Ceciel Chlon

Oil-filled polymer microcapsules for ultrasound-mediated delivery of lipophilic drugs

The use of ultrasound contrast agents as local drug delivery systems continues to grow. Current limitations are the amount of drug that can be incorporated as well as the efficiency of drug release upon insonification. This study focuses on the synthesis and characterisation of novel polymeric microcapsules for ultrasound-triggered delivery of lipophilic drugs. Microcapsules with a shell of fluorinated end-capped poly(L-lactic acid) were made through pre-mix membrane emulsification and contained, apart from a gaseous phase, different amounts of hexadecane oil as a drug-carrier reservoir. Mean number weighted diameters were between 1.22 microm and 1.31 microm. High-speed imaging at approximately 10 million fames per second showed that for low acoustic pressures (1 MHz, 0.24 MPa) microcapsules compressed but remained intact. At higher diagnostic pressures of 0.51 MPa, microcapsules cracked, thereby releasing the encapsulated gas and model lipophilic drug. Using conventional ultrasound B-mode imaging at a frequency of 2.5 MHz, a marked enhancement of scatter intensity over a tissue-mimicking phantom was observed for all differently loaded microcapsules. The partially oil-filled microcapsules with high drug loads and well-defined acoustic activation thresholds have great potential for ultrasound-triggered local delivery of lipophilic drugs under ultrasound image-guidance.

Focused ultrasound and microbubbles for enhanced extravasation

The permeability of blood vessels for albumin can be altered by using ultrasound and polymer or lipid-shelled microbubbles. The region in which the microbubbles were destroyed with focused ultrasound was quantified in gel phantoms as a function of pressure, number of cycles and type of microbubble. At 2MPa the destruction took place in a fairly wide area for a lipid-shelled agent, while for polymer-shelled agents at this setting, distinct destruction spots with a radius of only 1mm were obtained. When microbubbles with a thicker shell were used, the pressure above which the bubbles were destroyed shifts to higher values. In vivo both lipid and polymer microbubbles increased the extravasation of the albumin binding dye Evans Blue, especially in muscle leading to about 6-8% of the injected dose to extravasate per gram muscle tissue 30 min after start of the treatment, while no Evans Blue could be detected in muscle in the absence of microbubbles. Variation in the time between ultrasound treatment and Evans Blue injection, demonstrated that the time window for promoting extravasation is at least an hour at the settings used. In MC38 tumors, extravasation already occurred without ultrasound and only a trend towards enhancement with about a factor of 2 could be established with a maximum percentage injected dose per gram of 3%. Ultrasound mediated microbubble destruction especially enhances the extravasation in the highly vascularized outer part of the MC38 tumor and adjacent muscle and would, therefore, be most useful for release of, for instance, anti-angiogenic drugs.

Effect of molecular weight, crystallinity, and hydrophobicity on the acoustic activation of polymer-shelled ultrasound contrast agents.

Polymer-shelled microbubbles are applied as ultrasound contrast agents. To investigate the effect of the polymer on microbubble preparation and acoustic properties, polylactides with systematic variations in molecular weight, crystallinity, and end-group hydrophobicity were used. Polymer-shelled cyclodecane filled capsules were prepared by emulsification, and the cyclodecane was removed by lyophilization to obtain hollow capsules. Complete removal of cyclodecane from the microcapsules was only achieved for short chain (about M(w) 6000) crystalline polymers. The pressure threshold for acoustic destruction of the microbubbles was found to increase with molecular weight. Noncrystalline polymers showed a higher threshold for destruction than crystalline polymers. Hydrophobically modified short chain crystalline polymers showed the steepest increase in acoustic destruction after the threshold as a function of the applied pressure, which is a favorable characteristic for ultrasound mediated drug delivery. Microcapsules made with such polymers had an inhomogeneous surface including pores through which cyclodecane was lyophilized efficiently.

Ultrafast vapourization dynamics of laser-activated polymeric microcapsules

Precision control of vapourization, both in space and time, has many potential applications; however, the physical mechanisms underlying controlled boiling are not well understood. The reason is the combined microscopic length scales and ultrashort timescales associated with the initiation and subsequent dynamical behaviour of the vapour bubbles formed. Here we study the nanoseconds vapour bubble dynamics of laser-heated single oil-filled microcapsules using coupled optical and acoustic detection. Pulsed laser excitation leads to vapour formation and collapse, and a simple physical model captures the observed radial dynamics and resulting acoustic pressures. Continuous wave laser excitation leads to a sequence of vapourization/condensation cycles, the result of absorbing microcapsule fragments moving in and out of the laser beam. A model incorporating thermal diffusion from the capsule shell into the oil core and surrounding water reveals the mechanisms behind the onset of vapourization. Excellent agreement is observed between the modelled dynamics and experiment.

9B-5 Ultrasound Therapy with Drug Loaded Microcapsules

A novel class of acoustically active microcapsules that partially contain drug-bearing oil with gas offers great potential for localized drug release. The acoustical activation threshold and rate of such microcapsules with different oil-to-gas ratios were measured in vitro. Drug loaded microcapsules were prepared from which the anti-cancer drug paclitaxel was delivered in murine tumors. For recovered mice each with two tumors on the left and right hind legs, the growth rate of the acoustically treated tumors was substantially reduced in comparison to that of the untreated tumor on the same mouse. These initial results showed that lipophilic drugs can be packaged in microcapsules and released locally using focused ultrasound.

Influence of shell properties on high-frequency ultrasound imaging and drug delivery using polymer-shelled microbubbles

This two-part study investigated shell rupture of ultrasound contrast agents (UCAs) under static overpressure conditions and the subharmonic component from UCAs subjected to 20-MHz tonebursts. Five different polylactide-shelled UCAs with shell-thickness-to-radius ratios (STRRs) of 7.5, 30, 40, 65, and 100 nm/¿m were subjected to static overpressure in a glycerol-filled test chamber. A video microscope imaged the UCAs as pressure varied from 2 to 330 kPa over 90 min. Images were postprocessed to obtain the pressure threshold for rupture and the diameter of individual microbubbles. Backscatter from individual UCAs was investigated by flowing a dilute UCA solution through a wall-less flow phantom placed at the geometric focus of a 20-MHz transducer. UCAs were subjected to 10- and 20-cycle tonebursts of acoustic pressures ranging from 0.3 to 2.3 MPa. A method based on singular-value decomposition (SVD) was employed to obtain a cumulative subharmonic score (SHS). Different UCA types exhibited distinctly different rupture thresholds that were linearly related to their STRR, but uncorrelated with UCA size. The rupture threshold for the UCAs with an STRR = 100 nm/μm was more than 4 times greater than the UCAs with an STRR = 7.5 nm/μm. The polymer-shelled UCAs produced substantial subharmonic response but the subharmonic response to 20- MHz excitation did not correlate with STRRs or UCA-rupture pressures. The 20-cycle excitation resulted in an SHS that was 2 to 3 times that of UCAs excited with 10-cycle tonebursts.

Monodisperse polymeric particles prepared by ink-jet printing: Double emulsions, hydrogels and polymer mixtures

Submerged ink-jetting produces a monodisperse emulsion that can be converted into monodisperse particles. As the initial droplet size is known and the final particle size can be easily measured, such a method can be used to quantify the shrinkage and the swelling of polymer particles made from double emulsions, polymer mixtures and hydrogel forming polymers. It is found that at the same starting concentration and initial emulsion droplet size poly-lactide-co-glycolide particles made from an ink-jetted emulsion have the same size as particles ink-jetted from a solution, however with a more porous structure. The total pore volume, however, is negligible compared to the polymer volume of the particle. If polymers containing a poly-ethylene glycol block are included, particles with internal porosity are formed, even if no double emulsion process is applied. Still the final particle size is the same. Only if typical hydrogel forming polymers are used, in which water is distributed more homogeneously, significantly higher particle diameters are found; for a four-arm PEG-poly-caprolacton a degree of swelling of 3.3 is found.

Oil-filled polymeric ultrasound contrast agent as local drug delivery system for lipophilic drugs

Novel polymeric microcapsules, filled with a mixture of gas and oil, were produced and their potential as ultrasound contrast agent-based drug delivery system for lipophilic drugs was investigated. Microcapsules were synthesized that contained either no oil, were almost half-filled with oil, or were almost completely filled with oil. Mean number weighted diameters were between 1.22 and 1.31 mum. At a low MI (1 MHz, P_ of 0.24 MPa), microcapsules typically compressed without cracking. At a high MI (1 MHz, P_ of 0.51 MPa), microcapsules cracked, thereby releasing their content. Guidance and monitoring of therapy will be possible because the microcapsules were echogenic and stable at low MI. These microcapsules therefore have great potential as local drug delivery system for lipophilic drugs.

Characterization of an innovative drug carrying photoacoustic contrast agent: fluorescent polymer microcapsules

Local drug delivery is studied to cross biological barriers and reduce the systemic side effects. Multimodal agents are being developed to combine step-response activation with the monitoring of its triggered release. The release can be triggered by acoustical or thermal means, e.g. using thermo-sensitive liposomes. Here we design an optically triggered microcarrier with well-controlled release precision and, in addition, a strong acoustic response in the far field, making the carrier a highly specific photoacoustic agent. The novel biocompatible microcapsules with a shell of fluorinated poly-L-lactic acid mixed with a fluorescent dye were produced with hexadecane oil core as drug-carrier reservoir. Single capsules were excited by a pulsed laser and their responses were monitored through combined ultra-high-speed imaging and sensitive acoustic detection. The experiments support a model where the polymer heats up through dye absorption thereby inducing the shell destruction and the vaporization of the surr...

Ultrasound and Microbubble Mediated Doxil Delivery in a Murine Breast Cancer Model: Therapeutic Efficacy Dependence on Tumor Growth Rate

The effect of tumor growth rate and treatment repeats are examined as parameters in pressure-mediated ultrasound treatments with microbubbles and Doxil in a murine breast cancer model. For this purpose, mice with a tumor doubling time of 8 and 13 days respectively received either a single or two ultrasound treatments (at 1 MHz/1 MPa) in conjunction with Definity microbubbles (1:1 dilution) and Doxil (3 mg/Kg dose) after the tumor size reached 150 mm3. The tumor model was generated using MDA-MB-231-luc cells implanted into the lower mammary fat pad of SCID beige mice. At 15 days post-treatment, tumor size was reduced by 3±18%, 8±14%, and 20±10% as compared to control for the Doxil only, ultrasound + microbubbles + Doxil (single treatment), and ultrasound + microbubbles + Doxil (2 treatments) groups, respectively, in the mice with the slower growing tumors. The mice with the faster growing tumor yielded tumor size reductions of 46±27%, 71±10%, and 61±26%, respectively, for the same groups. We hypothesize that treatment efficacy is dependent on the dynamics of the tumor itself, even within the same cell line.