The ultraviolet-visible absorption spectrum of camptothecin (CPT) has been been recorded in water solution at pH 5.3, where the equilibrium of the different CPT forms is shifted toward the lactonic one.TD-DFT computations lead to a remarkable reproduction of the experimental spectrum only upon addition of explicit water molecules in interactions with specific moieties of the camptothecine molecule.Molecular dynamics simulations enforcing boundary periodic conditions for CPT embedded with 865 water molecules with a force field derived from DFT computations show that the experimental spectrum is actually due to the contribution of CPT molecules with different salvation patterns.A similar solvent effect is observed for several CPT derivatives, chemically modified in different positions, including the clinically relevant SN-38 and topotecan drugs.The quantitative agreement between TDDFT computations and experiments permits to pick-up specific spectroscopic signatures diagnostic of the drug environment that can be used to monitor the drug-DNA/protein interaction.