LIPID BILAYER PARTITIONING AND STABILITY OF CAMPTOTHECIN DRUGS

The intense intrinsic fluorescence emissions from several clinically r elevant camptothecin drugs have been exploited in order to determine ( 1) the structural basis of drug binding to lipid bilayers, (2) the lip id bilayer stability of each drug's alpha-hydroxylactone moiety, a pha rmacophore which is essential for antitumor activity, and (3) the site of drug binding in the bilayer. Equilibrium affinities of camptotheci n and related congeners for small unilamellar vesicles composed of ele ctroneutral dimyristoylphosphatidylcholine (DMPC) or negatively-charge d dimyristoylphosphatidylglycerol (DMPG) were determined using the met hod of fluorescence anisotropy titration. Experiments were conducted i n phosphate-buffered saline (PBS) at 37-degrees-C and overall associat ion constants (K values) were determined. Of the seven compounds studi ed, the new compound 9-chloro-10,11-methylenedioxy-(20S)-camptothecin (CMC) was found to display the highest membrane affinities (K(DMPC) = 400 M-1, K(DMPG) = 320 M-1), followed by 10,11-methylenedioxycamptothe cin and camptothecin, which exhibited K(DMPC) and K(DMPG) values of 10 0 M-1 or greater. Topotecan displayed markedly reduced binding to lipi d bilayers (K(DMPC) = 10 M-1, K(DMPG) = 50 M-1). HPLC assays were subs equently employed to assess the relative stabilities of the lactone ri ng of membrane-bound drugs. Our results clearly indicate that lipid bi layer interactions stabilize the lactone moiety of camptothecin drugs. In comparison to half-lives in PBS (37-degrees-C) of 17 and 19 min fo r camptothecin and CMC, respectively, DMPC- or DMPG-bound drugs were f ound to be stable even for periods up to 72 h. Iodide quenching data i ndicate that membrane-bound camptothecin intercalates between the lipi d acyl chains, in a protected environment well removed from the aqueou s interface. In this manner lipid bilayer interactions stabilize the l actone ring structure of camptothecins and thereby conserve the biolog ically active form of each medication.