While revolutionary for liquid tumors, CAR-T cells struggle to attack solid tumors. The tumor's 'microenvironment'—a complex ecosystem of blood vessels, immune cells, and supportive structures—acts as a physical and biological barrier that prevents the engineered T-cells from reaching their target.

Dr. Patrick Baeuerle suggests that instead of engineering complex co-stimulatory signals into T-cell engagers, a more effective strategy is to combine them with standard-of-care treatments like chemotherapy or ADCs. This approach dramatically augments efficacy and has already prompted multiple Phase 3 trials.

Developing CAR T-cell therapies for solid tumors is difficult because many tumor-associated antigens are also expressed on normal tissues. This creates a significant risk of "on-target, off-tumor" effects, causing severe toxicity. Mitigating this risk, for instance with engineered "kill switches," is as crucial as preserving the therapy's efficacy.

Companies like VIR are making progress with masked T-cell engagers that limit systemic toxicity like cytokine release syndrome (CRS). This approach, which concentrates efficacy at the tumor site, could be the key to unlocking the broad potential of T-cell engagers beyond hematologic malignancies into the much larger solid tumor market.

Unlike CAR-T therapies that rely on a limited number of engineered cells, T-cell engagers activate the body's entire T-cell repertoire. This vast pool of effector cells makes exhaustion a negligible issue, as only a small fraction is engaged at any time, ensuring a sustained attack on cancer cells.

Next-generation CELMoDs (iberdomide, mezagdomide) show promise beyond direct tumor killing. Their ability to reduce or reverse T-cell exhaustion suggests they could be used to 'prime' a patient's immune system, potentially restoring function before or after treatment with a CAR-T or bispecific T-cell engager.

Many promising solid tumor antigens (e.g., PSMA, HER2) are also on normal tissues, making them too toxic for T-cell engagers. By using masks that are cleaved only in the tumor microenvironment, these "dirty" targets become viable, dramatically expanding the therapeutic landscape for solid cancers.

To combat immunosuppressive "cold" tumors, new trispecific antibodies are emerging. Unlike standard T-cell engagers that only provide the primary CD3 activation signal, these drugs also deliver the crucial co-stimulatory signal (e.g., via CD28), ensuring full T-cell activation in microenvironments where this second signal is naturally absent.

For solid tumors, the critical design hurdle for T-cell engagers is achieving selectivity. Most target antigens are also expressed at low levels on healthy cells, so molecules must be engineered to attack tumors with high antigen expression while sparing healthy tissue to avoid on-target, off-tumor toxicity.