Medical ultrasound is a highly valuable diagnostic tool, especially when compared with other imaging modalities. It is a noninvasive, real-time, portable, extremely safe method compared with X-ray and inexpensive relative to magnetic resonance imaging. However, ultrasound is limited in its ability to distinguish between diseased and normal tissue. This limitation has led to the development of contrast agents. We have produced novel poly (lactic-co-glycolic) acid air-filled microcapsules that work well as ultrasound contrast agents, giving up to 24 and 25 dB enhancement when insonated in the medical imaging range at 5 and 7.5 MHz, respectively. The capsules were fabricated by modifying a double emulsion method to encapsulate camphor in the oil phase and ammonium carbonate in the aqueous phase, and later sublime the encapsulated material, leaving voids capable of being filled with a gas in their place. The role of the surfactant, poly vinyl alcohol, solution temperature, was studied and found to play an important role in the morphology of the capsules, altering their acoustic response.

A polymer ultrasound contrast agent (UCA) developed in our lab has been shown to greatly reduce in size when exposed to ultrasound, resulting in nanoparticles less than 400 nm in diameter capable of escaping the leaky vasculature of a tumor to provide a sustained release of drug. Previous studies with the hydrophilic drug doxorubicin (DOX) demonstrated enhanced drug delivery to tumors when triggered with ultrasound. However the therapeutic potential has been limited due to the relatively low payload of DOX. This study compares the effects of loading the hydrophobic drug paclitaxel (PTX) on the agent’s acoustic properties, drug payload, tumoricidal activity, and the ability to deliver drugs through 400 nm pores. A maximum payload of 129.46 ± 1.80 μg PTX/mg UCA (encapsulation efficiency 71.92 ± 0.99 %) was achieved, 20 times greater than the maximum payload of DOX (6.2 μg/mg), while maintaining the acoustic properties. In vitro, the tumoricidal activity of paclitaxel loaded UCA exposed to ultrasound was significantly greater than controls not exposed to ultrasound (p<0.0016). This study has shown that PTX loaded UCA triggered with focused ultrasound have the potential to provide a targeted and sustained delivery of drug to tumors.

An echogenic, intravenous drug delivery platform is proposed in which an encapsulated chemotherapeutic can travel to a desired location and drug delivery can be triggered using external, focused ultrasound at the area of interest. Three methods of loading poly lactic acid (PLA) shelled ultrasound contrast agents (UCA) with doxorubicin are presented. Effects on encapsulation efficiency, in vitro enhancement, stability, particle size, morphology and release during UCA rupture are compared by loading method and drug concentration. An agent containing doxorubicin within the shell was selected as an ideal candidate for future hepatocellular carcinoma studies. The agent achieved a maximal drug load of 6.2 mg Dox/g PLA with an encapsulation efficiency of 20.5%, showed a smooth surface morphology and tight size distribution (poly dispersity index = 0.309) with a peak size of 1865 nm. Acoustically, the agent provided 19 dB of enhancement in vitro at a dosage of 10 µg/ml, with a half life of over 15 mins. In vivo, the agent provided ultrasound enhancement of 13.4 ± 1.6 dB within the ascending aorta of New Zealand rabbits at a dose of 0.15 ml/kg. While the drug-incorporated agent is thought to be well suited for future drug delivery experiments, this study has shown that agent properties can be tailored for specific applications based on choice of drug loading method.

The subharmonic emission from insonified contrast microbubbles was used to create a new imaging modality called Subharmonic Imaging. The subharmonic response of contrast microbubbles to ultrasound pulses was first investigated for determining adequate acoustic transmit parameters. Subharmonic A-lines and gray scale images were then obtained using a laboratory pulse-echo system in vitro and a modified ultrasound scanner in vivo. Excellent suppression of all backscattered signals other than from contrast microbubbles was achieved for subharmonic A-lines in vitro while further optimization is required for in vivo gray scale subharmonic images.

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