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Cell-mediated drug delivery is an emerging strategy for targeted drug delivery to the central nervous system (CNS), where specific cells are used as carriers to transport drugs or active agents across the blood-brain barrier (BBB) and into the CNS. The use of cells as drug carriers presents an opportunity to overcome the major challenge associated with CNS drug delivery, that is, the highly selective and impermeable nature of the BBB, which restricts the passage of most drugs from blood to brain tissue.

The blood-brain barrier is a specialized structure formed by tight junctions between the endothelial cells lining the blood vessels in the brain, along with astrocytes and pericytes. It is a highly selective barrier that prevents the passage of harmful substances into the CNS while still allowing the transport of essential nutrients and molecules. The BBB is a significant obstacle in drug delivery to the CNS as it impedes the majority of therapeutic agents from entering the brain tissue, making the treatment of neurological disorders more challenging.

Various types of cells have been explored as carriers for cell-mediated drug delivery, including stem cells, macrophages, and genetically modified cells. The choice of carrier cells depends on their ability to infiltrate the BBB and deliver the drug to the specific site of action within the CNS. Some characteristics of ideal carrier cells include low immunogenicity, high target specificity, ability to carry and release encapsulated drugs or therapeutic agents, and in vivo imaging capabilities.

Carrier cells can be engineered to transport the drugs across the BBB by exploiting the natural trafficking mechanisms of these cells. In particular, some cells such as macrophages can cross the BBB via a process called diapedesis, wherein the cells migrate through the tight junctions of the endothelial layer. Additionally, the carrier cells can be modified to express receptors for specific endogenous transport systems present at the BBB, facilitating receptor-mediated transcytosis. This allows for the encapsulated drug to be transported across the BBB efficiently and specifically.

For successful cell-mediated drug delivery to the CNS, drugs or therapeutic agents must be encapsulated and loaded into the carrier cell, ideally without affecting the cell's viability and function. Various loading approaches include simple incubation, electroporation, and genetic modification to produce the therapeutic protein intracellularly. After reaching the target site, the drug must be released from the carrier cells in an appropriate manner, either by direct exocytosis or targeted cell death leading to drug release and uptake by the adjacent cells.

Cell-mediated drug delivery to the CNS has potential applications in treating various neurological disorders, including neurodegenerative diseases (such as Alzheimer's and Parkinson's), brain tumors, and neuroinflammatory conditions like multiple sclerosis. Current research is focused on developing highly specific and efficient technologies for successful drug delivery and evaluating the safety and efficacy of these techniques for clinical applications. In conclusion, cell-mediated drug delivery is a promising strategy to overcome the challenges associated with delivering therapeutic agents to the central nervous system. By utilizing cells as carriers, this approach enables targeted and efficient drug delivery across the blood-brain barrier, paving the way for innovative therapies for neurological disorders.