Molecular Dynamics Simulations and Experimental Studies of Binding and Mobility of 7-tert-Butyldimethylsilyl-10-hydroxycamptothecin and Its 20(S)-4-Aminobutyrate Ester in DMPC Membranes

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• 作者：Tian-Xiang Xiang ; Zhi-Qiang Jiang ; Lin Song ; Bradley D. Anderson
• 刊名：Molecular Pharmaceutics
• 出版年：2006
• 出版时间：October 2006
• 年：2006
• 卷：3
• 期：5
• 页码：589 - 600
• 全文大小：618K
• 年卷期：v.3,no.5(October 2006)
• ISSN：1543-8392

文摘

The enhanced permeability and retention of liposomes in solid tumors makesliposomal formulations attractive for the targeting of various antitumor agents. This study exploresthe binding, orientation, and dynamic properties of a potent topoisomerase I inhibitor, 7-tert-butyldimethylsilyl-10-hydroxycamptothecin (DB-67), and its 20(S)-4-aminobutyrate ester prodrug(DB-67-AB) in DMPC liposomes by molecular dynamics (MD) simulations and experimentalstudies. MD simulations of an all-atom and fully hydrated liquid-crystalline bilayer (2 × 36 DMPClipids) containing single molecules of DB-67 and DB-67-AB were conducted for up to 50 ns.Membrane/water partition coefficients for DB-67 and DB-67-AB vs pH were determined byultracentrifugation. Fluorescence spectra and/or steady-state anisotropies were measured invarious solvents and in DMPC liposomes. Kinetics for the reversible DB-67 lactone ring-openingin the presence and absence of DMPC liposomes were determined by HPLC with fluorescencedetection. During the entire simulation time both DB-67 and DB-67-AB were located on thebilayer membrane near the polar ester groups of DMPC. The average depth of penetration forDB-67 and DB-67-AB was similar (12.4-13.2 Å) with the prodrug's protonated amino groupstrongly solvated by surface water and lipid phosphate groups. Binding and fluorescenceexperiments revealed only a modest reduction in the binding affinity upon attachment of theionized 4-aminobutyrate group onto DB-67. The binding microenvironment polarity resemblesthat of a polar solvent such as EtOH and DMSO. Kinetics experiments confirmed that DB-67lactone hydrolysis is inhibited in the presence of DMPC liposomes, consistent with the reducedexposure of its lactone ring to water, as observed in the simulations. Both bound DB-67 andbound DB-67-AB have nonrandom orientations and reduced mobility in the membrane, especiallyfor diffusion normal to the bilayer surface, and rotational relaxation, both of which are 2 ordersof magnitude slower than in bulk water. MD simulations correctly predicted the high bindingaffinities for DB-67-AB to DMPC bilayers, protection of bound DB-67 toward lactone hydrolysis,and the lack of a substantial reduction in binding for the 20(S)-4-aminobutyrate prodrug of DB-67.