passive targeting

Passive targeting is like letting leaves collect naturally in the slow eddy of a stream rather than placing them there by hand. The carrier is not given any special address label; instead it accumulates at a site because of the body's own anatomy and physiology.

The classic example is in tumours and inflamed tissue, where blood vessels are abnormally leaky and lymphatic drainage is poor. Nanoparticles and other carriers in roughly the 10–200 nm size range can escape these leaky vessels and become trapped, building up over time. Carrier surface properties also matter: particles that escape rapid filtration by the spleen and uptake by the liver's phagocytes circulate longer and so have more chances to reach the target.

Because it relies on chance physiology rather than active steering, passive targeting is modest and variable. The leaky-vessel effect differs between tumour types, between patients, and even within one tumour, so the amount of drug that actually reaches the target is often small and unpredictable. It is a foundation that active targeting strategies are usually layered on top of, not a guarantee of selectivity.