Drug delivery compositions for specific delivery of a drug to a tumor are described. These compositions include a core for sequestering the drug and a shell to which a ligand is attached for delivery of a drug to target cells. Since normal cells may also be targeted by the ligand, the compositions embed the ligand in the shell until the localized conditions surrounding the tumor cause the ligand to be displayed on the surface of the shell. One composition exhibits shrinkage of the shell at tumor pH, whereas another composition exhibits extension of the ligand at tumor pH. Still another composition causes the ligand to be exhibited at an elevated temperature.