Checkpoint inhibitors have transformed cancer care, but colorectal cancer remains a reminder that a validated target does not guarantee a responsive tumor. Most colorectal cancers are microsatellite stable (MSS), a molecular subtype that tends to carry fewer immunogenic mutations and fewer tumor-infiltrating T cells. As a result, the dramatic responses seen with PD-1 blockade in mismatch repair-deficient disease rarely extend to the much larger MSS population.
Researchers at Nanyang Technological University, Singapore, are approaching that problem from a different direction. Instead of using antibodies to block PD-1 and PD-L1 after the proteins reach the cell surface, they developed peptide-based microdroplets that deliver small interfering RNA (siRNA) into T cells and colorectal cancer cells, reducing production of the checkpoint proteins themselves.
In a mouse model of MSS colorectal cancer, the dual treatment inhibited tumor growth by about 67%, with activity comparable to combined anti-PD-1 and anti-PD-L1 antibodies used as a benchmark. The work was published in Biomaterials.
Two cell types, two RNA payloads
Delivering RNA into T cells is notoriously difficult. They are small, non-phagocytic cells and are considerably less permissive to many nonviral delivery systems than tumor cells.
The NTU team addressed that problem using peptide coacervates, liquid-like droplets formed through phase separation, and built two formulations around the biology of the PD-1/PD-L1 interaction.
One carried siRNA against PD-1 and was coated with anti-CD3 antibodies to promote uptake by T cells. The second delivered siRNA against PD-L1 into colorectal cancer cells. The microdroplets encapsulated the RNA while allowing the targeting antibodies to remain concentrated at their surface, creating a relatively simple cell-specific delivery architecture.
“Our approach works from inside both types of cells,” senior author Ali Miserez, PhD, said. “The siRNA destroys the instructions they need to make these proteins.”
In primary human T cells, the targeted formulation reduced PD-1 expression by roughly 40% to 45%. In colorectal cancer cells, PD-L1 silencing was considerably stronger. When the two cell types were cultured together, simultaneous silencing increased T-cell proliferation and activation and drove substantially more tumor-cell apoptosis than either intervention alone.
Putting a ‘cold’ tumor under pressure
The in vivo test was deliberately challenging. The researchers used CT26 tumors as an immunocompetent model of MSS colorectal cancer, a setting where conventional checkpoint blockade has limited clinical relevance.
Intratumoral delivery of both siRNAs reduced average tumor volume by 67% relative to controls. Tumors also contained more CD3-positive T cells and higher levels of the effector cytokines IFN-γ and TNF-α. The data suggest that silencing both sides of the checkpoint interaction can reshape local immune activity rather than simply removing one inhibitory signal.
That distinction could matter in heterogeneous tumors. RNA interference is programmable: once an effective delivery system exists, changing the therapeutic target may be considerably easier than developing an entirely new antibody.
The economics are also part of the appeal. NTU researchers estimate that an siRNA-based treatment could ultimately cost substantially less to manufacture than monoclonal antibodies, although that remains speculative until the platform is scaled and clinically developed.
The delivery problem is not solved yet
The major limitation is route of administration. The approximately micron-sized coacervates were injected directly into tumors, sidestepping the stability, biodistribution and tissue-targeting problems that would accompany intravenous delivery. The study also used a subcutaneous mouse tumor model rather than an orthotopic colorectal cancer model.
Those constraints make this a platform study rather than a near-term replacement for checkpoint antibodies.
Still, it addresses two problems at once: how to get RNA efficiently into T cells, and how to modulate immune checkpoints on both sides of the tumor–immune interface. For MSS colorectal cancer, where checkpoint therapy has struggled to gain traction, that combination makes the delivery technology at least as interesting as the targets it carries.
