In recent years, the interest in cannabidiol (CBD) has increased because of the lack of psychoactive properties. However, CBD has low solubility and bioavailability, variable pharmacokinetics profiles, poor stability, and a pronounced presystemic metabolism. CBD nanoformulations include nanosuspensions, polymeric micelles and nanoparticles, hybrid nanoparticles jelled in cross-linked chitosan, and numerous nanosized lipid formulations, including nanostructured lipid carriers, vesicles, SNEEDS, nanoemulsions, and microemulsions. Nanoformulations have resulted in high CBD solubility, encapsulation efficiency, and stability, and sustained CBD release. Some studies assessed the increased Cmax and AUC and decreased Tmax. A rational evaluation of the studies reported in this review evidences how some of them are very preliminary and should be completed before performing clinical trials. Almost all the developed nanoparticles have simple architectures, are well-known and safe nanocarriers, or are even simple nanosuspensions. In addition, the conventional routes of administration are generally investigated. As a consequence, many of these studies are almost ready for forthcoming clinical translations. Some of the developed nanosystems are very promising for a plethora of therapeutic opportunities because of the versatility in terms of the release, the crossing of physiological barriers, and the number of possible routes of administration.

1. Introduction

Cannabis sativa L. (Cannabaceae), commonly known as hemp, is a plant that has been used for food, fibre, and as a medicinal herbal drug for several millennia. Probably originating from Central Asia, hemp was domesticated very early throughout Eurasia, and before its classification as a narcotic, insights concerning its medicinal use date back to The Lancet and BMJ in the late 1800s.

Phytocannabinoids represent the characteristic constituents of hemp, and they are widely accumulated in the glandular trichomes on the female inflorescences of C. sativa. They represent a unique group of isoprenoid polyketides, having a resorcinol moiety, mainly represented by cannabidiol (CBD) and Δ9-tetrahydrocannabinol (THC) 

The domesticated forms of C. sativa have different CBD and THC contents, which are used to discriminate its industrial, medicinal, or recreational end uses, and to classify the species in the subtaxa.

Accordingly, three chemotypes based on the THC/CBD ratio are now available: the first having high-content THC (CBD/THC ratio: 0.00–0.05), a second one with high-content CBD (CBD/THC ratio: 15–25), and a third one with a CBD/THC ratio of 0.5–3.0

The pharmacological and toxicological interest in phytocannabinoids is due to the presence in the human body of the endocannabinoid system, which supports many physiological processes and has regulatory effects in a plethora of apparently distinct disorders, including neurological, cancer, and cardiovascular diseases.

The potential therapeutic activity of phytocannabinoids is principally related to their association with G-protein-coupled cannabinoid receptors denominated type 1 (CB1) and type 2 (CB2). However, other imperative networks include endogenous cannabinoids, such as anandamide, proliferator-activated receptor, transient receptor potential channels, and various orphan receptors (i.e., GPR55 and GPR18).

Up to now, the United States Food and Drug Administration (FDA), European Medicines Agency (EMA), and other regulatory agencies have approved a few C. sativa-derived drugs. Sativex is an oral spray dosage form that is currently legal for sale in over 25 countries. It contains both CBD and THC as the primary constituents (a blend of extracts high in THC and CBD in a roughly 1:1 ratio), and it is produced from natural cannabis without synthetic ingredients. The drug’s primary application is the managing of severe pain, and principally pain caused by sclerosis. On 17 February 2016, the European Commission granted an orphan designation (EU/3/16/1621) for delta-9-tetrahydrocannabinol and cannabidiol from extracts of the Cannabis sativa L. plant for the treatment of glioma. Some formulations are nowadays on the market under the brand names of Marinol and Dronabinol, both sold in solid dosage form (capsules) and containing as the active principle the synthetic form of THC. The principal uses of these medicines is to limit chemotherapy-induced nausea and to treat AIDS-induced anorexia and other related symptoms. Epidiolex is a further formulation on the market, sold as a liquid dosage form, and CBD (only botanically derived) is the active principle. It is indicated for the treatment of Lennox–Gastaut syndrome and Dravet syndrome in patients from two years of age. It is also used to treat tuberous sclerosis complex with other epilepsy treatments in patients aged two years and above. Both are very rare diseases, and accordingly, Epidiolex was designated as an “orphan medicine” by the EMA.

2. CBD: A Promising Drug Molecule with Biopharmaceutical Issues

CBD is the first isolated cannabinoid that dates back to the late 1800s, and it was obtained as a crystalline molecule by acetylation from a crude narcotic red oil distilled from the resin of Indian hemp. Indeed, over 500 different compounds have been identified from Cannabis sativa. In particular, about 130 cannabinoids have been identified, about 120 terpenoids, 42 noncannabinoid phenolics, 34 flavonoids, 3 sterols, and 2 alkaloids. Cannabinoids are in the leaves, flowers, resin, stembarks, and roots. Noncannabinoid phenols, including stilbenoids, phenanthrenes, lignans, and phenolic amides, are present in the leaves, flowers, stems, hemp pectin, resin, fruit, seeds, and roots. Terpenoids represent the second largest class of constituents, and they are also responsible for the typical smell of the plant. They include monoterpenes and sesquiterpenes, which are typical essential-oil constituents, in addition to diterpenes and triterpene. They are distributed in the leaves, flowers, stembarks, and roots. The leaves, flowers, seeds, and fruit contain flavonoids, while sterols and alkaloids are distributed in the stembarks, roots, and leaves. A very high content of oil, represented by saturated and unsaturated fatty acids and their glycerol esters, is contained in the seeds.

Among the cannabinoids, THC and CBD represent the principal constituents and the main psychoactive and non-psychoactive cannabinoids, respectively.

In recent years, the interest in CBD has increased because of the lack of psychoactive properties and its easy availability. Indeed, it can be obtained by purifying Cannabis sativa L. extracts, or by chemical synthesis. The interest is also due to the astonishing plethora of possible uses in therapy.

CBD has an affinity with more than 65 molecular targets. In particular, it acts as a serotonin 1A receptor (5-HT1A), regulating the cannabinoid-related receptors G-protein-coupled receptor 55 (GPR55) and transient receptor potential vanilloid 1 (TRPV1). In addition, CBD acts on the type 1 equilibrative nucleoside transporter (ETN1), fatty acid-binding protein (FABP), nuclear factor erythroid 2-related factor 2 (NRF2), voltage-activated T-type calcium channels, adenosine and glycine receptors, mu and delta opioid receptors, and voltage-dependent anion channel 1 (VDAC1), among the most relevant.

Recent reports suggest that CBD elevates the levels of the endocannabinoid anandamide (AEA) when administered to humans, suggesting that phytocannabinoids target the cellular proteins involved in endocannabinoid clearance. On the one hand, fatty acid-binding proteins (FABPs) are intracellular proteins that mediate AEA transport to its catabolic enzyme fatty acid amide hydrolase (FAAH). On the other hand, CBD inhibits the degradation, via FAAH, and uptake of endocannabinoids, resulting in an increase in endocannabinoid–receptor binding. Once it is inside the cell, anandamide is broken down by FAAH, a metabolic enzyme, as part of its natural molecular lifecycle. However, CBD interferes with this process by reducing anandamide’s access to FABP transport molecules, and by delaying the endocannabinoid passage into the cell’s interior.

Although CBD has little binding affinity for either of the two cannabinoid receptors (CB1 and CB2), CBD modulates several noncannabinoid receptors and ion channels. CBD also acts through various receptor-independent pathways; for example, by delaying the “reuptake” of endogenous neurotransmitters (such as anandamide and adenosine), and by enhancing or inhibiting the binding action of certain G-protein-coupled receptors.

At high concentrations, CBD directly activates the 5-HT1A (hydroxytryptamine) serotonin receptor, thereby conferring an antianxiety effect. This G-protein-coupled receptor is implicated in a range of biological and neurological processes, including (but not limited to) anxiety, addiction, appetite, sleep, pain perception, nausea, and vomiting. CBD also binds to TRPV1 receptors, which also function as ion channels. TRPV1 is well known to mediate pain perception, inflammation, and body temperature. Whereas CBD directly activates the 5-HT1A serotonin receptor and several TRPV ion channels, some studies indicate that CBD functions as an antagonist that blocks or deactivates another G-protein-coupled receptor known as GPR5, which is widely expressed in the brain, and especially in the cerebellum. GPR5 is involved in modulating blood pressure and bone density, among other physiological processes. Finally, GPR55 is expressed in various types of cancer, and when activated, it also promotes cancer cell proliferation.

Furthermore, CBD also exerts an anticancer effect by activating peroxisome proliferator-activated receptors (PPARs) that are situated on the surface of the cell’s nucleus. The activation of the receptor known as PPAR-gamma has an antiproliferative effect, as well as an ability to induce tumour regression in human lung cancer cell lines. PPAR-gamma activation degrades amyloid-beta plaque, which is a key molecule linked to the development of Alzheimer’s disease. Therefore, CBD can play a clinical role in patients with Alzheimer’s disease. PPAR receptors also regulate the genes that are involved in energy homeostasis, lipid uptake, insulin sensitivity, and other metabolic functions. Diabetics, accordingly, may benefit from a CBD-rich treatment regimen.

CBD functions as an anandamide reuptake and breakdown inhibitor, thereby raising the endocannabinoid levels in the brain’s synapses. Enhancing the endocannabinoid tone via reuptake inhibition may be a key mechanism whereby CBD confers neuroprotective effects against seizures, as well as many other health benefits.

CBD’s anti-inflammatory and antianxiety effects are in part attributable to its inhibition of adenosine reuptake. By delaying the reuptake of this neurotransmitter, CBD boosts the adenosine levels in the brain, which regulates the adenosine receptor activity. A1A and A2A adenosine receptors play significant roles in cardiovascular function, regulating the myocardial oxygen consumption and coronary blood flow. These receptors have broad anti-inflammatory effects throughout the body.

CBD is also an allosteric receptor modulator, which means that it can either enhance or inhibit how a receptor transmits a signal by changing the shape of the receptor. CBD interacts with the GABA-A receptor in a way that enhances the receptor binding affinity for its principal endogenous agonist, gamma-aminobutyric acid (GABA), which is the main inhibitory neurotransmitter in the mammalian central nervous system. Hence, CBD reduces anxiety by changing the shape of the GABA-A receptor in a way that amplifies the natural calming effect of GABA.

Moreover, CBD is a “negative allosteric modulator” of the CB1 receptor, which is mainly present in the brain and central nervous system. CBD does not bind to the CB1 receptor directly, as THC does; however, it interacts allosterically with CB1, modifying the shape of the receptor in a way that weakens the ability of THC to bind to CB1.

As a negative allosteric modulator of the CB1 receptor, CBD-rich products with little THC can convey therapeutic benefits without having a euphoric or dysphoric effect.

According to all these studies, many properties have been reported for CBD, and namely, antiepileptic and anxiolytic properties, together with the prevention of Parkinson’s disease, schizophrenia and psychosis, Alzheimer’s disease, and numerous kinds of tumors. In addition, CBD has pronounced anti-inflammatory activity, potent antimicrobial activities, and great effectiveness in skin-related diseases, with a worthy safety profile.

However, the numerous promising clinical uses of CBD, and substantial issues, including the low bioavailability, variable pharmacokinetic profiles, and poor stability, limit the success of CBD as a medicine. CBD has very low water solubility (12.6 mg/L) and high lipophilicity (logP of 6.3), where logP is the logarithm of a drug’s partition coefficient between n-octanol and water. In addition, CBD has weak acidic properties (pKa 9.1), a melting point of 67 °C, and a molar mass of 314 g/mol. Therefore, CBD can be classified as a Class II drug of the Biopharmaceutics Classification System (BCS), which makes it a poorly water-soluble and highly permeable drug, which is eliminated by metabolism. The literature describes the presystemic metabolism as a result of phase I oxidation, which is mainly due to CYP3A4 and CYP2C19, and phase II glucuronidation via UGT1A9. Consequently, according to the World Health Organization, the CBD oral bioavailability is approximately only 6%.

Due to the highly lipophilic nature, CBD is generally supplied as an oily or alcoholic formulation, either in soft-gel capsules, liquid solution, sublingual drops, or as an oromucosal spray. In addition, studies investigating the oral and oromucosal delivery of CBD and THC at equimolar concentrations in humans have evidenced high inter-/intra-individual variability. Finally, CBD can precipitate in the gastrointestinal tract, with a subsequent poor absorption rate.

The very low CBD oral bioavailability of CBD is due to irregular absorption because of the high lipophilicity, instability in the stomach acidic environment, and pronounced first-pass liver metabolism. The reported time to peak plasma concentration is between 1 and 4 h. After the administration of oromucosal spray (20 mg), the Cmax was 2.4 ng/mL of CBD, and the half-life was between 1.4 and 10.9 h. For the commercial preparations Epidiolex and Sativex, similar pharmacokinetic parameters are reported, but Sativex, administered as an oromucosal spray, provides a faster onset of action. Additionally, the bioavailability of inhaled CBD is much more (about 31%), but it is strongly dependent on the inhalation method and breath duration. Finally, after topical administration, an accumulation of CBD in the stratum corneum occurs without penetration to the deeper tissue layers. Indeed, although CBD has a suitable molecular weight (314.46 Da), its high logP value strongly limits transdermal delivery. A further issue of CBD is the low stability: it is easily degraded by light and auto-oxidation, and at room temperature.

3. Nanosized Strategies to Improve Biopharmaceutical Performances of Class II Drugs

According to the scientific data reported in Paragraph 2, CBD is a typical drug of the Class II Biopharmaceutics Classification System (BCS), which is widely accepted today in the academic, industrial, and regulatory worlds.

In the last decade, the studies to enhance the solubility of Class II drugs through delivery strategies have increased because almost 90% of the new drugs are poorly soluble compounds. The bioavailability of Class II drugs is related to the dissolution rate, which is ultimately related to the solubility. Consequently, an increased solubility produces an improved bioavailability in vivo. Many strategies can be used to optimise the dissolution rate, including the production of amorphous powders, and supramolecular complexes with cyclodextrins, micronisation, and nanonisation.

Nanonisation is a process to reduce the particle size of the active pharmaceutical ingredient (API) to a nanometre size range. In fact, according to the Noyes–Whitney equation, a decrease in the particle size of a drug results in an increase in its surface area, and thus the dissolution rate will increase proportionally, resulting in the better absorption of poorly soluble drugs. Additionally, the API could be formulated using nanoscale carriers, including vesicles and micelles, lipid nanoparticles, nano- and microemulsions, and polymeric and inorganic nanoparticles. These nanosystems aid in preventing drugs from being tarnished in the gastrointestinal region, and they help the delivery of Class II drugs to their target locations. A main characteristic of these nanodrug delivery systems is the high versatility in their administration routes, including the parenteral, oral, nasal, pulmonary, ocular, and transdermal routes.

The choice of the nanometre range for drug delivery loaded with Class II drugs is also linked to their ability to either cross biological barriers themselves, or to allow loaded drugs to cross them. This is a key factor for some specialised barriers, such as barriers between the blood and neural tissues, and in particular, the blood–brain barrier (BBB), which are very difficult to cross by drugs because of their function for maintaining the homeostasis of the brain by regulating the chemical environment, immune cell transport, and entry of xenobiotics. The BBB represents the most important factor limiting the development of drugs for the central nervous system, and it is characterized by relatively impermeable endothelial cells with tight junctions, enzymatic activity, and active efflux transport systems. The tight junctions formed by brain microvascular endothelial cells regulate paracellular transport, whereas transcellular transport is regulated by specialised transporters, pumps, and receptors. Nanocarriers can open the tight junctions between endothelial cells, transcytosis through the endothelial cell layer, and endocytosis by endothelial cells, releasing the drug inside the cell. Furthermore, coating agents, such as polysorbates, inhibit the transmembrane efflux systems (i.e., P-glycoprotein). Various novel drug delivery systems, such as liposomes, microspheres, polymeric nanoparticles, lipid nanoparticles, and inorganic systems, have been proposed to overcome the limitations imposed by the BBB.

4. Nanosized Drug Delivery Systems Loaded with CBD

Nanosized drug delivery systems, which received their appellation from their nanometre (nm) size (generally from a few nanometres up to some hundreds of nanometres), have increased considerably in the literature and market over the last two decades. Two approaches are generally reported: nanosuspensions, and nanovectors, which are characterised by the huge loading properties of the drug and are considered easily taken up by cells.

Nanosuspension formation can improve the solubility, permeability, and ultimately, the bioavailability. An increase in the surface area to volume ratio will result in increased cellular uptake due to the 100-fold size reduction. Nanosuspensions present a substantial enhancement of the in vivo bioavailability, they are easy to prepare, and it is easy to scale up the developed nanoformulations.

Nanosuspensions are formulated by dispersing insoluble drugs in aqueous media in the presence of appropriate excipients, followed by particle size reduction in a media mill, wherein the drug particles are broken down via bead collision. They generally need surfactants or hydrophilic polymers during the formulation to impede agglomeration and cake formation. The advantages after oral or other routes of administration are numerous, and they are represented by fast dissolution rates with a consequently improved bioavailability; even the limits are represented by the absence of vectors, with a consequent lack of controlled release, passive or active targeting, and limited protection from physical or chemical degradation.

In addition, nanovectors represent very encouraging tools because, like nanosuspensions, they are characterised by a huge surface area to volume ratio, and their ability of encapsulation efficiency allows for an extended circulation time, reduced clearance rates, increased physical and chemical stabilities, improved cell uptake, and an optimised pharmacokinetic profile. A further auspicious feature of using nanovectors is to deliver the drugs to the target site via active or passive methods due to the flexibility of the functionalisation of their surface with hydrophilic polymers or targeting ligands (i.e., peptides or small molecules, such as folic acid or antibodies). Targeting confers an increased selectivity towards the targeted cells/tissues/organs to improve their therapeutic efficacy.

According to the nature of the nanomaterials, the nanovectors can be classified as polymeric, lipid-based, and inorganic nanocarriers. Polymeric nanovectors can be derived from natural (polysaccharides and proteins) or synthetic polymers, and they can be further subclassified into biodegradable and non-biodegradable polymers. The lipid-based nanovectors include microemulsions and nanoemulsions, vesicles, solid lipid nanocarriers, and nanostructured lipid carriers. Lastly, the inorganic nanocarriers include many nanostructures, including quantum dots, carbon nanotubes, and gold, magnetic, and silica nanoparticles.