Preclinical efficacy of a systemically-administered, second-generation STING agonist that promotes antitumour immunity in combination with radiotherapy

This study demonstrates that the systemically-administered, second-generation STING agonist BI-1703880 is well-tolerated and synergizes with radiotherapy to enhance antitumor immunity and immune memory in preclinical models, particularly when combined with immune checkpoint inhibitors.

The Gist

Imagine your body's immune system as a highly trained security force. Its job is to patrol your body, find intruders (like cancer cells), and eliminate them. However, cancer is a tricky enemy. It often wears a "disguise" that makes it invisible to the security force, or it builds a "force field" that keeps the guards away.

This paper describes a new strategy to wake up the security force and help it defeat the cancer. The researchers tested a new drug called BI-1703880 (let's call it the "Alarm Bell") and combined it with Radiotherapy (the "Demolition Crew").

Here is the story of what they found, broken down into simple steps:

1. The Problem: The Alarm is Broken

In a healthy body, when a cell is damaged, it releases a tiny signal (like a smoke alarm) that tells the immune system, "Hey, something is wrong here!" Cancer cells are good at hiding this signal. Even when we use radiotherapy to damage the cancer, the immune system sometimes doesn't show up to finish the job because the "alarm" isn't loud enough.

2. The New Tool: The "Alarm Bell" (STING Agonist)

The researchers developed a new drug, BI-1703880, which acts like a super-charged smoke alarm.

• How it works: When you inject this drug into the bloodstream, it triggers a specific sensor inside cells called STING. Think of STING as the main control panel for the immune system.
• The Reaction: When the drug hits the control panel, it screams, "INTRUDER ALERT!" This causes the body to release a flood of chemical messengers (cytokines) that wake up the immune cells, specifically the CD8+ T-cells (the "special forces" that hunt down cancer).

3. The Catch: Don't Ring the Bell Too Loud

The researchers first tried ringing the bell very loudly (a high dose of the drug).

• The Result: It worked too well! The immune system went into a frenzy, causing massive inflammation and damaging healthy tissue. It was like setting off a fire alarm that also started a fire. The mice got very sick.
• The Fix: They realized they needed to ring the bell gently but repeatedly. They found that low doses given over time were safe and kept the immune system alert without causing a panic attack.

4. The Perfect Team-Up: Demolition + Alarm

The real magic happened when they combined the Demolition Crew (Radiotherapy) with the Alarm Bell (Low-dose Drug).

• The Strategy: First, they used radiotherapy to smash the cancer cells. This is like breaking the walls of the enemy's fortress.
• The Synergy: When the walls break, the cancer cells spill their "secrets" (tumor antigens) into the open. Then, the low-dose Alarm Bell goes off.
• The Outcome: The immune system sees the spilled secrets, hears the alarm, and realizes, "Oh! That's the enemy!" The "special forces" (T-cells) rush in, recognize the cancer, and destroy it.

The Analogy: Imagine trying to catch a thief in a dark house.

• Radiotherapy turns on the lights and breaks the thief's hiding spot.
• The Drug is the siren that alerts the police.
• Together, the police can see the thief clearly and catch them. Alone, the lights might just make the thief run, or the siren might just scare them without catching them.

5. The "Exhausted" Guards and the Final Boost

The researchers noticed something interesting: After the combined treatment, the cancer cells tried to fight back. They put up "Do Not Enter" signs (proteins like PD-L1) to tell the immune guards, "Stop, I am friendly!" This made the guards tired and lazy (exhausted).

To fix this, they added a third ingredient: Checkpoint Inhibitors (drugs that rip down the "Do Not Enter" signs).

• The Result: With the signs removed, the tired guards woke up, got angry, and finished the job. In many of the mice, this four-part combination (Radiotherapy + Alarm Bell + Two types of Sign-Removers) completely cured the cancer and even gave the mice immunity against the cancer returning later.

6. Why This Matters for Humans

• Safety: The study showed that you don't need a massive, toxic dose of the drug to work; a small, repeated dose is safer and effective.
• Versatility: This combination worked on many different types of "cancer models" (thyroid, head/neck, colon), suggesting it could work on many different human cancers.
• The Future: This research provides a strong reason to test this combination in human clinical trials. It suggests that by combining standard radiation with this new "Alarm Bell" drug and existing immune-boosting drugs, we might be able to turn "cold" tumors (that ignore the immune system) into "hot" tumors (that are attacked and destroyed).

In Summary:
This paper is about finding the right volume for the alarm bell. By using a gentle, repeated dose of a new drug alongside standard radiation, and then removing the cancer's "stop signs," the researchers created a powerful team-up that taught the body's immune system to hunt down and destroy cancer, offering hope for more effective and less toxic cancer treatments in the future.

Technical Summary

1. Problem Statement

While immune checkpoint inhibitors (ICIs) have transformed cancer care, many tumors remain refractory, particularly those lacking functional tumor-specific CD8+ T cells. First-generation STING agonists (cyclic dinucleotides) showed promise in preclinical models but were limited to intratumoral injection due to poor systemic stability and toxicity. Second-generation STING agonists capable of systemic delivery are needed to target multiple lesions and lymphoid tissues simultaneously. However, high-dose systemic administration often leads to severe cytokine release syndrome (CRS) and toxicity, limiting their therapeutic window. The challenge is to identify a dosing strategy that activates the STING pathway sufficiently to drive antitumor immunity without causing lethal systemic inflammation, potentially by combining it with standard-of-care therapies like radiotherapy (RT).

2. Methodology

The study utilized a comprehensive preclinical approach involving in vitro, ex vivo, and multiple in vivo murine models:

• Compound: BI-1703880, a novel synthetic, non-nucleotide STING agonist designed for intravenous (IV) delivery.
• In Vitro/Ex Vivo Validation:
  • Used human THP-1 monocytes (Wild-type vs. STING-Knockout) and primary human PBMC-derived dendritic cells (DCs).
  • Assessed pathway activation via Western blot (phosphorylation of STING, TBK1, IRF3) and RNA sequencing.
  • Tested activity across five human STING variants (WT, REF, HAQ, AQ, Q) using reporter assays.
  • Measured cytokine release (IFNs, TNFα, chemokines) via multiplex assays.
• In Vivo Models:
  • Tumor Models: Subcutaneous and orthotopic implants in immunocompetent mice (C57Bl/6, C3H) and immunodeficient NSG mice. Models included TBPt-4C4 (thyroid), mEER (head and neck), MC38 (colorectal), TC1, AT84, TBP-B79, and TKP-4831.
  • Dosing Schedules: Compared high-dose (20 µmol/kg) vs. low-dose (7 µmol/kg) IV/IP administration. Evaluated single-agent vs. combination with RT (2 fractions of 8 Gy).
  • Combination Therapies: Tested RT + STINGa alone, and RT + STINGa + ICIs (anti-PD-1 and/or anti-CTLA-4).
  • Immune Depletion: Used anti-CD8 antibodies and NSG mice to determine immune dependency.
• Analytical Techniques:
  • Flow Cytometry & Spectral Cytometry: Used "Tocky" (Nr4a3-timer) transgenic mice to track temporal TCR signaling dynamics. Analyzed activation/exhaustion markers (PD-1, CTLA-4, TIM-3, Ki67, Granzyme B).
  • Transcriptomics: Bulk RNA sequencing of tumors to profile immune infiltration and pathway activation.
  • Histology: H&E staining, immunohistochemistry (CD8, CD4, CD31), and in situ hybridization (IFNβ).

3. Key Contributions

• Characterization of BI-1703880: Demonstrated that BI-1703880 is a potent, selective, and pan-variant active STING agonist in human and murine cells.
• Dose Optimization: Established that while high-dose monotherapy causes rapid tumor necrosis and high cytokine toxicity, repeated low-dose systemic administration is well-tolerated and induces transient, beneficial immune activation.
• Synergistic Mechanism: Defined the synergistic mechanism where RT induces antigen release and STINGa activates innate immunity, creating a "cold-to-hot" tumor microenvironment (TME).
• Triple/Quadruple Therapy Rationale: Provided preclinical evidence that the combination of RT + STINGa upregulates immune checkpoints (PD-1/PD-L1, CTLA-4), creating a specific vulnerability that can be exploited by adding ICIs to achieve tumor cures in large, resistant tumors.

4. Key Results

• Pathway Activation: BI-1703880 induced dose-dependent phosphorylation of STING, TBK1, and IRF3, leading to the upregulation of Type I/II interferons and pro-inflammatory cytokines (IFNα, IFNβ, CXCL10, CCL2, CCL5). This occurred specifically in STING-expressing cells.
• Monotherapy Toxicity vs. Efficacy:
  • High-dose (20 µmol/kg) single-agent therapy caused massive tumor necrosis and hemorrhage but resulted in high systemic cytokine levels (1000–60,000 pg/mL), suggesting toxicity upon repeated dosing.
  • Low-dose (7 µmol/kg) repeated administration was well-tolerated, induced transient cytokine peaks (returning to baseline within 24h), and provided moderate tumor growth control.
• RT + STINGa Synergy:
  • The combination of sub-therapeutic RT (16 Gy total) and low-dose STINGa significantly improved tumor control and survival compared to either monotherapy across multiple tumor types.
  • Immune Dependency: The therapeutic effect was abolished in NSG mice and significantly reduced in CD8+ T-cell-depleted mice, confirming the mechanism is CD8+ T-cell dependent.
  • Immune Memory: Mice cured by the combination therapy were protected against re-challenge, indicating the generation of protective immune memory.
• TME Remodeling:
  • RT/STINGa treatment increased CD8+ T-cell infiltration and the CD8+/Treg ratio.
  • It upregulated expression of PD-1, PD-L1, and CTLA-4 on T cells and tumor cells.
  • Using Tocky mice, the study identified a specific cluster of CD8+ T cells ("Cluster 3") undergoing persistent antigen signaling, expressing high levels of activation markers (CD69, Ki67) and exhaustion markers (PD-1, TIM-3, ICOS).
• Quadruple Therapy Efficacy:
  • Adding anti-PD-1 and anti-CTLA-4 to the RT/STINGa backbone converted partial responses into complete tumor cures in large, previously incurable tumors (e.g., 5/6 and 6/6 cures in TBPt-4C4 and mEER models, respectively).
  • This quadruple therapy increased Granzyme B and Perforin expression, indicating enhanced cytotoxic function of T cells without necessarily changing their proliferation or activation phenotype, suggesting a reversal of exhaustion.
• Abscopal Effect: While a significant reduction in unirradiated contralateral tumors was not observed, there was a measurable increase in CD8+ T-cell infiltration in unirradiated tumors, suggesting a systemic immune boost.

5. Significance

• Clinical Translation: The study provides a strong rationale for the ongoing Phase I clinical trial (NCT05471856) of BI-1703880. It supports a dosing strategy of repeated low-dose systemic administration to avoid CRS while maximizing immune activation.
• Combination Strategy: The data strongly supports combining systemic STING agonists with radiotherapy to prime the immune system, followed by immune checkpoint blockade to sustain the response. This "RT-STING-ICI" axis addresses the challenge of treating "cold" tumors and overcoming resistance in "hot" but exhausted tumors.
• Mechanistic Insight: The use of Tocky mice offers novel insights into the temporal dynamics of T-cell exhaustion and activation in the context of STING agonism, suggesting that the timing of ICI administration relative to STING activation is critical for maximizing efficacy.
• Therapeutic Window: The findings define a therapeutic window where low-dose STING agonism acts as an adjuvant to RT, enhancing antigen presentation and T-cell priming without the toxicity associated with high-dose monotherapy.