(586e) Drug-Loaded Core-Shell Microcapsules with Solid and Porous Silica Corks for Ultrasound-Triggered Drug Delivery

Background: Drug delivery systems typically exhibit a release profile that either remains constant or decays over time. In addition, non-specific distribution of drug within the body increases systemic side effects, ultimately lowering the allowable dose and thus reducing efficacy [1]. The key challenge here is the lack of control over the drug after it is administered. On the other hand, frequent repeated administrations of small doses place an increased painful burden on the patient. To tackle these shortcomings, novel “smart” drug delivery systems are being developed to prevent non-specific release in circulation until drug release is triggered at a local disease site [1]. Specifically, there is a strong motivation to achieve pulsatile release which can be triggered to an “on” state in the presence of a stimulus and fall into an “off” state in the absence of a stimulus.

There are two separate routes to trigger drug release. The first approach is to take advantage of endogenous stimuli related to unique microenvironments created by different diseases. For example, in cases of rheumatoid arthritis, there is a localized temperature increase due to innate inflammatory responses [2]. The second approach relies on exogenous stimuli that are applied externally, including magnetic fields, light, heat, or X-rays. External stimuli enable drug release to be triggered at any site, even if no physiological gradient exists [2].

Ultrasound offers advantages over other exogenous stimuli. Since ultrasound activation is primarily mechanically driven, drug release is typically not heavily reliant on the chemical properties of the material. However, the speed at which ultrasound waves travel through a material is dependent on its compressibility and density [3]. In addition, ultrasound is widely used in the medical field and is generally regarded as safe because it does not involve ionizing radiation such as X-rays [3].

Herein, we describe the fabrication of a drug loaded core-shell microcapsule with silica “corks” embedded in a poly (D,L-lactide-co-glycolic acid) (PLGA) shell [5]. The drastic difference in mechanical properties of the polymer and the silica results in the corks being “popped-off” upon the application of ultrasound to allow for drug release from the core; no drug is released until ultrasound is applied. The microcapsules thus offer potential to achieve pulsatile, on-demand drug release with “on” and “off” states rather than a one-time burst release. However, previous iterations of such devices with solid corks indicated that the corks pop out to leave a permanent pore in the capsule. It would be ideal for the cork to vibrate in place, creating space for the drug to escape during the application of ultrasound, and then fall back into place to cease drug release once ultrasound is removed. It was hypothesized that introducing pores into the silica cork would dampen the effect of the ultrasound wave, with the air or water that fills the pores serving to attenuate the ultrasound wave [3]. This could prevent the corks from leaving the shell entirely and instead provide solely enough energy to vibrate in place.

Methods: Non-porous silica microbeads, 3-10 μm diameter, were purchased from Corpuscular Inc (Cold Springs, NY) and were subsequently aminated by dropwise addition of (3-aminopropyl) triethoxysilane (APTES). Porous silica particles with a diameter of 5 μm, pore diameter of 100Å, and surface functionalised with a primary amine (APTES) were purchased from Glantreo (Cork, Ireland). Both cork materials were dispersed at 1 w/w% in a 5 w/v% PLGA solution in dichloromethane (DCM). A 2.5w/v% aqueous bovine serum albumin (BSA) solution was used as the model therapeutic encapsulated inside the microcapsule.

An immersion coaxial electrospraying set-up was used to fabricate the microcapsules. The set-up consisted of (a) a double syringe pump with syringe 1 containing the BSA solution and syringe 2 containing the PLGA-cork solution, (b) a high voltage generator set to 18 V, (c) a coaxial needle (inner needle = core, outer needle = shell) immersed in an aqueous collection bath, and (d) a grounded collection plate. Flow rates of 0.3 mL/h for the inner core and 3 mL/h for the shell phase were used.

Visualization of the microcapsules carried out via confocal microscopy (Nikon Eclipse ME600 in combination with a Nikon D-Eclipse C1). The PLGA shell phase was doped with hydrophobic Rhodamine B (magenta), while fluorescein isothiocyanate (FITC) was grafted to residual amine groups on the microcorks (green). Microcapsule shells were imaged using a 543 nm wavelength laser and a 590 nm wavelength detector, while the corks were imaged using a 488 nm wavelength laser and a 515 nm wavelength detector. For subsequent release studies, BSA was doped with FITC to use quantify drug release.

Drug release studies were conducted by suspending microcapsules in a cell strainer and immersing the strainer in a 6-well plate filled with MIQ water. The released BSA escapes the cell strainer into the well plate for collection while the microcapsules remain within the strainer. A polyacrylamide gel tissue mimic was placed on top of the cell strainer to better reproduce the speed at which ultrasound waves travel through human tissue. A short-term drug release study was carried out on the microcapsules with solid silica microcorks in which an ultrasound pulse was applied for 15 minutes every hour over the course of one day. Samples were obtained from the release bath every 30 minutes. A long-term drug release study was carried out using the microcapsules with porous silica microcorks. In this study, a 20-minute ultrasound pulse was applied every 24 hours over an entire week. An Olympus 3.5 MHz immersion ultrasound probe connected to an Olympus 5077PR pulse generator was used for each ultrasound pulse.

Results: Immersion coaxial electrospraying resulted in the successful formation of drug loaded core-shell microcapsules with silica corks embedded in the shell. The average diameter of the porous silica microcapsules was 125 ± 5 μm (n=12), while the average diameter of the solid silica microcapsules was 140 ± 25 μm (n=12). Representative images of the microcapsules are provided in the attached figure.

The short-term BSA release study on the solid silica microcapsules confirmed drug release increases upon ultrasound pulse, as noted by the increase in slope on the cumulative drug release graph after each ultrasound pulse (indicated by red arrows, seen in Figure 1a). However, drug release also continues between ultrasound pulses. This type of release profile is expected if the solid silica corks popped completely out of the shell due to the ultrasound pulse, allowing drug to exit from the core continuously after the first ultrasound. In contrast, the release profile of the porous silica microcorks is shown in Figure 1b. Significantly less drug is released at each timepoint tested, despite the potential for slow diffusion-driven drug release through the porous silica corks even without ultrasound activation. Furthermore, significant pulse-like drug delivery was observed after every other ultrasound pulse applied, i.e. every 48 hours. Together, these results suggest that the porous silica corks can largely remain in the microcapsules, with the trapped air or water in the pores sufficiently dampening the ultrasound wave such that the porous corks do not permanently leave the shell and drug release can be retarded between ultrasound pulses.

Significance: On-demand drug release using ultrasound is typically achieved by ultrasound waves triggering micro/nanoparticle destruction resulting in release of encapsulated drug cargo. However, while controlling the location and time of drug release overcomes several prominent obstacles of drug delivery, no subsequent control over drug release can be achieved after the trigger is applied. In this work, we designed a system in which a cork can pop out of the shell and/or be vibrated within a shell but the shell itself is not destroyed, offering potential for introducing “on-off” intervals to the release kinetics. This eliminates the need for repeated injections, substantially improving patients’ quality of life. Additionally, ultrasound is pain-free, non-invasive, and safe to surrounding tissues. Using porous silica corks offers potential to expand the therapeutic capabilities of this system by enabling dual-drug delivery from the core and the pores of the cork.

References:

[1] D. Liu, F. Yang, F. Xiong, and N. Gu, “The smart drug delivery system and its clinical potential,” Theranostics, vol. 6, no. 9, pp. 1306–1323, 2016, doi: 10.7150/thno.14858.

[2] X. Li, E. C. Montague, A. Pollinzi, A. Lofts, and T. Hoare, “Design of Smart Size-, Surface-, and Shape-Switching Nanoparticles to Improve Therapeutic Efficacy,” Small, vol. 18, no. 6, pp. 1–29, 2022, doi: 10.1002/smll.202104632.

[3] “Ultrasound,” Lecture. Washington University, Biomedical Engineering. 2016.

[4] T. Boissenot, A. Bordat, E. Fattal, and N. Tsapis, “Ultrasound-triggered drug delivery for cancer treatment using drug delivery systems: From theoretical considerations to practical applications,” J. Control. Release, vol. 241, pp. 144–163, 2016, doi:

[5] T. Hoare, J. Dorogin, D. Koff, and A. Singh, “Stimulus Activated Cork-Shell Capsules,” U.S. Patent Application 17/320,446, May 14, 2021.