In the world of cancer immunotherapy, the combination of immune checkpoint inhibitors, such as anti-PD-1 and anti-CTLA-4, has revolutionized the treatment of various types of cancer. However, this therapeutic approach is not without its limitations. The long-lasting antitumor responses it can generate—particularly in diseases like melanoma, lung cancer, or hepatocellular carcinoma—are often compromised by the appearance of immune-related adverse events (irAEs), which force treatment interruption or the use of immunosuppressants such as steroids, which in turn reduce therapeutic efficacy. In this complex clinical scenario, where effectiveness must be balanced with toxicity, a study led by Gang Xue and collaborators (2024) offers an innovative and potentially transformative perspective: the use of clofazimine as an adjuvant agent to enhance dual anti-PD-1/CTLA-4 immunotherapy while significantly reducing its toxicity.
The researchers conducted an extensive screening of approximately 3,000 FDA-approved drugs using murine organotypic tumor spheroids (OTS), which preserve the architecture and immune components of the tumor microenvironment. This experimental model, highly representative of real tumor physiology, enabled the identification of clofazimine, a traditional antibiotic used to treat multibacillary leprosy, as a compound that not only improves the efficacy of dual immunotherapy but also possesses immunomodulatory properties capable of reducing severe side effects such as colitis, neurotoxicity, and immunotherapy-induced myocarditis.
Perhaps the most surprising result is that clofazimine does not act like traditional steroids, which, although they reduce inflammation, also impair antitumor immune responses. Instead, this drug enhanced the cytotoxic activity of CD8+ T cells by activating the E2F1-TCF1 pathway, a molecular axis that promotes the persistence of memory-stem-like T cells while reducing immune exhaustion. In simpler terms, it helps the immune cells that attack the tumor maintain their ability to “remember” and effectively respond to cancer over time, rather than becoming fatigued or losing their potency.
In addition, clofazimine showed a remarkable capacity to suppress the expansion of pathogenic Th17 cells, which are a type of T cell associated with excessive inflammatory responses and autoimmune diseases. Simply put, this means it helps reduce the kind of uncontrolled immune response that causes collateral damage in the body—such as that seen in irAEs.
The research extended beyond in vitro analysis. Findings were validated in murine models of colorectal cancer (MC38), melanoma (D4M.3A), B-cell lymphoma (A20), and lung carcinoma (LL/2), as well as in orthotopic models and patient-derived organoids. In every case, the combination of ICB plus clofazimine clearly outperformed conventional treatment, achieving complete remissions and tumor-free survival rates exceeding 80%. Even under experimental conditions where autoimmune-like diseases were induced (colitis, experimental autoimmune encephalomyelitis, and myocarditis), co-administration of clofazimine reduced mortality and significantly improved clinical outcomes in treated animals, without compromising antitumor effects.
One of the study’s most remarkable aspects is the direct comparison between clofazimine and standard clinical strategies to mitigate adverse effects, such as reducing anti-CTLA-4 dosage or using high-dose methylprednisolone. While these alternatives provided only partial or short-term benefits, only clofazimine consistently maintained robust therapeutic efficacy along with sustained reduction of adverse events. For example, in a myocarditis model induced with TnI peptide, clofazimine rescued 80% of the animals, compared to just 50% with high-dose steroids. Furthermore, histopathological and cardiac function analyses revealed less immune infiltration, reduced myocardial necrosis, and notably preserved ventricular function in clofazimine-treated groups.
From a molecular standpoint, the authors conducted an in-depth analysis of tumor-infiltrating CD8+ T cells using single-cell RNA sequencing (scRNA-seq), ATAC-seq, and luciferase assays. These studies showed that the ICB + clofazimine combination induced a transcriptional signature rich in cytotoxic and early-memory characteristics, with upregulation of genes like Tcf7, Lef1, Gzmb, and Ifng, and downregulation of exhaustion markers like Pdcd1 and Lag3. This means that T cells treated with clofazimine are not only better equipped to destroy cancer cells but also maintain their functional vitality longer—improving their ability to control or completely eradicate the tumor.
Figure 1. A high-throughput screening of ~3,000 FDA-approved drugs identified clofazimine as a potent adjuvant to dual anti-PD-1 and anti-CTLA-4 therapy. Clofazimine promotes CD8⁺ T cell-mediated tumor eradication and modulates CD4⁺ T cell responses via the E2F1-TCF1 pathway, reducing irAEs without compromising efficacy.
Beyond the sophisticated techniques and experimental models, this work proposes a concrete clinical strategy: reconsidering the repositioning of known, safe, and accessible drugs as a rapid and effective way to address the main challenges of modern immunotherapy. With its well-established pharmacological profile and clinical history, clofazimine emerges as a promising candidate for human clinical trials as an adjuvant to aggressive immunotherapeutic regimens—offering a broader and safer therapeutic window for patients who previously had to suspend or alter their treatment due to severe toxicities.
Thus, this study by Xue et al. not only provides a potentially practical and effective solution to a pressing clinical problem but also opens the door to rethinking the role of combination therapies in immuno-oncology. In an era where personalized medicine and immunotherapy lead the way toward more effective treatments, the strategic inclusion of drugs with dual functions—immunomodulatory and antitumor—could represent a turning point in the treatment of advanced cancer.
Main Reference:
Xue G, Li X, Kalim M, Fang J, Jiang Z, Zheng N, Wang Z, Li X, Abdelrahim M, He Z, Nikiforov M, Jin G, Lu Y. Clinical drug screening reveals clofazimine potentiates the efficacy while reducing the toxicity of anti-PD-1 and CTLA-4 immunotherapy. Cancer Cell. 2024 May 13;42(5):780-796.e6. doi: 10.1016/j.ccell.2024.03.001. Epub 2024 Mar 21. PMID: 38518774; PMCID: PMC11756590.
Other References:
Gao X, McDermott DF. Ipilimumab in combination with nivolumab for the treatment of renal cell carcinoma. Expert Opin Biol Ther. 2018 Sep;18(9):947-957. doi: 10.1080/14712598.2018.1513485. Epub 2018 Aug 30. PMID: 30124333; PMCID: PMC6289271.
Rotte A. Combination of CTLA-4 and PD-1 blockers for treatment of cancer. J Exp Clin Cancer Res. 2019 Jun 13;38(1):255. doi: 10.1186/s13046-019-1259-z. PMID: 31196207; PMCID: PMC6567914.
Hao C, Tian J, Liu H, Li F, Niu H, Zhu B. Efficacy and safety of anti-PD-1 and anti-PD-1 combined with anti-CTLA-4 immunotherapy to advanced melanoma: A systematic review and meta-analysis of randomized controlled trials. Medicine (Baltimore). 2017 Jun;96(26):e7325. doi: 10.1097/MD.0000000000007325. PMID: 28658143; PMCID: PMC5500065.
