Background

Immune checkpoint inhibitors (ICIs) have revolutionized cancer treatment by activating the patient's immune system to attack cancer cells. However, in solid tumors such as pancreatic and colorectal cancer, the efficacy of ICIs is limited to a subset of patients. A key reason for this poor response rate is the unique physical structure of the tumor microenvironment (TME). Tumor blood vessels are disorganized and have leaky walls, unlike normal blood vessels, allowing substances to easily leak out. This leads to severe hypoxia, as blood cannot circulate properly, and the formation of a barrier that prevents the penetration of anticancer drugs and immune cells such as CD8+ T cells. The medical community has focused on nitric oxide (NO) as a substance that can normalize blood vessels and alleviate hypoxia. However, systemic administration has significant side effects because NO has a very short half-life and rapidly diffuses in the gaseous state, causing severe hypotension.

Key Findings

The research team, led by Professor Xiaolong Liang at Peking University Third Hospital, focused on the characteristics of Escherichia coli Nissle 1917 (EcN), which targets and proliferates in hypoxic areas with dense cancer cells. This is an attempt to overcome the limitations by using EcN as a delivery vehicle. The team genetically engineered the bacteria to continuously release NO in vivo and used it as a therapeutic delivery vehicle. Escherichia coli originally regulates L-arginine production through its own feedback inhibition mechanism. The researchers removed the Arginine repressor (ArgR) gene to allow unlimited production of L-arginine. They then constructed an artificial metabolic circuit (EcN-NO) that simultaneously expresses argininosuccinate synthase (ArgG), argininosuccinate lyase (ArgH), and Bacillus subtilis nitric oxide synthase (BsNOS). EcN-NO, equipped with the artificial circuit, continuously produces NO using L-arginine as a raw material in the tumor. In mouse model experiments, the bacteria stably colonized the tumor tissue. NO release was confirmed in MC38, CT26 (colorectal cancer), B16 (melanoma), and Hepa1-6 (hepatocellular carcinoma) models. Continuous NO supply contributed to normalizing the twisted blood vessels around cancer cells and alleviating hypoxia. As a result, the penetration of drugs, dendritic cells (DCs), and CD8+ T cells was maximized. Indeed, immune cell infiltration increased significantly, and when combined with anti-PD-L1, an immune checkpoint inhibitor, it showed a synergistic effect in strongly inhibiting tumor growth.

Significance and Prospects

This study is significant in that it precisely controls the physical limitations of the TME, which was previously almost impossible to deliver drugs to, by using microbial synthetic circuits. It demonstrates that synthetic biology technology can be used as a direct tool for cancer treatment. However, there are several barriers to overcome before it can be applied to humans. In particular, safety is critical. Since live bacteria are directly injected into blood vessels or tumors, it is necessary to carefully evaluate safety to prevent excessive immune responses such as sepsis in the patient's body. It is also essential to confirm that the exogenous genetic circuit functions stably without mutation in the patient's complex in vivo environment.