Nanotechnology and cancer therapeutics: delivering on the hype?

Nanomedicine is an umbrella term that can apply to a broad range of therapies and has gained significant traction over the past decade. Usually defined as within the range of 1–100 nm (1 nm = 1 × 10^-9 m), nanomaterials have shown promise in improving drug delivery and treatment outcomes, particularly in cancer. The field is continuing to grow rapidly but clinical translation appears to have stalled, with fewer nanotechnology-based therapies gaining approval in the last couple of years [1].

The field is rife with interdisciplinary collaboration between biophysics, chemical engineering, materials science, molecular biology and oncology. This eagerness to collaborate is understandable when considering the advantages over conventional therapy: nanoparticle-based delivery systems have been shown to improve the stability, bioavailability, and precision targeting of biologics; they allow co-delivery of multiple drugs to specific intracellular sites, including nucleic acids; and they have enabled transfer of drugs across the blood–brain barrier – a conventionally difficult task. The use of nanomedicine in cancer has shown superiority in some indications and replaced conventional drug treatments with improved efficacy, reduced toxicity, and novel treatment mechanisms [2,3].

Despite these advantages, it has proven difficult to translate the delivery systems into consistently effective cancer therapies. Results from previous clinical trials show a high degree of interpatient heterogeneity, indicating that there are still significant challenges to overcome. These include a proper understanding of the ‘enhanced permeability and retention effect’ and how it can vary, bioaccumulation and organ drug exposure, hypersensitivity immune reactions, and even quality control in manufacturing [4]. Regardless, nanotechnology remains highly attractive to both pragmatic and optimistic researchers, and as these issues are better understood the field is sure to improve.

Lipid-based nanomaterials

Research into lipid-based nanotechnology has received the most attention to date. It was the first nanoparticle-based delivery system to be developed in 1965 and has since been used as a carrier for multiple drugs and cancer types. Research currently focuses on improvements to the basic structure of lipid-based nanomaterials and how they can accumulate and release the carried drugs more effectively [1].

Lipid-based delivery systems mainly refer to the use of liposomes, solid lipid nanoparticles or nanostructured lipid carriers to deliver drug treatments to the active site (Figure 1) [5]. Liposome systems make up the majority of nanotechnology-based cancer treatments on the market and consist of a hydrophilic core enclosed within a hydrophobic phospholipid bilayer. They typically range between 20 and 1000 nm in size and can carry both hydrophilic (in the core) and hydrophobic drugs (in the bilayer), with co-delivery and controlled release being two attractive applications. The size and number of bilayers can be adjusted to alter both the half-life and load amount, which can include combinations of chemical drugs and nucleic acid polymers [2]. The system also allows for the surface layer to be readily modified for improved bioavailability and drug targeting: PEGylation is one well-known modification that significantly extends the half-life of biological therapies, but otherwise, binding various peptides to the surface can help improve bioavailability, immune evasion, and enable drug targeting [2,6].

For many of the same reasons, solid lipid nanoparticles are attractive to cancer researchers; however, they comprise a cationic phospholipid monolayer and a solid, non-aqueous core. They must be smaller than liposomes due to their structure, at around 1–100 nm, but the delivery function is the same as liposomal formulations. Additionally, the surface layer of lipid nanoparticles can be modified in much the same way as in liposomes while benefitting from better stability and prolonged release. Nanostructured lipid carriers are the most recent iteration of lipid-based nanomaterials and combine attractive qualities of liposomes and lipid nanoparticles. They still comprise the outer lipid monolayer to improve biocompatibility and non-immunogenicity, but the core matrix is composed of both solid and liquid lipids to improve stability and allow for more flexibility and capacity to accommodate different types of drugs. Nanostructured lipid carriers can be delivered in more ways due to their higher stability, and have gained attention in recent years as a viable anti-cancer drug carrier [2,7].

Doxil^® (Caelyx^® in Europe) was the first liposomal formulation to be approved for clinical use (encapsulated doxorubicin for the treatment of Kaposi's Sarcoma, 1995). The PEGylation of lipids on its outer surface conferred greater solubility and a high hydrodynamic volume; water molecules cluster around the liposome more readily, which protect it from interactions with proteolytic cells and increase its solubility in the blood [8]. Doxil demonstrated improved biocompatibility and immune evasion, while decreasing toxicity and renal clearance. Other PEGylated stealth liposomes were subsequently developed that all essentially served as delivery systems to improve the blood circulation time of a chemotherapeutic. There has been little evidence to show that stealth liposomes improve efficacy, but they have improved survival outcomes and response rates [1,8,9].

Following the clinical and market success of Doxil, other liposomal formulations that have gained market approval include; DepoCyt^® (carrying hydrophilic cytarabine for the treatment of neoplastic meningitis, 1999), Marqibo^® (hydrophobic vincristine used for hematologic malignancies, 2012), Onivyde^® (hydrophilic irinotecan for metastatic pancreatic cancer, 2015) and Vyxeos^® (daunorubicin and cytarabine for acute myeloid leukemia, 2017). The latter generated excitement as a first for encapsulating two drugs for co-delivery, allowing their synergy to be exploited for better outcomes [8–10].

The majority of preclinical research and ongoing clinical trials focus on improving active cancer targeting of stealth liposomes using surface-bound ligands or controlled release mechanisms. Some of these use antibody conjugates to increase accumulation around the target cells, e.g., HER2-targeted liposomal doxorubicin for patients with advanced HER2+ breast cancer (NCT02213744). However, thus far, there have been few noteworthy developments in this approach. The above trial was actually terminated due to a lack of observable benefits; target accessibility, expression and optimal ligand density have all proved difficult challenges to overcome [2,8,9].

Despite this, anti-EGFR-IL-dox is progressing well in trials using surface bound anti-EGFR antibodies for improved targeting towards advanced triple-negative breast cancer (NCT02833766) and high-grade gliomas (NCT03603379). Additionally, Mebiopharm Co., Ltd (Tokyo, Japan) currently have five drug products under development, one of which is a liposomal formulation encapsulating oxaliplatin in a phase II trial for the treatment of gastric and gastroesophageal junction cancer (NCT00964080). Their products differ as they aim to increase bioaccumulation around tumors via transferrin ligand expression on the liposomal surface, which binds to the more readily expressed transferrin receptors on tumor cells [8–10].

EndoTAG-1 for the treatment of locally advanced or metastatic pancreatic cancer uses a slightly different mechanism and is progressing well through Phase III trials, with positive results expected soon (NCT03126435). EndoTAG-1 encapsulates paclitaxel (a hydrophobic chemotherapeutic) in a positively charged lipid membrane, and uses electrostatic attraction to negatively charged angiogenic endothelial cells for tumor targeting. Angiogenesis is a hallmark of cancer during which tumor endothelial cells become negatively charged, making EndoTAG-1 an attractive delivery mechanism for cancers overexpressing angiogenic factors [11].

ThermoDox^® is another noteworthy product that recently completed phase III trials for the treatment of hepatocellular carcinoma with positive results (NCT00617981). This treatment uses a thermosensitive controlled released mechanism to deliver encapsulated doxorubicin to tumors. During this process, ThermoDox passively accumulates inside tumors via capillaries. The tumor is then heated to 40–45°C, triggering the release of the drug. This mechanism ensures that, regardless of the distribution of ThemoDox throughout the body, doxorubicin is only released within the tumor.

Finally, LiPlaCis^® has recently started a phase II study for advanced solid tumors and incorporates an innovative enzyme-based tumor-triggered release mechanism (NCT01861496). Cisplatin is encapsulated in a liposome designed to be particularly sensitive to the secretory phospholipase A2 enzyme, which is present on tumors. Once LiPlaCis accumulates at the tumor, the liposome is readily degraded and cisplatin is released into the tumor microenvironment [10].

Delivery of gene products

All of these examples encapsulate a chemotherapeutic approach to cancer treatment; however, immunotherapy is another field in cancer research that has exploded over the past decade, and recent progress is encouraging for the nanotechnology-driven delivery of immune system agonists. Nanotechnologies using RNA interference and mRNA-based technologies are viewed as easier to translate to the clinic than other immunotherapeutic strategies, and nanostructured lipid carriers are utilized more readily as the delivery system than liposomes. mRNA therapies can stimulate the innate immune system, and they are relatively easy to produce on a large scale. They are also readily degraded by serum endonucleases so require protection for efficient delivery.

A broad range of commercial gene therapies delivered using nanostructured lipid carriers are under development, but none have been approved for clinical use yet. Many of these use tumor-associated or tumor-specific antigens to prime the immune system against cancer. Figure 2 shows the mechanism of mRNA uptake and subsequent presentation of the translated antigen product on MHC classes I and II [12]. Antigen presentation on MHC class I triggers cell-mediated immunity and activation of cytotoxic CD8⁺ T cells, whereas presentation on MHC class II triggers humoral immunity and B-cell-driven production of tumor-associated or -specific antibodies. These mechanisms can be complimented with immunomodulatory therapies, such as immune checkpoint inhibitors. Immune activation using coding mRNA is more potent and flexible compared with antigen peptides. This flexibility means a wider range of tumor-specific T-cell epitopes can be targeted [7,12].

RNA interference technologies are receiving a similar degree of research interest but have also proved difficult to translate to the clinic. Rather than directly triggering immune reactions, short RNA sequences target oncogenes or downregulate immune-suppressive proteins to induce an antitumor response. Short hairpin RNA (shRNA), small-interfering RNA (siRNA), and miRNA are all being investigated as interference technologies. Some examples of promising targets include STAT3, PD-L1 and TGF-β/IL-10Ra [7].

The amount of research into lipid-based delivery of gene products, compared with the minimal number of advanced stage trials and regulatory approvals, indicates there are some intrinsic issues with the delivery system that must be investigated further. One of these is that, although the structure of manufactured lipid nanocarriers is highly similar to micelles or lipid nanoparticles produced by cells, the synthesized lipid constituents in manufactured lipid formulations are considered exogenous. This leads to unintended cellular metabolism or immune responses that interfere with the intended gene regulation of the nucleic acids being delivered. Non-specific gene regulation and unintended immune activation reduce therapeutic efficacy, so future developments need to investigate the off-target effects of functional lipids used in such delivery systems [7].

Paclitaxel in nanotechnology

Paclitaxel is a chemotherapeutic that started the second wave of commercialized nanomedicines following liposomes. It has been a common therapy since 1994 and has been formulated as nanoparticle albumin-bound paclitaxel (nab-paclitaxel, Abraxane^®) since 2005. Originally, paclitaxel was administered along with toxic solvents to facilitate delivery, and it was the elimination of this toxicity that drove the development of the nanomedicine. The nab platform also led to a marked increase in transport of paclitaxel across endothelial cells compared with conventional administration due to the albumin receptor-mediated drug transport [1].

To date, Abraxane has been approved for the management of advanced breast cancer, pancreatic cancer, and advanced non-small-cell lung cancer (NSCLC), with other combinations currently in trials. Nab-rapamycin (ABI-009) uses a similar mechanism and is under investigation in early trials for the treatment of pediatric patients with recurrent or refractory solid tumors (NCT02975882). The platform is being investigated for other tumor types but appears to have stalled due to similar limitations around immunogenicity, the toxicity of solvents used in manufacturing, and the relatively uncontrolled nature of accumulation and drug release [1,13].

Metallic nanoparticles

Platinum has historically been one of the most exploited metals in cancer treatments, the first being cisplatin, followed by a wealth of other platinum-based cancer drugs. Such therapies are known to be highly toxic to non-cancer cells and so delivering sufficient concentrations to malignant cells has proved limiting for overall effectiveness. Nanotechnology, therefore, presents an attractive solution to improve precision targeting and delivery of these well-studied cancer killers, particularly with some of the targeting and ‘stealth’ mechanisms described above. Nanoparticles containing metal ions, such as iron, copper, gold and silver, conjugated with peptides or nucleic acids, have gained traction in clinical trials [2,14].

Metallic nanoparticles usually consist of a metallic crystalline core and a protective shell of organic ligand molecules. They can trigger apoptosis and ferroptosis pathways intrinsically or extrinsically; inhibit angiogenesis via VEGF/VEGFR-mediated signaling mechanisms; inhibit metastasis and inflammatory cytokine release; and act as immunogens triggering certain immune responses. They are most typically used as theranostics and for magnetic imaging in cancer but advances in precision targeting have improved clinical translation as a therapy as well [2,14].

Toxicity remains a significant issue with metallic nanoparticles, and the various in vivo mechanisms of these toxic effects demonstrate why many such treatments stall at the preclinical stage. The main mechanism of cell death from metallic nanoparticles is the altered redox balance and the resultant apoptotic effect on cells, both malignant and not. The similarity in size between metallic nanoparticles and regular biological molecules means they can enter cells more readily, trigger redox reactions, and cause uncontrolled cytotoxic effects in healthy cells. Furthermore, the generation of reactive oxygen species, even in target cells, can affect neighboring cellular structures and lead to further toxicity. Bioaccumluation can also vary greatly between lymph nodes, bone marrow, the liver and kidneys, and the brain, which makes systemic administration of the treatment difficult [2,15].

NanoTherm^® is the only approved cancer therapy which uses iron oxide nanoparticles and is used to target glioblastoma. This therapy involves intratumoral delivery and subsequent heating by an alternating magnetic field, which causes cell death. AuroLase^® is a similar therapy currently in pilot trials for the treatment of prostate cancer (NCT04240639). CYT-6091 (Aurimune^™) is a colloidal gold (gold suspended in a fluid) nanoparticle therapy currently under investigation in multiple solid tumors – gold nanoparticles are sPEGylated and bound to TNF-α. It has been shown to accumulate more specifically in tumors and avoid hypotension, which had been a dose-limiting side effect with TNF (NCT00356980) [1,16].

What's on the horizon?

Plenty of researchers would agree that the massive potential of nanomedicine for cancer therapeutics has not been fully realized. Some platforms have reached market authorization as cancer treatments but require further research and fine-tuning for broader translation to the clinic, whilst others are stuck with toxicity and tolerability issues in clinical trials, and some are yet to leave the laboratory.

Carbon nanomaterials, including nanotubes and nanohorns, have received a wealth of study as drug delivery platforms for chemical agents (doxorubicin, paclitaxel) but are still undergoing preclinical studies into precise treatment mechanisms and toxicology. Nanoemulsions are colloidal formulations using emulsions to suspend anticancer drugs as solid spheres with an amorphous and lipophilic surface. Some viable treatments have been tested but understudied issues with manufacturing and the pressure placed on the active ingredient during preparation have hindered clinical translation [2,14]. Meanwhile, oncolytic virotherapy has demonstrated such success and received such focused translational research that it has practically become a separate field of study [17].

Generally, passive cancer-targeting of nanomedicines, including nanoparticle-bound platforms, lipid nanocarriers, and metallic nanoparticles, have been based on the enhanced permeability and retention (EPR) effect. The phenomenon is based on the theory that the abnormal growth of tumor cells disrupts the surrounding microenvironment and produces abnormal fluid transport dynamics, so large molecules accumulate more readily [16]. However, empirical evidence and a fundamental understanding of how the EPR effect influences bioaccumulation, extravasation, and endocytosis of drugs into tumor cells are broadly lacking. Numerous nanotechnologies prove attractive in principle and in cell studies, but a fundamental lack of understanding of how the EPR effect impacts drug delivery and cell death causes them to fall short in human and in vivo studies. Improvement of this singular issue could greatly ameliorate translation of nanotechnology-based cancer therapy [1,18,19].

Although preclinical research into the accumulation and clearance of nanoparticles should take precedence, using models, such as organoids and lab-on-a-chip, some preliminary clinical studies show the value of investigating intra- and inter-patient heterogeneity [20]. This suggests heterogeneous outcomes are linked with heterogeneous biology and certainly warrants further research with theranostics, companion imaging, and serum/tissue biomarker data [1].

It appears likely that the effectiveness of currently available nanomedicine platforms and marketed therapies can be improved via a focus on patient selection, but it can't be denied that basic research and a greater understanding are required to reap the benefits of nanotechnology in developing cancer treatments.