**1. Introduction**

Citrus essential oils are widely used in perfumery due to their very pleasant aroma. Nevertheless, because of the high volatility, the aroma is quickly lost [1], hindering its cosmetic application. Regarding medicinal properties, antioxidant, antifungal and anticancer ones are reported [2]. Moreover, citrus oil would be interesting to inflammatory skin diseases [3] and to disorders generating oxygen-generating species [4, 5]. Then, citrus oils could be used as adjuvants in the treatment of several skin disorders.

In their composition, essential oils have terpenes [6] which are employed in pharmaceutics formulations to provide a transdermal release due to its ability to increase skin permeation [7]. Despite its limited use, transdermal route may be used to overcome the drawbacks of the injectable routes or as an alternative to the oral route allowing a systemic drug delivery through the skin [8]. Additionally, as several factors affect oil chemical composition [9], it is crucial to select an adequate oil source to obtain a suitable phytochemical composition. Hence, in the development of therapeutic products plant-ingredient based, it is required to associate strategies of a correct plant cultivation together with an adequate pharmaceutical study [3, 4].

Nanoparticulate systems are promising approaches to dermatological [10] and systemic-diseases treatment [11]. Accordingly, this chapter addresses nanoformulations containing citrus terpenes or citrus essential oils targeting skin delivery. Initially, aspects related to chemical composition will be briefly displayed. Lastly, the most recent contributions of nanocarriers containing terpenes or essential oils from citrus for skin or transdermal delivery will be shown.

## **2. Citrus oil composition**

The essential oils composition varies among different species and cultivars, rootstocks, structure (of leaf, flower or fruit), according to the extraction method, edaphoclimatic conditions and agronomic management employed at harvest time. In general, the constituents of the essential oil present in the peel of citrus fruits are monoterpenes, followed by sesquiterpenes and phenylpropanoids [12, 13].

Oranges fruit juice is the main source of citrus oils. Besides, the peel of young green fruits removed during the manual thinning of mandarin fruits is also an origin of essential oil. Moreover, leaves and flower buds are other sources of oils, by steam distillation or hydrodistillation extraction methods. Oils obtained from flower buds are known as petitgrain (*petit grain* in French means "small grain") as an allusive name to the leaves and buds of the bitter orange (*Citrus aurantium*). However, petitgrain oil can be extracted from leaves of any citrus species [12, 14, 15].

In most citrus oils, the major terpene is d-limonene, accounting for a percentage of up to 98%. Other compounds of importance, such as γ-terpinene, geraniol, citral, valencene, α-pinene, sabinene, myrcene and linalool, are present in variable amounts [12]. Essential oils extracted from the peel of sweet oranges (*Citrus sinensis*), grapefruit (*Citrus paradisi*), pummelo (*Citrus grandis*), clementines (*Citrus clementina*), satsumas (*Citrus unshiu*) and tangors (hybrids of orange with tangerines) have a limonene content higher than 90% [16–19]. On the other hand, the contents of limonene and linalyl acetate are similar (~35%) in bergamot oil (*Citrus bergamia*) [18]. Elseways, limonene content ranges from around 70 to 75% in Cai and Montenegrina mandarin (*Citrus deliciosa*) [20].

With respect to essential oils extracted from citrus leaves (petitgrain), there is a greater variation between species. A greater amount of methyl-N-methylanthranilate is found in *Citrus deliciosa* species. Contrarily, *Citrus reticulata*, *Citrus reshni* and tangors seem to accumulate higher contents of linalool and sabinene. Diversely, in *Citrus sunki*, beta-pinene is the most important terpene [14, 15].

## **3. Nanocarriers bearing citrus oils or citrus terpenes**

Nanopharmaceuticals either contain terpenes or essential oils. The use of isolated terpenes is desirable at times [11–14] instead of essential oil. Concerning citrus oils, they are related to skin irritation and low stability due to their volatility. To overcome these limitations, nanoencapsulation of these oils has been reported [21]. Thereby, the loading of essential oils in microemulsions allows to reduce adverse effects [21], and the encapsulation of citrus oil in ethosomes reduces oil volatilization [22].

Nanodrug delivery systems include polymeric nanoparticles, liposomes, ethosomes, nanoemulsions, solid lipid nanoparticles, nanostructured lipid carriers and microemulsions [23]. **Figure 1** shows the most important nanometric carriers bearing citrus oils or citrus terpenes. Polymeric nanoparticles are composed of polymers and surfactants in which different drugs can be encapsulated. Liposomes are vesicular systems containing phospholipids [26]. Other vesicular carriers have been developed over the years including ethosomes [11, 27], invasomes [28, 29], transfersomes [11] and bilosomes [30], and they all have other ingredients than phospholipids [25, 26]. For instance, in bilosomes, there are bile salts [30].

Nanoemulsions contain an oily core and surfactants in their composition. Solid lipid nanoparticles (SLN) and nanostructured lipid carriers (NLC) are both formed of lipids where SLN and NLC contain, respectively, solid lipids and a mixture of solid and liquid lipids (**Figure 1**) [26]. Elseways, microemulsions are similar to nanoemulsions as they have an oil phase, an aqueous phase, and surfactants (**Figure 1**) [31, 32]. Nonetheless, microemulsions are thermodynamically stable systems [33], unlike nanoemulsions [32]. These drug delivery systems are mainly employed to entrap oily substances and therefore are suitable for essential oils [10, 21, 22, 34, 35].

Previously to nanoencapsulation, essential oils are extracted [4] or at times commercial essential oils can be used [10, 22, 35]. Either terpenes or essential oils are employed in nanostructures as a therapeutic ingredient [4, 21, 34] or as skin permeation enhancers [36–40]. About terpenes, limonene provides a drug release into the skin [36] or a drug transdermal release [39–41].

Limonene is frequently used in liposomes to obtain invasomes. Invasomes are vesicular systems composed of terpenes and alcohol [24] which favour the skin penetration/permeation of drugs [42]. Apart from that, when topical application of drugs is mentioned both permeation and penetration may occur. Permeation is the passage of drugs from one skin layer to another skin layer (e.g. from the epidermis to the dermis). Oppositely, penetration is when a drug reaches one skin layer (e.g. the stratum corneum). Skin penetration of drugs always precedes permeation. Once skin permeation is reached, drugs may reach systemic circulation [43].

In general, nanocarriers show a biphasic pattern with an initial burst drug release followed by a sustained release. The initial release is due to the outermost drug

#### **Figure 1.**

*Citrus essential oils or citrus terpenes loaded in nanocarriers targeting transdermal or skin delivery. Source: [24, 25]. Created in BioRender.*

location in the nanocarrier. It is expected that at least some part of the drug will be adsorbed to the nanostructure, favouring its burst release. Drugs located more internally in the nanocarrier will show a gradual release providing the prolonged release effect. The combination of initial burst release and prolonged release is interesting because it allows a pharmacological activity just after skin application along to a prolonged effect [30, 39], even for several hours [30, 35, 38].

Furthermore, ingredients of nanometric carriers must be carefully selected considering the concentrations of each ingredient. From an optimized formulation, subsequent efficacy and safety studies can be conducted [30, 38, 44]. An optimized formulation can be determined by physicochemical assays including particle size, zeta potential and encapsulation efficiency [10, 11, 30, 35, 37, 45]. Also, optimization also implies evaluation of formulation stability over a period of weeks or months at different temperatures [7–49].

Concerning nanometric systems intended for skin application, both a localized effect on the skin [11, 31, 34, 36, 37] or a systemic effect can occur [29, 41, 45]. A topical effect is desirable to skin diseases treatment [11, 31, 36, 37] while a systemic effect is desirable to treat a more serious disease whose source is in another organ (i.e. not the skin) [30, 41, 45]. For topical delivery, skin penetration and/or permeation is desired [36]. Nevertheless, for systemic delivery, permeation is mandatory as it precedes drug release into the circulatory stream [30, 39]. Additionally, in view of the trend of using natural compounds, nanoparticles are also applied to increase skin penetration of polyphenols [36].

Since the discovery of new drugs is very expensive and very time consuming [50], the use of penetration enhancers improves biopharmaceutical characteristics of drugs (e.g. low aqueous solubility) [38, 44]. Hence, combination of physical and chemical permeation methods is proposed as a strategy to increase skin permeability of drugs [39, 45].

#### **3.1 Citrus oils loaded into nanocarriers**

Most citrus oils were encapsulated in lipid carriers, as shown in **Table 1**. Also, spanlastics, ethosomes and transfersomes were employed as carriers due to their ability to increase skin penetration. These carriers are entitled as elastic liposomes, variations of conventional liposomes [51]. Since conventional liposomes (**Figure 1**) generally provide skin penetration only at the outermost skin layer, elastic liposomes are used to improve skin penetration into the innermost layers of the skin [52]. In this sense, elastic liposomes are composed of penetration enhancers such as ethanol and surfactants [7]. Ethosomes and transfersomes are vesicular systems containing ethanol and surfactants, respectively [11]. On the other hand, spanlastics are elastic liposomes bearing the surfactants span 60 and polysorbate 80 [51].

Recently, lipid nanocapsules were proposed as blending the features of polymeric nanoparticles and of liposomes. In its composition, there is lecithin, an ingredient commonly used for liposomes. This carrier resembles polymeric nanoparticles as it contains an oil phase and an aqueous phase containing hydrophilic surfactants [53]. In the case of lipid nanocapsules, polyethylene glycol derivatives are employed as hydrophilic surfactants. This new carrier may be more attractive than traditional polymeric nanoparticles in increasing the skin permeation [34].

Essential oils content varies according to the type of nanocarrier (**Table 1**). Most studies employed low essential oils content [10, 22, 35]. However, for lipid


#### **Table 1.**

*Nanocarriers bearing citrus essential oil.*

nanocapsules [34] and microemulsions [31], a higher concentration was used. Although a high oil concentration may provide a formulation with a lower physical stability [22], a higher oil content may be needed to ensure nanocarrier formation (e.g. lipid nanocapsules) [34]. Therefore, it is fundamental to ensure a suitable oil content as it may influence nanosystems performance.

As for physicochemical characterization, essential oil content influenced particle size. The higher the oil content the higher particle size [22]. Diversely, the lower the concentration of surfactants the higher the particle size [10, 38, 41], which is related to the stabilizing effect of surfactants. A higher surfactant concentration improves stability, reduces particle size, and prevents coalescence [54]. Zeta potential, another important assay, also is also indicative of stability [38]. It provides information about the surface electrical charge, and it is affected by anionic and cationic surfactants. Cationic surfactant will impart a positive zeta potential, while anionic surfactant will provide a negative zeta potential [47]. Regards to encapsulation efficiency, it only applies when a drug is entrapped inside a nanocarrier [31, 47]. A high encapsulation efficiency means that a high percentage of drug is encapsulated. In that regard, a high encapsulation efficiency was obtained for luteolinloaded in bergamot oil nanoemulsions, a desirable effect as luteolin is protected from degradation [47].

Another strategy is the development of cationic carriers to increase skin permeation. Luteolin-loaded in cationic nanoemulsions had a better skin permeation regarding luteolin-loaded anionic nanoemulsions. Since there is electrostatic interaction between the skin and cationic nanocarrier, the skin adhesion is favoured. Consequently, there is a greater permeation of luteolin. Nonetheless, luteolin loaded in anionic nanoemulsions had a higher skin permeation than non-nanotechnological luteolin suspensions. Although the use of cationic carrier usually provides the best skin penetration, anionic carrier are also potential carriers are they may have a better performance than non-nanotechnological formulations [47].

Citrus essential oils are related to phototoxic reactions due to the presence of furocoumarins [55]. Nonetheless, for vitiligo, this phototoxic reaction can be desirable to stimulate skin repigmentation [10]. In this sense, bergamot oil-loaded lipid carriers had an increased skin repigmentation in vitiligo patients [10, 35] due to the increased skin permeation of bergamot oil loaded into nanocarriers [35]. In view of vitiligo treatment limitations and treatment-related side effects [10], vesicular lipid carriers are a promising treatment for vitiligo patients.

Further, orange oil entrapped into nanocarriers provided a greater antifungal activity [34], probably due to oil protection from volatilization. Likewise, orange oil encapsulation also promoted an increase in aroma durability, a desirable effect in perfumery [22]. As citrus essential oils are top notes [1], its encapsulation may be an interesting alternative in the development of perfumes with greater durability.

Newly, essential oils from *Citrus aurantifolia and C. reticulata* were loaded in microemulsions*.* Citrus oils loaded in microemulsions were less irritating to the skin than undiluted oil, due to oil-controlled release provided by microemulsions [4]. Although limonene may cause change in epidermis structure to promote skin penetration [56], citrus essential oils-loaded microemulsions were less irritant than undiluted oils [4].

#### **3.2 Citrus terpenes loaded into nanocarriers**

Limonene is the most employed terpene in drug delivery systems [29, 37, 39, 44] while linalool is least frequent [48, 57]. Encapsulation of linalool improves its stability [48, 57] and consequently increases its antioxidant activity since the terpene itself has radical-scavenging ability [58]. The higher radical-scavenging ability is important to ensure a more effective skin treatment as oxidative stress can worse dermatological diseases [59, 60].

**Table 2** summarizes the main outcomes of terpenes entrapped into nanocarriers. As the log P of limonene is greater than of linalool, the former is a more suitable ingredient for lipophilic drugs [64]. In fact, a higher encapsulation efficiency is reported to nanocarriers bearing limonene and lipophilic drugs [29, 39, 44]. As to terpenes content, it is variable, between 0.47% [46] and 5% [49]. As can be noted by comparison of **Tables 1** and **2**, a lower terpenes content is employed regarding essential oils content.

Moreover, particle size [24, 39, 44] and drug release [37, 45] are affected by limonene content. Particle size is a key feature in skin permeation. In general, carriers with smaller particle sizes are more efficient in promoting permeation [65]. As for drug release, a higher limonene content caused a lower drug release and possibly generating a prolonged release [37]. Limonene is then the most suitable terpene to provide a greater release of lipophilic drugs due to its lower boiling point [37].

Several studies have shown a better performance of limonene entrapped into lipid carriers [28, 37, 39, 63] for transdermal drug release [37, 39, 44] or drug topical release [11, 36]. Despite nanosystems itselves also having the ability to promote skin penetration [66], nanoencapsulation of limonene [40, 63] further increases the penetration of drug into skin [40, 63]. In this sense, limonene-pegylated liposomes improved skin permeation of galantamine in relation to pegylated liposomes [40].

About carriers containing terpenes, invasomes [29, 39], bilosomes [30], microemulsions [31] and nanoemulsions [36] are described. Invasomes provide a greater drug skin permeation [39, 63] than conventional liposomes [63] and other nanocarriers [29]. Solid lipid nanoparticles and nanostructured lipid carriers promote an occlusive effect which causes an increase drug penetration [65]. In turn, lornoxicam-loaded in bilosomes had a


#### **Table 2.**

*Nanocarriers bearing citrus terpenes.*

transdermal release resulting in a better anti-inflammatory effect. As bilosomes have bile salts in their composition, a higher skin permeability can be achieved [30]. Oppositely, nanoemulsions and microemulsions are used to increase solubility of low-solubility drugs which results in an enhanced topical delivery [31, 36].

Moreover, nanoemulsions [36, 61, 62], transfersomes [11], invasomes [63] and microemulsions [61] are also employed for purposes of skin topical effect. Phenylethyl resorcinol loaded in elastic liposomes had a lower tyrosinase activity along with a higher skin deposition [63]. Transfersomes improved skin deposition of cyclosporin and would be an interesting topical treatment for psoriasis [11]. Resveratrol nanoemulsions [36] and astaxanthin self-nanoemulsifying systems [61] had a higher photostability [36] and a higher antioxidant ability and then can be used in the treatment of different skin conditions [61]. Additionally, due to limonene, there was an improved deposition in skin layers to astaxanthin [61] and resveratrol [36] loaded in delivery systems.

Concerning systemic delivery, nanocarriers can improve drug bioavailability [28, 44], the amount of drug that reaches the circulatory stream and is directly related to the pharmacological effect [67]. Avanafil loaded-invasomes transdermal films [28], raloxifene in transfersomes gels [44] and dapsone-loaded invasomes [37] had a higher bioavailability than non-nanotechnological formulations. The increased bioavailability suggests a possible use of skin as an alternative route of drug administration, notably for drugs with low oral bioavailability [29, 44].

Other report showed different performance of ibuprofen-microemulsions and ibuprofen-nanoemulsions for topical delivery. Microemulsions provided a greater drug release while nanoemulsions were the most effective regarding skin penetration. The better skin penetration for nanoemulsions probably is due to its higher terpene content, and the higher drug release for microemulsions is caused by its higher interfacial area. Aside from this, nanoemulsions also had a higher cell viability. Hence, nanoemulsions were the most appropriate carriers for cutaneous delivery [62].

Recently, a skin pre-treatment with ultrasound increased cutaneous deposition of agomelatine-loaded invasomes [39]. Similarly, microneedling pre-treatment increased the permeation of tizanidine-loaded invasomes [45]. Therefore, a combination of physical and chemical methods of skin penetration has been designed to improve transdermal delivery [68].

## **4. Conclusion**

Citrus plants are cultivated on all continents, and derived products are used worldwide. Although citrus oils are being widely used as fragrances, they have limited use in dermatological treatments. A trend over plant ingredients loaded in nanocarriers for topical products is seen due to the several benefits provided by vegetal phytocompounds. In addition, further development of products bearing plant ingredients can be increased by its nanoencapsulation in drug delivery systems.

A few studies report clinical trials of nanoformulations based on essential oils or on citrus terpenes. It is expected that more safety and efficacy on humans will be conducted so that new nanomedicines become available. Hence, the quality of life of patients having diseases whose treatment is side effects associated may be improved. Likewise, the improvement of transdermal drug release by combination of several skin penetration enhancers may be a potential approach to develop less invasive pharmacological treatments.

*Citrus Essential Oils and Nanosystems towards Skin Delivery DOI: http://dx.doi.org/10.5772/intechopen.110406*
