Targeting the HSP90β/VEGFR2 Axis in Radiation-Induced Brain Injury: the Novel Inhibitor KDR2-2 Ameliorates Neurovascular Dysfunction

Radiation therapy is a powerful tool for treating brain tumors, using high-energy beams to destroy cancer cells. However, the beams cannot always distinguish perfectly between the tumor and the healthy tissue surrounding it. When healthy brain tissue receives too much radiation, it can develop a severe complication known as radiation-induced brain injury. This condition causes blood vessels in the brain to become leaky and disorganized, leading to swelling, tissue death, and a loss of cognitive function. For decades, the medical community has relied on a drug called bevacizumab to treat this problem. This drug works by blocking a specific protein called VEGF, which normally signals blood vessels to grow. By blocking this signal, the drug attempts to calm the chaotic blood vessel activity. Yet, for many patients, this treatment offers only temporary relief. The swelling often returns once the drug is stopped, and some patients do not respond to it at all, leaving them with few options and a difficult prognosis.

A team of researchers at Sun Yat-sen University has now identified a different way to approach this problem, one that targets the machinery inside the blood vessel cells themselves rather than just the external signal. They tested a new drug called KDR2-2 in mice that had received radiation to a specific part of their brain, mimicking the human condition. The study found that this new drug was significantly more effective than the standard treatment. It not only reduced the size of the damaged areas and restored normal blood flow but also repaired the physical structure of the blood vessels. The researchers discovered that radiation triggers a specific chain reaction inside the cells that standard drugs miss. They found that radiation causes a protein called HSP90β to activate a specific switch on the blood vessel receptor, turning it on even without the usual signal. By blocking this specific switch, the new drug stopped the damage at its source.

The journey to this discovery began with a simple observation in the lab. The researchers compared how well different drugs could stop the growth of blood vessel cells that had been exposed to radiation. While the standard drug, bevacizumab, and other existing treatments showed some effect, the new compound, KDR2-2, stood out. It was far more potent at suppressing the activity of the blood vessel receptor. To understand why this was happening, the team looked at the brain tissue of mice that had been treated with the new drug versus those treated with the standard care. Using advanced imaging, they saw that the mice receiving KDR2-2 had much smaller areas of brain damage. Their blood flow was restored to near-normal levels, and the blood vessels themselves looked healthy and organized, rather than the dilated and sparse vessels seen in untreated mice. The drug also reduced the leakage of fluid into the brain tissue, a key sign of injury, and protected the brain's nerve cells from dying.

To find the root cause of this difference, the scientists examined the molecular changes happening inside the cells. They knew that radiation usually causes the body to produce a protein called VEGF, which then binds to a receptor on blood vessels to make them leaky. Bevacizumab works by grabbing onto VEGF before it can reach the receptor. However, the researchers suspected that radiation might be activating the receptor through a different, hidden path. They analyzed the proteins inside the cells using a technique that acts like a high-resolution snapshot of chemical activity. They found that while the usual signals were present, radiation had triggered a very specific chemical change on the receptor that the standard drug could not stop. This change involved a specific spot on the receptor, known as Y1214, which became highly active.

The investigation then turned to what was causing this specific spot to light up. The researchers discovered that a heat shock protein, called HSP90β, was the key player. Under normal conditions, this protein helps other proteins fold into the correct shape. However, after radiation, this protein became chemically modified at a specific spot, S226, and began to drive the activation of the blood vessel receptor directly. It was as if the radiation had flipped a switch on the HSP90β protein, which then forced the blood vessel receptor to turn on, regardless of whether the usual VEGF signal was present. This explained why blocking VEGF alone was not enough; the receptor was being activated from the inside out.

To prove this connection, the team performed a series of precise experiments. They reduced the amount of HSP90β in the cells and found that the radiation-induced activation of the receptor stopped. They also created a version of the receptor that could not be activated at that specific spot and saw that the cells remained healthy even after radiation. These results confirmed that the HSP90β protein was driving the damage through a unique pathway that bypassed the usual signals. The new drug, KDR2-2, was able to block this pathway effectively, preventing the receptor from turning on and stopping the cascade of damage that leads to brain injury.

The study suggests that for patients who do not respond to current treatments, targeting this specific internal mechanism could offer a new hope. The researchers noted that the drug they tested is already being studied in early clinical trials for a different eye condition, which means its safety profile is already being evaluated in humans. While the study was conducted in mice and cells, the findings provide a clear roadmap for understanding why radiation injury is so difficult to treat and point to a specific molecular target that could be blocked to prevent it. The work highlights that sometimes, to stop a disease, one must look beyond the obvious signals and find the hidden switches that keep the damage going. By understanding and blocking these switches, doctors may soon be able to offer better protection to the brains of those undergoing life-saving radiation therapy.