Utilization of pectin as a polysaccharide microcarrier for drug delivery

Pectin is a polysaccharide of natural origin, which has been widely exploited in the food and pharmaceutical industry due to its biodegradable and biocompatible nature. It is commonly used as an efficient texturizer and stabilizer in different food products due to its high stability and also as a pharmaceutical excipient in various drug formulations due to its valuable characteristics. The utilization of pectin as a drug carrier for achieving controlled drug release in pharmaceutical dosage forms has been extensively researched recently. Pectin has the potential for targeted drug delivery due to its degradation by colonic microorganisms, making it a popular subject of investigation for biomedical applications. The developed in recent years pectin microparticle systems have several advantages over classical pectin gel formulations. These advantages include higher drug loading efficiency, reduced burst drug release, and the ability to better control the drug release and limit the polymer swelling. This review outlines the recent developments of pectin as a microcarrier in the production of drug delivery systems, which include the properties of the polysaccharide material essential for microparticle production, specific formulation methods with their key technological parameters, influencing the final microparticle product and potential applications of the proposed pectin microsized formulations.


Figure 1
Polymer microparticle characteristics and advantages as drug delivery systems Pectins are biopolymers commonly employed in the food and pharmaceutical industries, possessing diverse applications. They function as thickening agents, gelling agents, and colloidal stabilizers. Furthermore, they serve as an excellent matrix for delivering drugs, proteins, and cells [5,6]. The following review is focused on pectin chemical structure and properties related to the development of drug-delivery systems, methods for the formulation of pectin microparticles, their characteristics and biomedical uses.

Chemical structure and properties of pectins
Pectin, a non-toxic natural polysaccharide, is mainly isolated from citrus fruits and apples. It is commonly used in the food industry as a viscosity-enhancing or gelling agent [7]. The chemical structure of pectin is composed of Dgalacturonic acid residues linearly bound through α-1,4-bonds ( Figure 3). These residues are partially esterified with methanol or acetylated and may contain smaller amounts of rhamnose residues in the main chain as well as other neutral monosaccharides such as arabinose, galactose, xylose, etc. that are found in the side chains [8]. Pectic polymers vary in uronic acid and neutral sugar content, glycosidic bond type, degree of esterification, acetylation, amide content, and molecular mass. There are two main groups of pectin polysaccharides classified based on their chemical structure: acid pectin polysaccharides (homogalacturonan (HG), xylogalacturonan (XHG), apiogalacturonan (AHG), rhamnogalacturonan I (RG I), and rhamnogalacturonan II (RG II)) and neutral pectin polysaccharides (galactans, arabinans, and arabinogalactans (AG)) [9]. The ratio of esterified to free acid groups in pectin's structure determines its degree of esterification (DE), which affects properties like solubility and gel formation. Pectin can be classified as having a high (HM; DE>50%) or low (LM; DE<50%) methoxylation degree. Pectins with high methoxylation form gels at high concentrations, while low methoxylation pectins only form gels in the presence of divalent ions [10]. These polysaccharides remain stable in acidic environments, even at high temperatures. However, their molecules form aggregates in low pH media that dissociate in neutral conditions [11]. Pectin resists protease and amylase in the upper gastrointestinal tract (GIT) but is broken down by microorganisms in the colon [5].

Figure 3 Pectin chemical structure
Pectin polysaccharides are incorporated in a complex way into plant cell walls. A pretreatment specific to the raw material facilitates separation from the cell and inactivates enzymes. Some common methods include mechanically degrading plant material with hot ethyl alcohol or acetone, washing with sodium deoxycholate, dimethyl sulfoxide, or a phenol/acetic acid/water mixture, and enzymatic hydrolysis. In industrial productions, pectin is typically extracted from agricultural by-products using a dilute mineral acid with a pH between 1.5 and 3.6, at a temperature of 60 to 100°C, for a duration of 1 to 6 hours. One drawback of the traditional method for pectin extraction is the environmental harm it can cause, such as acid effluent discharge. However, new and alternative methods, such as ultrasonic, microwave, and enzymatic extraction, have been developed in the "green industry". The application of enzymatic and ultrasonic extraction is an effective method for extracting pectin. These methods reduce the amount of solvents used and increase the polysaccharide yield while maintaining the selectivity of the hydrolysis process [12,13]. After being extracted, pectic polysaccharides undergo purification and fractionation using chromatographic techniques such as ion exchange, gel, or affinity chromatography. Ion exchange chromatography separates the polymer molecule based on the number of dissociated carboxylic groups. Pectic polysaccharides can be fractionated by ion exchange chromatography and depending on their esterification degree and the content of covalently bound neutral sugars. The pectin polysaccharides that are separated through ion exchange chromatography are subsequently purified using size-exclusion chromatography (SEC), which separates the polymer based on its molecular mass [12,13].

Preparation of pectin-based microparticles
Pectin has good mucoadhesive properties, attaching easily to mucin and mucosal surfaces in the body, making it a useful drug-carrier. Numerous studies have demonstrated the successful formation of drug-loaded microparticles using this polymer (Table 1, Figure 4) [5,6]. Hybrid microparticles are commonly used in polymeric drug delivery systems that involve pectin as a carrier. This is achieved by combining pectin with other polymers. Hydrophilic polymers like alginate or gellan gum are commonly used to create composite matrices that provide mucoadhesion, stability, and protection for active substances incorporated in them [14,15]. Only a small percentage of microspheres are made solely of pectin, and even fewer have a polysaccharide surface coating on the particles [16]. The main purpose of pectin microparticles is to achieve controlled release of medicinal substances for diseases of the colon and digestive tract, targeting antitumor, anti-inflammatory, or antibacterial action. Such drug-delivery systems have been used for diabetes, bone diseases with regenerative function, and nasal drug delivery [6]. The emulsion technique is a reliable method for creating pectin microspheres. The polymer and the drug are dissolved in an organic solvent, emulsified in water, and microparticles are formed upon evaporation of the organic phase. Various emulsions can be made with pectin, such as ethanol in oil [32], water-in-oil (W/O) [33], or oil-in-water (O/W) [34,35]. The choice of a microsystem development technique depends on the drug substance properties and the intended application.
Usually, a water-in-oil emulsion is made by mixing a pectin solution with the drug and vegetable oil, such as corn or sunflower oil, and an emulsifier stabilizes the formed droplets. Stirring rate is essential for the size and shape of pectin microparticles [36]. The polymer concentration, emulsifier, stirring speed, duration and temperature affect polymer microparticle characteristics obtained through the emulsion technique. A higher polymer concentration increases the dispersed phase's viscosity, leading to larger droplets and microparticles [37]. Studies show that increasing stirring speed reduces microstructure size [38]. Pectin microparticle preparation by emulsion technique with solvent evaporation is a longer process. Duration depends on the solvent evaporation rate and the heating may affect drug substance stability.

Figure 4 Pectin-based microparticles as drug delivery systems
Spray drying is an effective method for developing pectin microstructures with optimized particle characteristics like size, distribution, shape, and morphology. It is a single-stage process that directly transforms liquid material into dry particles. The small droplets produced increase the surface area/volume ratio of the liquid, leading to rapid solvent evaporation. The interaction time between the sprayed droplets and hot gas is brief, which does not affect the stability of the substances used [39,40]. The concentration of the starting material for spray drying, including the polymer and drug substance, significantly impacts the characteristics of the resulting microparticles. Therefore, it is crucial to pay close attention to this parameter, as well as the speed of the peristaltic pump, amount of compressed gas, temperature, and airflow rate [41,42]. Dosage forms with sustained drug release have been obtained using this technique, such as pectin microparticles for delivering albendazole, folic acid, and melatonin [28,43,44].
Complex coacervation is a technique where microspheres are formed by ionic interaction between oppositely charged polymer solutions, resulting in phase separation. A wide range of natural and semisynthetic polymers can be used in this technique. Studies indicate that higher polymer concentration leads to larger microparticles and higher drug entrapment efficiency, while low concentration results in microstructures with low density, wide size distribution, and rapid drug release. Achieving opposite charges between polymers is critical, and the ideal pH for effective phase separation varies based on the polymers used. Microparticle size and distribution depend on stirring speed and duration during coacervation, with excessive stirring leading to smaller microspheres and reduced drug incorporation efficiency [45][46][47][48]. By the complex coacervation method, controlled drug-release pectin microspheres can be designed e.g. microparticles with acetaminophen [49], various essential oils can be microencapsulated [50], and the resulting structures are usually resistant to mechanical stress and high temperature [51].
Although various other techniques have been described, the most common method for creating pectin microparticles involves ionotropic gelling of the polysaccharide [22,52,53]. Like alginate, pectin forms a gel structure in the presence of calcium, zinc, or copper ions due to strong ionic bonds between cations and galacturonic acid from the polymer structure [54,55]. The rapid swelling and dissolution of pectin in body fluids limit its use as a sustained-release vehicle for medicine. Esposito et al. suggest crosslinking the polymer to create a long-acting dosage form [56]. They evaluated the possibility of obtaining pectin microparticles loaded with the antibiotics metronidazole and tetracycline. Calcium chloride was used as an ionic crosslinking agent to limit the solubility of the polymer. Scientists have shown that by modifying pectin, microparticles of pectin with desirable morphological characteristics and sizes can be obtained to serve as promising modified-release drug carriers. Lasco et al. proposed using chlorhexidine as an alternative to divalent ions to gel pectin [57]. The chlorhexidine acts as a reagent to form microparticles and a crosslinking agent for the polysaccharide, while also serving as an antiseptic. According to the report, the drug-polymer interaction was very strong, limiting the drug release. To improve the release, the authors added zinc ions to the optimal chlorohexidine microparticle model. The zinc ions compete with the drug substance for pectin binding, reducing the bonds between chlorhexidine and pectin. This produced a physically weaker gel structure that facilitates drug release.
An important feature of pectin is its ability to create electrostatic complexes with oppositely charged macromolecules, including polysaccharides, nucleic acids and proteins. Complexation by electrostatic bonds leads to the formation of stable colloidal structures that combine the advantages of the polymers used [58]. A number of studies have used polyelectrolyte complexes of pectin with chitosan, alginate, and other natural polysaccharides as carriers for drugloaded microparticles. For instance, hybrid microspheres of pectin and chitosan have been developed for oral insulin delivery. The resulting systems showed good stability at the acidic pH of the stomach and a sustained release of the incorporated drug substance at pH 6.8 [58].
Polyelectrolyte complexes of pectin and chitosan of size 5-10 μm have been developed by the spray drying method at different ratios of the two polymers (1:9, 1:1, 9:1). They were designed carriers for nasal administration of tacrinedrug substance, used in the treatment of Alzheimer's disease. The results showed that the different amount of the two polysaccharides in the proposed models affected their wetting after administration, their mucoadhesive properties and drug absorption [59]. Hybrid microparticles of pectin and chitosan have been studied as carriers of resveratrol [22], vancomycin [23], nicin [60], metamizole [24], etc. Polyelectrolyte binding between pectin and alginate has also been well studied, in the formation of microsystems for the delivery of proanthocyanidins [61], polyphenols [62] and other biologically active substances. Other possible complexes include pectin-gelatin [63], pectin-casein [64], pectin-albumin [65], pectin-starch [66].
Pectin is widely used in the development of drug-delivery systems for targeted delivery in the colon. As already mentioned, pectin is less susceptible to degradation in the gastrointestinal tract compared to other natural polysaccharides. Its lysis occurs in the large intestine under the influence of enzymes secreted by the microorganisms there. This makes the polysaccharide a promising drug carrier for targeted delivery and modified drug release. Dashora et al., for example, have developed pectin microparticles loaded with prednisolone for topical treatment of ulcerative colitis [67]. An emulsion technique with solvent evaporation was applied, varying different technological parameters such as stirring time and rate, polymer and emulsifier concentration, in order to obtain microspheres with uniform size. In vitro solubility tests performed in gastric and intestinal juice mimicking media showed limited release of prednisolone (30-45%) over 4 hours. In the presence of rat ileocecal content, which is rich in enzymes from the large intestine, the polymer released up to 80% of the drug substance included in it. Unlike conventional oral formulations, in which prednisolone is quickly absorbed and does not reach the desired site in GIT, the proposed microsystems of pectin can effectively deliver the drug to the colon, providing local action and limiting its systemic side effects.

Conclusion
Pectin is an extremely promising excipient for the pharmaceutical industry, owing to its exceptional ability to deliver a wide range of drugs for controlled release applications. The widely adopted techniques for manufacturing pectin-based microparticle systems can be used to fabricate dosage forms with various morphology and characteristics. Moreover, pectin can serve as a reliable carrier for directing drugs to specific sites in the organism, thereby providing a localized treatment or a systemic action. The recent rapid development of micro-and nanotechnologies has opened up new possibilities for pectin as a drug carrier and has outlined this polysaccharide as a preferred polymer for different biomedical applications.