Antibody drugs have reshaped modern medicine over the past several decades, especially in diseases like cystic fibrosis. Early progress came from identifying the genetic mutations behind the condition, followed by therapies that partially correct the faulty protein. While these treatments improve outcomes, they do not fully restore normal function. The latest advance pushes this progress further by extending the reach of antibody-based technology inside the cell, where many disease processes actually begin.

This approach uses a nanobody—a smaller, more compact antibody fragment—since full antibodies cannot cross the cell membrane and instead bind only to targets on the cell surface. The nanobody enters cells and stabilizes a mutated cystic fibrosis protein from within, helping it function properly. In cystic fibrosis, the protein that regulates salt and water transport is misshapen, and current drugs improve its folding or activity but do not fully correct it. Earlier efforts explored intracellular nanobodies using engineered systems, including AI-guided designs , but the central challenge remains: delivering a protein into cells in an active form. Here, that challenge is addressed by enabling the nanobody itself to cross the membrane.

New research builds on precision protein targeting and membrane transport by combining a nanobody with a molecular tag designed to carry it across the cell membrane. This hybrid molecule reaches an intracellular protein long considered difficult to fix and restores much of its function. When paired with existing drugs, the effect approaches near-normal activity in patient-derived cells, suggesting a powerful new way to extend the reach of current treatments.

Turning Nanobodies Into Intracellular Drugs

Nanobodies are engineered to bind tightly to specific proteins, much like traditional antibodies. Their small size makes them more stable and better at penetrating tissues than larger antibodies.

To transport the nanobody inside cells, it fuses to a short cell-penetrating peptide comprised of 10 positively charged arginine amino acids. These peptides interact with the negatively charged surface of cells and are taken up into the cell, carrying the nanobody with them. A portion of the protein then escapes into the cell’s interior, where the nanobody can reach its target. Without this peptide, the nanobody cannot enter cells at all.

Fixing a Misfolded Protein in Cystic Fibrosis

Cystic fibrosis is caused by a mutation that distorts a protein called CFTR, preventing the protein from folding correctly and reaching the cell surface, where it regulates chloride ion movement. The result is thick, sticky mucus in the lungs and chronic respiratory disease.

The nanobody used in this study binds directly to a portion of CFTR that becomes unstable due to this mutation. Once inside the cell, it stabilizes the misfolded protein, allowing it to mature properly and travel to the cell membrane. There, CFTR can resume its role as a functional channel.

Nanobody-treated cells produce more mature CFTR protein and display increased levels of the protein at the cell surface. Most importantly, the restored protein is functional: chloride transport, an essential process disrupted in cystic fibrosis, increases as more of the nanobody enters the cell.

A Boost to Existing Therapies

While existing small-molecule drugs stabilize other regions of the protein, the nanobody targets a specific domain directly affected by the mutation. When combined, the two strategies reinforce each other.

In cell models, pairing the nanobody with approved drug combinations leads to a substantial increase in CFTR activity beyond what either approach achieves alone. In patient-derived airway cells, this combined treatment restores function to nearly normal levels—reaching close to 90% of what is observed in healthy cells.

Crossing the Cell Membrane

Delivering protein drugs into cells has long been a major challenge in medicine. Many experimental approaches rely on gene therapy to force cells to produce therapeutic proteins internally, a strategy that introduces its own complexities and risks.

This approach instead delivers the protein directly, without altering the cell’s genetic material. The cell-penetrating peptide carries the nanobody into the cell, allowing it to interact with its target.

This delivery also works in primary human airway cells, which are typically much harder to manipulate. Even in these more realistic models, the nanobody enters cells and improves CFTR function.

While this nanobody targets CFTR, the implications extend much further. Cell-permeable nanobodies offer a potential way to target malfunctioning proteins inside cells, a key feature in disease from rare genetic disorders to common conditions. By combining precise protein recognition with the ability to cross cellular barriers, they open the possibility of correcting disease mechanisms at their source.

Further work will refine delivery, especially in complex environments like the lung, where mucus can block access to cells. But the principle is now clear: protein drugs can be designed to reach targets inside cells. Moving protein drugs inside cells expands what these therapies can reach—and what they may one day be able to treat.

This is the second article in a series of nanobodies for medical applications.