Abstract
Nanoparticles are used in medicine to deliver drugs, for imaging, for vaccination and for local heating of tissue (tumor thermotherapy). If malignant tissue shall be addressed, it is of prime importance to direct the nanoparticles to their target. This can be accomplished by making use of physical effects (e.g., the EPR effect: enhanced permeation and retention) or by chemical modification of the nanoparticles to specifically recognize cells or tissues. The efficiency of the targeting can be assessed by in vitro cell culture experiments and also in vivo in animal experiments. As they are closest to the practical clinical application, in vivo imaging methods are particularly suitable to monitor the targeting. In general, a limited colloid-chemical stability of the nanoparticles in a biological environment and the formation of a protein corona around the nanoparticle may constrain their targeting ability. The current state of such targeting strategies is reviewed and discussed.