title: Synthesis, acoustic stability, and pharmacologic activities of papaverine-loaded echogenic liposomes for ultrasound controlled drug delivery.
abstract: BACKGROUND
  development of encapsulated therapeutics that could be released upon ultrasound exposure has strong implications for enhancing drug effects at the target site. We have developed echogenic liposomes (ELIP) suitable for ultrasound imaging of blood flow and ultrasound-mediated intravascular drug release. Papaverine was chosen as the test drug because its clinical application requires high concentration in the target vascular bed but low concentration in the systemic circulation.
  
  
  METHODS
  the procedure for preparation of standard ELIP was modified by including Papaverine hydrochloride in the lipid hydration solution, followed by three freeze-thaw cycles to increase encapsulation of the drug. Sizing and encapsulation pharmacokinetics were performed using a Coulter counter and a phosphodiesterase activity assay. Stability of Papaverine-loaded ELIP (PELIP) was monitored with a clinical diagnostic ultrasound scanner equipped with a linear array transducer at a center frequency of 4.5 MHz by assessing the mean digital intensity within a region of interest over time. The stability of PELIP was compared to those of standard ELIP and Optison.
  
  
  RESULTS
  relative to standard ELIP, PELIP were larger (median diameter = 1.88 +/- 0.10 microm for PELIP vs 1.08 +/- 0.15 microm for ELIP) and had lower Mean Gray Scale Values (MGSV) (92 +/- 24.8 for PELIP compared to 142.3 +/- 10.7 for ELIP at lipid concentrations of 50 microg/ml). The maximum loading efficiency and mean encapsulated concentration were 24% +/- 7% and 2.1 +/- 0.7 mg/ml, respectively. Papaverine retained its phosphodiesterase inhibitory activity when associated with PELIP. Furthermore, a fraction of this activity remained latent until released by dissolution of liposomal membranes with detergent. The stability of both PELIP and standard ELIP were similar, but both are greater than that of Optison.
  
  
  CONCLUSIONS
  our results suggest that PELIP have desirable physical, biochemical, biological, and acoustic characteristics for potential in vivo administration and ultrasound-controlled drug delivery.
authors: Division of Cardiology, Department of Internal Medicine, The University of Texas Health Science Center at Houston, Houston, Texas 77030, USA. patrick.kee@uth.tmc.edu
authors: Patrick H
authors: PH
authors: Kee
authors: Todd A
authors: TA
authors: Abruzzo
authors: Denise A B
authors: DA
authors: Smith
authors: Jonathan A
authors: JA
authors: Kopechek
authors: Boyu
authors: B
authors: Wang
authors: Shaoling L
authors: SL
authors: Huang
authors: Robert C
authors: RC
authors: MacDonald
authors: Christy K
authors: CK
authors: Holland
authors: David D
authors: DD
authors: McPherson
mesh_terms: D000162:Acoustics; D000818:Animals; D002417:Cattle; D004339:Drug Compounding; D016503:Drug Delivery Systems; D008081:Liposomes / Q000737:chemistry / Q000378:metabolism; D010208:Papaverine / Q000737:chemistry / Q000378:metabolism; D010316:Particle Size; D010726:Phosphodiesterase Inhibitors / Q000737:chemistry / Q000378:metabolism; D010727:Phosphoric Diester Hydrolases / Q000378:metabolism; D014465:Ultrasonics
chemical_list: D008081:Liposomes; D010726:Phosphodiesterase Inhibitors; D010208:Papaverine; D010727:Phosphoric Diester Hydrolases