Transition metal-mediated liposomal encapsulation of irinotecan (CPT-11) stabilizes the drug in the therapeutically active lactone conformation.
OBJECTIVE
To determine whether entrapped transition metals could mediate the active encapsulation of the anticancer drug irinotecan into preformed liposomes. Further, to establish that metal complexation could stabilize liposomal irinotecan in the therapeutically active lactone conformation.


METHODS
Irinotecan was added to preformed 1,2-distearoyl-sn-glycero-phosphocholine/cholesterol (DSPC/chol) liposomes prepared in CuSO4, ZnSO4, MnSO4, or CoSO4 solutions, and drug encapsulation was determined over time. The roles of the transmembrane pH gradient and internal pH were evaluated. TLC and HPLC were used to monitor drug stability and liposome morphology was assessed by cryo-TEM.


RESULTS
Irinotecan was rapidly and efficiently loaded into preformed liposomes prepared in unbuffered (approximately pH 3.5) 300 mM CuSO4 or ZnSO4. For Cu-containing liposomes, results suggested that irinotecan loading occurred when the interior pH and the exterior pH were matched; however, addition of nigericin to collapse any residual transmembrane pH gradient inhibited irinotecan loading. Greater than 90% of the encapsulated drug was in its active lactone form and cryo-TEM analysis indicated dark intravesicular electron-dense spots.


CONCLUSIONS
Irinotecan is stably entrapped in the active lactone conformation within preformed copper-containing liposomes as a result of metal-drug complexation.
Ramsay
Euan
E
Department of Advanced Therapeutics, BC Cancer Agency, 675 West 10th Avenue, Vancouver, BC, Canada, V5Z 1L3. eramsay@bccrc.ca
Alnajim
Jehan
J
Anantha
Malathi
M
Taggar
Aman
A
Thomas
Anitha
A
Edwards
Katarina
K
Karlsson
Göran
G
Webb
Murray
M
Bally
Marcel
M
D000972:Antineoplastic Agents, Phytogenic / Q000008:administration & dosage* / Q000737:chemistry*; D002166:Camptothecin / Q000008:administration & dosage / Q000031:analogs & derivatives* / Q000737:chemistry; D002264:Carboxylic Acids / Q000737:chemistry; D002851:Chromatography, High Pressure Liquid; D002855:Chromatography, Thin Layer; D003300:Copper / Q000737:chemistry; D020285:Cryoelectron Microscopy; D004337:Drug Carriers; D004339:Drug Compounding; D006863:Hydrogen-Ion Concentration; D000077146:Irinotecan; D007783:Lactones; D008081:Liposomes; D028561:Transition Elements / Q000737:chemistry*
D000972:Antineoplastic Agents, Phytogenic; D002264:Carboxylic Acids; D004337:Drug Carriers; D007783:Lactones; D008081:Liposomes; D028561:Transition Elements; D000077146:Irinotecan; D003300:Copper; D002166:Camptothecin