Nano-sized exosomes originated from cells with a diameter of 40~100 nm can carry and transport "cargo" in cell-to-cell communication. They are important mediators of intercellular communication and regulators of the cellular niche. Their altered characteristics in many diseases suggest their important roles in diseases diagnosis and therapy, thus prompting the idea of using exosomes as drug delivery vehicles. As naturally occurring endogenous carriers of drugs, exosomes have unique advantages such as limited immunogenicity, great stability in blood, high delivery efficiency, targeting ability, and the improvement of enhanced permeability and retention effect (EPR). So far, genetic drugs, anticancer drugs, anti-inflammatory drugs and etc. have been successfully delivered by exosomes. In these cases, exosomes contribute to improving transfection efficiency of gene and reducing their side effects, as well as protecting therapeutic drugs from clearance by human bodies. This review covers the latest developments in the field of exosome-based drug delivery systems with regard to "cargo" species. Key components of exosome-based drug delivery system, such as the sources and purification methods of exosomes, choice of therapeutic cargo, loading methods, and administration routes are briefly discussed. At last, the challenges of exosomes as drug carriers in clinical practice are raised and the possible solutions are proposed.

Contents
1 Exosomes and their biofunction
2 Exosomes as drug carriers
2.1 Key components of exosomes as drug carriers
2.2 Exosomes as genetic drug carriers
2.3 Exosomes as anticancer drug carriers
2.4 Others
3 Conclusion and outlook