d Original Contribution EFFECT OF ULTRASOUND PARAMETERS ON THE RELEASE OF LIPOSOMAL CALCEIN MERCY AFADZI,* y CATHARINA DE L. DAVIES,* YNGVE H. HANSEN,* TONNI JOHANSEN, y ØYVIND K. STANDAL, y RUNE HANSEN, yz SVEIN-ERIK M  ASØY , y ESBEN A. NILSSEN, x and BJØRN ANGELSEN y * Department of Physics; y Department of Circulation and Medical Imaging, The Norwegian University of Science and Technology, Trondheim, Norway; z Department of Medical Technology SINTEF Technology and Society, Trondheim, Norway; and x Epitarget AS, Oslo, Norway (Received 1 July 2011; revised 17 November 2011; in final form 27 November 2011) Abstract—The ultrasound exposure parameters that maximize drug release from dierucoyl-phosphatidylcholine (DEPC)-based liposomes were studied using two transducers operating at 300 kHz and 1 MHz. Fluorescent calcein was used as a model drug, and the release from liposomes in solution was measured using a spectrophotometer. The release of calcein was more efficient at 300 kHz than at 1 MHz, with thresholds of peak negative pressures of 0.9 MPa and 1.9 MPa, respectively. Above this threshold, the release increased with increasing peak negative pressure, mechanical index (MI), and duty cycle. The amount of drug released followed first-order kinetics and increased with exposure time to a maximal release. To increase the release further, the MI had to be increased. The results demonstrate that the MI and the overall exposure time are the major parameters that determine the drug’s release. The drug’s release is probably due to mechanical (cavitation) rather than thermal effects, and that was also confirmed by the detection of hydroxide radicals. (E-mail: mercy.afadzi@ntnu.no) Ó 2012 World Federation for Ultrasound in Medicine & Biology. Key Words: Ultrasound parameters, Mechanical index, Drug release, Liposomes, Cavitation. INTRODUCTION The main limitation associated with conventional chemo- therapy is the poor therapeutic index caused by the high level of toxicity in healthy tissues (Drummond et al. 1999). Successful cancer therapy requires that cytotoxic drugs reach the tumor cells and inactivate them with minimal damage to normal tissue. To reduce the exposure of normal tissue, cytotoxic drugs should be selectively delivered to tumor tissue. This may be achieved by encap- sulating the drug in a particulate carrier, such as a lipo- some, micelle, or other nanoparticle (Allen 1997; Barenholz 2001, 2007; Torchilin 2005; Liu et al. 2006). For an effective therapeutic effect, the carrier should remain stable in the circulatory system with an adequate amount of drug and then release the drug at a sufficient rate once the nanoparticle is at the tumor site (Huang and McDonald 2004). Because of the hyper- permeable, fenestrated nature of tumor vessels (Yuan et al. 1994; Bae 2009), nanoparticles with diameters of approximately 100 nm are typically able to cross the capillary wall and accumulate in the tumor interstitium. However, the distribution of the nanoparticles and the drug is heterogeneous within the tumor tissue (Vaage et al. 1997; Davies et al. 2004; Bae 2009). Large areas of the tumor are not reached by the drug because of the heterogeneous fenestration of the tumor blood vessels and poor penetration through the extracellular matrix. There is also a challenge in controlling the localiza- tion and drug-release kinetics of intravenously injected nanoparticles so as to obtain sufficient drug concentrations at the target site. Triggered mechanisms, both chemical and physical, for controlling the release of the drug may be used to overcome this challenge. Ultrasound has been shown to improve both the release and the biodistribution of the drug (Huang and McDonald 2004; Rapoport 2007; Steinberg et al. 2007; Schroeder et al. 2007, 2009;). Ultrasound is of special interest because it is noninvasive, can be controlled both spatially and Address correspondence to: Mercy Afadzi, M.Sc., Department of Physics, The Norwegian University of Science and Technology, Høgskoleringen 5, 7491 Trondheim, Norway. E-mail: mercy.afadzi@ ntnu.no 476 Ultrasound in Med. & Biol., Vol. 38, No. 3, pp. 476–486, 2012 Copyright Ó 2012 World Federation for Ultrasound in Medicine & Biology Printed in the USA. All rights reserved 0301-5629/$ - see front matter doi:10.1016/j.ultrasmedbio.2011.11.017