Targeted drug delivery systems refer to the methods used for delivering drugs to specific cells, tissues, or organs of the body, while minimizing their exposure to other parts. The primary aim of targeted drug delivery systems is to improve the therapeutic value of drugs by delivering them at the specific site of action, increasing their potency and reducing side effects.

There are several different types of targeted drug delivery systems, depending on the chosen mechanism of action. Some of the most commonly used systems include:

1. Ligand-receptor targeting: In this method, a specific ligand molecule is linked to the drug molecule, which binds to a complementary receptor on the surface of the target cells. This helps the drug to be taken up selectively by the target cells only. Antibody-based therapies and small-molecule inhibitors are examples of this system.

2. Magnetic targeting: Iron oxide nanoparticles are attached to the drug molecules, and a magnetic field is applied to concentrate the drug at the target site.

3. Nanoparticle-based delivery: Drug molecules are encapsulated within nanoparticles, which can be engineered to have specific surface properties and target certain cells or tissues. Polymeric and liposomal carriers are commonly used for this system.

4. pH-triggered delivery: Some drugs are designed to be activated or released only under certain conditions, such as the acidity levels within specific cells. pH-triggered delivery systems use these conditions to trigger the release of the drug specifically in target cells.

5. Gene-based delivery: Gene therapy is a type of targeted drug delivery system, where the genes responsible for certain disorders are substituted or repaired.

Overall, targeted drug delivery systems offer immense potential for improving the therapeutic value of drugs and reducing their side effects, allowing for more targeted and efficient treatment of diseases.