The tumor therapeutics effect of chemotherapeutant, proteins and genes could be improved when they are loaded in nano-drug carriers. But in some cases the therapeutics effects are modest due to the poor penetration and distribution of nano-carriers in the heterogeneous tumor tissue. During the recent years, increasing efforts have been dedicated to improve the intra-tumor penetration by controlling the size, surface zeta potential, targeting agents, and shape of nano-drug carriers. This paper reviews the progress in enhancement of intra-tumor penetration and distribution of nano-drug carriers through the modification of their components, structures and physicochemical properties. The challenges of nano-drug carriers in tumor treatment are raised and the possible solutions are proposed.

Contents
1 Introduction
2 Inorganic nano-drug carriers
2.1 Meso-porous silicon nano-drug carriers
2.2 Gold nano-drug carriers
3 Organic nano-drug carriers
3.1 Micelles
3.2 Dendrimers
3.3 Liposomes
3.4 Polymer nanocapsule
4 Conclusion