Two major factors involved in the regulation of leukocyte production and function are currently used in clinical practice: recombinant human (rh) granulocyte colony-stimulating factor (G-CSF, filgrastim(Drug information on filgrastim) [Neupogen]) and granulocyte-macrophage colonystimulating factor (GM-CSF, sargramostim(Drug information on sargramostim) [Leukine]). A thorough, clinically relevant review of the biology of these two factors is available.[1,2] It is often overlooked that G-CSF and GM-CSF differ significantly in their roles in hematopoiesis and the regulation of mature effector cell function, and hence require individual evaluation of their clinical utility. Although human interleukin (IL)-3 and monocyte colony-stimulating factor have effects on leukocytes and have been cloned and introduced into clinical trials, they are not currently in clinical use.
- Granulocyte Colony-Stimulating Factor-G-CSF is probably the primary regulator of both basal neutrophil production and increased production and release of neutrophils from the marrow in response to infection ("emergency hematopoiesis") in mammals. Endogenous G-CSF levels rise during neutropenia or infection, and a mechanism for the feedback regulation of G-CSF levels by neutrophils has recently been described.[3] Mature neutrophils have functional receptors, and G-CSF induces activation and priming of neutrophils in several in vitro assays, and increases neutrophil migration to tissues without increased adhesion to endothelial surfaces.[4] G-CSF is also an antiapoptotic factor, prolonging neutrophil survival. Worldwide, two "unmodified" forms of rhG-CSF are in clinical use: filgrastim, which is expressed in Escherichia coli, and lenograstim(Drug information on lenograstim), which is expressed in yeast cells and is available outside the United States. Following subcutaneous administration, rhG-CSF has a terminal serum half-life of approximately 2 to 5 hours and is associated with a dose-related increase in circulating neutrophils due to increased production and release from the marrow, and to increased neutrophil survival.[5,6] Neutrophils produced in response to rhG-CSF therapy function normally or "supernormally" in in vitro function assays.[7] Toxicities of rhG-CSF include bone pain and rare cases of Sweet's syndrome (neutrophilic dermatosis). Several cases of steroid-responsive interstitial pulmonary infiltrates have been reported in rhG-CSF-treated patients with hematologic malignancies.[8] Recently, a pegylated preparation of filgrastim that appears to be cleared by neutrophils and their marrow precursors and to have a prolonged serum half-life (particularly in neutropenic patients) has been introduced into clinical practice.[9] This preparation does not appear to be associated with any additional toxicities or an increase in the incidence or severity of bone pain.
- Granulocyte-Macrophage Colony- Stimulating Factor-GM-CSF does not appear to be required for basal hematopoiesis. In vitro studies suggest that at least some of its apparent hematopoietic effects on the neutrophil lineage are mediated through its effects on phagocytic accessory cells and synergy with G-CSF.[10,11] Studies in knockout mice have demonstrated that GM-CSF has a role in pulmonary homeostasis, which may be due, in part, to a role in the generation of antigen-presenting cells in the lungs.[12] Serum GM-CSF levels do not increase during neutropenia or infection. Mature neutrophils, eosinophils, and monocyte/macrophages have receptors for GM-CSF; in vitro, GM-CSF primes neutrophils to enhanced performance in function assays. Unlike G-CSF, GM-CSF increases adhesion of neutrophils to endothelial surfaces and inhibits neutrophil migration across inflamed endothelial surfaces.[4] Two rhGM-CSF preparations are currently in clinical use: sargramostim, expressed in yeast, and molgramostim(Drug information on molgramostim) (Leucomax), expressed in bacteria. The subcutaneous or intravenous injection of rhGM-CSF is associated with a dose-related increase in circulating eosinophils and neutrophils, and a terminal serum half-life of approximately 2½ hours.[13] Circulating neutrophils taken from patients treated with rhGM-CSF function normally or supernormally in in vitro assays. However, some evidence suggests that in vivo migration of neutrophils to extravascular sites is impaired during rhGM-CSF therapy-an observation that is consistent with in vitro migration studies and that may be relevant to the efficacy of this factor in the prevention of infections.[14] Toxicities of rhGM-CSF include bone pain, rare cases of Sweet's syndrome, fevers, myalgias, serositis, and eosinophilic pneumonia.[15] Some investigators have concluded that these influenza-like symptoms occur more frequently when bacteria-derived rhGM-CSF is used; however, randomized comparative trials of the two preparations have not been reported. In addition, a transient pulmonary leukoagglutination syndrome characterized by hypoxia and hypotension following the first dose of rhGM-CSF has been observed, presumably related to the effects of this factor on neutrophil adhesion.[16]
Several randomized trials have evaluated multicycle chemotherapy with or without rhG-CSF, usually administered subcutaneously at a daily dose of approximately 5 μg/kg before the onset of neutropenia. In adequately powered studies, the results have been consistent, with a significant reduction in the duration of neutropenia for all chemotherapy cycles and a 50% or greater reduction in the risk of clinically significant infections.[19-29] These results have been observed when adjustments in chemotherapy doses were permitted for neutrophil counts or infection (which may bias the study against the overall efficacy of rhG-CSF),[20-22,27] when solid tumors[19,21,23] or nonmyeloid hematologic malignancies[20,22,24-28] were studied (in trials that included pediatric[24] and elderly[25] patient populations), and when prophylactic oral antibiotics were administered to all study patients.[20] Moreover, in the only well-powered randomized comparison of the efficacy of two myeloid growth factors for the prevention of infection during chemotherapy, rhG-CSF was superior to leridistim, a G-CSF/IL-3 fusion molecule.[30] The consistent efficacy of rhG-CSF has been matched by a consistent safety profile, with no evidence of an increased rate of tumor progression or stem cell depletion in clinical trials to date. Trials of rhGM-CSF to Prevent Infection
The administraton of rhGM-CSF has also been associated with a reduction in the duration of neutropenia in patients receiving chemotherapy.[ 31] However, randomized, controlled clinical trials of the effects of rhGM-CSF on the incidence of infection during multicycle chemotherapy have been fewer in number and have generally involved smaller numbers of patients than those that studied rhG-CSF. Moreover, the results of these trials have been inconsistent.[32-38] Two studies have reported a decreased incidence of infection associated with rhGM-CSF therapy-one in an efficacy-evaluable subset of adult patients with lymphoma treated with a daily dose of 400 μg/d for 7 days per cycle,[32] and the other, a small study of 11 children treated for 2 weeks per cycle.[35] Three studies in adults have reported a decreased incidence of infection limited to the first chemotherapy cycle despite continued rhGM-CSF therapy in subsequent cycles.[33,34,36] Three trials-one in adult breast cancer patients[37] and two in pediatric patients[38,39]-have failed to detect a difference in the risk of infection associated with rhGM-CSF therapy. These trials have also yielded varying assessments of the toxicity of rhGM-CSF in this setting, with four trials reporting no significant increase in adverse events,[32,36-38] and other studies suggesting an increased incidence of thrombocytopenia[33,35,39] or of symptoms, including fever, myalgia, edema, and rash.[33-35] In one study, 14% of patients required discontinuation of rhGM-CSF therapy due to toxicity.[34] The variations in efficacy and toxicity results cannot be completely explained by differences in rhGM-CSF preparation, dose, or schedule, or in statistical issues associated with trial design or analysis. To date, the efficacy of rhGM-CSF in the prevention of infection during multicycle chemotherapy has not been established, and if it is effective, the optimal dose and schedule are unknown. One dose-finding study suggested that 10 μg/kg/d would be appropriate for testing in a randomized trial.[33] Practical Issues
Although the safety and efficacy of rhG-CSF has been clearly demonstrated in this setting, practical issues remain. First, because rhG-CSF has not been shown to improve survival, cost-effectiveness is an important consideration. For prevention of infection, the impact of rhG-CSF therapy on cost depends on the cost of the drug, the risk of infection without the use of rhG-CSF, the risk of infection with rhG-CSF, and the cost of treating the infections. Several studies that used a combination of clinical data and modeled assumptions have concluded that the break-even point, at which rhG-CSF therapy becomes less expensive than leaving patients untreated, occurs when the risk of febrile neutropenia is greater than approximately 20%. In addition to assumptions about patients who have experienced a serious neutropenic infection during a prior chemotherapy cycle (secondary prophylaxis) and patients who are initiating myelosuppressive therapy (primary prophylaxis), a useful model for predicting a cost-effective degree of risk based on neutrophil counts during previous chemotherapy cycles has been published.[40] Some clinicians, in an attempt to decrease costs, initiate rhG-CSF therapy only when the neutrophil count has nadired, and thus, fewer than one-half of the doses used in all the positive randomized trials are administered. No evidence suggests that this practice provides any decrease in the risk of infection, and preclinical data predict that it is ineffective. One randomized trial has failed to demonstrate any benefit in terms of infection risk, antibiotic use, or length of hospitalization.[ 41] Paradoxically, this misguided practice increases overall cost by adding the cost of rhG-CSF for 5 or 6 days per cycle without decreasing the risk or cost of infections. Some clinicians administer myeloid growth factors to raise prechemotherapy neutrophil counts when they do not meet their established criteria for initiating the next chemotherapy cycle. This approach should be discouraged, because there is no evidence that temporarily raising neutrophil counts before administering chemotherapy provides any benefit to patients. If the goal of chemotherapy is cure, and delay of such therapy is clinically unacceptable, chemotherapy administered on time, with the growth factor given beginning 1 day thereafter, has been shown to reduce the risk of infection and is a more rational use of the resource. Finally, some evidence suggests that the administration of myeloid growth factors during chemoradiotherapy may be associated with increased thrombocytopenia.[42] Dose and Schedule Issues
There is still legitimate debate regarding the most cost-effective dose and schedule of rhG-CSF. Would a dose of rhG-CSF other than 5 μg/kg/d be more appropriate? The results of one randomized trial suggest that 2 μg/kg/d provides similar results in terms of neutropenia and infection, obviously at a cost savings.[43] Another study failed to find any advantage of μg/kg/d compared to 5 μg/kg/d in a population of pediatric patients.[44] Should rhG-CSF be administered every day? In one trial, 5 μg/kg/d for 5 days a week provided protection against infection, with cost and convenience advantages for patients.[25] Although daily administration of 5 μg/kg remains the most exhaustively studied and validated approach to rhG-CSF use, there may be more costeffective alternatives.
- Pegylated rhG-CSF-Further investigation of the optimal dose and schedule of rhG-CSF has been rendered less important by the introduction of pegylated rhG-CSF (pegfilgrastim [Neulasta]). Because it appears to be cleared by mature neutrophils and their precursors, this factor can be considered "self-regulating" and once-per-cycle dosing is effective. The efficacy and safety of this preparation has been demonstrated in randomized trials that compared it to conventional rhG-CSF therapy.[45] The pegylated preparation is similarly effective when administered at a fixed dose of 6 mg given 24 hours after chemotherapy or when administered using weight-based dosing.[46] A substantial trend in the data suggests that pegylated rhG-CSF may actually be superior to rhG-CSF in terms of protection against infection. The major issues remaining for future study are the safety of administering pegylated rhG-CSF on the same day as chemotherapy and use of pegylated rhG-CSF to support every-2-week dose-dense chemotherapy.
A major change began with the recognition that hematopoietic progenitor cells circulate in increased numbers during myeloid growth factor therapy, followed by the discovery that it was feasible to harvest sufficient quantities of these cells with leukapheresis to provide an alternative and potentially superior graft compared to traditional bone marrow. Peripheral blood progenitor cells (PBPC) harvested during therapy with rhG-CSF[70-74] or rhGM-CSF[70,75-78] and used as hematopoietic support were shown to be associated with significantly more rapid neutrophil and platelet engraftment compared to bone marrow. This advantage of PBPC over marrow may be due to priming of the harvested progenitors. Rapid engraftment profiles have similarly been observed when rhG-CSF-primed marrow is employed.[79] The relatively few published studies comparing engraftment associated with rhG-CSF- vs rhGM-CSF-mobilized PBPCs have suggested that rhG-CSF may be superior to rhGM-CSF in terms of efficacy and/or toxicity.[80,81] The consistency of engraftment is related to the quantity of CD34-positive PBPCs infused. Improved yields are obtained when myeloid growth factors are administered following chemotherapy, although chemotherapy is usually not necessary for adequate graft acquisition even in heavily pretreated patients.[82,83] This approach is not feasible for normal donors in allogeneic transplantation. Several trials have addressed the optimal dose of rhG-CSF dose for an adequate graft. For patients not receiving chemotherapy, rhG-CSF at a dose of 10 μg/kg/d with harvesting commencing on day 5 may produce better yields than lower doses.[84] Splitting the daily dose and administering rhG-CSF twice daily may[85] or may not[86] improve the yield of CD34- positive cells. Very high-dose rhG-CSF (12 μg/kg twice daily) may be superior to 10 μg/kg/d.[87] For patients mobilized with chemotherapy plus rhG-CSF, 16 μg/kg/d is associated with greater cell yields than 8 μg/kg/d, although the increased cell dose may not translate into faster engraftment.[88] Similar dose-finding studies have not been reported for rhGM-CSF in PBPC harvesting. The combination of rhG-CSF and rhGM-CSF is an effective mobilizing regimen,[89] but no studies have demonstrated that this approach is superior to single-factor mobilization in terms of benefit to patients. Initial data suggest that pegylated rhG-CSF is an excellent PBPC mobilizer, and clinical trials are in progress. Graft-vs-Host Disease
Although growth-factor-mobilized PBPCs are clearly superior to unprimed marrow for autologous hematopoietic support, there was initial concern that PBPCs would not provide the sustained long-term donor engraftment required for allogeneic transplants or might change the risk of graft-vs-host disease in this setting. Some studies of mobilized PBPCs for allogeneic transplantation have found that these grafts are durable and not associated with increased graft-vs-host disease.[90-92] However, an increase in the prevalence of anti-HLA alloantibodies has been observed among patients who received PBPC allografts,[ 93] and one controlled trial found an increased incidence of chronic graft-vs-host disease in this population.[94] The use of growth-factor-primed bone marrow may confer all of the engraftment advantages associated with PBPCs without increasing graftvs- host disease.[95] The use of PBPCs as opposed to bone marrow in the unrelated allogeneic transplant setting has not been associated with increases in graft-vs-host disease.[96] Although many large transplant centers now routinely use PBPCs for allografting, and there are clear advantages to donors in terms of morbidity and to recipients in terms of engraftment kinetics, controversy remains regarding the impact of this practice on the severity of the graft-vs-host reaction. Posttransplant Growth Factor Therapy
Are myeloid growth factors beneficial when administered following PBPC transplantation? In sufficiently powered randomized clinical trials, rhG-CSF, at a daily dose of 5 μg/kg or less until neutrophil recovery, has reduced the duration of neutropenia, and in some cases, decreased the length of hospitalization and overall costs.[97-101] It may be possible to delay the initiation of growth factor therapy until day 6 posttransplant without compromising efficacy.[102] Few data exist on the efficacy of rhGM-CSF during recovery from PBPC transplant. Ongoing Research
The use of myeloid growth factors continues to transform bone marrow transplantation. Mobilized PBPCs have made the procedure safer and less costly and have replaced marrow as the preferred source of cellular support. Equally importantly, the use of this flexible graft source has made several promising graft manipulations more feasible-for example, tumor cell purging, ex vivo expansion,[103,104] "split grafts" for multiple cycles of high-dose therapy, selective removal of particular effector cells to reduce or manage graft-vs-host reactions, acquisition of dendritic cell precursors for anticancer vaccine therapy,[105] and gene therapy. Therapy with rhG-CSF has been reported to produce benefits similar to those achieved with donor leukocyte infusions in patients who relapse following allogeneic transplantation.[ 106,107] It is reasonable to expect one or more of these lines of research to yield results that will enable important additions to standard practice in transplantation.
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