To date, the only CBD formulation approved by the FDA for the treatment of rare forms of epilepsy is Epidiolex^®, CBD in an oral solution (100 mg/mL), with maximum recommended doses of 20 mg/kg/daily. Currently, there are numerous clinical trials of CBD for the treatment of different disorders, including palliative care in cancers, where CBD is delivered predominantly as an oil solution, orally, or via inhalations (https://clinicaltrials.gov/ct2/results?cond=&term=cannabidiol&cntry=&state=&city=&dist=, accessed on 14 February 2022).

Intravenous CBD injection is an alternative delivery method, which prevents GI degradation and has demonstrated better bioavailability. It was tested and compared with other delivery methods in studies in humans and mice (Table 2) [142,143,145]. Intravenous administration caused higher CBD plasma levels than oral administration [145], smoking [142], or inhalation [143] (Table 2). In healthy volunteers, the injection of a 20 mg/kg dose resulted in a rapid rise in the plasma concentration, ranging from 358 to 972 ng/mL (1–3 μM), which was approximately five times higher than by smoking [142]. Although these concentrations are close to the cytotoxicity range reported for some tumors (discussed in Section 2), plasma CBD levels had dropped drastically within 1 h of administration [142]. Similar results were obtained in a murine model, with an immediate plasmatic concentration rise to 3000 ng/mL (approx. 10 μM), when 10 mg/kg was injected, followed by a rapid (within 1 h) tenfold drop [145].

Thus, any administration route of free CBD resulted in a transient rise in the plasmatic drug level, where only the maximal levels are comparable to cytotoxic concentrations. Importantly, bioavailability in cancer tissue is expected to be significantly lower than in plasma and highly variable, depending on the cancer type, tumor size, geometry, and vascularization. On the other hand, achieved plasma concentrations are sufficient to cause undesirable side effects (Section 6). Thus, increasing the dose of pure CBD by any administration method should not be considered as an appropriate strategy for CBD delivery for cancer treatment. Instead, alternative formulations, aimed to increase CBD’s stability and its specific targeting to the cancer tissue, should be developed.

Multiple nanoformulations have been proposed to overcome the delivery challenges of hydrophobic unstable drugs such as CBD. There are several excellent comprehensive reviews discussing in detail the best approaches to design nanocarriers (NC) for cancer therapeutics [146,147,148]. There are various important criteria that should be taken into consideration. NC should be composed of biocompatible nontoxic and non-immunogenic materials. According to their chemical structure, NP can be categorized into different groups, such as inorganic, polymeric, liposomas, nanomicelles, etc. In inorganic nanoparticles, the core is composed of metal or metal oxide (silver or gold are frequently used). Polymeric NC are produced using a conjugation of several polymers with desirable characteristics. Liposomes are nanoparticles with an aqueous interior part, surrounded by one or more concentric bilayers of amphipathic lipids (e.g., phospholipids). The design of such NC can be developed according to therapeutic requirements. Their diameter ranges normally from 1 nm to several μM. Consequently, such liposomes can be distributed in the bloodstream (smallest capillary diameter is approximately 5–6 μM) and accumulated in the target tumors. The ultra-filterable range of less than 200 nm provides the possibility for sterilization. Covalent linkage of NC to polyethylene glycol (PEG), so-called PEGylation, decreased significantly their immunogenicity. Moreover, such a modification changes the physicochemical and hydrodynamic properties, which results in a prolonged circulation time and reduced renal clearance [149]. NC easily incorporate drug molecules and form a barrier around therapeutic agents, preventing the premature drug interaction with body fluids and immune cells before their delivery to the target site. A precise design, which takes into consideration the material, size, and shape of NC, may provide drug release in a controlled and predictable fashion. This approach is also useful for the delivery of two or more drugs simultaneously, which can be very useful for cancer treatment, considering multi-drug chemotherapeutic protocols. Moreover, the nature of the core molecules may provide the possibility to combine both hydrophobic and hydrophilic drugs at the same time. In liposomes, hydrophobic drugs are incorporated into the lipid membrane, whereas hydrophilic compounds are present within the central aqueous cavity.

Target-specific drug delivery can significantly decrease side effects and increase the therapeutic index of encapsulated drugs. Passive and active targeting of nanoparticles can be used for cancer therapy. Passive targeting is possible due to the phenomenon known as the enhanced permeability and retention (EPR) effect in solid tumors [147,150,151,152,153]. In rapidly growing tumor tissue, characterized by the overexpression of vascular endothelial growth factor (VEGF), the microvasculature is characterized by a chaotic ramification with enhanced endothelial porosity or fenestration, in contrast to the tighter endothelial structures of normal capillaries. As a result of the changed cytoarchitecture, the blood flow is slower, and, due to the high porosity, tumor capillaries are leaky. Both these factors ensure the retention of enlarged particles, such as NC, in tumors. In hematological malignances, the bone marrow (BM) leukemic niche is the target tissue. Blood vessels supplying BM (sinusoids) possess the fenestrations and are semipermeable, providing favorable conditions for the accumulation of NC [154]. At the same time, the EPR effect was reported to provide a relatively modest, twofold enhancement of the nanodrug retention in tumor tissues, when compared with healthy organs [155].

The surface of NC can be modified to improve their targeting to tumors. A variety of ligands/antibodies to specific antigens, expressed by cancer cells, can be proposed for NC surface engineering [146]. Dual-action CXCR4-targeting liposomes were developed and proposed for drug delivery and the simultaneous blockage of the CXCR4/CXCL12 axis for leukemia treatment [156]. HER2-targeted liposomes were accumulated in the tumor tissue of patients with HER2-positive breast cancer [157]. The RGD (arginyl/glycyl/aspartic acid) motif was proposed to target integrins to tumor cells [158]. Anionic liposomes were shown to accumulate in BM and were then predominantly adsorbed by leukemic cells [154]. Hyaluronic acid, which shows a high binding affinity for the CD44 adhesion molecule, is present at enhanced concentrations in a variety of tumors and was also proposed for NC modification [159,160]. Experimental trials of novel delivery methods for CBD in cancer therapy are still scarce but have demonstrated promising results (Table 3) [161,162,163,164,165,166,167,168].

Gold PEGylated nanodrones were proposed recently to target lung cancer with cannabinoids and radiosensitizers [161]. The efficiency of two administration routes, inhalation and intravenous, was tested in transgenic mouse models bearing lung adenocarcinoma. The particle size (100 nm) was optimized to ensure an increased circulation time and efficient tumor uptake. Additionally, drones were functionalized with the RGD (arginyl/glycyl/aspartic acid) motif to target integrin receptors on the lung tumor cells’ surface. Both administration routes provided efficient nanodrone penetration into the tumor tissue, but the inhalation route was more promising for this tumor type. CBD was proposed to be conjugated to the amine groups present on the PEG. However, CBD-conjugated drones have not been tested yet.

The efficiency of a micellar delivery system for targeting cannabinoids to cancer tissue was tested in a murine model of triple-negative breast cancer [162]. In this case, micelles were loaded with the synthetic cannabinoid WIN55,212-2. The average micelle size was 152 nm, ensuring their accumulation in the tumor by the EPR. WIN, being conjugated with the micellar system, efficiently inhibited tumor growth. Remarkably, predominant micelle accumulation in the tumor was demonstrated, indicating the viability of the micellar system for its use with cannabinoids.

CBD-loaded poly-ε-caprolactone microparticles, as an alternative delivery system for long-term CBD administration, demonstrated their efficiency in inhibiting glioblastoma growth and tumor angiogenesis in a murine xenograft model [163].

More recently, poly-(lactic-co-glycolic acid), PLGA, microparticles, loaded with CBD, were tested for their potential to improve the conventional chemotherapy of breast and ovarian cancers [164,165]. PLGA is approved by the FDA for use in parenteral release systems. The mean particle size was around 25 μM, with a high entrapment efficiency in the tumor tissue. Particles were sterilized by gamma irradiation (25 kGy). Since sterilization accelerates the polymer erosion, a CBD:polymer ratio (10:100) was selected to ensure a durable release profile. Remarkably, a single administration of this formulation ensures the antitumor activity in vitro for at least 10 days. CBD-loaded microparticles were effective as a monotherapy, but synergism with DEX (breast cancer) and paclitaxel (breast and ovarian cancer) allowed a more pronounced effect at a single administration. However, a particle size in the μM range is not suitable for intravenous injections, because only particles smaller than 5 μM can freely circulate in the bloodstream and reach the tumor site. Afterwards, PLGA CBD-loaded nanocarriers for i.p. administration in ovarian cancer treatment were developed, which demonstrated improved CBD stability, its long-lasting release, internalization by cancer cells, and anticancer efficiency [165].

Drug delivery to brain malignancies such as glioma/glioblastoma is restricted by the blood–brain barrier (BBB). Aparicio-Blanco and colleagues proposed the original strategy of non-immunologic BBB targeting using NC decorated (functionalized) with CBD [166,167]. They elaborated small lipid nanoparticles with a size range of 10–100 nm, carrying CBD on their surface, which were able to pass through the BBB. CBD-decorated particles were suggested to target the brain endothelium, which expresses different surface molecules able to bind CBD, namely the CB1 receptor, the G-protein-coupled receptor 55 (GPR55), and serotonin receptor 5-HT. After the brain endothelium transcytosis, these particles were expected to target glioma cells overexpressing CB1/2 receptors. As far as CBD was reported to be cytotoxic for glioma, lipid nanoparticles were loaded with CBD and tested as prolonged-release carriers for glioma therapy [166,167]. This strategy was demonstrated to enhance the glioma targeting, and a combination of CBD loading with CBD functionalization significantly reduced the IC₅₀ values. CBD decoration was confirmed to enhance the passage of lipid nanoparticles across the BBB both in vitro (human brain endothelial hCMEC/D3 cells) and in vivo (mouse glioma xenograft models).

An RGD proteinoid polymer was synthesized and used to encapsulate CBD [168]. Resulting nanoparticles inhibited tumor growth in xenograft mouse models of colorectal and breast cancer and were proposed for further trials.

The possibility of the delivery of two or more drugs simultaneously by nanocarriers is of special interest for the inclusion of CBD into chemotherapeutic protocols, taking into the account the fact that CBD improves the effect of various anticancer drugs. Importantly, there are several anticancer drugs that are already clinically used in liposomal formulations for chemotherapeutic protocols [169]. Among them are doxorubicin (Doxil^®, 1995 and Myocet^®, 2000), danourobicin (DaunoXome^®, 1996), cytarabine (Depocyt^®, 1999), mifamurtide (Mepact^®, 2004), vincristine (Marquibo^®, 2012), and irinotecan (Onivyde^TM, 2015). Recently, pure CBD, encapsulated in a lipid bilayer for enhanced CBD delivery (liposomal CBD), was developed by InnoCanFarma (https://www.newsfilecorp.com/release/72614/Innocan-Pharma-Announces-Successful-Production-of-CBD-Loaded-Liposomes-under-Aseptic-Conditions, accessed on 14 February 2022).