CLDN18.2 Testing May Influence Advances in the Gastric Cancer Treatment Paradigm

Kevin M. Waters, MD, PhD

Determining the potential for Claudin18.2 (CLDN18.2) as a target in patients with gastric cancer begins with defining and disseminating effective testing and interpretation strategies, according to Kevin M. Waters, MD, PhD.

“As we learn more about the stain and how to interpret it, it will be incumbent on us [as pathologists] to become competent [in testing for CLDN18.2] to give [oncologists] the information they need to make treatment decisions for their patients,” Waters said.

The ongoing phase 3 SPOTLIGHT trial (NCT03504397) is evaluating the efficacy and safety of zolbetuximab, a monoclonal antibody targeting CLDN18.2, plus mFOLFOX6 vs placebo plus mFOLFOX6 in patients with previously untreated CLDN18.2-positive, HER2-negative, locally advanced unresectable or metastatic gastric or gastroesophageal junction (GEJ) adenocarcinoma. Primary findings from this study demonstrated a median progression-free survival of 10.61 months (95% CI, 8.90-12.48) in the zolbetuximab arm vs 8.67 months (95% CI, 8.21-10.28) in the placebo arm.¹

In response to clinical trials evaluating CLDN18.2-targeted agents, Waters coauthored a study evaluating CLDN18.2 expression in distal esophagus, GEJ, and stomach samples with intestinal metaplasia, dysplasia, and adenocarcinoma. This study found no significant differences in CLDN18.2 expression across dysplasia categories in the GEJ or stomach. Additionally, 80% of the gastric neuroendocrine tumors were CLDN18.2 positive, and 57.5% of the gastric neuroendocrine tumors met clinical trial inclusion criteria. Conversely, 0.62% of non-gastric neuroendocrine tumors showed staining for CLDN18.2 (P < .001).²

In an interview with OncLive^®, Waters discussed key findings from this study that reinforced the rationale for CLDN18.2 testing, explained the prevalence of this protein in gastric cancers, and highlighted how this potential therapeutic target represents the evolving role of pathology in cancer diagnosis and care.

Waters is the director of the Anatomic and Clinical Pathology Residency Program and an assistant professor of Pathology and Laboratory Medicine at Cedars-Sinai Medical Center in Los Angeles, California.

OncLive^®: What is the rationale behind testing for CLDN18.2 expression in patients with gastric cancer?

Waters: CLDN18.2 is a membranous protein expressed in normal gastric tissue. CLDN18.2 is [also] expressed in a portion of gastric and upper GEJ adenocarcinomas. There’s now a targeted antibody for CLDN18.2 that is undergoing clinical trials. The inclusion criteria for these trials includes expression of CLDN18.2 in the tissue. We use immunohistochemical [IHC] tests to gauge CLDN18.2 expression levels in these tumors.

How prevalent is CLDN18.2 in patients with gastric cancer? Are there any tumor types that have particularly high rates of CLDN18.2 expression?

In gastric and esophageal adenocarcinomas, [CLDN18.2] is expressed in most tumors, upward of 70%. Some of these adenocarcinomas have low levels of expression, and some have more expression. Some clinical trials require moderate to marked expression, upward of 50% [of tumor cells], whereas the most recent trial, the [SPOTLIGHT] trial, wanted moderate to strong expression at least 75% [of tumor cells].

[As clinical trials] require more and more expression, the prevalence of expression that meets that threshold goes down. Around 30% or 40% of these tumors end up meeting that [75%] threshold.

Other tumors [outside of gastric cancer] express some CLDN18.2. Pancreatic cancer expresses it at lower levels than [gastric cancer], and some gynecologic tumors [may also express it].

How is CLDN18.2 testing currently conducted?

It’s an IHC test. An antibody that has a brown chromogen attached to it will bind to CLDN18.2 on the tissue. When we look at the slide, we can see how much brown there is in a membranous staining outside on the cell border. That will tell us how much expression there is on the tumor cells.

Right now, all the staining is being performed centrally for clinical trials. [After a potential FDA approval of zolbetuximab], we will need to see how they [will require this testing to be conducted], with [either] outside labs, reference laboratories, or however it is distributed.

At Cedars Sinai, you were an investigator on a study that evaluated CLDN18.2 expression in esophageal, GEJ, and stomach cancers and precursor lesions. What was the goal of this research?

I learned that [zolbetuximab] was under study and that staining for the claudin protein would be part of the inclusion criteria for the [SPOTLIGHT] study. I thought it would be fun to get ahold of the antibody and stain [for CLDN18.2] a variety of tissues. Within the stomach, and even in the GEJ, there is progression from normal mucosa to intestinal metaplasia, dysplasia, and then ultimately to neoplasia. We thought it would be interesting to stain these different steps in the process to learn more about how the protein worked, before it potentially became a target for treatment.

Could you expand on the key findings of this study?

We got the antibody to work. It had a clean staining; it stained normal gastric tissue the way it was supposed to. It also stained adenocarcinomas at similar levels to what we were seeing in the reported trial data.

Interestingly, in the precursor lesions, we found that intestinal metaplasia would stain in some instances, but not in others. For instance, in autoimmune metaplastic atrophic gastritis, it almost never stained the intestinal metaplasia. We learned more about how intestinal metaplasia might differ from patient to patient and case to case.

We also saw that it stains at similar levels across the different steps in the process of intestinal metaplasia to dysplasia and neoplasia, but that wasn’t always concordant within cases. [For instance], cases where the adenocarcinoma stained did not always correlate with what we saw in the precursor lesions. You [likely] need the adenocarcinoma itself for the ultimate result.

The other interesting finding was that while we were looking at these cases of autoimmune metaplastic gastritis, neuroendocrine hyperplasia stained strong with CLDN18.2. Based on that, we stained several neuroendocrine tumors and found that gastric neuroendocrine tumors stain strongly for CLDN18.2 in many cases. We also stained some non-gastric neuroendocrine tumors, and they, for the most part, did not stain.

[CLDN18.2] ended up being a sensitive and specific marker of gastric origin in neuroendocrine tumors. If [a patient had] a metastasis, this could help pathologists to diagnostically figure out if [the disease was of] gastric origin. If these antibodies are effective, [that] could [potentially] be interesting.

What advances remain to be seen with CLDN18.2 testing?

All the testing so far for inclusion in the trials has been done in central locations, where the labs are extremely standardized, and limited observers are interpreting the stains. If [zolbetuximab] is approved and the testing becomes wider, labs must be able to validate it and show similar staining patterns to what was done centrally. That will add [the need for] more people to interpret the stains. We want to know if there is interobserver variability in how the stain is interpreted and read [to ensure] the right patients are receiving treatment down the line.

The other interesting [aspect of] these targeted monoclonal antibodies is [tumor] heterogeneity. If some of the tumor is positive [for CLDN18.2] and some of the tumor is negative, how does that result affect how the potential drug works?

Since this testing is currently only part of clinical trials, how could it continue to progress in clinical practice?

At this point, it’s unclear how it will be rolled out. It could be limited to central lab tests at reference laboratories, or they could roll it out widely to select hospitals. That’s where [giving the labs the ability to] validate the antibodies will be important, [along with ensuring we] as pathologists are confident in interpreting the stain. That’s where it’s also important that everybody learns how the stain works in their own lab. With other targeted antibodies such as HER2, it’s incumbent on pathologists to become comfortable with how their lab handles that antibody, because it will stain differently from lab to lab. You want to be confident in your interpretation.

What is your main message regarding the role of testing for and targeting CLDN18.2 in gastric cancer?

As pathologists, we always enjoy learning how to interpret these stains. Often, we’re tasked with characterizing disease, such as where it came from, its area of spread, how it’s spreading, and where it has gone. It’s oftentimes bad news. However, when we can learn how to interpret stains that give [information to] our oncologists, surgeons, and our whole clinical team fight the cancer for their patients, it’s exciting for us.

References

1. Shitara K, Lordick F, Bang YJ, et al. Zolbetuximab plus mFOLFOX6 in patients with CLDN18.2-positive, HER2-negative, untreated, locally advanced unresectable or metastatic gastric or gastro-oesophageal junction adenocarcinoma (SPOTLIGHT): a multicentre, randomised, double-blind, phase 3 trial. Lancet. Published online April 14, 2023. doi:10.1016/S0140-6736(23)00620-7
2. Wong MT, Singhi AD, Larson BK, et al. Claudin-18. Pathol Lab Med. Published online August 17, 2022. doi:10.5858/arpa.2021-0428-OA