Influence of Excipients on the Release Kinetics and Antioxidant Activity of Encapsulated Propolis Phenolic Compounds

Abstract

Propolis is a widely studied natural raw material, the composition of which varies depending on the plant origin, harvest season, geographical area, climate and bee species. This large variety of chemical composition limits the use of propolis extracts in the pharmaceutical industry, which makes it difficult to ensure standardization of the raw material. One of the challenges that limit the modeling of oral pharmaceutical forms with propolis extract is the limited solubility and bioavailability of active compounds. Solid dispersion technology is commonly used in the production of oral capsules. The aim of this study is to evaluate the influence of different materials (HPMC, poloxamer and β-cyclodextrin) on the dissolution kinetics of phenolic compounds of propolis dry extract contained in capsules and their antioxidant activity in vitro. Analysis of the selected formulations showed that the major phenolic compounds detected in the propolis extract were also present in the dissolution medium samples. The auxiliary polymeric materials selected for the capsules formed a prerequisite for the dissolution kinetics profile. The addition of poloxamer and cyclodextrin increased the solubility and dissolution kinetics of hydrophobic propolis compounds in the test media. The addition of HPMC prolonged the dissolution kinetics of propolis active compounds. The antioxidant activity of the tested samples depends on the concentration of active compounds in the receptor medium by both the ABTS and DPPH methods.

Introduction

Propolis is a resinous mixture produced by bees, which bees collect from plants and use to protect their hive. Propolis is a widely studied raw material with a wide range of chemical compositions that vary depending on several parameters, such as plant origin, harvesting seasons, geography, type of bee flora, climatic changes and bee species at the collection site, and its biological properties are widely described in the scientific literature [1,2,3,4,5]. More than 300 different compounds have been isolated and identified from this natural product [6].

The main part of the propolis raw material consists of resins, rich in phenolic compounds, which determine the wide range of biological effects [7,8,9]. Numerous studies conducted on various propolis samples have shown that the main secondary metabolites are phenolic substances, especially flavonoids belonging to different subclasses, such as flavanones, flavones, flavonols and dihydroflavonols, which constitute more than 50% of the weight of propolis [7,10,11]. A wide matrix of chemical composition is directly associated with the biological effects of propolis. Propolis raw material is used to strengthen the immune system, treat colds due to its antibacterial and antiviral effects, as a remedy for skin problems, to treat small ulcers and aphthae in the oral cavity or finally restore the balance of the gastric mucosa [1,5,6,11,12,13]. The antioxidant activity of propolis mainly depends on the flavonoid and polyphenol fraction [14]. The high antioxidant activity of these compounds is associated with their ability to donate hydrogen atoms and electrons of the aromatic hydroxyl group to free radicals and the possibility of charge delocalization in the double-bond system of the aromatic ring [15].

The commercial use of propolis in pharmaceutical products is most limited by the large variation in chemical composition, which is determined by the diversity of botanical origin, variability of plant precursors, seasonality and climatic conditions, which makes it difficult to ensure the standardization and traceability of the raw material [16]. Dry propolis extracts obtained by controlled extraction and drying technologies (vacuum or spray drying) allow the stabilizing of phenolic compounds and ensure dosing accuracy [17]. By encapsulating the dry extract of propolis, it is possible to modulate the release of active ingredients in a targeted manner, both by protecting them from degradation in the digestive tract and by controlling diffusion mechanisms, which is relevant when developing oral systems with modified or local biological effects. One of the problems that must be addressed when modeling oral pharmaceutical forms with propolis extract is the limited solubility of active compounds [18]. Propolis resins are hydrophobic and include highly valuable compounds, such as flavonoids and phenolic acids, which have are low polarity and are characterized by poor solubility in water. Scientific research data reveal that propolis active substances are extremely poorly soluble in aqueous media; the resinous structure and wax components block the effective release of phenolic compounds into water, which limits their use in pharmaceuticals [13,19,20].

Propolis active compounds (phenolic compounds) are characterized by limited oral bioavailability, which is mainly determined by their poor solubility in aqueous media. It can be stated that poor solubility in water together with intensive metabolism in the digestive tract are the main barriers determining poor bioavailability [21,22,23,24,25]. To overcome these obstacles, scientists propose the use of cyclodextrins, which increase solubility and protect the active components of propolis from biotransformation in the body [26]. Solid dispersion (SD) technology is often used in the production of capsules. Scientific studies confirm that poloxamers are suitable for increasing the bioavailability of propolis. Their use allows for the successful transformation of hydrophobic propolis extract into a well-absorbed oral form (capsules), ensuring a more effective therapeutic effect. Poloxamer acts as a carrier that not only physically disperses hydrophobic particles of poor solubility, but also reduces their crystallinity, turning them into an amorphous form. This allows a faster release of active ingredients and their penetration through intestinal membranes [27].

Cyclodextrins can also be used as excipients in capsule formulations to improve the solubility and dissolution rate of poorly water-soluble compounds. By forming complexes, hydrophobic components are partially encapsulated in the cyclodextrin cavity, while the hydrophilic outer layer facilitates dispersion in aqueous gastrointestinal fluids. This reduces aggregation and increases the dissolution rate [28,29,30]. Extended release is another way to more effectively absorb poorly soluble active ingredients. HPMC (hydroxypropyl methylcellulose) is a widely used hydrophilic matrix polymer, which, due to its ability to form a viscous gel barrier, ensures a controlled and prolonged release of active ingredients [31,32,33]. The aim of this study is to evaluate and compare the influence of different excipients (HPMC, poloxamer and β-cyclodextrin) on the dissolution kinetics of encapsulated propolis dry extract phenolic compounds and their antioxidant activity in vitro.

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Materials

All reagents, standards, and solvents were used at analytical grade. Caffeic acid (≥98%, HPLC) was purchased from Sigma-Aldrich Chemie GmbH (Steinheim, Germany). D(+)-glucose monohydrate, starch, hypromellose (HPMC, USP testing specifications), poloxamer 407 (Kolliphor P 407, Ph. Eur./USP/NF pharmaceutical grade, containing 71.5–74.9% oxyethylene), β-cyclodextrin (≥97% purity, bio-reagent grade), and PROSOLV SMCC 50 were obtained from Sigma-Aldrich GmbH (Buchs, SG, Switzerland), respectively, and all were used as excipients. Ethanol (96%) was purchased from AB “Vilniaus degtine” (Vilnius, Lithuania). Phosphate-buffered saline (PBS) (pH 7.4) was obtained from Gibco (Paisley, UK). Ultrapure water was produced using a water purification system Milli-Q (Millipore, Arlington, MA, USA). Chromatographic grade acetonitrile and trifluoroacetic acid (TFA, ≥99.0% purity) were obtained from Sigma-Aldrich Chemie GmbH (Steinheim, Germany). Crude propolis was harvested during the autumn of 2024 from Raseiniai region, Lithuania.

Jokubaite, M.; Marksa, M.; Nefodov, O.; Sakharova, T.; Ramanauskiene, K. Influence of Excipients on the Release Kinetics and Antioxidant Activity of Encapsulated Propolis Phenolic Compounds. Antioxidants 2026, 15, 767. https://doi.org/10.3390/antiox15060767