FDA Grants Fast Track Designation to Novel Immunotherapy Combination for MSS Metastatic Colorectal Cancer

1. Key Highlights

• A novel combination therapy consisting of an anti-CTLA-4 monoclonal antibody and a PD-1 inhibitor has been granted Fast Track designation by the U.S. Food and Drug Administration (FDA) following promising efficacy and favourable safety results from a Phase II clinical study.

• The anti-CTLA-4 antibody features a tumour microenvironment-selective activation (“masked”) design, which may allow higher dosing while reducing systemic toxicity.

• This approach offers a potential new immunotherapy strategy for patients with microsatellite stable (MSS) metastatic colorectal cancer, a group that traditionally responds poorly to conventional immune checkpoint inhibitors.

2. Background of the Fast Track Designation

The FDA recently granted Fast Track designation to muzastotug (ADG126) in combination with pembrolizumab for the treatment of adult patients with MSS metastatic colorectal cancer (mCRC) without active liver metastases.

The decision was based on efficacy and safety data from an ongoing Phase II clinical trial, with updated findings presented at the 2025 ASCO Annual Meeting. Fast Track designation is intended to accelerate the development and regulatory review of innovative therapies that demonstrate the potential to address significant unmet medical needs.

Unlike microsatellite instability-high (MSI-H) colorectal cancer, MSS mCRC generally shows limited response to conventional immune checkpoint inhibitors, and effective immunotherapy options have long been lacking.

Muzastotug is a masked anti-CTLA-4 IgG1 monoclonal antibody with several key design features:

• A cleavable masking peptide allows the antibody to be selectively activated within the tumour microenvironment.
• Binding to specific epitopes blocks CTLA-4 function, enhancing T-cell activation.
• Enhanced antibody-dependent cellular cytotoxicity (ADCC) or phagocytosis may help reduce the level of immunosuppressive regulatory T cells within the tumour environment.

This design may allow the therapy to be administered at higher doses when combined with PD‑1 inhibitors, theoretically improving efficacy while reducing systemic immune‑related toxicity. As a result, it provides a new immunomodulatory pathway for MSS colorectal cancer patients.

3. Current Clinical Development

The ongoing randomised Phase II study is designed as follows:

Patients are randomly assigned to receive muzastotug at 10 mg/kg or 20 mg/kg, in combination with pembrolizumab.

The treatment follows an induction-maintenance dosing strategy. No monotherapy control arm is included, and each dose group plans to enrol approximately 30 patients.

Eligibility criteria include:

• Adults aged 18 years or older with MSS metastatic colorectal cancer
• ECOG performance status of 0–1
 • At least one measurable lesion according to RECIST 1.1 criteria
 • Adequate organ function
• Availability of tumour tissue samples collected within the two years prior to enrolment

The primary study endpoints include maximum tolerated dose (MTD), recommended Phase II dose, safety, tolerability, and preliminary anti-tumour activity.

4. Efficacy and Safety Data

Data presented at the ASCO Annual Meeting showed the following results:

In the 10 mg/kg cohort (n=29):

• Objective response rate (ORR): 17%
 • Disease control rate (DCR): 76%
 • 6-month clinical benefit rate: 38%
 • Median progression-free survival (PFS): 4.8 months
 • Median duration of response (DOR): 6.2 months

In the 20 mg/kg cohort (n=21):

• ORR: 29%
 • DCR: 81%
 • Median PFS: not yet reached
 • 6-month PFS rate: approximately 50%

At a median follow-up of 17.8 months, the overall median overall survival (OS) reached 19.4 months.

5. Safety Profile

Treatment-related adverse events were relatively common in both dose groups but were generally manageable.

Common adverse reactions included:

• Pruritus (itching)
• Diarrhoea
• Hypothyroidism
• Fatigue
• Adrenal insufficiency

Importantly:

• No dose-limiting toxicities were observed
 • No grade 4 or grade 5 treatment-related adverse events occurred
 • The treatment discontinuation rate due to adverse events was approximately 4%

6. Next Steps

Based on the current study findings and regulatory feedback, a registrational Phase III clinical trial is expected to begin in 2027.

The primary endpoint will be overall survival (OS), with secondary endpoints including:

• Progression-free survival (PFS)
• Duration of response (DOR)
• Objective response rate (ORR)

The control arm will use the current standard-of-care treatment.

In the field of MSS metastatic colorectal cancer, where immunotherapy has historically been difficult to advance, combination immunotherapy strategies based on masked anti-CTLA-4 antibodies are gradually demonstrating clinical potential.

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