Transdermal drug delivery (TDD) has become a viable alternative to conventional routes of drug administration since it can avoid the hepatic first pass effect, improve the compliance of patients, decrease the administration frequency, and reduce the gastrointestinal side effects. Despite its great potential, delivery of most drug molecules via a transdermal route remains one of the major challenges in the development of transdermal drug delivery systems (TDDS). The principal barrier to TDD is located in the stratum corneum (SC), the outermost layer of the skin, thereby limiting percutaneous absorption . The SC is composed of 15~20 layers of flattened cells with no nuclei and cell organelles separated by an intercellular lipid domain. The structure of the SC can be described in terms of a so-called “brick-and-mortar” model, with the keratin-filled corneocytes as the bricks and the intercellular lipids as the mortar. The lipids, comprised of 50% ceramides, 25% cholesterol, 15% free fatty acids, as well as low levels of phospholipids , are organized in orderly-arranged lamellar layers and thus form an impermeable barrier to drug diffusion [2,3,4,5].
There are mainly three possible routes for percutaneous penetration of drug molecules, which include intracellular diffusion across the SC corneocytes, permeation through the SC intercellular lipid spaces, and penetration through skin appendages . Among these options, the scientific community agrees that the intercellular lipid domain of the SC is the main pathway for the skin penetration of most drug molecules [1,7].
To achieve therapeutically effective drug levels at the proper site following TDD, the barrier properties of the SC must be modified to enable sufficient drug permeation. A lot of approaches have been used to alter the SC barrier properties, and the most commonly applied approach is the application of penetration enhancers (PEs), which have been used in TDDS since the 1960s . Until now, efforts have been directed at identifying desirable PEs which possess safe yet effective properties.
Due to their high enhancement effect and low skin irritation, terpenes of natural origin are now receiving much attention in pharmaceutical and cosmetic formulations as PEs [8,9]. Terpenes, primarily extracted from medicinal plants, are volatile compounds with molecular components that are composed of only carbon, hydrogen and oxygen atoms. The basic chemical structure of terpenes consists of a number of repeated isoprene (C5H8) units which are used to classify terpenes. They are generally regarded to be safer compared to synthetic Pes which include surfactants, fatty acids/esters, and solvents . Furthermore, a few terpenes (e.g., 1,8-cineole, menthol, and menthone) are included in the list of Generally Recognized As Safe (GRAS) agents issued by the US Food and Drug Administration .
This review aims to give an overview of terpenes as PEs for use in TDD, which will be helpful to researchers working on TDDS in the selection of a suitable terpene.
2. Skin Penetration Enhancement Effect
Many publications have already provided substantial evidence that terpenes are capable of enhancing percutaneous absorption [11,12,13,14,15,16].
Compared to conventional synthetic PEs (e.g., oleica acid, azone, dimethyl sulfoxide (DMSO), ethanol), natural terpenes have been shown to improve the permeation of both lipophilic and hydrophilic compounds. One study was focused on bulfalin which is a drug molecule suitable for TDD (molecular weight = 386.5, log P = 2.78). The feasibility of using different PEs to reduce the permeation barrier was evaluated. The results of skin permeation studies of bulfalin demonstrated that terpenes (1,8-cnieole, d-limonene, and l-menthol) were the most effective among different PEs. The enhancement ratios (ERs) of 5% 1,8-cineole, d-limonene, and l-menthol were determined to be 17.1, 22.2, and 15.3, respectively. In comparison, other synthetic PEs at the same concentration increased the flux of bulfalin by less than 6-fold. The ER values were determined to be 5.1, 5.2, 2.5, 2.4, 1.4, and 5.3 for oleic acid, lauric acid, SDS, azone, ethyl oleate, and ethyl lauric acid, respectively . In another study, different PEs were incorporated into the gel to improve the skin permeation of hydrophilic lidocaine hydrochloride. The ER values of DMSO, urea, sodium lauryl sulfate (SLS), and menthol were determined to be 1.13, 1.72, 2.59, and 3.72, respectively . In addition, the synergistic effect of terpenes and iontophoresis has been demonstrated and utilized to increase the percutaneous absorption of oligonucleotides .
The effects of PEs on the bioavailability of meloxicam (MLX) gels were investigated and compared after TDD to rabbits. After application of the control gel without PE, the drug was detectable but not quantifiable in plasma. In contrast, after administration of the gel containing 5% menthol, MLX appeared in plasma immediately and reached the maximum peak concentration in about 4 h. It was found that the 5% menthol gel delivered 3.93 ± 0.85 mg of MLX into the systemic circulation compared to 1.41 ± 0.24 mg of MLX delivered by 1% oleic acid gel .
Using 1% 1,8-cineole as a PE, valsartan transdermal gel was prepared and evaluated. The pre-clinical evaluation of the antihypertensive efficacy of the valsartan gel was carried out using experimental hypertensive rats. The gel was applied to the rat abdominal skin area and the blood pressure values from the tail were recorded at different intervals up to 24 h. The valsartan gel containing 1% 1,8-cineole was found to reduce the blood pressure remarkably (p < 0.001) to about the normal value and maintain its level for 24 h. Conversely, no reduction in blood pressure was observed with the control gel without a PE .
Propranolol hydrochloride (PH) can be used to treat infantile hemangiomas. To formulate the PH gel, nine terpenes were compared and 3% farnesol was found to be the most effective. The final PH gel used hydroxypropyl methylcellulose (HPMC) as the matrix material and used 3% farnesol as the PE. In clinical tests, the PH gel was proved to be an effective treatment option for superficial infantile hemangioma considering its wonderful clinic efficacy without obvious side effects .
The terpenes used to increase drug penetration are summarized in Table 1. As PEs, the most commonly used terpenes include 1,8-cineole, menthol, limonene, menthone, nerolidol, and others. It should be noted that 25 out of 28 (89.29%) terpenes are oxygen-containing terpenes.
The penetration enhancement effect of these terpenes is summarized in Table 2.
It should be emphasized that the penetration enhancement effect of terpenes on the SC may be different in different vehicle systems due to the differences in physico-chemical properties of these solvents and their interactions with the SC . Co-solvents like propylene glycol (PG) or ethanol have synergistic effects when added to the terpenes. In addition, other factors including skin type, pH values, and formulation ingredients should also be taken into account as the sources of experimental variabilities.
3. Mechanism of Action
It is widely agreed that PEs may enhance the skin penetration of a drug molecule by acting on the SC intercellular lipids via extraction or fluidization and/or by increasing the SC partitioning of the drug and/or by modifying the keratinized protein conformations . As demonstrated in Table 2, terpenes possess high enhancement activity for both hydrophilic and lipophilic drugs even at low concentration. It seemed that terpenes interact with SC components by more than one mechanism, although their interactions with SC intercellular lipids could be the key mechanism.
3.1. Effect on SC Lipids
The effect of terpenes on SC lipids mainly involves the interactions at two sites, namely the lipophilic tails of the intercellular lipid and the polar head groups, affecting both lipoidal intercellular and polar transcellular pathways. Nowadays, the former route has attracted more attention.
To elucidate the mechanism of action, attenuated total reflection-fourier transform infrared spectroscopy (ATR-FTIR) or FT-IR spectrometry studies were often applied to investigate the biophysical alterations of the skin barrier which could be attributed to its capability of obtaining the conformation information of the SC lipids and keratins. Stretching peaks near 2850 cm−1 (C-H symmetric stretching absorbance frequency peak), 2920 cm−1 (C-H asymmetric stretching absorbance frequency peak), 1640 cm−1 (Amide I), and 1540 cm−1 (Amide II) are usually detected following the administration of terpenes to the SC . The shift to a higher frequency of C-H stretching peaks (~2850 and ~2920 cm−1) occurs when methylene groups of the SC lipid alkyl chains change from trans to gauche conformation, indicating the perturbation of SC lipids. The stronger the perturbation, the higher the C-H stretching peak position. The areas and heights of these two peaks (~2850 and ~2920 cm−1) are proportional to the amount of the SC lipids. So any extraction of the lipids by terpenes results in a decrease of peak area and peak height.
Both menthol and menthone, the classic terpenes as PEs, can enhance the skin permeability by extracting SC lipids [31,32]. Revealed by ATR-FTIR studies, compared with the control, the shift of asymmetric or symmetric C-H stretching to higher wave number was observed after treatment with menthol or menthone, despite the fact that the alteration of these peak positions was relatively weak. The results indicated that menthol and menthone could slightly interact with the lipophilic tails of skin lipids, which could contribute to the transdermal absorption of lipophilic drugs. Remarkably, menthol and menthone resulted in the significant decrease of peak areas of C-H stretching absorption peaks, indicating that they could directly extract part of the SC lipids to weaken the skin permeability barrier provided by the SC lipids. Moreover, no significant difference in the peak positions nor peak areas of two amide bonds could be observed after treatment with menthol and menthone, suggesting they had little effect on the keratin in corneocytes . In addition, it was demonstrated that the capacity of menthone in disturbing and extracting lipids was higher than that of menthol and azone .
Similar results were obtained with other terpenes [28,29]. There were obvious differences in the FT-IR spectra of the control and the terpene-treated (1,8-cineole, 1,4-cineole, rose oxide, safranal, and valencene) SC samples. Considering the peak height and area of asymmetric and symmetric C-H stretching peaks, these terpenes were demonstrated to enhance permeation of valsartan by directly extracting SC lipids . However, they did not fluidize the SC lipids as the peak shift to a higher wave number was not observed .
For other terpenes, the mechanism might be reversed. ATR-FTIR study results showed nerolidol produced significant blue shift of asymmetric and symmetric C-H stretching peaks. But the decrease of peak heights and areas for CH2 asymmetric and symmetric stretching peaks were statistically insignificant. It could be concluded that nerolidol fluidized rather than extracted the SC lipids . In order to further investigate the interaction between SC lipids and terpenes, molecular dynamics simulations could be used to reveal the detailed mechanism .
3.2. Effect on Hydrogen Bond Connection
A large number of ceramides are tightly arranged in the SC lipid bilayer via hydrogen bonding. It is the hydrogen bond connection that forms the network at the head of ceramide. The hydrogen bonding makes the lipid bilayer strong and stable, and it is needed in order to maintain the barrier trait of the SC. The tight network may be loosened by the terpenes with a functional group that can donate or accept a hydrogen bond.
It was found that menthol had the better penetration enhancement effect on ligustrazine hydrochloride than that of menthone. The structures of menthol and menthone only differed by the attached group. The hydroxy group of menthol forms a hydrogen bond with an amide group, which is much easier to form than with the ketone group of menthone. This loosens the network of SC to improve the permeation flux of the drug .
ATR-FTIR studies using a simple SC lipid model have revealed that the presence of 1,8-cineole and L-menthol reduces the amide I stretching frequency, indicating that they act mainly on polar lipid headgroups and break inter- and intra-lammellar hydrogen bonding networks [10,35].
Among the terpenes evaluated (menthol, nerol, camphor, methyl salicylate), nerol was found to produce the highest level of disruption of the SC lamellae. A hydroxyl group in the terpene molecule can form a hydrogen bond, leading to disruption of existing hydrogen bonds between the ceramide head groups in the SC bilayer .
3.3. Effect on SC Partition of Drugs
The partition of drug molecules into the SC is the first step of transdermal drug delivery, and it lays down the foundation for penetration enhancement. Therefore, increasing the partition coefficient has become one of the action mechanisms of PEs . A positive correlation between terpene uptake (menthol, thymol, carvacrol, menthone and cineole) into the SC intercellular lipid and β-estradiol partitioning enhancement was found. This indicated that terpene dissolved in the intercellular lipid domain can help to improve drug partitioning into the SC .
To measure the SC partition, the dried SC sheets were pulverized into powders. The partition coefficient of propranolol hydrochloride in the SC powder with terpenes ((+)-borneol, (+)-camphor and α-bisabolol) was found to be significantly higher than that with vehicle (p < 0.05). It is suggested that the interaction between the drug and terpenes via hydrogen bonding contributes to the enhancement of the partition coefficient. Following treatment with terpenes, the concentration of PH in the SC increased as a result of a molecular complex formation between the drug and the terpene .
Modelling studies suggest that either hydrocarbon or oxygen-containing terpenes could form complexes with drugs. It was proposed that hydrocarbon terpenes could interact with drug molecules by donor/acceptor interactions, van der Waals forces, and HBD (hydrogen bond donor)-π interactions, while oxygen-containing terpenes could interact with drugs by forming hydrogen bonds .
The effect on the SC partition may depend on the lipophilicity of the drug. A series of model drugs with a wide span of lipophilicity, namely indometacin (log P = 3.80), lidocaine (log P = 2.56), aspirin (log P = 1.23), antipyrine (log P = 0.23), tegafur (log P = −0.48), and 5-fluorouracil (log P = −0.95) were employed to study the penetration enhancement effects of camphor. The enhancement ratios of the SC/vehicle partition coefficients of model drugs were measured to be 1.68 (indometacin), 2.04 (lidocaine), 1.21 (aspirin), 0.98 (antipyrine), 1.05 (tegafur), and 0.96 (5-fluorouracil). It was indicated that lipophilic camphor could facilitate the partition of lipophilic drugs into the SC .
3.4. Effect on Physiological Reactions
Indeed, some terpenes can induce physiological reactions in the living skin, such as vasodilatation and increase of skin temperature, that can affect their efficacy as PEs.
Menthol’s ability to chemically trigger the cold-sensitive transient receptor potential cation channel subfamily M member 8 (TRPM8) receptors in the skin is responsible for the well-known cooling sensation after its application to the skin. In addition, menthol can stimulate skin nociceptors and initiate an axon reflex with subsequent release of vasodilator peptides . Moreover, an increase in skin temperature has been found after dermal administration of a mixture containing menthol .
Alkaloid, any of a class of naturally occurring organic nitrogen-containing bases. Alkaloids have diverse and important physiological effects on humans and other animals. Well-known alkaloids include morphine, strychnine, quinine, ephedrine, and nicotine.
Alkaloids are found primarily in plants and are especially common in certain families of flowering plants. More than 3,000 different types of alkaloids have been identified in a total of more than 4,000 plant species. In general, a given species contains only a few kinds of alkaloids, though both the opium poppy (Papaver somniferum) and the ergot fungus (Claviceps) each contain about 30 different types. Certain plant families are particularly rich in alkaloids; all plants of the poppy family (Papaveraceae) are thought to contain them, for example. The Ranunculaceae (buttercups), Solanaceae (nightshades), and Amaryllidaceae (amaryllis) are other prominent alkaloid-containing families. A few alkaloids have been found in animal species, such as the New World beaver (Castor canadensis) and poison-dart frogs (Phyllobates). Ergot and a few other fungi also produce them.
The function of alkaloids in plants is not yet understood. It has been suggested that they are simply waste products of plants’ metabolic processes, but evidence suggests that they may serve specific biological functions. In some plants, the concentration of alkaloids increases just prior to seed formation and then drops off when the seed is ripe, suggesting that alkaloids may play a role in this process. Alkaloids may also protect some plants from destruction by certain insect species.
The chemical structures of alkaloids are extremely variable. Generally, an alkaloid contains at least one nitrogen atom in an amine-type structure—i.e., one derived from ammonia by replacing hydrogen atoms with hydrogen-carbon groups called hydrocarbons. This or another nitrogen atom can be active as a base in acid-base reactions. The name alkaloid (“alkali-like”) was originally applied to the substances because, like the inorganic alkalis, they react with acids to form salts. Most alkaloids have one or more of their nitrogen atoms as part of a ring of atoms, frequently called a cyclic system. Alkaloid names generally end in the suffix -ine, a reference to their chemical classification as amines. In their pure form most alkaloids are colourless, nonvolatile, crystalline solids. They also tend to have a bitter taste.
Interest in the alkaloids stems from the wide variety of physiological effects (both wanted and unwanted) they produce in humans and other animals. Their use dates back to ancient civilizations, but scientific study of the chemicals had to await the growth of organic chemistry, for not until simple organic bases were understood could the intricate structure of the alkaloids be unraveled. The first alkaloid to be isolated and crystallized was the potent active constituent of the opium poppy, morphine, in about 1804.
Alkaloids are often classified on the basis of their chemical structure. For example, those alkaloids that contain a ring system called indole are known as indole alkaloids. On this basis, the principal classes of alkaloids are the pyrrolidines, pyridines, tropanes, pyrrolizidines, isoquinolines, indoles, quinolines, and the terpenoids and steroids. Alternatively, alkaloids can be classified according to the biological system in which they occur. For example, the opium alkaloids occur in the opium poppy (Papaver somniferum). This dual classification system actually produces little confusion because there is a rough correlation between the chemical types of alkaloids and their biological distribution.
The medicinal properties of alkaloids are quite diverse. Morphine is a powerful narcotic used for the relief of pain, though its addictive properties limit its usefulness. Codeine, the methyl ether derivative of morphine found in the opium poppy, is an excellent analgesic that is relatively nonaddictive. Certain alkaloids act as cardiac or respiratory stimulants. Quinidine, which is obtained from plants of the genus Cinchona, is used to treat arrhythmias, or irregular rhythms of the heartbeat. Many alkaloids affect respiration, but in a complicated manner such that severe respiratory depression may follow stimulation. The drug lobeline (from Lobelia inflata) is safer in this respect and is therefore clinically useful. Ergonovine (from the fungus Claviceps purpurea) and ephedrine (from Ephedra species) act as blood-vessel constrictors. Ergonovine is used to reduce uterine hemorrhage after childbirth, and ephedrine is used to relieve the discomfort of common colds, sinusitis, hay fever, and bronchial asthma.
Many alkaloids possess local anesthetic properties, though clinically they are seldom used for this purpose. Cocaine (from Erythroxylon coca) is a very potent local anesthetic. Quinine (from Cinchona species) is a powerful antimalarial agent that was formerly the drug of choice for treating that disease, though it has been largely replaced by less toxic and more effective synthetic drugs. The alkaloid tubocurarine is the active ingredient in the South American arrow poison, curare (obtained from Chondrodendron tomentosum), and is used as a muscle relaxant in surgery. Two alkaloids, vincristine and vinblastine (from Vinca rosea), are widely used as chemotherapeutic agents in the treatment of many types of cancer.
Nicotine obtained from the tobacco plant (Nicotiana tabacum) is the principal alkaloid and chief addictive ingredient of the tobacco smoked in cigarettes, cigars, and pipes. Some alkaloids are illicit drugs and poisons. These include the hallucinogenic drugs mescaline (from Anhalonium species) and psilocybin (from Psilocybe mexicana). Synthetic derivatives of the alkaloids morphine and lysergic acid (from C. purpurea) produce heroin and LSD, respectively. The alkaloid coniine is the active component of the poison hemlock (Conium maculatum). Strychnine (from Strychnos species) is another powerful poison.
Special methods have been developed for isolating commercially useful alkaloids. In most cases, plant tissue is processed to obtain aqueous solutions of the alkaloids. The alkaloids are then recovered from the solution by a process called extraction, which involves dissolving some components of the mixture with reagents. Different alkaloids must then be separated and purified from the mixture. Chromatography may be used to take advantage of the different degrees of adsorption of the various alkaloids on solid material such as alumina or silica. Alkaloids in crystalline form may be obtained using certain solvents.