Fluorescence spectral properties of the anticancer drug topotecan by steady-state and frequency domain fluorometry with one-photon and multi-photon excitation

Topotecan is an antitumor agent with activity against a variety of cancers. We examined the steady-state and time-resolved fluorescence spectral prope rties of topotecan with one- and two-photon excitation. Topotecan was found to display a high two-photon cross section near 20 GM for wavelengths with in the fundamental output of a Ti:sapphire laser, 800-880 nm, In frozen sol ution the anisotropies of topotecan are near the theoretical maxima for one -photon and two-photon excitation with colinear electronic transitions. The intensity and anisotropy decays of topotecan fluorescence were found to be homogeneous (single exponentials) in phosphate-buffered saline and propyle ne glycol. The steady-state and time-resolved data indicate that topotecan binds to a double-helical DNA oligomer d(AT)(10) resulting in increased ani sotropies and multiexponential intensity and anisotropy decays, Subnanoseco nd components in the anisotropy decay of the DNA-topotecan complex suggest loose binding of the drug to DNA, Loose binding of topotecan to DNA is also revealed by accessibility of topotecan to collisional quenching by iodide.