EPR Effect vs Active Targeting

Nanoparticle design

Particle size optimal (~80 nm)

Blood flow speed 1.0×

Live concentration

Passive · EPR-only

Healthy tissue

Tumor tissue

In cancer cells

Active · ligand-targeted

Healthy tissue

Tumor tissue

In cancer cells

How it works

Healthy capillaries have a tight, continuous endothelial lining — almost nothing leaks through into surrounding tissue. Tumor blood vessels grow chaotically and are riddled with abnormal gaps (glowing rings on the right-hand vessel), so appropriately-sized nanoparticles slip out and accumulate in tumor tissue just by circulating in the blood. That's the enhanced permeability and retention (EPR) effect — it needs no targeting mechanism at all.

Both particle colors leak through the tumor gaps at the same rate, because EPR depends only on size and vessel geometry. The difference shows up next: gold particles carry surface ligands that bind receptors cancer cells overexpress, so once they're in tumor tissue they actively dock and get internalized. Blue particles have no ligands — they accumulate loosely in tumor tissue but drift back into circulation without ever specifically entering a cancer cell.

Particle size — too small and particles leak everywhere, including healthy tissue (non-specific); too large and they can't fit through even the leaky tumor gaps.
Tumor tissue vs in cancer cells — the key distinction: EPR fills the tumor's extracellular space passively, but only active ligand-binding drives uptake inside the cancer cells themselves.

Passive (EPR-only) Active (ligand-targeted) Healthy tissue (tight vessel) Tumor tissue (leaky vessel) Cancer cell Leaky vessel gap

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EPR Effect vs Active Ligand Targeting