First in vivo evaluation of the radiolabelled Hsp90 inhibitor [11C]Onalespib for cancer and brain PET imaging

Inside every living cell, a team of molecular helpers works tirelessly to ensure that proteins—the building blocks of life—fold into their correct shapes and stay that way. Without these helpers, proteins would tangle and fail, causing the cell to malfunction. One of the most important of these helpers is a protein called Hsp90. While it is essential for normal cell function, cancer cells often hijack Hsp90 to survive and grow, using it to protect the very proteins that drive their uncontrolled division. Because of this, scientists have long hoped to find a way to see exactly where Hsp90 is active inside the body. If doctors could take a picture of these molecular helpers, they could better understand how a tumor is behaving and whether a treatment designed to block Hsp90 is actually reaching its target.

For years, researchers have tried to create a special camera for the inside of the body, using a technique called PET scanning. This method involves injecting a tiny amount of a radioactive substance that sticks to specific targets, allowing a scanner to see where it goes. However, creating a camera that works well for Hsp90 has been difficult. Many previous attempts resulted in images that were too blurry, or the camera substance disappeared from the body too quickly, or it got stuck in the wrong places, like the liver or kidneys, making it hard to see the tumor. The search for a clear, reliable image of Hsp90 in both cancer and the brain has remained a significant challenge in medical science.

A team of researchers at KU Leuven in Belgium has taken a fresh look at this problem by testing a new candidate molecule called [11C]Onalespib. This molecule is a modified version of a drug already known to block Hsp90, designed specifically to be visible to a PET scanner. The scientists wanted to know if this new tracer could successfully find and stick to Hsp90 in living animals, and whether it could cross the blood-brain barrier to image the brain, a region that is notoriously difficult to scan. To find out, they first created the radioactive version of the molecule in their lab, ensuring it was pure and safe to use. They then tested how well it stuck to Hsp90 in slices of tissue from different types of cancer, including breast, brain, and prostate tumors, as well as in healthy brain tissue.

The results from the tissue slices were encouraging. When the researchers placed the radioactive tracer on these thin slices, it bound strongly to the Hsp90 proteins. To prove that this binding was specific and not just random sticking, they added other known Hsp90 blockers to the mix. When these blockers were present, the radioactive tracer could no longer attach, confirming that it was indeed targeting the correct molecular helper. This worked well across all the cancer types tested and even in healthy brain tissue, suggesting that the molecule has the right shape to find its target. The researchers also tested the molecule on living cancer cells in a dish, where it was absorbed by the cells in a way that could be stopped by adding blockers, further confirming its ability to find Hsp90.

However, when the researchers moved from the lab bench to living mice, the picture became more complicated. They injected the tracer into mice that had grown human tumors and watched where it went using a PET scanner. In the mice with brain tumors, the tracer did not enter the brain in any significant amount. It stayed mostly in the blood and the body's filtering organs, such as the kidneys and liver, which is a common route for the body to clear foreign substances. While the tracer did show up in the brain tissue when they looked at the slices under a microscope, it failed to cross the blood-brain barrier in a living animal. This is a crucial distinction, as it suggests that while the molecule can bind to Hsp90 in the brain, it cannot get there on its own when injected into the bloodstream.

The study also revealed that the molecule breaks down very quickly once it enters the bloodstream. Within just ten minutes of injection, less than a quarter of the tracer remained in its original, intact form; the rest had already been transformed into different chemical byproducts by the body. This rapid change explains why the tracer did not accumulate well in the tumors or the brain. In the mice with brain tumors, the tracer showed only a moderate amount of uptake, and in mice with breast cancer tumors, the uptake was very low. The researchers observed that the tracer was cleared from the body primarily through the kidneys and the liver, which is typical for many small-molecule drugs but limits the time available to take a clear picture.

Despite these challenges, the study provides a clear map of where this new tracer stands. It proved that the molecule can be made safely and that it has the correct shape to recognize and bind to Hsp90 in both cancer and brain tissue. The failure to see the tracer in the brain of living mice, however, contradicts earlier reports that suggested the drug could easily cross into the brain. This finding raises questions about whether this specific molecule is the right choice for imaging brain diseases or for treating brain tumors, as a drug that cannot reach the brain cannot effectively treat it. The rapid breakdown of the molecule in the blood also suggests that future versions of this tracer would need to be chemically modified to last longer in the body.

The researchers concluded that while [11C]Onalespib is a promising starting point because it binds specifically to its target, it is not yet ready for use as a medical imaging tool. The molecule's inability to penetrate the brain and its quick disappearance from the bloodstream mean that it cannot yet provide the clear images needed for diagnosis or treatment monitoring. The study highlights that the path to a successful medical tracer is often a process of trial and error, where understanding what does not work is just as important as finding what does. The team plans to use these findings to design new versions of the molecule that might stay intact longer and be able to cross the blood-brain barrier, continuing the search for a clear window into the molecular world of cancer and the brain.