The prodrug idea
Every system so far wraps a drug in a carrier. A prodrug does something different: it changes the drug molecule itself. You attach a chemical group that makes the molecule inactive and gives it better delivery properties; once inside the body, an enzyme or a chemical reaction snips the group off and the active drug appears. The patient swallows a disguise; the body unmasks it. This is a chemical route to the same goals — better bioavailability, or true site-specific delivery.
- Improve absorption. Add a group that raises lipid solubility — a higher log P — so a poorly absorbed drug crosses membranes, then is cleaved back to the active form (many ACE-inhibitor esters work this way).
- Boost solubility. A water-soluble promoiety can rescue a drug with poor solubility for an injectable, then drop off in the blood.
- Dodge first-pass loss. A prodrug stable through the gut wall and liver can sidestep the first-pass effect that destroys the parent drug.
- Target a site. If the unmasking enzyme is concentrated in one tissue, the active drug appears mainly there — a chemical form of drug targeting (some colon and tumour prodrugs rely on local enzymes).
Choosing among the tools
You now have a whole toolbox: passive EPR targeting, active targeting with a ligand, mucoadhesion, gastroretention, stimuli-responsive release, implants and depots, and prodrugs. They are not rivals — they are layers you combine. The right combination always starts from the same three questions: where must the drug act, what is special about that site, and how long must the action last?
A real anticancer nanomedicine shows the layering. The carrier is sized for EPR to get it into the tumour passively; it is PEGylated to circulate long enough; a ligand adds active targeting so it is taken into tumour cells once there; and a pH-sensitive linker gives stimuli-responsive release inside the acidic cell, all on top of controlled release from the matrix. Four ideas from this track stacked into one particle.