Cytokines, typically ranging in size from 5 to 25 kDa, are primarily produced by immune cells in response to infection, inflammation, injury, or various stimuli. They are also produced by a variety of other cells, including fibroblasts, epithelial cells, endothelial cells, and stromal cells. Cytokines mediate intercellular communication and play a crucial role in promoting immunity and either promoting or inhibiting cell differentiation and proliferation. They also play an important role in the tumor microenvironment (TME), where they coordinate immune and inflammatory responses. Due to their regulatory effects on the immune system and their impact on tumor progression within the TME, certain cytokines have been considered potential cancer immunotherapies. In fact, cytokines have a long history as anticancer agents, dating back to the 1970s, with interferon-α and interleukin-2 being the first cytokines used for cancer treatment. Since then, numerous preclinical experiments and studies have confirmed the anticancer activity of cytokines. Several cytokines, including granulocyte-macrophage colony-stimulating factor (GM-CSF), interferon-γ, interleukin-15, interleukin-12, and interleukin-21, are currently in clinical trials.
Current Status of Cytokine Clinical Research
As of June 2023, interferon is the most actively tested drug in clinical trials, followed by IL-2, GM-CSF, TGF-β, IL-12, and IL-15. Anticancer cytokines are primarily used in hematological cancers, pan-cancer diseases, melanoma, renal cell carcinoma, neuroblastoma/glioblastoma, and breast cancer. While some cytokines, such as interferon, exhibit direct anticancer effects through antiproliferative effects, most anticancer cytokines do not specifically target cancer cells. Instead, they act on the broader tumor microenvironment and are therefore developed for pan-cancer indications. Because cytokines have low direct cytotoxicity against cancer cells, most clinical trials are actively combining cytokines with other therapies, rather than using them alone, to maximize therapeutic efficacy. Furthermore, with the exception of TGF-β, cytokines are primarily developed as agonists, exerting their anticancer effects by activating the tumor microenvironment.
Figure 1. Trends in Clinical Trials for Cytokines. (Song K., 2024)
IL-2 is a key cytokine currently being developed for anticancer immunotherapy. IL-2 is a small, glycosylated protein of 15.5 kDa with a four-bundle α-helical structure. It is primarily produced by antigen-activated CD4+ and CD8+ T cells. It is also produced by naive T cells, NK cells, and dendritic cells. Secreted IL-2 binds to an allosteric receptor complex composed of three distinct subunits: IL-2Rα (CD25), IL-2Rβ (CD122), and a shared γ chain (CD132). It is primarily expressed on activated T lymphocytes, regulatory T cells, mature dendritic cells, and B cells.
The effects of IL-2 depend on IL-2 levels. Low levels of IL-2 primarily promote the differentiation of CD4+ T cells into follicular helper or central memory T cells. In particular, IL-2 plays a key role in the differentiation, expansion, and activation of effector CD8+ T cells and NK cells in the tumor microenvironment, thereby exerting anticancer effects. IL-2 was approved by the U.S. Food and Drug Administration (FDA) in 1992 for the treatment of metastatic renal cell carcinoma and subsequently approved for the treatment of transplanted melanoma in 1998. Since then, IL-2 has continued to develop as an immune anticancer cytokine in various cancer types.
IL-12 is a 75 kDa cytokine composed of two subunits, p40 and p35. It is believed to exhibit anticancer activity primarily by inducing the proliferation of NK cells and cytotoxic CD8+ T cells. However, IL-12 has limitations in clinical application, as existing research indicates that it can cause systemic inflammation and sometimes severe myelotoxicity and hepatotoxicity.
Among anticancer cytokines, IL-12 did not enter active development until 2010, after which its development as an anticancer cytokine has rapidly increased. Notably, over half of the drugs developed since 2010 have primarily been gene therapy. Although IL-2 has been experimentally demonstrated to have anticancer effects, systemic administration of recombinant proteins in clinical trials has yielded modest efficacy at acceptable doses and has been associated with relatively high toxicity. Therefore, to address this issue, efforts are underway to develop drug forms through gene therapy that target IL-12 to the tumor microenvironment to minimize systemic exposure. Since 2005, the introduction of these new technologies has led to a steady increase in the number of IL-12 drugs entering clinical trials.
IL-15 is one of the most promising and potent anticancer cytokines. IL-15 is a 14-15 kDa cytokine that plays a key role in both innate and adaptive immune responses. Like IL-2, IL-15 has four α-helices and shares the IL-2 receptor subunits IL-2Rβ (CD122) and the common γ chain (CD132) with IL-2. Therefore, IL-15 functions very similarly to IL-2 on lymphocytes and natural killer (NK) cells. Furthermore, one of the key downstream targets of the JAK/STAT pathway activated by IL-15 is NK cells. IL-15 can enhance NK cell proliferation and cytotoxicity by upregulating the expression of molecules such as perforin and granzyme B, thereby inducing apoptosis in cancer cells. IL-15 can also activate other immune cells, such as cytotoxic CD8+ and memory T cells, as well as dendritic cells, which contribute to anti-tumor responses.
Like IL-12, IL-15 has been actively developed since 2010 for hematologic cancers and various solid tumors. Since 2010, various technology platforms have been used to develop the anticancer cytokine IL-15, while development of native IL-15 has been relatively limited. Specifically, the main development approaches involve the creation of fusion proteins combining IL-15 muteins with Fc, as well as gene therapy to induce IL-15 expression in CAR-NK cells.
IL-21 is a cytokine that shares a common gamma receptor with IL-2 and IL-15. IL-21 promotes B cell differentiation into plasma cells, regulates immunoglobulin production, controls the proliferation and/or effector function of CD4+ and CD8+ T cells, and restricts the differentiation of Tregs. It has garnered considerable attention as a potential anticancer immunotherapy due to its ability to stimulate the immune system and enhance anti-tumor responses. Compared to other cytokines, the clinical development of IL-21 as an anti-tumor agent has been less active. Clinical development of IL-21 began in 2004, and as of July 2023, 14 clinical trials were underway, primarily targeting hematologic cancers, pan-oncology, melanoma, and renal cell carcinoma. Eleven of these trials utilized the native form of IL-21, and three are developing IL-21 as an albumin fusion protein.
| Cat.No. | Product Name | Price |
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| CLKO-1766 | IL21 KO Cell Lysate-HeLa | Inquiry |
| CLOE-2067 | Rat Il21 (His) Insect Cell Lysate | Inquiry |
| CSC-DC007574 | Panoply™ Human IL21 Knockdown Stable Cell Line | Inquiry |
| CSC-RT1911 | IL21 Knockout Cell Line-HeLa | Inquiry |
| CSC-SC007574 | Panoply™ Human IL21 Over-expressing Stable Cell Line | Inquiry |
| AD08039Z | Human IL21 adenoviral particles | Inquiry |
| LV15393L | human IL21 (NM_021803) lentivirus particles | Inquiry |
| LV15394L | human IL21 (NM_001207006) lentivirus particles | Inquiry |
Interferons
Interferons are a class of cytokines with a long history of anticancer activity. They are primarily classified into three types: type I, which includes IFN-α, IFN-β, IFN-ω, and IFN-τ; type II, which includes IFN-γ; and the more recently recognized type III, which includes IFN-λ. IFN-α was the first anticancer cytokine approved by the FDA in 1986 for the treatment of hairy cell leukemia. Since then, IFN-α has been used in patients with chronic myeloid leukemia and some solid tumors, such as melanoma and carcinoma, as well as hairy cell leukemia.
IFNs are the third most actively studied cytokine in cancer therapy, after IL-2 and GM-CSF. However, since the early 2000s, clinical trials of interferon for cancer indications have continued to decline significantly compared to other cytokines. Cancer indications do not differ significantly from those for other cytokines, primarily involving hematologic cancers, pan-oncology, melanoma, and renal cell carcinoma. Most clinical trials have been conducted with the natural form of the cytokine. In addition to the natural form of interferon, pegylation of interferon is an important direction, which is mainly used to extend the half-life of interferon and improve the efficacy of clinical trials.
Transforming Growth Factor-β
TGF-β is secreted by many cancer cells and plays a key role in cancer progression by participating in cancer proliferation and tumor microenvironment (TME) immunity. Since TGF-β induces cancer cell invasiveness, stemness, and treatment resistance, and contributes to the generation of regulatory T cells in the TME, it promotes anti-tumor immune suppression. Therefore, inhibition of TGF-β signaling is considered a key approach to improve the efficacy of current and future immunotherapies. TGF-β has been actively in clinical trials since 2015, mainly for pan-tumor indications. More than 66% of the trials focus on the development of anti-TGF-β antibodies and fusion proteins.
Challenges of Cytokines as Anticancer Therapies
A major factor limiting the clinical application of cytokine therapy is the complexity of the cytokine immune response. The mechanisms of action and interactions of cytokines are not fully understood. Cytokine networks can exhibit a high degree of redundancy, with multiple cytokines often activating the same downstream pathways. Furthermore, cytokines can exert pleiotropic effects, simultaneously regulating multiple cellular functions within the tumor microenvironment (TME), making their actions highly complex. Even the same cytokine can exhibit proliferative or antiproliferative effects depending on its interactions within the surrounding environment. While this complexity enables exquisite control of the immune response, it can also lead to inadequate efficacy due to compensatory mechanisms within the TME.
On the other hand, systemic toxicities, such as vascular leak syndrome, central nervous system toxicity, and cardiotoxicity, also pose challenges and limit the use of adequate doses for clinical applications. Furthermore, short in vivo half-lives not only reduce drug efficacy but also impair patient compliance with frequent dosing.
Conclusion
Despite several limitations, anticancer cytokine therapy is emerging as a new frontier in anticancer immunotherapy. Scientists are employing various technological platforms to address the challenges associated with anticancer cytokine therapy. With the approval of a new generation of cytokine drugs (such as N-803), cancer immunotherapy is expected to usher in a safer and more efficient "cytokine 2.0 era".
Reference
Song K. Current development status of cytokines for cancer immunotherapy. Biomolecules & therapeutics, 2024, 32(1): 13.