active targeting

If passive targeting is letting a package drift to the right neighbourhood, active targeting is putting a precise street address on it so it docks at one particular door. Molecules on the carrier's surface recognise and bind to markers that are abundant on the target cells.

These surface molecules, called ligands, can be antibodies, antibody fragments, peptides, sugars, vitamins such as folate, or aptamers. They bind to receptors or antigens that are over-expressed on diseased cells, for example folate receptors or HER2 on certain cancer cells. Binding often triggers the cell to internalise the carrier, delivering its payload inside, which is valuable for drugs that must act intracellularly.

An important subtlety is that active targeting usually does not increase how much drug reaches the tumour overall, that is still governed by blood flow and vessel leakiness. What it changes is the fate of the carrier once it arrives: it improves binding to and uptake by the right cells. Adding ligands also makes the product more complex to manufacture and can shorten circulation time if the ligands are recognised by the immune system.