Figure 1.

Illustration of size-dependent antitumor drug delivery. stage 1: extravasation based on the tumor vascular pore size selection (Three arrows indicate that less phagocytosis by MPS, longer circulation time in bloodstream, and higher extraversion at tumor vasculature are beneficial for enhancing passive targeting of tumor by EPR effect); stage 2: antitumor drug delivery in the highly permeable tumor tissue; stage 3: antitumor drug delivery in the poorly permeable tumor tissue; stage 4: tumor cell uptake and subsequent intracellular drug release. pH ~7.4 for normal tumor tissue and blood stream, ~6.5 for tumor tissue, ~5.5 for endosome, ~5.0 for lysosome.