Addition of Pegylated Megakaryocyte Growth Development Factor (pegMGDF) to G-CSF Improves the Mobilization of Primitive Hemopoietic Cells.

We have established a nonhuman primate (NHP) model to test novel agents for their ability to mobilize hemopoietic progenitors and stem cells. Both recombinant thrombopoietin and the truncated form, MGDF, have been shown in Phase 1 trials to increase the number of hemopoietic progenitors in the peripheral blood. We have explored this further in our NHP model by using the combination of pegMGDF with G-CSF and performed comparisons with other cytokines including G-CSF alone, pegylated G-CSF (pegG-CSF) and the combination of G-CSF + Stem Cell Factor (SCF). Male baboons aged between 7 and 14 years of age received cytokines as follows: 1. G-CSF alone 100mcg/kg/day S/C for 5 days; 2. pegG-CSF, single dose 300mcg/kg S/C; 3. G-CSF 100mcg/kg/day S/C + SCF 50mcg/kg/day S/C for 5 days; and 4. pegMGDF 1mcg/kg S/C second daily for 10 days + G-CSF 100mcg/kg/day S/C for 5 days starting 5 days after the pegMGDF. To control for inter-individual variation and allow a direct comparison, animals receiving G-CSF+pegMGDF were also mobilized with G-CSF+SCF with a minimum period of 12 weeks between mobilisations. Blood counts, peripheral blood (PB) CD34 positive cells and colony forming cells (CFC) were quantified at baseline and at day 5 after G-CSF. NOD/SCID repopulating cell (SRC) frequency was quantified at baseline from PBMNCs and on day 5 using cells harvested by leucapheresis. Baseline PB CD34 and CFC counts were < 2/μL and <1.6/μL respectively in all animals.The median peak PB CD34 count was 12/μL, 12.5/μL, 52/μL and 48/μL for G-CSF (n=8), pegG-CSF (n=4), G-CSF+SCF (n=9) and G-CSF+pegMGDF (n=4) respectively. The peak CD34 +ve count after mobilization with G+pegMGDF was significantly higher than that after mobilization with G-CSF alone using the non-parametric Mann Whitney test (p=0.049). The median peak PB CFC count was 6.5/μL, 8/μL, 32/μL and 28.5/μL for G-CSF (n=4), Peg G-CSF (n=4), G-CSF+SCF (n=5) and G-CSF+pegMGDF (n=4) respectively. The median SRC frequency fold-increase from baseline was 12, 7.5, 42 and 53 for G-CSF (n=4), pegG-CSF (n=4), G-CSF+SCF (n=2), and G-CSF+pegMGDF (n=3) respectively. Although there was a trend for an increase in peak CFC and SRC using G+pegMGDF, the differences were not statistically significant due to smaller sample numbers (p=0.34 and p=0.4 respectively). In the 4 animals that received both G-CSF+SCF and G-CSF+pegMGDF the peak PB CD34 and CFC were similar in each animal. The mean CD34 count was 48.3 ± 3.7/μL and 51 ± 8.5 μL and the mean CFC count 32 ± 5.7/μL and 28.5 ± 3/μL for G-CSF+SCF and G-CSF+MGDF respectively. SRC data for this cohort is currently available for two animals. The mean SRC frequency fold-increase was 42 and 58.5 respectively for G-CSF+SCF and G-CSF+pegMGDF respectively. These data suggest that the addition of pegMGDF to G-CSF increases primitive hemopoietic cell numbers compared to G-CSF alone and to a degree comparable to G-CSF plus SCF. This is of relevance in light of the recent clinical development of novel thrombopoietic proteins.