Introduction

Inulin, a starchy substance found in plants like wheat, leeks, artichokes, and asparagus, is primarily extracted from chicory roots. As a prebiotic fiber, it isn’t digested by human enzymes and remains in the bowel, promoting beneficial bacteria growth. Approved by the FDA as a dietary fiber, inulin enhances food’s nutritional value, reduces calorie content, and improves the stability of foams and emulsions. It forms a gel-like structure in liquids, enhancing texture and moisture retention.

The processing history and degree of polymerization significantly affect inulin’s physicochemical behavior, which is crucial for its functionality in various applications. Unlike common saccharides like glucose and lactose, inulin has a higher molecular weight, resulting in higher glass transition and melting temperatures, and greater viscosity but lower solubility. The glass transition is the process where an amorphous material (like glass or certain polymers) changes from a hard and brittle state to a more flexible, rubbery state as the temperature increases. Its (2 → 1) linked-d-fructosyl backbone provides high molecular flexibility, leading to relatively low glass transition and melting temperatures compared to other oligo- and polysaccharides. These properties can be advantageous or disadvantageous depending on the application.

Inulin is not metabolized by humans, making it useful for applications like kidney function determination and colonic targeting, utilizing microbiota in the colon. It is also preferred in pharmaceutical applications where reducing groups are undesirable, unlike glucose and lactose.

Pharmaceutical Applications

1. Stabilization

1.1. Anhydrobiosis

Inulin plays a significant role in drought protection for plants by stabilizing cell membranes. Studies have shown that inulin increases in stress-resilient plants and helps stabilize membranes during dehydration. Researchers have found that inulin inserts between lipid bilayers, prevent vesicle fusion and increase membrane stability. This effect is stronger than other polysaccharides due to inulin’s hydrophobic nature and its ability to interact deeply with membrane lipids.

Inulin’s flexibility and small furanose rings help it overcome steric hindrance, enhancing its interaction with membranes. It also lowers the gel-to-liquid crystalline phase transition temperature, further stabilizing membrane structure. According to the vitrification theory, inulin stabilizes membranes by forming a glass-like state, maintaining structural integrity under stress. The water replacement theory suggests that inulin replaces water molecules around polar residues in membrane phospholipids and proteins, preserving membrane structure and function in the absence of water.

1.2. Protein stabilization

The mechanisms behind membrane stabilization in nature and protein stabilization in pharmaceuticals are similar, involving vitrification and water-replacement theories. When the glass transition temperature (Tg) is significantly higher than the storage temperature, water replacement is the dominant mechanism; otherwise, vitrification limits stability. Inulin, compared to trehalose, has similar hygroscopicity (the ability to attract and hold water molecules from the environment), crystallizes less rapidly, and has higher Tg values, making it an effective protein stabilizer under various conditions.

Inulin has been successfully used to stabilize proteins during spray-drying, freeze-drying, and spray-freeze drying. It has fewer reducing groups and higher molecular flexibility, which enhances its interaction with proteins. Inulin’s effectiveness as a stabilizer is influenced by its molecular weight and storage conditions, often outperforming other saccharides like dextran and trehalose.

Overall, inulin is a reliable stabilizer for proteins in the dry state, providing better stability under certain conditions compared to smaller sugars.

1.3. Other stabilization

Inulin has been used to stabilize various pharmaceutically relevant systems, including PEGylated liposomes, polyethylenimine-based polyplexes, lipoplexes, polymersomes, influenza virosomes, whole inactivated influenza virus, recombinant adenovirus, and THC.

Inulin provides stabilization against both chemical and physical degradation. It has been shown to protect PEGylated lipoplexes during lyophilization and storage, outperforming dextran. Inulin also preserves the physicochemical characteristics of doxorubicin-loaded nanopolymersomes better than other excipients. Additionally, it maintains the stability and potency of influenza virosomes and recombinant adenovirus during freeze-drying and storage.

Incorporating THC into an inulin matrix significantly increases its stability, with spray-freeze drying offering even better results than freeze-drying. Overall, inulin is a promising stabilizer for various pharmaceutical applications.

2. Drug delivery

Inulin, a natural polysaccharide, is gaining attention for controlled drug delivery systems due to its non-toxic and biodegradable properties. Unlike synthetic polymers, inulin provides stability, targeting, and stealth properties—allowing nanoparticles to evade immune system detection—essential for optimal drug delivery. Its flexible molecular structure makes it versatile for various applications.

Inulin is chemically stable in the gastrointestinal tract until it reaches the colon, where it is digested by gut bacteria, making it suitable for colon-targeted drug delivery. Its solubility varies with chain length, allowing for use in both particulate delivery systems and kidney function testing.

Inulin has been used to stabilize proteins, improve the dissolution of poorly soluble drugs, and as a vaccine adjuvant. It is effective in various drug delivery systems, including hydrogels, micelles, liposomes, prodrugs, conjugates, microparticles, nanoparticles, and solid dispersions. Its ability to undergo chemical modification enhances its potential as a multifunctional drug delivery scaffold.

3. Physiological and disease-modifying effects

3.1. Systemic

3.1.1. Vaccine Adjuvant

Inulin, particularly in its crystalline forms ( γ- and δ-inulin), has been shown to enhance immune responses when used as a vaccine adjuvant. These forms of inulin are virtually insoluble at body temperature and consist of high molecular weight fractions.  γ-inulin specifically activates the alternative complement pathway, while the more soluble forms (α and β) are biologically inactive and even hindered pathway activation by γ- inulin. δ-inulin, which is more immunoactive than γ-inulin, has been used to enhance the potency of various vaccines, including those for influenza and hepatitis B. Additionally, soluble inulin microparticles have demonstrated potential as adjuvants, achieving robust immune responses and outperforming traditional aluminum salts.

3.1.2. Kidney Function

Inulin is widely used as a diagnostic tool for kidney function testing due to its unique properties. It is distributed over the extracellular volume and is not metabolized, making it ideal for measuring glomerular filtration rate (GFR). Inulin is excreted exclusively via glomerular filtration and is not reabsorbed by the renal tubules, providing an accurate measure of GFR. This method involves administering inulin intravenously and measuring its concentration in both urine and plasma. However, detecting inulin in biological matrices can be challenging, requiring precise analytical techniques to ensure accurate results.

3.2. Gastro-Intestinal Tract

3.2.1. Constipation

Inulin is commonly used as a dietary fiber and prebiotic, known for its ability to improve stool frequency and relieve constipation. It positively affects gut flora and mobility, providing health benefits across different age groups. Studies have shown that high molecular weight inulin increases stool frequency, making it effective against constipation in both newborns and elderly patients. By promoting a gut microbiota similar to that associated with breastfeeding, inulin helps improve digestive health and overall well-being.

3.2.2. Inflammatory Bowel Disease & Colon Cancer

Inulin has demonstrated both local and systemic immunomodulatory effects, making it a promising candidate for managing inflammatory bowel disease and potentially reducing the risk of colon cancer. Its local effects are primarily due to its prebiotic action, stimulating the growth of beneficial gut bacteria. Studies have shown that inulin can reduce chemically induced pre-neoplastic lesions and tumors in the colon of mice and rats, with long-chain inulins being more effective. However, further clinical research is needed to confirm these benefits in humans and fully understand inulin’s potential in treating these conditions.

4. Permeability research using FITC-inulin

FITC-inulin provides quantitative data on the transport and permeability of healthy and diseased tissues in real-time using intravital fluorescence microscopy. This technique is highly sensitive, detecting concentrations as low as 1 μg/mL in tissue fluids. It has been used to study the permeability of intestinal epithelial cell monolayers, colonic epithelial tight junctions post-irradiation, and the disruption of liver and gut tissues. Additionally, it assesses the barrier function in bioartificial kidney devices. Research has shown an excellent correlation between creatinine clearance and FITC-inulin clearance in mice.

References

• Maarten A. Mensink, Henderik W. Frijlink, Kees van der Voort Maarschalk, Wouter L.J. Hinrichs, Inulin, a flexible oligosaccharide. II: Review of its pharmaceutical applications, Carbohydrate Polymers, Volume 134, 2015, Pages 418-428, https://doi.org/10.1016/j.carbpol.2015.08.022.
• Afinjuomo, F.; Abdella, S.; Youssef, S.H.; Song, Y.; Garg, S. Inulin and Its Application in Drug Delivery. Pharmaceuticals 2021, 14, 855. https://doi.org/10.3390/ph14090855
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