Overview

Claudin 18.2 is a tight-junction protein normally hidden inside the healthy stomach. In gastric cancer that arrangement breaks down: the protein becomes exposed on the tumor cell surface at high density across a meaningful proportion of tumors. That switch from hidden to exposed makes Claudin 18.2 an exploitable therapeutic target and its detection by immunohistochemistry (IHC) a decisive assay. This article explains the molecular basis of the target, the IHC principle, and how the result selects patients for Claudin 18.2-directed antibody therapy such as zolbetuximab.

Why Claudin 18.2 Becomes a Usable Target

Claudins are the building blocks of tight junctions, the seals that keep gastric acid from leaking between epithelial cells. Claudin 18.2, the isoform found in the stomach, is normally confined to the junctional region where an antibody cannot reach it. In gastric adenocarcinoma the junction architecture is perturbed, exposing Claudin 18.2 on the tumor surface at higher density. Because normal tissues outside the stomach express little of it, the differential between tumor and normal tissue is wide — a favorable property for an antibody target.

The Principle of the IHC Detection Method

Immunohistochemistry locates a protein in tissue by letting a labeled antibody bind it and making the binding visible under a microscope. For Claudin 18.2, a formalin-fixed, paraffin-embedded (FFPE) gastric biopsy is sectioned, the antigen is retrieved, and a Claudin 18.2-specific primary antibody is applied. After a wash step, a detection system amplifies the signal, and a chromogen produces a brown precipitate at the binding sites. The pathologist then reads membranous staining intensity and the percentage of stained tumor cells, yielding a semi-quantitative score rather than a simple positive or negative call.

Scoring and Eligibility for Claudin 18.2-Directed Therapy

Eligibility for zolbetuximab — the anti-Claudin 18.2 monoclonal antibody evaluated in gastric cancer trials — depends on the IHC score meeting a threshold of intensity and extent. In pivotal study designs, tumors are eligible when they show moderate-to-strong (2+ or 3+) membranous staining in a specified minimum percentage of viable tumor cells. The assay is therefore a companion-diagnostic-style gate, and the laboratory must control antibody clone, staining platform, and scoring criteria to keep results reproducible.

For a distributor or lab sourcing a Claudin 18.2 IHC assay, clinical relevance depends on standardization. Confirm the antibody clone, whether the assay is cleared for companion diagnostic use in your market, the validated threshold, and the availability of external proficiency testing. Because a borderline score changes patient eligibility, the analytical reproducibility of the readout is a procurement-critical specification.

FAQ

Q: Why does Claudin 18.2 become a target in gastric cancer when it is a normal stomach protein? A: In healthy stomach it is hidden inside tight junctions at low density, but in gastric tumors the junction architecture breaks down, exposing Claudin 18.2 at high density on the tumor cell surface where an antibody can bind it.

Q: What does the IHC result actually measure? A: The assay measures the intensity of membranous staining (often scored 0 to 3+) and the percentage of viable tumor cells showing it; eligibility for zolbetuximab depends on both meeting a defined threshold.

Q: Can a borderline Claudin 18.2 IHC score change a patient's treatment eligibility? A: Yes. Because the threshold requires a minimum intensity and a minimum percentage of stained cells, a small scoring difference can place the same tumor on either side of the line, which is why standardized controls and scoring are essential.