(596b) Controlled Rupture of Drug-Encapsulated Ultrasound Contrast Agents

Ultrasound contrast agents using micron-sized bubbles (less than 8 µm in diameter) have the potential to be drug delivery vehicles in addition to providing targeted molecular imaging.  However, several challenges remain in terms of producing monodisperse microbubbles stable against gas dissolution for improving blood circulation times. In the case of drug delivery, it is crucial to control destruction to release encapsulated contents only to the targeted area with minimal prior passive leakage of drug. By using a photopolymerizable diacetylene lipid as a component in a microbubble shell matrix, the surface properties such as elasticity or resistivity against gas dissolution can be tuned,² which therefore enables imaging at one frequency and drug release in the area of interest at a different frequency. Drug-encapsulated monodisperse bubbles are created using a double emulsion microfluidic technique to increase drug loading capacity.

Polymerizable lipid mixtures, consisting of 10 mol% of ethylene glycol diacetylene lipids (h-PEG₁PCDA), 15 mol% of PEG-diacetylene lipids (m-PEG₅₀₀₀-PCDA) and L-α-phosphatidylcholine, hydro soy PC were used. The microfluidic flow focusing device designs were adapted from Hettiarachchi et al.^3,4 Paclitaxel in triacetin (0.1mg/ml) was used as a test drug of which  1 mol% was fluorescently labeled (Oregon Green 488-paclitaxel).

The size distribution of the polymerized shell microbubbles (PSM) produced by microfluidic flow focusing technique and commercially available microbubbles (Vevo MicroMarker, VMM) was observed to be 2.2 µm with 8% polydispersity, and 2.3 µm with 36% polydispersity, respectively. The PSM showed a significantly slower decrease in intensity of gray-scale ultrasound image brightness than VMM or nonpolymerizable soy PC shell microbubbles (NSM). The half-life (t_H the time for the ultrasound signal to drop 6 dB in brightness) for the 25%DA was t_H = 5 min, and for both VMM and NSM, t_H < 1 min. The brightness of the 25%DA decreased by 14 dB after 15 min, whereas the VMM brightness decreased by 36 dB, suggesting rapid microbubble destruction. Additionally, bubbles polymerized to different extents (t_polym) showed variable destruction rates at different ultrasound power levels. This suggests that polymerization can not only provide passive bubble longevity but also tunable rupture capabilities. The double emulsion microfluidic device produced the paclitaxel encapsulated microbubbles. The FITC-filtered image shows the location of paclitaxel within the shell. These results indicate that the dissolution of microbubbles in the bloodstream or under ultrasound stimulation is tunable by varying the fraction of polymerizable lipid or polymerization time. Therefore, these tunable microbubbles have the potential to be customized ultrasound contrast agents for targeted molecular imaging and therapeutic treatment applications.

References: [1] Park et al. Langmuir 2012, 28, 3766-3772. [2] Hettiarachchi, K.et al.  Lab Chip 2007, 7, 463-8. [3] Hettiarachchi, K.et al.  Biotechnol. Prog. 2009, 25, 938-945.