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Chemotherapy-induced thrombocytopenia can lead to chemotherapy treatment delays or dose reductions. The ability of romiplostim, a thrombopoietin (TPO) mimetic, to promote platelet recovery in a mouse model of multicycle chemotherapy/radiation therapy (CRT)-induced thrombocytopenia was examined. In humans, an inverse relationship between platelet counts and endogenous TPO (eTPO) concentration exists. In a CRT mouse model, eTPO was not elevated during the first 5 days after CRT treatment (the "eTPO gap"), then increased to a peak 10 days after each CRT treatment in an inverse relationship to platelet counts seen in humans. To bridge the eTPO gap, mice were treated with 10–1,000 μg/kg of romiplostim on day 0, 1, or 2 after CRT. In some mice, the romiplostim dose was approximately divided over 3 days. Platelet recovery occurred faster with romiplostim in most conditions tested. Romiplostim doses of ≥100 μg/kg given on day 0 significantly lessened the platelet nadir. Fractionating the dose over 3 days did not appear to confer a large advantage. These data may provide a rationale for clinical studies of romiplostim in chemotherapy-induced thrombocytopenia.

We previously reported an increased incidence of thrombotic toxicities in Sprague-Dawley rats administered the highest dose level of a hyperglycosylated analog of recombinant human erythropoietin (AMG 114) for 1 month as not solely dependent on high hematocrit (HCT). Thereafter, we identified increased erythropoiesis as a prothrombotic risk factor increased in the AMG 114 high-dose group with thrombotic toxicities, compared to a low-dose group with no toxicities but similar HCT. Here, we identified pleiotropic cytokines as prothrombotic factors associated with AMG 114 dose level. Before a high HCT was achieved, rats in the AMG 114 high, but not the low-dose group, had imbalanced hemostasis (increased von Willebrand factor and prothrombin time, decreased antithrombin III) coexistent with cytokines implicated in thrombosis: monocyte chemotactic protein 1 (MCP-1), MCP-3, tissue inhibitor of metalloproteinases 1, macrophage inhibitory protein-2, oncostatin M, T-cell-specific protein, stem cell factor, vascular endothelial growth factor, and interleukin-11. While no unique pathway to erythropoiesis stimulating agent-related thrombosis was identified, cytokines associated with increased erythropoiesis contributed to a prothrombotic intravascular environment in the AMG 114 high-dose group, but not in lower dose groups with a similar high HCT.

For targets that are homogenously expressed, such as CD19 on cells of the B lymphocyte lineage, immunotherapies can be highly effective. Targeting CD19 with blinatumomab, a CD19/CD3 bispecific antibody construct (BiTE®), or with chimeric antigen receptor T cells (CAR-T) has shown great promise for treating certain CD19-positive hematological malignancies. In contrast, solid tumors with heterogeneous expression of the tumor-associated antigen (TAA) may present a challenge for targeted therapies. To prevent escape of TAA-negative cancer cells, immunotherapies with a local bystander effect would be beneficial. As a model to investigate BiTE®-mediated bystander killing in the solid tumor setting, we used epidermal growth factor receptor (EGFR) as a target. We measured lysis of EGFR-negative populations in vitro and in vivo when co-cultured with EGFR-positive cells, human T cells and an EGFR/CD3 BiTE® antibody construct. Bystander EGFR-negative cells were efficiently lysed by BiTE®-activated T cells only when proximal to EGFR-positive cells. Our mechanistic analysis suggests that cytokines released by BiTE®-activated T-cells induced upregulation of ICAM-1 and FAS on EGFR-negative bystander cells, contributing to T cell-induced bystander cell lysis.

Therapeutic use of recombinant human cytokines in humans can result in the generation of drug-specific antibodies. To predetermine the maximum potential effects of a granulocyte colony-stimulating factor (G-CSF) neutralizing auto-immunoglobulin G (auto-IgG) response during recombinant human G-CSF therapy, we developed a mouse model of mouse G-CSF (mG-CSF) neutralizing auto-IgG response. Mice were immunized and boosted with mG-CSF chemically conjugated to either keyhole limpet hemocyanin or ovalbumin on an alternating schedule. Sera were analyzed for mG-CSF-specific titers and full blood counts were performed on a Technicon H-1E. On day 252, tissues were collected for histology. IgG was protein A affinity purified from pooled mG-CSF autoimmune sera. Mice immunized with mG-CSF conjugates produced mG-CSF-specific auto-IgG responses that lasted for the length of the study. Significant neutropenia (p(max) < 0.004) was concurrent with the rise in mG-CSF-specific IgG titers. However, neutrophil counts remained at approximately 20% of preimmunization levels through day 252. Endogenous mG-CSF neutralizing auto-IgG had no significant effect on hemoglobin, erythrocyte, lymphocyte, eosinophil, basophil, and platelet counts, and had minor, transient, or no effects on monocyte counts. Bone marrow colony assays from mG-CSF autoimmune mice demonstrated no significant effect of G-CSF neutralization on the numbers or proliferative capacity of preneutrophil lineage progenitors. Purified IgG from mG-CSF autoimmune mice neutralized mG-CSF in vitro. High-titer G-CSF neutralizing auto-IgG in adult mice partially inhibited steady-state granulopoiesis and had little or no effect on steady-state levels of other hematopoietic cells.

Granulocyte colony-stimulating factor (G-CSF) has proven effective in the prophylaxis of chemotherapy-induced neutropenia and as a mobilizer of peripheral blood progenitor cells. The longevity of G-CSF action is limited by its removal from the body by two mechanisms. The first is thought to be mediated via receptors (receptor mediated clearance [RMC]) predominantly on neutrophils, the second process is likely the result of renal clearance. With the intention of developing a novel form of Filgrastim (r-met HuG-CSF) with a sustained duration of action in vivo, a new derivative named SD/01 has been made by association of Filgrastim with poly(ethylene glycol). The desired properties of this new agent would include a prolonged duration of action sufficient to cover a complete single course of chemotherapy. SD/01 is shown here to sustain significantly elevated neutrophil counts in hematopoietically normal mice for 5 days. In neutropenic mice effects were noted for at least 9 days, accompanying a significant reduction in the duration of chemotherapy induced neutropenia. Normal human volunteers showed higher than baseline ANC for around 9 to 10 days after a single injection of SD/01. Data from these normal volunteers also indicate that mobilization of CD34+ cells and progenitors may occur in a more timely manner and to around the same absolute numbers as with repeated daily injections of unmodified Filgrastim. These data indicate that SD/01 represents an efficacious novel form of Filgrastim with actions sustained for between one and two weeks from a single injection.

The effect of chronic expression of flt3 ligand (FL) on in vivo hematopoiesis was studied. Retroviral vector-mediated gene transfer was used in a mouse model of bone marrow transplantation to enforce expression of mouse FL cDNA in hematopoietic tissues. As early as 2 weeks posttransplantation, peripheral blood white blood cell counts in FL-overexpressing recipients were significantly elevated compared with controls. With the exception of eosinophils, all nucleated cell lineages studied were similarly affected in these animals. Experimental animals also exhibited severe anemia and progressive loss of marrow-derived erythropoiesis. All of the FL-overexpressing animals, but none of the controls, died between 10 and 13 weeks posttransplantation. Upon histological examination, severe splenomegaly was noted, with progressive fibrosis and infiltration by abnormal lymphoreticular cells. Abnormal cell infiltration also occurred in other organ systems, including bone marrow and liver. In situ immunocytochemistry on liver sections showed that the cellular infiltrate was CD3+/NLDC145+/CD11c+, but B220− and F4/80−, suggestive of a mixed infiltrate of dendritic cells and activated T lymphocytes. Infiltration of splenic blood vessel perivascular spaces resulted in vascular compression and eventual occlusion, leading to splenic necrosis consistent with infarction. These results show that FL can affect both myeloid and lymphoid cell lineages in vivo and further demonstrate the potential toxicity of in vivo treatment with FL.

We have studied the potential of combination growth factor treatment with GM-CSF and megakaryocyte growth and development factor (MGDF) to stimulate hematopoietic recovery in mice following bone marrow transplantation. More rapid recovery of neutrophils occurred in mice treated with recombinant murine (rm)GM-CSF plus pegylated recombinant human (PEG-rHu)MGDF than carrier treated controls, however this recovery was equivalent to the effect of treatment with rmGM-CSF alone. PEG-rHuMGDF stimulated a more rapid recovery of platelets with no effect on neutrophil recovery. At the two tested doses of rmGM-CSF (72 and 200 μg/kg/day) the platelet recovery was inferior to that in carrier treated mice. Also, the addition of rmGM-CSF to PEG-rHuMGDF had a dose-related negative impact on platelet recovery compared to PEG-rHuMGDF alone. These data suggest that the use of combination therapy in some clinical indications may lead to unexpected results. Furthermore, careful dosage studies may be necessary to identify the full potential of combined growth factors to obtain additive or synergistic effects on multilineage hematopoietic reconstitution in vivo.

We recently reported results that erythropoiesis-stimulating agent (ESA)–related thrombotic toxicities in preclinical species were not solely dependent on a high hematocrit (HCT) but also associated with increased ESA dose level, dose frequency, and dosing duration. In this article, we conclude that sequelae of an increased magnitude of ESA-stimulated erythropoiesis potentially contributed to thrombosis in the highest ESA dose groups. The results were obtained from two investigative studies we conducted in Sprague-Dawley rats administered a low (no thrombotic toxicities) or high (with thrombotic toxicities) dose level of a hyperglycosylated analog of recombinant human erythropoietin (AMG 114), 3 times weekly for up to 9 days or for 1 month. Despite similarly increased HCT at both dose levels, animals in the high-dose group had an increased magnitude of erythropoiesis measured by spleen weights, splenic erythropoiesis, and circulating reticulocytes. Resulting prothrombotic risk factors identified predominantly or uniquely in the high-dose group were higher numbers of immature reticulocytes and nucleated red blood cells in circulation, severe functional iron deficiency, and increased intravascular destruction of iron-deficient reticulocyte/red blood cells. No thrombotic events were detected in rats dosed up to 9 days suggesting a sustained high HCT is a requisite cofactor for development of ESA-related thrombotic toxicities.

We have investigated the potential of PEGylated recombinant human megakaryocyte growth and development factor (PEG-rHuMGDF), a molecule related to thrombopoietin (mpl ligand or TPO) in minimizing the thrombocytopenia associated with hematopoietic ablation and peripheral blood progenitor cell (PBPC) transplant. Irradiated mice that received PBPC mobilized by PEG-rHuMGDF or granulocyte colony-stimulating factor (G-CSF) had a reduced number of thrombocytopenic days with platelets below 100 x 10(6) per mL of blood. Recipients of unmobilized PBPC had a 9 day thrombocytopenic phase which was shortened to 7 days if they were given granulocyte-macrophage colony-stimulating factor (GM-CSF)- mobilized PBPC. This was further reduced to 2 or 3 days of thrombocytopenia in recipients of G-CSF- or PEG-MGDF-mobilized PBPC. Despite our observation that PEG-rHuMGDF is a relatively modest stimulator of the mobilization of myeloid progenitors to the blood, MGDF-mobilized PBPC do effect accelerated recovery of platelets after transplantation. However, the most effective use of PEG-rHuMGDF is when it is given during the recovery phase after PBPC transplantation to hematopoietically ablated mice. Posttransplant treatment with PEG- rHuMGDF reduces thrombocytopenia to a single day or less, in recipients of most types of PBPC. Mice that were treated during the first 2 weeks after PBPC transplant with PEG-rHuMGDF had 1 thrombocytopenic day compared to 9 days in carrier-treated recipients of unmobilized PBPC and 2 to 3 days in carrier-treated recipients of the optimally mobilized PBPC from G-CSF or G-CSF/PEG-rHuMGDF treated donors. In groups where PEG-rHuMGDF was included in the mobilization protocol and used to treat recipients as well thrombocytopenia was effectively eliminated. These data show that PEG-rHuMGDF is a highly effective agent in eliminating the thrombocytopenia associated with PBPC transplantation.

Megakaryocyte growth and development factor (MGDF) has recently been identified as a ligand for the c-mpl receptor. Using retroviral- mediated gene transfer, MGDF has been overexpressed in mice to evaluate the systematic effects due to chronic exposure to this growth factor. MGDF overexpressing mice had more rapid platelet recovery than control mice after transplantation. Following this recovery, the platelet levels continued increasing to fourfold to eightfold above normal baseline levels and remained elevated (five-fold above control mice) in these animals, which are alive and well at more than 4 months posttransplantation. Increased megakaryocyte numbers were detected in a number of organs in these mice including bone marrow, spleen, liver, and lymph nodes. Prolonged overexpression of MGDF led to decreased marrow hematopoiesis, especially erythropoiesis, with a shift to extramedullary hematopoiesis in the spleen and liver. All the MGDF overexpressing mice analyzed to date developed myelofibrosis and osteosclerosis, possibly induced by megakaryocyte and platelet produced cytokines. No significant effect on other hematopoietic lineages was seen in the MGDF overexpressing mice, showing that the stimulatory effect of MGDF in vivo is restricted to the megakaryocyte lineage.

Mobilized peripheral blood progenitor cells (PBPC) have been shown to provide rapid engraftment in patients given high-dose chemotherapy. PBPC contain cells with long-term engraftment potential as shown in animal models. In this study we have further analyzed mobilized PBPC for their ability to support serial transplantation of irradiated mice. Transplantation of recombinant human granulocyte colony-stimulating factor (rhG-CSF) plus recombinant rat stem cell factor (rrSCF) mobilized PBPC resulted in 98% donor engraftment of primary recipients at 12 to 14 months post-transplantation. Bone marrow (BM) cells from these primary recipients were harvested and transplanted into secondary recipients. At 6 months posttransplantation, all surviving secondary recipients had donor engraftment. Polymerase chain reaction (PCR) analysis showed greater than 90% male cells in spleens, thymuses, and lymph nodes. Myeloid colonies from BM cells of secondary recipients demonstrated granulocyte/macrophage colony-forming cells (GM-CFC) of male origin in all animals. In comparison, transplantation of rhG-CSF mobilized PBPC resulted in decreased male engraftment in secondary recipients. BM cells from secondary recipients, who originally received PBPC mobilized by the combination of rrSCF and rhG-CSF, were further passaged to tertiary female recipients. At 6 months posttransplantation, 90% of animals had male-derived hematopoiesis by whole-blood PCR analysis. These data showed that PBPC mobilized with rhG-CSF plus rrSCF contained cells that are transplantable and able to maintain hematopoiesis for more than 26 months, suggesting that the mobilized long-term reconstituting stem cells (LTRC) have extensive proliferative potential and resemble those that reside in the BM. In addition, the data demonstrated increased mobilization of LTRC with rhG-CSF plus rrSCF compared to rhG-CSF alone.

Aurora kinases A and B play essential roles in multiple stages of mitosis and are frequently overexpressed in a subset of human cancers, including acute myeloid leukemia (AML) (Ikezoe T et al, 2007). AMG 900, a potent and highly selective small molecule inhibitor of aurora kinases, is currently in Phase 1 clinical testing in adult patients with AML. In this study, we report the preclinical effects of AMG 900 in AML cell lines. We show that AMG 900 inhibits the phosphorylation of Histone H3 on serine-10 (a proximal substrate of aurora-B) leading to aborted cell division, apoptosis and/or polyploidy. We evaluated the activity of AMG 900 and two other well-characterized aurora kinase inhibitors [AZD1152-hQPA (B-selective AKI) and MLN8054 (A-selective AKI)] in a panel of AML cell lines. AMG 900 inhibited proliferation in all 10 cell lines at single-digit nanomolar concentrations. At effective concentrations, AMG 900 and AZD1152-hQPA showed similar cellular phenotypes, indicating that the activity of AMG 900 may occur through inhibition of aurora-B. A subset of cell lines sensitive to AMG 900 and MLN8054 were insensitive to AZD1152-hQPA, suggesting that AMG 900 may be less susceptible to resistance mediated by drug-efflux (Grundy M et al, 2011). Two AMG 900 oral dosing schedules are being evaluated in the ongoing AML clinical trial; patients receive either 4 or 7 consecutive daily doses followed by a drug holiday in 14-day cycles. In this study, we evaluated the in vivo anti-proliferative effects of AMG 900 using the two dose schedules in the skeleton of NOD/SCID IL2γnull mice bearing MOLM-13 (AML) cells expressing luciferase. To assess tumor cell proliferation in vivo, we used 18FLT (radioactive thymidine analog) PET/CT imaging, a technique that has been used to monitor early treatment response in the bone marrow of AML patients (Vanderhoek M et al, 2011). Mice were imaged for luciferase activity and 18FLT uptake before treatment and at multiple time-points during the drug holiday phase within the 14-day cycle. While the two AMG 900 dosing schedules resulted in a similar decrease in tumor burden across study time points (as measured by luciferase activity), they differed in the timing of skeletal 18FLT responses. Mice administered AMG 900 showed an attenuated skeletal 18FLT uptake compared with the vehicle group, followed by an 18FLT flare. This 18FLT flare event is notably higher using the AMG 900 4-day schedule, although the cumulative dose is similar for both schedules. This difference in 18FLT flaring may indicate the schedules differ in the duration and/or level of target inhibition in the skeletal tumor and bone marrow cells. Mice treated with sunitinib (positive control agent) did not show a skeletal 18FLT flare during the drug holiday, suggesting its mode of action is distinct from that of AMG 900. At the end of the study, mouse bone marrow was assessed for tumor burden by flow cytometry. Mice treated with AMG 900 showed a significant decrease in tumor burden compared with the vehicle group. Interestingly, the mice administered AMG 900 7-day schedule showed the most suppression of tumor growth compared with either AMG 900 4-day schedule or sunitinib. Together, our preclinical studies demonstrate that AMG 900 is a potent inhibitor of aurora kinases that robustly suppresses the growth of AML cells in vitro and in vivo. Furthermore, we highlight the utility of in vivo imaging to monitor AMG 900 drug action, which may help to inform future dose scheduling and drug combination studies. Disclosures: Cheung: Amgen Inc: Employment, Equity Ownership. Juan:Amgen Inc.: Employment, Equity Ownership. Wayne:Amgen Inc.: Employment, Equity Ownership. Hanestad:Amgen Inc.: Employment, Equity Ownership. Keegan:Amgen Inc.: Employment, Equity Ownership. Huard:Amgen Inc.: Employment, Equity Ownership. McElroy:Amgen Inc.: Employment, Equity Ownership. Stanton:Amgen Inc.: Employment, Equity Ownership. Bush:Amgen Inc.: Employment, Equity Ownership. Kendall:Amgen Inc.: Employment, Equity Ownership. Radinsky:Amgen Inc.: Employment, Equity Ownership. Abella:Amgen Inc. : Employment, Equity Ownership. Pieslor:Amgen Inc.: Employment, Equity Ownership. Friberg:Amgen Inc.: Employment, Equity Ownership. Coxon:Amgen Inc.: Employment, Equity Ownership. Gamelin:Amgen Inc: Employment, Equity Ownership. Payton:Amgen Inc.: Employment, Equity Ownership. Off Label Use: AMG 900 is currently in phase 1 clinical development, there is no approved label.

Megakaryocyte growth and development factor (MGDF) is a recently characterized ligand for the cell surface receptor mpl. We have evaluated the effects of polyethylene glycollated recombinant human MGDF (PEG-rHuMGDF) on recovery of hematopoietic cells in mice following bone marrow transplantation (BMT) to support lethal irradiation. Mice treated with PEG-rHuMGDF (50 micrograms/kg/d) had accelerated recovery of platelet numbers compared with BMT mice treated with carrier or recombinant human granulocyte colony-stimulating factor (rHuG-CSF, 72 or 200 micrograms/kg/d). In contrast, PEG-rHuMGDF had no effect on white blood cell (WBC) or red blood cell (RBC) recovery. As previously reported, animals treated with rHuG-CSF had an enhanced recovery of WBC but not platelet or RBC levels. Interestingly, BMT receipient mice treated with the combination of PEG-rHuMGDF and rHuG-CSF showed simultaneous enhanced recovery of both leukocytes and platelets. PEGylated rHuMGDF was found to be considerably more potent than non- PEGylated rHuMGDF in this setting. PEG-rHuMGDF is an effective growth factor for enhancing platelet recovery in mice following BMT either alone or in combination with rHuG-CSF. It will be of interest to evaluate in a clinical setting the ratios of PEG-rHuMGDF and rHuG-CSF for simultaneous administration of these factors and accelerated recovery of both leukocytes and platelets.

Introduction: Recombinant human erythropoietin (rHuEPO) has become a standard treatment for the alleviation of anemia resulting from chronic renal failure and chemotherapy induced anemia. Treatment with rHuEPO corrects anemia, resulting in a broad range of clinical benefits including improved quality of life. Darbepoetin alfa (DA), a super-sialylated erythropoietic stimulating protein (ESP) with a 3-fold longer serum half-life than rHuEPO, is approved for use in the treatment of anemia. Another ESP investigated was a hyperglycosylated analog of DA having a greater number of carbohydrate chains and sialic acids (AMG205). Objective: Our intention was to develop and characterize new ESPs and examine their relative activity and ability to maintain normal hemoglobin levels in a mouse model with stable anemia (hemoglobin 2g/dL below initial levels, > 98 days). Methods and Results: Clearance of ESPs was reduced and in vivo potency was increased in proportion to number of carbohydrate chains; AMG205 was cleared the slowest and was the most potent of the 3 test molecules. A regimen of repeated low dose exposure to cisplatin (CDDP) was established that resulted in a stable reduction of hemoglobin levels by 2–3 g/dL (12.23±0.75g/dL). The hemoglobin range in control animals not treated with CDDP in this experiment was (14.53±0.57g/dL). Animals made anemic by repeated administration of CDDP were treated with rHuEPO (20μg/kg), DA (10μg/kg) or AMG205 (10μg/kg). Each group consisted of 15 mice. Following the initial injection, hemoglobin was measured every 2–3 days, and subsequent injections were administered when hemoglobin values were < 13 g/dL. Animals were so treated for 98 days. In this manner the number of injections of each ESP required to maintain normal hemoglobin was determined. Mice treated for 98 days required more injections of rHuEPO to maintain normal Hb levels than DA (13 vs.7 injections), while mice treated with AMG205 required 5 injections. Conclusion: Normal hemoglobin levels were maintained over a 3 month period with repeated administration of the ESPs investigated. The ESPs with increased carbohydrate content had reduced clearance, increased in vivo potency and could maintain hemoglobin within a normal range with fewer injections than rHuEPO.

Therapeutic use of cytokines in humans can result in the generation of antibodies that bind to the drug. In order to predict the potential effects of an autoantibody response to recombinant human G-CSF therapy, we developed a novel, reproducible mouse model of anti-G-CSF specific humoral immunity. Two groups of mice were immunized and boosted with mouse G-CSF (mG-CSF) chemically conjugated to either KLH or OVA on an alternating schedule. All mice immunized with the mG-CSF conjugates produced high titer, mG-CSF specific IgG responses that lasted for the length of the study (252 days). Significant neutropenia was concurrent with the rise in anti-mG-CSF IgG titers (p