Determination of intraliposomal pH and its effect on membrane partitioning and passive loading of a hydrophobic camptothecin, DB-67.
The purpose of this work was to study the effect of pH on the liposomal encapsulation of a model camptothecin anti-tumor agent, DB-67, by considering the state of ionization and bilayer membrane/water partitioning of the drug as a function of pH. A novel fluorescence method was developed to monitor intravesicular pH in liposomal formulations containing entrapped DB-67 by using the drug itself as a pH indicator. Fluorescence spectra were recorded in aqueous buffers and liposomes and used to estimate the ionization constant of the A-ring phenol of DB-67 (pKappa(a2)) and shifts in ionization constants ( pKappa (a1) and pKappa(a2) ) due to membrane binding. Bilayer/water partitioning studies by equilibrium dialysis were employed to show that DB-67 is highly membrane bound over the entire pH range examined though binding decreases with an increase in pH. The observed ionization constants of membrane-bound DB-67 obtained from the equilibrium dialysis experiments were consistent with observations from fluorescence measurements and previous permeability results. The pH dependence of DB-67 loading using a passive loading technique was found to reflect the pH dependence of membrane binding of the drug. This results in poor encapsulation efficiency of DB-67 at high pH, necessitating further development of formulation strategies to improve loading efficiency.
Department of Pharmaceutical Sciences, College of Pharmacy, University of Kentucky, Lexington, KY 40506, United States.
Vijay
V
Joguparthi
Shaoxin
S
Feng
Bradley D
BD
Anderson
D000972:Antineoplastic Agents, Phytogenic / Q000737:chemistry*; D002166:Camptothecin / Q000031:analogs & derivatives* / Q000737:chemistry; D002626:Chemistry, Pharmaceutical; D004339:Drug Compounding; D006863:Hydrogen-Ion Concentration; D057927:Hydrophobic and Hydrophilic Interactions; D008051:Lipid Bilayers / Q000737:chemistry*; D008081:Liposomes; D008956:Models, Chemical; D017646:Organosilicon Compounds / Q000737:chemistry*; D010539:Permeability; D015203:Reproducibility of Results; D012995:Solubility; D013050:Spectrometry, Fluorescence; D013678:Technology, Pharmaceutical / Q000379:methods; D013997:Time Factors; D014867:Water / Q000737:chemistry
D000972:Antineoplastic Agents, Phytogenic; D008051:Lipid Bilayers; D008081:Liposomes; D017646:Organosilicon Compounds; D014867:Water; C415575:7-tert-butyldimethylsilyl-10-hydroxycamptothecin; D002166:Camptothecin