Topotecan (TOP), a water-soluble derivative of camptothecin, is a potent antitumor agent that is receiving growing attention for the treatment of several types of cancer. However, one of the major constraints in the clinical use of this drug is its inactivation at the physiological pH of 7.4. Mesoporous silica nanoparticles (MSNs) constitute promising nanocarriers to circumvent this issue. Herein TOP has been encapsulated into MSNs and the nanosystem has been provided with selectivity towards tumor cells, which permits releasing the active form of the molecule at the acidic cell compartments (endo/lysosomes; pH