The problem: a drug goes everywhere
When you take a dose of a conventional medicine, the drug substance dissolves, is absorbed, and the blood carries it to every tissue it can reach — healthy and diseased alike. The target organ might receive only a tiny fraction; the rest is, at best, wasted and, at worst, the source of side effects. A cancer drug that kills tumour cells will also harm fast-dividing healthy cells in the gut and bone marrow simply because it arrives there too.
Drug targeting is the attempt to change this. The goal is simple to state and hard to achieve: deliver more drug where it is needed and less where it is not. A well-targeted system can let you use a lower total dose, widen the gap between the helpful dose and the toxic dose, and improve bioavailability at the site that matters rather than in the bloodstream as a whole.
Two broad strategies
Targeting splits into two families. In passive targeting, you do not steer the drug; instead you exploit a difference in the body's plumbing so the carrier naturally accumulates where you want. The classic example is the leaky blood vessels of tumours, which we cover next. In active targeting, you decorate the carrier with a homing molecule — a ligand — that binds a receptor over-expressed on the target cells, so the carrier is grabbed once it arrives.
- Choose the destination. Decide which organ, tissue, or cell type should receive the drug — a tumour, the colon, an inflamed joint.
- Find a difference to exploit. Look for something unique about that site: leaky vessels, a special pH, a receptor, a local enzyme.
- Pick passive or active. Use plumbing differences for passive targeting, or a homing ligand for active targeting — often both together.
- Match the route. A route of administration that lands the carrier near the target (inhaled, intra-articular, intratumoural) makes every later step easier.