Role of Essential Oils in Drug Delivery Systems: Mechanisms, Applications and Future Perspectives

The purpose of drug delivery systems (DDS) is to deliver therapeutic agents in a controlled manner to achieve the most desirable pharmacological effects with the least side effects. The conventional dosage delivery systems, such as tablets, capsules, and injections, are prone to alterations in drug levels, which lead to the most ideal therapeutic actions. The new DDS, such as transdermal systems, nanoparticles, liposomes, and controlled-release formulations, have been developed due to the advances in pharmaceutical technology, enhancing drug bioavailability, targeting, and patient compliance. They are supposed to enhance the therapeutic index of drugs by passing the drugs to the target site of action precisely [1,2].

Regardless of these developments, traditional DDS continue to pose a number of limitations. Delivery of drugs orally, such as, is commonly linked with low bioavailability because of first-pass metabolism, enzyme degradation, and unpredictable absorption in the gastrointestinal tract. Similarly, parenteral administration is effective but could be invasive, painful and non-compliance by the patient. Moreover, most formulations contain synthetic excipients which may be toxic, irritating, or cause hypersensitivity. These drawbacks have motivated scientists to find safer and efficient ways of delivering drugs [3,4].

The importance of using natural excipients to make pharmaceutical formulations has gained interest over the past few years. Biodegradable, biocompatible and usually safe, natural excipients are found in plants, animals and minerals. These possess several functional properties such as emulsifying, stabilizing and permeation-enhancing properties. In addition, the trend towards green chemistry and sustainable development has added to the rapid pace of the use of natural materials in drug delivery systems. Not only do these excipients improve the performance of formulations, but also reduce the environmental impact of synthetic substances [5].

Volatile, aromatic compounds that are extracted out of plants, known as essential oils, have become promising natural excipients in DDS. These are a complex blend of terpenes, terpenoids and other bioactive components which exert various pharmacological actions, including antimicrobial, anti-inflammatory, antioxidant and penetration enhancing actions. Essential oils are lipophilic, and these have the capability of reacting with the biological membrane and increase the rate of drug permeation particularly in transdermal and topical delivery systems. They are multifunctional and therefore are good candidates to enhance the stability and efficacy of drugs [6,7].

Figure 1: Classification of essential oil-based drug delivery system

The purpose of this review is to conduct an in-depth study of the role of essential oils in current drug delivery systems, its mechanism of action, its application in various delivery systems, and its future. The possible use of essential oils as natural excipients in order to address the shortcomings of traditional DDS and to help to design safer, more effective, and sustainable pharmaceutical formulations is also mentioned in this review.

2. ESSENTIAL OILS: COMPOSITION AND PROPERTIES

Essential oils are volatile and complex mixtures of bioactive compounds derived in different parts of plants like flowers, leaves, bark, seeds and roots. They are mainly chemical compounds of low-molecular weight such as terpenes, terpenoids, phenolics, aldehydes, alcohols, esters, and ketones. The most common constituents include limonene, pinene and myrcene which are the constituents that give essential oils their common smell. Terpenoids (oxygenated derivatives of terpenes e.g., linalool, menthol) also affect their biological activity. Phenolic compounds such as thymol and carvacrol have been found to possess high antimicrobial and antioxidant properties and aldehydes, such as citral and cinnamaldehyde have been known to possess high antimicrobial and flavoring properties. The different ratios of different compounds in essential oils vary and are relative to the plant species, geographical location and the extraction process [8,9].

Essential oils have physicochemical characteristics that are important in determining their use in drug delivery systems. One of the characteristics is their high volatility since they contain low-boiling-point constituents. This characteristic allows easy evaporation but may present a challenge in stability of formulations. Essential oils are usually lipophilic, meaning that they readily dissolve in lipid membranes and enhance the drug permeation across biological barriers, such as the skin. Their insolubility in water necessitates the use of suitable carriers or emulsifying agents in the pharmaceutical preparations. Also, essential oils have high refractive indices and are susceptible to environmental conditions like light, heat and oxygen that may cause degradation and loss of activity with time [10,11] .

In terms of drug delivery, the lipophilicity and the low molecular weight of the constituents of the essential oils can cause the disruption of the lipid structure of the biological membranes, consequently, raising the permeability. This makes them particularly beneficial in topical and transdermal delivery systems of drugs as penetration enhancers. Besides, they have natural biological properties like antimicrobial, anti-inflammatory and antioxidant which can be utilized together as part of formulations. However, there are additional issues such as potential skin irritability, instability and unpredictability of composition, which must be put to the forefront during formulation development [12].

Some of the essential oils have undergone a lot of research on their pharmaceutical uses. One such example is the anti-inflammatory and penetration enhancing properties of eucalyptus oil (rich in 1,8-cineole) and the cooling, analgesic and penetration enhancing properties of peppermint oil (rich in menthol). Clove oil is a good antimicrobial and antioxidant with a high concentration of eugenol and can be used in medicine and preservation. One of such is lavender oil (including linalool and linalyl acetate) with its calming, antimicrobial, and anti-inflammatory properties. The foregoing shows how essential oils can be utilized in drug delivery systems with their potential to be multifunctional excipients [13-15].

Figure 2: Mechanistic pathways of essential oils in enhancing drug delivery across biological membranes.

4.2 Topical and Dermal Formulations

Essential oils are also popular in the topical preparations, such as creams, gels, and ointments, because of their antimicrobial, anti-inflammatory, and antioxidant effects. They enhance the absorption of drugs into the skin and at the same time they are therapeutic.

EO topical preparations have been successfully utilized in skin care such as acne, fungus and wound healing. One such is tea tree oil and lavender oil, which is extensively used in creams and gels because of their overall antimicrobial properties. In addition, essential oils are added to topical formulations to increase the sensory properties and patient compliance of topical formulations [25,26].

4.3 Nano-Based Drug Delivery Systems

The addition of essential oils to nano-based drug delivery systems has gained a lot of attention during the last several years. These systems enhance the stability, solubility and bioavailability of essential oils as well as incorporated drugs.

Figure 3: Nano-carriers used for essential oil-based drug delivery

Nanoemulsions

EO-based nanoemulsions are thermodynamically stable systems which increase the solubility and penetration of drugs. They have a superior surface area and drug absorption because of their small droplet size and hence, very effective in topical and oral delivery [27].

Liposomes

Liposomes are vesicles that encapsulate essential oils in form of phospholipid bi layers. The EO-loaded liposomes enhance drug targeting and decrease drug toxicity, and regulated drug release. They come in handy especially in the delivery of hydrophobic drugs [28].

Solid Lipid Nanoparticles (SLNs)

SLNs with essential oils have better stability and prolonged release of drugs. These carriers assure that the active components do not degrade and they enhance their penetration into the biological membranes [29].

4.4 Oral Drug Delivery

Essential oils have been shown to have a potential in oral drug delivery systems to increase the solubility and absorption of drugs that are insoluble in water. Their lipophilic nature enhances dissolution of drugs in the gastrointestinal fluids and increases the permeability of the membrane.

Additionally, the essential oils can inhibit the enzymes and efflux transporters in the metabolism thereby raising the bioavailability of drugs. Self-emulsifying drug delivery systems (SEDDS) are EO-based formulations which have been designed to improve oral drug delivery efficiency [30].

4.5 Antimicrobial and Wound Healing Systems

There is a broad use of essential oils in the antimicrobial and wound healing due to the ability to disrupt the cell membranes of microbes and inhibit their growth. They are included in drug delivery systems and enhance wound healing and prevention of infections.

EO loaded dressings, gels and films have shown a lot of effectiveness against a broad spectrum of pathogens such as bacteria and fungi. Moreover, they have anti-inflammatory and antioxidant effects, which promote quicker tissue regeneration and shorter healing period [31,32].

Table 4: Applications of Essential Oils in DDS

5. ADVANTAGES OF ESSENTIAL OILS IN DRUG DELIVERY SYSTEMS

Essential oils (EOs) have attracted a lot of interest as functional excipients in drug delivery systems because of their peculiar physicochemical and biological characteristics. Their biogenic properties, multifunctionality, and biocompatibility with biological systems are likely to be substitutes to synthetic agents.

5.1 Natural and Biodegradable

One of the best advantages of the products is the fact that essential oils are natural and biodegradable. EOs are derived out of plants and are usually regarded as environmentally friendly and have low environmental persistence in comparison to synthetic excipients. They are biocompatible, which minimizes the chances of toxicity in the long run and enables them to be used in the pharmaceutical and cosmeceutical industry [33].

5.2 Multifunctional Properties (Therapeutic + Delivery Role)

Essential oils are also applied in multiple ways because they can also be used as drugs and therapies. In addition to improving permeation and solubility of drugs, EOs possess innate antimicrobial, anti-inflammatory, antioxidant and analgesic properties. This multifunctional behavior can lead to synergistic treatment effects which will reduce the use of other active pharmaceutical ingredients and will enhance the overall treatment efficacy [34,35].

5.3 Reduced Need for Synthetic Enhancers

Natural permeation enhancers (essential oils) minimize the use of synthetic chemical permeation enhancers, which are typically associated with toxicity and irritation. EOs contain terpenes that can be used effectively to increase drug permeation through biological barriers without damaging skin integrity. This makes EOs the safer alternative in preparations such as transdermal patch and topical system [36].

6. LIMITATIONS AND CHALLENGES

Although essential oils have promising benefits, the use of essential oils in drug delivery systems is linked to a number of limitations that need to be overcome in order to make their use safe and effective.

6.1 Instability and Volatility

Essential oils are volatile and chemically unstable and therefore they may easily be damaged by light, heat, oxygen and moisture. Oxidation of EO components can cause loss of activity and produce by-products which can be toxic. This instability presents serious problems in the development of formulations and storage [37].

6.2 Skin Irritation and Toxicity

Although the majority of them can be said to be safe, essential oils can cause skin irritation, sensitization, and allergic reactions, especially when applied to the skin in large quantities, and during prolonged use. Certain of these components such as phenolic compounds may be cytotoxic. Therefore, the doses and safety of their incorporation in drug delivery systems should be optimized [38].

6.3 Lack of Standardization

A lack of standardization in composition and quality is one of the most important problems concerning essential oils. The variations in the plant species, geographical origin, harvesting condition and extraction methods can cause differences in the chemical profiles. This variability affects reproducibility, efficacy, and safety of EO-based formulations [39].

6.4 Regulatory Issues

The regulatory status on the use of essential oils in pharmaceuticals is complex. The majority of the essential oils are either categorized under cosmetic or traditional medicine and this creates confusion in sanctioning the use of the oils in drug delivery. Lack of clear regulatory frameworks and standardized testing procedures is a problem to their commercialization and clinical acceptance [40].

Table 5: Advantages vs Limitations

7. FUTURE PERSPECTIVES

The use of essential oils (EOs) in advanced drug delivery systems is an emerging field in pharmaceuticals. The trends in the future will probably be directed at enhancing stability, specificity and therapeutic performance of EO-based formulations through creation of new technological solutions since the interest in natural and multifunctional excipients is still increasing.

7.1 Nano-Enabled Essential Oil Systems

Nanotechnology can do a lot to reduce the shortcoming of essential oils, such as volatility, aqueous insolubility, and instability. The addition of EOs to nano-based carriers like nanoemulsions, liposomes, polymeric nanoparticles and solid lipid nanoparticles can be considered to enhance their stability, inhibit degradation and alter controlled release profiles.

Nano-encapsulation is also known to enhance targeted delivery and bioavailability of the essential oils and co-administered drugs. Recent research has shown that EO loaded nanocarriers have better pharmacokinetic characteristics and therapeutic effectiveness than traditional formulations [41,42].

7.2 Smart and Stimuli-Responsive Drug Delivery Systems

Another emerging trend in EO based formulations is the emergence of smart or stimuli responsive drug delivery systems. These systems are programmed to deliver drugs in reaction to certain physiological or external stimuli like pH, temperature, enzymes, or light.

These systems can also be enhanced by incorporating the controlled release of drugs with inherent therapeutic effects with the use of essential oils. To provide an example, active compounds may be localized to the location of infection or inflamed tissues using EO-loaded hydrogel or nanocarriers and eliminate side effects at the systemic level [43].

7.3 Combination Therapies

The combination of essential oils and conventional medications to bring about synergistic effects is a research area of the future, which is acquiring more and more interest. The essential oils can enhance permeability of drugs, avert microbial resistance, and reduce the quantity of drugs necessary.

The combination strategies are particularly promising in the treatment of infectious diseases, inflammation, and chronic conditions where the pharmacological effect of synthetic drugs can be complemented by EO components. The problems of drug resistance and other side effects of high dose treatment can also be addressed using this method [44].

7.4 Scope in Personalized Medicine

The concept of personalized medicine opens up new opportunities of utilizing essential oils in drug delivery. The development of pharmacogenomics and therapy design tailored to patients allows the development of personalized drug formulations.

Essential oils with their diverse chemical compositions and their diverse activities can be tailored to suit specific therapeutic requirements. The future generation of EO-based drug delivery systems can be customized to achieve optimal effectiveness and safety of a particular patient, considering the skin type, metabolic rate, and disease state [45].

8. CONCLUSION

Essential oils (EOs) have emerged as an attractive multifunctional agent in the modern drug delivery systems due to their unique physicochemical and biological properties. They can be used as alternative to conventional synthetic excipients because of their ability to enhance drug permeation, solubility, and provide inherent therapeutic functions such as antimicrobial, anti-inflammatory, and antioxidant. Inclusion of EOs in various delivery systems like transdermal systems, topical formulations, nano-based carriers and oral delivery systems have demonstrated remarkable improvements in drug bioavailability and therapeutic effects.

These are the advantages but nevertheless, there exist several challenges which limit their wide usage in pharmaceuticals. Chemical instability, volatility, potential irritation of the skin, lack of standardization of composition is yet another barrier to reproducibility and widespread application. Moreover, the regulatory frameworks of EO-based drug delivery system are not clearly laid down which is a barrier to clinical translation and commercialization. All these gaps suggest that there is a need to conduct more systematic research on the field of standardization, health evaluation, and long-term stability research.

Research in the future should be aimed at utilizing the essential oils with the latest technologies, such as nanocarriers, stimuli-responsive systems, and combination therapies to maximize the therapeutic effect. In addition, the use of essential oils as a customized medicine is a novel direction towards tailored methods of drug delivery. With scientific discovery and technological advances still on, the future of safe, effective and patient-centered drug delivery systems is immense with essential oils.

FUNDING SOURCES

The author(s) did not receive any financial assistance to conduct the research, write the article, and/or publish it.

CONFLICT OF INTEREST

The author(s) do not have any conflict of interest.

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