New Drug STF-1623 Shows Promise in Treating Solid Tumors

STF-1623 Transforming ‘Cold’ Tumors into ‘Hot’ Tumors

Cancer immunotherapy has changed the way certain cancers are treated, especially melanoma, lung, and blood cancers. However, many solid tumors remain resistant because they exist in “cold” tumor environments, where immune activity is suppressed. Traditional immunotherapies primarily target T cells, which are less effective in these conditions.

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A research team led by Lingyin Li, PhD, Stanford University, has developed a new drug candidate, STF-1623, designed to activate the innate immune system by preserving the inflammatory molecule cGAMP, which is essential for initiating immune responses. 

Tumors often overproduce the enzyme ENPP1, which breaks down cGAMP, allowing cancer cells to hide from immune detection. STF-1623 inhibits ENPP1, allowing cGAMP to build up and trigger the STING pathway, turning a tumor’s environment from cold to hot and enabling the immune system to attack effectively.

“What we’re trying to do here is help the body immunize itself against cancer by activating the innate immune system in the right place, at the tumor site,” said Lingyin Li.

How STF-1623 Works and Study Findings

In preclinical studies, STF-1623 successfully suppressed tumor growth in multiple mouse cancer models, including breast, pancreatic, colorectal, and glioblastoma cancers, without notable side effects. This precision is due to the drug’s ability to target ENPP1-rich tumor surfaces while quickly leaving other tissues.

The compound works by binding tightly to ENPP1, preventing it from destroying cGAMP. As cGAMP levels rise, nearby immune cells are activated through the STING pathway, sparking a stronger and localized anti-cancer immune response.

Unlike previous direct STING agonists, which risk overstimulating the immune system, STF-1623 works in a more controlled way by enhancing natural immune signals. Early results suggest STF-1623 may be most effective when combined with other cancer therapies, offering a potential new option for patients with hard-to-treat solid tumors.

Next Steps: Clinical Trials Ahead

The FDA has approved Phase I clinical trials for STF-1623, with patient recruitment expected to begin soon. If successful, this drug could represent a breakthrough for patients whose cancers do not respond to current immunotherapies.

This study, published in Cell Reports Medicine, demonstrates the first targeted approach to innate immune checkpoint inhibition for solid tumors, offering hope for a more personalized and effective cancer treatment strategy.

For More Information

Wang, S., et al. (2025). ENPP1 inhibitor with ultralong drug-target residence time as an innate immune checkpoint blockade cancer therapy. ARC Institute