The intense intrinsic fluorescence emissions from several clinically relevant camptothecin drugs have been exploited in order to determine (1) the structural basis of drug binding to lipid bilayers, (2) the lipid bilayer stability of each drug's alpha-hydroxylactone moiety, a pharmacophore which is essential for antitumor activity, and (3) the site of drug binding in the bilayer. Equilibrium affinities of camptothecin and related congeners for small unilamellar vesicles composed of electroneutral dimyristoylphosphatidylcholine (DMPC) or negatively-charged dimyristoylphosphatidylglycerol (DMPG) were determined using the method of fluorescence anisotropy titration. Experiments were conducted in phosphate-buffered saline (PBS) at 37 degrees C and overall association constants (K values) were determined. Of the seven compounds studied, the new compound 9-chloro-10,11-methylenedioxy-(20S)-camptothecin (CMC) was found to display the highest membrane affinities (KDMPC = 400 M-1, KDMPG = 320 M-1), followed by 10,11-methylenedioxy-camptothecin and camptothecin, which exhibited KDMPC and KDMPG values of 100 M-1 or greater. Topotecan displayed markedly reduced binding to lipid bilayers (KDMPC = 10 M-1, KDMPG = 50 M-1). HPLC assays were subsequently employed to assess the relative stabilities of the lactone ring of membrane-bound drugs. Our results clearly indicate that lipid bilayer interactions stabilize the lactone moiety of camptothecin drugs. In comparison to half-lives in PBS (37 degrees C) of 17 and 19 min for camptothecin and CMC, respectively, DMPC- or DMPG-bound drugs were found to be stable even for periods up to 72 h. Iodide quenching data indicate that membrane-bound camptothecin intercalates between the lipid acyl chains, in a protected environment well removed from the aqueous interface. In this manner lipid bilayer interactions stabilize the lactone ring structure of camptothecins and thereby conserve the biologically active form of each medication.