Transparent tumor microenvironment: Are liposomal nanoparticles sufficient for drug delivery to hypoxic regions and clonogenic cells?

Abstract Passive targeting of nanoparticles (NPs) mediated by Enhanced Permeability and Retention effect is essential to NP delivery to tumors. However, clinically, only limited success has been found due to inadequate intratumoral delivery. Herein, for the first time, we present direct a demonstration visualizing the spatial distribution of liposomal nanoparticles (NP1: Lip-Ru (∼126 nm), NP2: Lip-Ru (∼164 nm) and NP3: Lip-Ru (∼234 nm)) in relation to hypoxic (HIF1α) and clonogenic (CD44) cells in intact transparent tumor tissues. Quantitative verification indicated that NP1, NP2, and NP3 did not diffuse into the intratumoral region over 100 μm away from the blood vessel. However, the liposomal nanoparticles represented slight difference in the three dimensional distribution within 100 μm around the blood vessel. Spatial distribution of HIF1α cells in tumors ranged from 62 to ∼460 μm. By contrast, CD44 expressing cells were observed in regions adjacent and away from the vessels. However, the maximum distribution of CD44 was observed ∼300 μm from the nearest vessels. Thus, the transparent tumor model reveals the penetration limits of Lip-Ru, and beyond the maximum diffusion of Lip-Ru, tumors contain hypoxic and clonogenic cells that are viable.