A Field in Transition: T Cell Engagers

By Peter Jurich

September 24, 2026 | For years, the story of T cell engagers (TCE) in oncology followed a predictable arc: remarkable results in liquid tumors, stubborn failure in solid ones. The addition of a dedicated TCE track to this year's PEGS Europe reflects a growing conviction in the field that the second half of that story is changing. Dr. Aude Segaliny, founding scientist of Amberstone Biosciences and one of the program's speakers, thinks the timing is right. She has a clear view of why.

Part of it is simply the manufacturing argument. "An antibody is a product that you can well-define, well-characterize, a one-control molecule," she says. "It's an easier modality to use as a therapeutic for development." This is compared to cell therapies, which carry significant complexity in production and administration. But the more important shift, in her view, is mechanistic: "There are a few molecules right now in the clinic showing encouraging results that support conditional activation as a key strategy in the coming years."

The field has struggled since the first solid tumor TCE programs. Two obstacles compound each other. First, most solid tumor antigens are also expressed in healthy tissue, shrinking the therapeutic window. Second, the tumor microenvironment itself actively resists treatment.

"Solid tumors have a very tough microenvironment with a complex matrix the antibody has to penetrate," Segaliny explains. "And then you have a lot of other tough features like acidity, which we are leveraging with our technology, plus metabolites and other inhibitory factors that also inhibit T cell activity."

The resulting catch-22 is one every TCE developer knows intimately: aggressive T cell activation is what drives efficacy, but it's also what causes cytokine release syndrome and off-tumor toxicity.

"If you boost more of the immune response, you add toxicity," Segaliny says. "That's also where local activation and conditional activation is very critical. If one can achieve a very thin baseline of side effects, then we can start building additional layers on top."

The field's earlier answer was to tune down CD3 affinity, trading potency for a safer activation threshold. Segaliny sees that as a partial solution at best. The emerging approach—molecules that behave differently in the tumor microenvironment than in systemic circulation—is producing data she finds more compelling. She points to a protease-cleavable masked TCE in prostate cancer with early-stage data showing close to 50% objective response rates in a heavily pretreated, high-burden population, with grade one CRS rates even at high doses.

"That kind of data really shows that conditional activation can break the trade-off relationship between efficacy and safety," she says.

Amberstone's T-MATE platform engineers that conditional behavior differently: through pH rather than protease cleavage. The CD3-binding arm undergoes a conformational change based on acidity, functioning as a potent conventional engager at tumor pH and transitioning to a low-affinity state at physiological pH. "You can see it as a two-in-one molecule," Segaliny says. "At physiological pH—blood is 7.45—we basically don't have much T cell activation."

Crucially, the design accounts for the pH heterogeneity within tumors, maintaining activity across a range of acidic conditions. In non-human primate studies run side-by-side against a commercial TCE comparator, Amberstone has dosed T-MATE at levels roughly 200-fold higher without observing cytokine release. "We don't see any tox," she says flatly.

The reversibility is also a meaningful differentiator from protease-cleavable masked TCEs. In that approach, once the mask is cleaved, the molecule is permanently active, which is why those designs incorporate fast-clearance domains to limit systemic exposure. T-MATE's pH dependence is inherently reversible: a molecule that migrates from the tumor back into physiological tissue simply switches back to its low-affinity state. The safety mechanism is continuous rather than one-time.

A Phase 1 study is targeted for the first half of 2027, beginning in China before bridging to a U.S. extension Segaliny said, but she is candid about what the platform still needs. "We don't have clinical data yet, and that's the key."

The gap shapes how she reads the investment landscape. "Everyone is convinced by the modality itself. Everyone also knows there's still room for optimization. But the question is more which flavor is going to make the biggest difference, and we need to see more clinical data to conclude."

Multiple companies are now in the clinic with conditional strategies, each taking a different approach to the same underlying hypothesis: that decoupling the site of activation from the site of toxicity is what finally makes solid tumor T cell engagement work. The field will now look at the clinical data coming in and take stock of what's working.