drug targeting
Imagine a letter that finds only one mailbox in a whole city and ignores every other door. Drug targeting is the attempt to make a medicine behave that way: to concentrate where the disease is and stay away from healthy tissue, so the patient gets the benefit with fewer side effects.
Most conventional drugs spread throughout the body after they are absorbed, reaching the diseased site and many healthy organs alike. Targeting tries to raise the ratio of drug at the target relative to drug elsewhere. This is usually achieved by formulating the drug into a carrier (such as a nanoparticle, liposome, or microsphere) whose size, surface, or release behaviour steers it toward the intended site, or by chemically attaching a homing molecule that recognises a feature of the target cells.
Paul Ehrlich's century-old idea of a “magic bullet” captures the goal, but perfect targeting is rare in practice. Even the best systems deliver only a fraction of the dose to the target, the rest is cleared by the liver, spleen, and kidneys, and surface modifications can trigger immune recognition. Targeting is therefore better understood as shifting the balance of exposure rather than achieving absolute selectivity.
Targeting is often classified as first order (to an organ or tissue), second order (to a specific cell type), and third order (to a compartment inside the cell). Higher orders are harder to achieve.