G-CSF, G-CSF alone, and daniplestim alone treated cohorts were characterized by a mean peak value on days 7, 6, and 18, respectively. Mean peak values for PB-derived GM-CFC/mL were greater for MPO (5,427/mL) than for daniplestim+ G-CSF (3,534/mL), G-CSF alone (3,437/mL), or daniplestim alone (155/mL) treated cohorts. Mean peak values for CD34⁺ cells/mL were noted within day 4 to day 5 of cytokine administration: MPO (255/μL, day 5), daniplestim+G-CSF (47/μL, day 5), G-CSF (182/μL, day 4), and daniplestim (96/μL, day 5). Analysis of the mobilization data as area under the curve indicated that for total CFCs, GM-CFC, MK-CFC, or CD34⁺ cells, the MPO-treated areas under the curve were greater than those for all other experimental cohorts. A single, large-volume (3.0 x blood volume) leukapheresis at day 5 of MPO administration (PB: CD34⁺ cell/μL = 438 ± 140, CFC/mL = 5,170 ± 140) resulted in collection of sufficient CD34⁺ cells (4.31 x 10⁶/kg ± 1.08) and/or total CFCs (33.8 x 10⁴/kg ± 8.34) for autologous transplantation of the lethally irradiated host. The immunoselected CD34⁺ cells were transfused into autologous recipients (n = 8) at cell doses of 2 x 10⁶/kg (n = 5), and 4 x 10⁶/kg (n = 3) on the day of apheresis. Successful engraftment occurred with each cell dose. The data demonstrated that MPO is an effective and efficient mobilizer of PB progenitor cells and CD34⁺ cells, such that a single leukapheresis procedure results in collection of sufficient stem cells for transplantation and long term engraftment of lethally irradiated hosts.