Smart Microparticles for On-Demand Drug Delivery 

By Abid Neron

Abstract 

Drug delivery systems represent a cornerstone of contemporary medical research, particularly in addressing intractable diseases such as cancer and neurological disorders. Current treatment modalities often result in collateral damage to healthy cells, underscoring the need for targeted delivery methods. Microparticles have emerged as promising candidates for precise drug delivery. This study explores the use of PLGA microparticles, fabricated via double emulsion, for sustained vancomycin hydrochloride release. The microparticles were loaded with Indocyanine Green (ICG) for near-infrared–responsive (NIR) drug delivery. Process parameters were optimized for size, morphology, drug loading, and release kinetics. Anti-bacterial tests showed higher drug release and antibacterial efficiency when exposed to NIR light. Chitosan coating halted drug release in the absence of NIR light, demonstrating controlled, on-demand release. This system offers the potential for targeted and efficient drug delivery. 

Introduction

Drug delivery systems are a crucial component of the drug manufacturing process. An effective drug is rendered useless if it fails to reach its target area. Thus, a drug delivery vehicle must be manufactured to ensure that the drug is properly administered. Traditional drug delivery methods, including oral, topical, and injection, are effective, but all have certain disadvantages, namely, poor bioavailability, instantaneous release of the drug, and lack of site-specific targeting1. These disadvantages can be mediated by using higher concentrations or continuous intaking of the specified drug, which steadily uses more drugs for treatment and can cause damage to other areas. Novel drug delivery systems have been at the forefront of bioengineering research, as finding a more effective vehicle for a drug will increase its effectiveness, thus reducing the amount of the drug needed for treatment and potential side effects. 

Microcarriers are currently spearheading novel drug delivery systems. Most notably, much research is being conducted on microparticles. They are spherical particles that are 1 to 100 μm in diameter. Microparticles have been chosen as a suitable drug delivery vector as they can be tailored to the specific task since they are highly customizable. This degree of customization is due to the different fabrication processes and the numerous materials used to manufacture them. Further, multiple materials can be used in conjunction for a higher degree of complexity when desired. Most importantly among the properties of microparticles is their ability to hold drugs inside of them. An effective drug delivery vector must have numerous properties; among them are drug encapsulation, biodegradability, biocompatibility, sustained release, and implementation of surface modifications. Depending on the fabrication, microparticles can have all these properties. 

Background

The fabrication process is crucial for microparticles as it is the process that gives the particles their properties. Thus, when creating a microparticle for drug delivery, deliberate thought is put into the fabrication process and the desired properties. For instance, a microparticle made with a toxic polymer is rendered useless in the field of drug delivery as it will harm the patient. All microparticles made for drug delivery use a biocompatible polymer such as PLGA, Chitosan, Gelatin, and PCL. Further, surface modifications can be implemented to help increase uptake of the drug by the cells or to target certain cells. Targeted drug delivery increases the effectiveness of the treatment. Lack of site targeting can lead to the drug not working effectively or having undesirable consequences. The most infamous treatment that lacks targeting delivery is Chemotherapy. Chemotherapy targets rapidly dividing cells, a property of cancerous cells. However, it is also a property of hair, skin, and gastrointestinal tract cells. This causes multiple side effects that wouldn’t occur if Chemotherapy only targeted the cancerous cells. 

Microparticles are potential candidates for cancer treatment. They can be modified for targeted drug delivery because cancerous cells can be targeted since they express certain proteins and have special receptors. A biocompatible microparticle fabricated with an effective anti-cancer drug and a specific targeting ligand could revolutionize the way cancer is treated. While cancer treatment is a major application for microparticle-mediated drug delivery, other important applications include treatment for neurological disorders, diabetes, and respiratory and inflammatory diseases.

Poly Lactic-co-Glycolic Acid (PLGA) is the most widely used polymer for microparticle fabrication due to its biocompatibility and FDA approval. PLGA is a co-polymer, meaning it is made using two different monomers: Lactic Acid and Glycolic Acid. The ratio of both these monomers affects the properties of PLGA, giving an extra layer of customization in the fabrication process. Among the properties affected by the ratios is the rate of biodegradation, which increases with the increase of Glycolic Acid. In addition, it supports numerous fabrication methods as well as surface modifications and has good drug encapsulation properties. 

Near-infrared (NIR) light, which includes wavelengths from 800 to 2,500 nm, is invisible to the naked eye. Indocyanine Green (ICG) is a photothermal material and thus will react with certain wavelengths of light and convert light energy into heat energy. This central concept was utilized in fabricating the microparticles for targeted drug release, as the use of NIR light in PLGA-ICG particles should accelerate drug release, providing a vessel for on-demand delivery. To further prevent drug elution, chitosan-coated microparticles were fabricated as well. The chitosan should prevent drugs from eluting except when exposed to NIR light due to the photothermal reaction occurring. 

Methods

Microparticles Fabrication: 

The PLGA-ICG-Van. HCl microparticles were fabricated using the double emulsion method as shown in Figure 1, with the external phase being 1% PVA. 

Figure 1: Schematic for Fabrication of PLGA-ICG-Van.HCl microparticles 

 

Drug Release: 

To test for drug release from the MPs, 10 mg of fabricated PLGA MPs containing ICG and Van.HCl was placed into a 2 ml solution of phosphate-buffered saline (PBS) with a pH of 7.4 inside a Cytivia dialysis kit with a cut-off of 8 kDa. The microparticles were exposed to near-infrared (NIR) light at a wavelength of 808nm. The experiment set up is shown in Figure 2. Sample absorbance values were recorded, and a vancomycin hydrochloride standard curve was used to determine the drug’s concentration. 

Figure 2: Drug Release Set Up 

 

Anti-Bacterial Activity: 

Escherichia coli S17 (E. coli) was cultured in LB broth and incubated overnight at 37°C at 250 RPM for 24 hours. Fresh bacterial cultures were prepared for each antibacterial test using the previously refreshed culture. For the antibacterial test, a new batch of bacteria was cultured overnight until reaching an optical density at 600 nm (OD600) of 0.5, corresponding to approximately 8 × 10^8 cells/ml. In a microcentrifuge tube, 10 mg of microparticles were combined with 1 ml of LB broth and inoculated with 10 µl of the bacterial culture. Each microparticle group was tested in duplicate: one sample was exposed to NIR light at 1 W for 30 minutes, while the other sample was not exposed to NIR light. To ensure consistency, the unexposed sample was kept at room temperature for 30 minutes. Both samples were then incubated at 37°C for 2 hours. After incubation, 100 µl of each sample was spread onto an LB agar plate and incubated at 37°C for 24 hours to allow visualization of bacterial growth where bacterial colonies were counted. A control group prepared using the same protocol but without microparticles, was included for comparison. 

Results 

I: Microparticle Fabrication: 

Four different microparticle concentrations were fabricated: PVI2, PVI4, PVI5, and PVI6. Each variation had a different concentration of ICG. ICG concentrations can be found in Table 1. 225 mg of PLGA and 75 mg were used for the fabrication of all the microparticles. PVI4 was chosen to be coated with chitosan as it was found to have the highest drug release (Figure 3) and the highest antibacterial activity (Figure 4). 

Table 1: ICG Concentrations in Microparticle Variations 

Variation	ICG Concentration (mg)
PVI2	3.875
PVI4	38.75
PVI5	19.375
PVI6	7.75

 

II. Drug Release:

Vancomycin hydrochloride release for each microparticle variation with and without exposure to NIR light can be seen in Figure 3. NIR light-exposed samples consistently have a higher drug release. The PVI4 exposed to NIR light had the highest drug release, followed by the PVI4 sample not exposed to NIR light. 

Figure 3: Vancomycin Hydrochloride Release for Each Microparticle Variation 

 

III. Antibacterial Activity: 

Antibacterial activity, when subjected to microparticles, is utilized as a visualization of the microparticles’ drug release and to further verify the drug release results. It is used as an in vitro test for fabricated microparticles. Antibacterial activity is shown by the number of colonies present on the LB agar plate. The resulting colony counts are found in Figure 4. Chitosan-coated particles’ antibacterial activity can be seen in Figure 5. 

Figure 4: Bacterial Colonies Counted for each Microparticle Variation 

Figure 5: Bacterial Colonies Counted for Chitosan Coated PVI4 

Discussion 

The microparticles exhibited good drug encapsulation and drug release with varying results depending on the specific variation used. Fabrication of four variations of the microparticles with differing ICG content demonstrated the effects of ICG in drug release when subjected to NIR light. PVI4 had the highest drug release and antibacterial activity as the ICG accelerated the release and deterioration of the PLGA microparticles. The uncoated samples released vancomycin hydrochloride almost instantly when subjected to solution. To mitigate undesired drug release, chitosan-coated microparticles released less vancomycin hydrochloride when not exposed to NIR light. Meanwhile, when exposed to NIR light, the vancomycin hydrochloride was released in a larger concentration. The concentration of drug release is less than its uncoated counterpart (PVI4). Chitosan-coated PLGA – ICG microparticles are potentially an excellent candidate for targeted and on-demand drug release as the drug inside is trapped and released when subjected to NIR light. Both drug release and antibacterial activity further emphasize the potential applications of PLGA – ICG microparticles in drug delivery. 

Future Directions

Further research into the usage of unstable drugs such as alkylating agents must be undertaken. Cancer drugs are unstable and further modifications to the microparticles might be needed for the PLGA to hold these drugs. Moreover, further in vitro testing with mammalian cell lines is needed. Finally, specific cell targeting surface modifications on the microparticles will enhance the targeting of the microparticles to the desired cell. 

Acknowledgements 

I would like to thank Mr. Mishal Pokharel, a BMEBT Ph.D. student; without his guidance and expertise, this project wouldn’t have existed. He invited me into his lab and allowed me to assist with his Ph.D. project, for which these results were gathered for. I’d also like to thank Dr. Tracie Ferreira for being an extraordinary supervisor and always being open to my questions and new ideas. Finally, I’d like to thank the Office of Undergraduate Research and the Honors College for their generous support and for believing in me and my project. Their funding allowed the continuation of this project.