title: Thermosensitive liposomes: extravasation and release of contents in tumor microvascular networks.
abstract: OBJECTIVE
  The purpose of this study was to determine whether hyperthermic exposure would accelerate drug release from thermosensitive sterically stabilized liposomes and enhance their extravasation in tumor tissues.
  
  
  METHODS
  In vivo fluorescence video microscopy was used to measure the extravasation of liposomes, as well as release of their contents, in a rat skin flap window chamber containing a vascularized mammary adenocarcinoma under defined thermal conditions (34 degrees, 42 degrees, and 45 degrees C). Images of tissue areas containing multiple blood vessels were recorded via a SIT camera immediately before, and for up to 2 h after i.v. injection of two liposome populations with identical lipid composition: one liposome preparation was surface labeled with Rhodamine-PE (Rh-PE) and the other contained either Doxorubicin (Dox) or calcein at self-quenching concentrations. The light intensity of the entire tissue area was measured at 34 degrees C (the physiological temperature of the skin) for 1 h, and at 42 degrees or 45 degrees C for a second hour. These measurements were then used to calculate the fluorescent light intensity arising from each tracer (liposome surface label and the released contents) inside the vessel and in the interstitial region.
  
  
  RESULTS
  The calculated intensity of Rh-PE for the thermosensitive liposomes in the interstitial space (which represents the amount of extravasated liposomes) was low during the first hour, while temperature was maintained at 34 degrees C and increased to 47 times its level before heating, when the tumor was heated at 42 degrees or 45 degrees C for 1 h. The calculated intensity of the liposome contents (Dox) in the interstitial space was negligible at 34 degrees C, and increased by 38- and 76-fold, when the tumor was heated at 42 degrees and 45 degrees C for 1 h, respectively. Similar values were obtained when calcein was encapsulated in liposomes instead of Dox. A similar increase in liposome extravasation was seen with nonthermosensitive liposomes, but negligible release of Dox occurred when the window chamber was heated to 45 degrees C for 1 h. Extravasation of liposomes continued after heating was stopped, but content release stopped after removal of heat. Release of Dox from extravasated liposomes was also seen if heating was applied 24 h after liposome administration, but no further enhancement of liposome extravasation occurred in this case.
  
  
  CONCLUSIONS
  Our data suggest that hyperthermia can be used to selectively enhance both the delivery and the rate of release of drugs from thermosensitive liposomes to targeted tissues.
authors.affiliation: Cancer Research Institute and Department of Pharmacology, University of California SF, USA.
authors.forename: M H
authors.initials: MH
authors.lastname: Gaber
authors.forename: N Z
authors.initials: NZ
authors.lastname: Wu
authors.forename: K
authors.initials: K
authors.lastname: Hong
authors.forename: S K
authors.initials: SK
authors.lastname: Huang
authors.forename: M W
authors.initials: MW
authors.lastname: Dewhirst
authors.forename: D
authors.initials: D
authors.lastname: Papahadjopoulos
mesh_terms: D000818:Animals; D000903:Antibiotics, Antineoplastic / Q000493:pharmacokinetics*; D004317:Doxorubicin / Q000493:pharmacokinetics*; D004337:Drug Carriers; D005260:Female; D006358:Hot Temperature; D008081:Liposomes; D009374:Neoplasms, Experimental / Q000098:blood supply / Q000188:drug therapy*; D051381:Rats; D011916:Rats, Inbred F344
chemical_list: D000903:Antibiotics, Antineoplastic; D004337:Drug Carriers; D008081:Liposomes; D004317:Doxorubicin