title: Copper-topotecan complexation mediates drug accumulation into liposomes.
abstract: These studies describe the role of transition metal ions in the liposomal encapsulation of topotecan. Liposomes (1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) and cholesterol (CH) (55:45, mole ratio)) were prepared with manganese (Mn), copper (Cu), zinc (Zn) or cobalt (Co) ion gradients (metal inside). Subsequently, topotecan was added to the liposome exterior (final drug-to-lipid ratio (mol/mol) of 0.2) and drug encapsulation was measured as a function of time and temperature. No drug loading was achieved with liposomes containing Co or Zn. Topotecan could be encapsulated into Mn-containing liposomes only in the presence of the ionophore, A23187 suggesting that a transmembrane pH gradient was necessary. However, Cu-containing liposomes, in the presence or absence of an imposed pH gradient, efficiently encapsulated topotecan. It has been reported that Cu(II) can form transition metal complexes with camptothecin; therefore, the Cu-topotecan interaction was characterized in solution as a function of pH. These investigations demonstrated that topotecan inhibited formation of an insoluble Cu hydroxide precipitate. Cryo-TEM analysis of the topotecan-loaded Cu liposomes showed electron-dense intravesicular precipitates. Further studies demonstrated that only the active lactone form of the drug was encapsulated and this form predominated in Cu-containing liposomes. Copper complexation reactions define a viable methodology to prepare liposomal camptothecin formulations.
authors.affiliation: Department of Advanced Therapeutics, British Columbia Cancer Agency, 675 West 10th Ave., Vancouver, BC, Canada V5Z 1L3.
authors.forename: Amandeep S
authors.initials: AS
authors.lastname: Taggar
authors.forename: Jehan
authors.initials: J
authors.lastname: Alnajim
authors.forename: Malathi
authors.initials: M
authors.lastname: Anantha
authors.forename: Anitha
authors.initials: A
authors.lastname: Thomas
authors.forename: Murray
authors.initials: M
authors.lastname: Webb
authors.forename: Euan
authors.initials: E
authors.lastname: Ramsay
authors.forename: Marcel B
authors.initials: MB
authors.lastname: Bally
mesh_terms: D002021:Buffers; D000001:Calcimycin / Q000737:chemistry; D002413:Cations, Divalent / Q000737:chemistry; D011232:Chemical Precipitation; D002784:Cholesterol / Q000737:chemistry; D003300:Copper / Q000737:chemistry*; D020285:Cryoelectron Microscopy; D004317:Doxorubicin / Q000737:chemistry; D004339:Drug Compounding / Q000379:methods; D006863:Hydrogen-Ion Concentration; D007783:Lactones / Q000737:chemistry; D008081:Liposomes / Q000737:chemistry*; D017895:Manganese Compounds / Q000737:chemistry; D015394:Molecular Structure; D009550:Nigericin / Q000737:chemistry; D010713:Phosphatidylcholines / Q000737:chemistry; D018892:Proton-Motive Force; D013431:Sulfates / Q000737:chemistry; D019772:Topotecan / Q000737:chemistry*
chemical_list: D002021:Buffers; D002413:Cations, Divalent; D007783:Lactones; D008081:Liposomes; D017895:Manganese Compounds; D010713:Phosphatidylcholines; D013431:Sulfates; D000001:Calcimycin; D003300:Copper; D019772:Topotecan; D004317:Doxorubicin; D002784:Cholesterol; C010942:1,2-distearoyllecithin; D009550:Nigericin; C039798:manganese sulfate