The Oncologist, Vol. 12, No. suppl_2, 22-26, October 2007; doi:10.1634/theoncologist.12-S2-22
© 2007 AlphaMed Press
The Role of Sargramostim (rhGM-CSF) as Immunotherapy
Edmund K. Waller
Bone Marrow and Stem Cell Transplant Center, Winship Cancer Institute, Emory University, Atlanta, Georgia, USA
Key Words. Granuloctye-macrophage colony-stimulating factor • Cancer vaccines • Immunotherapy • Dendritic cells
Correspondence: Correspondence: Edmund K. Waller, M.D., Ph.D., Bone Marrow and Stem Cell Transplant Center, Winship Cancer Institute, Emory University, 1365-C Clifton Road NE, Atlanta, Georgia 30322, USA. Telephone: 404-727-4995; Fax: 404-712-9995; e-mail: ewaller{at}emory.edu
Received May 22, 2007;
accepted for publication July 20, 2007.
Disclosure: E.K.W. has acted as a consultant to Berlex.
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ABSTRACT
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GM-CSF stimulates the differentiation of hematopoietic progenitors to monocytes and neutrophils, and reduces the risk for febrile neutropenia in cancer patients. GM-CSF also has been shown to induce the differentiation of myeloid dendritic cells (DCs) that promote the development of T-helper type 1 (cellular) immune responses in cognate T cells. This review summarizes some of the immunological effects of GM-CSF relevant to antitumor immunity in cancer patients. GM-CSF has been used to augment the activity of rituximab in patients with follicular lymphoma and to induce autologous antitumor immunity in patients with hormone-refractory prostate cancer. GM-CSF causes upregulation of costimulatory molecule expression on leukemia blasts in vitro, enhancing their ability to present antigen to allogeneic T cells, and, in combination with interferon-
, can induce antitumor immune responses in patients whose acute leukemia has relapsed following allogeneic hematopoietic progenitor cell transplant. Tumor cells engineered to secrete GM-CSF are particularly effective as antitumor vaccines, and the addition of GM-CSF to standard vaccines may increase their effectiveness by recruiting DCs to the site of vaccination. However, a significant limitation in the use of GM-CSF as an immunostimulatory agent is that objective antitumor responses are infrequent, and are often not durable. Effective and durable antitumor immunity will likely require novel methods to eliminate counterregulatory immune responses that limit activation and expansion of cytotoxic T cells with antitumor activity.
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INTRODUCTION
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GM-CSF, which shares a great deal of sequence homology with G-CSF, may have biological activity in the myeloid lineage of the hematopoietic stem cell, including the development of both dendritic cells (DCs) and natural killer cells. During the development of DCs, the professional antigen-presenting cells that are the most critical for shaping immune responses, GM-CSF is clearly involved in differentiation of hematopoietic stem cells toward the myeloid DC type 1 (DC1) phenotype that is associated with polarization of T-helper type 1 cells. Although this is an oversimplification, phenotypically distinct DC populations can have functionally distinct impacts on naïve T cells encountering antigen for the first time. Purified subsets of DC1 cells incubated with alloantigen and then used to challenge naïve T cells produce very brisk T-cell proliferation, whereas purified DC2 cells, similarly treated, produce very modest T-cell proliferation [1]. This is corroborated by cytokine polarization data showing that the two subsets of DCs are generated by different patterns of cytokines and produce different types of immune responses.
GM-CSF, known as sargramostim when referring to the yeast-derived recombinant human GM-CSF (rhGM-CSF) product (Leukine®; Bayer HealthCare Pharmaceuticals Inc., Wayne, NJ), might be relevant to cancer patients in two broad areas: first, in the effects of GM-CSF on normal hematopoiesis, and second, in the effects of GM-CSF on cancer cells. In normal hematopoiesis, GM-CSF acts as a myeloid cytokine that can reduce febrile neutropenia. It can also generate monocytes and myeloid DCs, leading to overall greater immune responses, including immunity to cancer-associated antigens. GM-CSF might act on cancer cells through a role in cytokine priming. One area of interest in acute myelogenous leukemia (AML) has been the idea that these types of cytokines, when given before and during induction chemotherapy, might make leukemic blast cells more susceptible to the cytotoxic effects of chemotherapy. Recent data support that concept [2, 3]. GM-CSF may also cause terminal differentiation of cancer stem cells in myeloid neoplasms, reducing the number of self-renewing cells. If the cancer stem cell that is generating the large population of circulating blasts is targeted in this way, the cancer may still be cured, even though the bulk of the tumor is not immediately reduced. Finally, these cytokines, and GM-CSF in particular, may cause the leukemic blast cell to differentiate into better antigen-presenting cells that stimulate antitumor immune responses and make it a better target for immunotherapy. These possibilities for GM-CSF anticancer activity suggest a number of clinical trial designs to test its utility in cancer patients (Table 1).
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GM-CSF TO REDUCE FEBRILE NEUTROPENIA
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In a meta-analysis published in 2004, researchers summarized several studies that evaluated the impact of colony-stimulating factors (CSFs) on febrile neutropenia and other factors in children with cancer [4]. CSFs were associated with a 20% reduction in febrile neutropenia overall, including two studies that used GM-CSF [4]. The effect in AML is likely to be similarly modest.
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GM-CSF TO ENHANCE THE ANTIGEN-PRESENTING QUALITIES OF CANCER CELLS
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Primary AML blasts cultured in vitro with GM-CSF demonstrate an upregulation of CD80/CD86 and greater susceptibility to cytotoxicity from allogeneic T cells [2]. In this experiment, rhGM-CSF (sargramostim) and tumor necrosis factor- (PeproTech Inc., Rocky Hill, NJ), with or without additional cytokines, enhanced alloantigen presentation twofold in blasts from 10 of 12 AML patients compared with blasts cultured with media alone [2]. This approach, GM-CSF plus other cytokines, in this case interferon- (IFN-), has now been tested in the clinic in AML and acute lymphocytic leukemia patients who relapsed after allogeneic transplantation and were not considered good candidates for any established therapies [3]. Overall, only a minority of patients are able to have a second transplantation. Of 100 subjects who had relapsed in this study, only 13 underwent a second transplantation, for reasons of donor availability, timing, and other factors. In these patients, donor lymphocyte transfusions produced some prolongation of survival compared with chemotherapy, but they were not durable remissions. A few patients (n = 7) were treated with a combination of GM-CSF and IFN- to try to stimulate a donor antitumor immune response, and they appear to have promising survival [3].
In one case, a 23-year-old man with acute lymphocytic leukemia who underwent matched related-donor transplantation during his second complete remission suffered an early relapse diagnosed on bone marrow aspirate 100 days post-transplantation. Immunosuppression was withdrawn, and the disease progressed. He was treated with rhGM-CSF (500 µg thrice weekly) plus rhIFN-2b (1.5 M units pegylated IFN-2b weekly), which induced a complete remission documented by bone marrow biopsy 4 weeks later. The patient developed grade 4 skin graft-versus-host disease that was responsive to steroids. He achieved a complete remission at 18 months postrelapse and returned to college on low-dose prednisone (M. L. Arellano, M.D., personal communication, September 2006). This approach is now being studied prospectively in a clinical trial [5].
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GM-CSF TO STIMULATE AUTOLOGOUS IMMUNE RESPONSES
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The ability of GM-CSF to stimulate autologous immune responses has been explored in several types of cancer, including renal cell cancer, prostate cancer, melanoma, and lymphoma. Cytokines might be combined to stimulate different aspects of the immune system. For example, GM-CSF demonstrates activity toward natural killer cells, neutrophils, macrophages, DCs, and Fc-
and complement receptors. This pattern of activity has led researchers to question whether there is a role for GM-CSF in combination with other cytokine therapy, specifically interleukin-2 (IL-2), with or without IFN, in treating metastatic renal cell carcinoma. This has been explored in phase II clinical studies, with partial response rates of <10% and, in general, no complete remissions [6–9]. A review of these and other phase II trials [10–12] reveals no convincing improvement with use of GM-CSF in renal cell carcinoma.
For treating leukemia, researchers have considered a potential synergy of GM-CSF with rituximab therapy, with the hope that GM-CSF may upregulate Fc- receptors present on effector cells. Several small phase I/II studies combining rituximab with other cytokine therapy (IFN- [13, 14], IL-2 [15], IL-12 [16], and G-CSF [17]) had a range of complete remission rates, 5%–35%, with median durations of response ranging from >8 months to nearly 2 years (Kaplan–Meier estimates) [13–17]. Two studies have looked at the addition of GM-CSF to rituximab in patients with follicular lymphoma [18, 19]. Complete remission rates reported at the 2005 American Society of Hematology meeting were 36% (of 38 evaluable lymphoma patients) [18] and 45% (of 33 intent-to-treat patients) [19], with median times to progression of 18.7 and 16.7 months, respectively [18, 19]. This combination may be worth pursuing in a randomized clinical trial.
The use of GM-CSF to stimulate autologous immunity to cancer has been studied extensively in prostate cancer. When men are diagnosed with prostate cancer, initial treatment blocks the activity of testosterone, surgically or chemically, but this approach produces a temporary remission. Levels of prostate-specific antigen (PSA), a marker for prostate cancer, eventually begin to rise as clinical disease progresses. In two sequential phase II trials, Small and colleagues looked at whether rhGM-CSF might have immunologic activity in hormone-refractory prostate cancer (HRPC) [20]. Cohort I received rhGM-CSF (sargramostim) at a dose of 250 µg/m2 per day for 2 weeks of each 28-day cycle. Ten of 22 evaluable patients (45%) had a reduction in PSA levels, and five had at least two consecutive reductions (measured daily). The median duration of response was 3.5 months. Cohort II received the same induction therapy followed by maintenance therapy (also 250 µg/m2 per day) three times per week until disease progression occurred. Twelve of the 13 patients in this cohort (92%) had a reduction in PSA levels; the median decline was 32%. One patient had a 99% drop in PSA lasting >14 months and improvement on bone scans [20].
In a more recent study, 30 patients received rhGM-CSF (sargramostim) at a dose of 250 µg/m2 per day for 2 weeks of each 28-day cycle until disease progression was noted [21]. A PSA decline of 50% was observed in 3 of 29 evaluable patients, for an overall response rate of 10%. Prior to treatment with rhGM-CSF, the median PSA doubling time was 8.4 months, with a range of 2.3–24.7 months. After treatment with rhGM-CSF, the PSA doubling time increased to a median of 15 months, with a maximum of 85 months, that is, >7 years [21]. Seven patients remained free from disease progression at a median of 5 years after starting rhGM-CSF therapy [22]. Patients receiving long-term rhGM-CSF therapy tended to have a lower initial disease stage, Gleason score, and pretreatment PSA level. The number of circulating monocytes and DCs increased after 14 days of rhGM-CSF treatment but returned to baseline levels during the 14-day nontreatment period [22]. Taken together, these data suggest some pharmacologic activity for GM-CSF in HRPC.
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GM-CSF AS A TUMOR VACCINE ADJUVANT
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GM-CSF has been combined in a variety of ways with cancer vaccines; for example, it has been administered with irradiated autologous or allogeneic melanoma cells and transduced into these types of cells [10, 23]. GM-CSF has also been combined with melanoma peptides to enhance immune responses [24, 25], employed as a DNA vaccine coexpressing melanoma-associated antigen and cytokine [26], and used to culture antigen-presenting cells for peptide pulse [24]. A variety of tumor types, both hematological cancers and solid tumors, have been shown in phase I /II studies to respond to adoptive therapy using antigen-primed DCs, including lymphoma [27], myeloma [28], colon cancer [29], prostate cancer [30], and melanoma [31]. Research by Small and colleagues, for example, evaluated immunotherapy of HRPC with antigen-loaded DCs [30]. The DCs had been loaded ex vivo with a recombinant fusion protein composed of prostate-specific acid phosphatase (PAP) linked chemically to GM-CSF. These antigen-loaded DCs exhibited upregulated expression of CD86, CD54, CD40, and HLA-DR. Patients who had evidence of an immune response to PAP had a longer progression-free survival interval than patients with no evidence of immune response, suggesting that induction of immunity was associated with clinical response [30].
Additional studies have been performed in patients with metastatic melanoma. In a phase I/II study of GM-CSF-transduced autologous melanoma cells, establishment of a primary tumor cell culture and transduction with GM-CSF was successful for 56 (88%) of 64 tumor samples [32]. Because of rapid disease progression, however, only 38 patients entered and 28 completed the vaccination protocol. Patients were randomly assigned to receive either 5 x 106 (standard dose, n = 14) or 5 x 107 (high dose, n = 14) transduced tumor cells per vaccination in three vaccinations at 3-week intervals. The vaccine was administered as two intracutaneous injections and two s.c. injections. Systemic toxicity included fever of grade <2 after 30% of the vaccinations and mild-to-moderate headache after 33%. Fatigue and generalized pruritus occurred in 20% of patients. A long-term disease-free survival duration of >5 years was observed for six patients among the 28 who completed the three vaccinations. Two patients in the high-dose tumor-cell vaccine group developed vitiligo lesions and remained disease free for >84 and >67 months, respectively [32].
In a second study, Palucka and colleagues generated autologous DCs by culturing monocytes with rhGM-CSF (sargramostim) and IL-4 and then activated the cells by additional culture with tumor necrosis factor and CD40 ligand [33]. Eight vaccines were administered at monthly intervals as three s.c. injections in arms or legs near draining lymph nodes. Half of the 20 evaluable patients who completed the trial had evidence of T-cell proliferation. Two of the 20 patients had durable objective clinical responses, with one complete response and one partial response [31]. The concept of vaccinating melanoma patients is now part of an ongoing randomized trial that includes sargramostim as an adjuvant [34].
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SUMMARY OF GM-CSF IN CANCER
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GM-CSF has been shown to stimulate monocytes and neutrophils, reducing the risk for febrile neutropenia in patients with cancer. It also augments the numbers and activity of DCs, enhancing cellular immunity. GM-CSF-producing cancer cells have served as sources of effective tumor vaccines, and systemic GM-CSF may cause differentiation of leukemic blasts and stimulate immune responses to other cancers. Several limitations exist, however, in the use of GM-CSF in patients with cancer. Immune responses appear to be necessary, but not sufficient, for clinical responses. Objective clinical responses occur infrequently, and clinical responses are often not durable. Finally, it is still unclear why dramatic durable responses occur in some patients but not others.
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ACKNOWLEDGMENTS
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This manuscript is based on a lecture delivered by the author, in October 2006, as part of an acute myelogenous leukemia consensus meeting that occurred in Dallas, Texas, sponsored by the Curry Rockefeller group. The author would like to thank Mary E. King, Ph.D., for her assistance in development of the manuscript.
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Abstract
Introduction
