How Do Drugs Reach Our Brain? The Blood-Brain Barrier and New Delivery Systems

Our brain is one of the best-protected organs in our body. This protection system prevents many harmful substances from reaching the brain. However, the same system also makes it difficult for some drugs to enter the brain. For this reason, scientists are developing targeted drug delivery systems that can be used especially in the treatment of brain diseases. This article examines in detail the journey of drugs in the body and how the blood-brain barrier works.

When we take a drug orally, it embarks on a longer and more complex journey than we might think. It first reaches the stomach, then is absorbed from the intestines into the bloodstream. Subsequently, it is transported to different parts of the body through the circulatory system. We can liken this process to a cargo distribution network. Blood vessels act like roads in our body; drug molecules use these roads to reach different organs. However, not every drug can pass into every tissue with the same ease. Especially when it comes to the brain, this journey becomes much more complicated. Because the brain has a strong protection system against external substances.

Our brain is one of the most sensitive organs in our body; our thoughts, memory, movements, and vital functions are managed by the brain. Therefore, it has a special defense system against harmful substances. One of the most important parts of this system is the blood-brain barrier. We can think of the blood-brain barrier as a high-security checkpoint that only allows certain substances to pass through. Special cells that form the blood vessels in the brain and the supporting structures around these cells filter many substances from the blood. Thus, only some molecules that the brain needs or is allowed to pass can reach the brain.

The blood-brain barrier does not completely block every substance. For example, some molecules that the brain needs, such as oxygen and glucose, can cross this barrier. Some molecules are transported to the brain via special transport systems. However, many drug molecules cannot reach the brain because they are not small enough or do not have the appropriate chemical properties. In general, small and fat-soluble molecules can cross the barrier more easily. Some substances can also be taken in a controlled manner through special carrier proteins on the cell surface. This raises an important question for scientists: If a drug cannot reach the brain, how can effective treatment of neurological diseases such as Alzheimer's, Parkinson's, brain tumors, or cerebrovascular diseases be possible?

In recent years, targeted drug delivery systems have been developed to direct drugs to the area of need rather than randomly distributing them in the body. These systems can use very small carrier structures called nanoparticles. These carriers are designed to protect drug molecules and deliver them to the target tissue. Scientists are trying to add special molecules to the surface of these carriers so that they can recognize specific cells or some transport systems in the blood-brain barrier. Thus, the aim is to ensure that the drug reaches the right target, reduce side effects, and increase treatment efficacy. These new-generation approaches, called smart drug delivery systems, aim to transport drugs in a more controlled manner. In the future, much more precise treatment methods await us thanks to nanotechnology, bioengineering, and artificial intelligence-supported systems.