BSC9 a betulin–acetazolamide conjugate enhances radiosensitivity in 2D and 3D HNSCC models

In the battle against cancer, one of the most stubborn obstacles is a lack of oxygen. Solid tumors often grow so fast that their blood vessels cannot keep up, leaving the core of the tumor starved for air. This state of low oxygen, known as hypoxia, acts as a shield for cancer cells. It makes them resistant to radiation therapy, which relies on oxygen to help create the destructive forces that kill cells. To make matters worse, these hypoxic cells produce a specific protein on their surface that helps them survive and adapt to this harsh environment. Scientists have long sought a way to target these hidden, resistant cells without harming the healthy tissue around them. The challenge lies in finding a treatment that can penetrate the tumor's oxygen-starved depths and strike a blow where standard therapies fail.

A team of researchers at Martin Luther University Halle-Wittenberg has developed a new approach to this problem by creating a single molecule that performs two jobs at once. They combined a drug known to block the survival protein found in low-oxygen zones with a natural compound derived from birch bark that is toxic to cancer cells. They named this new hybrid molecule BSC9. The researchers tested this compound on head and neck cancer cells, which are a common type of tumor known for developing resistance to treatment. Their goal was to see if this dual-action molecule could make these stubborn cancer cells more vulnerable to radiation, particularly in the oxygen-poor areas where they usually hide.

The scientists began by growing the cancer cells in flat, two-dimensional layers in a lab dish, a standard method for initial testing. They exposed these cells to BSC9 under normal oxygen conditions and then under low-oxygen conditions that mimic the tumor core. The results showed that the compound killed the cancer cells effectively in both environments, but it was particularly potent when oxygen was scarce. The researchers then moved to a more complex test that better resembles a real tumor. They grew the cells into tiny, three-dimensional balls, or spheroids. To distinguish between the outer layers of the ball, which have plenty of oxygen, and the inner core, which is oxygen-starved, they used a clever trick. They labeled the outer cells with a red glow and the inner cells with a green glow. This allowed them to watch exactly what happened to each part of the tumor after treatment.

When they treated these glowing spheroids with BSC9 and then exposed them to radiation, the results were striking. The compound made the cancer cells much more sensitive to the radiation, meaning a lower dose of radiation could kill them. This effect was most dramatic in the green-glowing inner core, the very place where cancer cells usually survive radiation therapy. The researchers found that the compound worked by disrupting the internal machinery of the cells. It caused a massive buildup of harmful reactive molecules inside the cells, damaged their DNA, and caused their energy-producing structures, called mitochondria, to fail. This collapse of the cell's internal power and structural integrity led to the death of the cancer cells.

To understand how this happened, the team looked at the genetic instructions inside the cells. They found that BSC9 triggered a complex chain of events that changed how the cells behaved. The compound appeared to activate pathways related to cell death and stress while shutting down the signals that help cells grow and move. Interestingly, while the natural birch bark component of the drug is known to lower levels of the main protein that controls the response to low oxygen, the combined drug did not lower this protein in the same way. Instead, it seemed to decrease the expression of the specific survival protein on the surface of the cell, likely by promoting its degradation, while the other part of the molecule attacked the cell's internal machinery. This suggests that the two parts of the drug work together in a coordinated fashion, with one part acting as a guide to the hypoxic zone and the other delivering a lethal blow.

The study confirms that this new molecule can overcome the resistance that low-oxygen cancer cells typically show against radiation in laboratory settings. By using the two-colored spheroid model, the researchers were able to show that the treatment works specifically in the difficult-to-reach, oxygen-poor regions of a tumor model. While the compound has not yet been tested in living animals or humans, and direct evidence linking its mechanism specifically to CA IX-dependent targeting remains to be established, the laboratory results provide a strong foundation for further development. The work demonstrates that combining a drug that targets a specific survival mechanism with a substance that naturally kills cancer cells is a promising strategy. It offers a potential new way to treat tumors that have become resistant to standard radiation therapy, giving hope that the hidden, shielded cells within a tumor can finally be reached and destroyed.