Targeted drugs increase efficacy and reduce side-effects and costs – by reducing the volume of medication required. Man-made delivery vehicles, such as polymer and lipid particles tagged with targeting molecules, often fail to meet clinical expectations. But disguising therapeutics as human cells could offer a better approach, by camouflaging them from the immune system and taking advantage of the tissue targeting abilities of certain cells.
In September 2015, nanoengineer Liangfang Zhang and coworkers at the University of California, US, reported coating polymer nanoparticles with membranes from human platelets – blood cells that target injured blood vessels to stop bleeding. They found that the coated nanoparticles could evade the immune system in rodents and had platelet-like binding properties that could be used for targeted drug delivery.1
To make the nanoparticles, the researchers took advantage of differences in charge distribution across platelet membranes to bind the inner surface to negatively charged nanoparticle cores. This created 100nm diameter particles with the outer surface of the platelet membrane facing the right way: outwards.
Zhang treated a rat model of the blood vessel thickening disorder restenosis, which can occur after corrective surgery on blood vessels, with platelet-mimicking nanoparticles loaded with the therapeutic agent docetaxel. The platelet-coated nanoparticles selectively bound to damaged blood vessels, and reduced thickening by a factor of four, compared with docetaxel alone.
The group also tested the ability of their nanoparticles to target certain bacteria that bind to platelets and use them as shields to hide from the immune system. One example is methicillin-resistant Staphylococcus aureus (MRSA) bacteria. Mice infected with MRSA were given one-sixth of the clinical dose of the antibiotic vancomycin loaded in platelet-coated nanoparticles. This resulted in bacteria counts in the liver and spleen that were up to a thousand times lower than in mice treated with the clinical dose of vancomycin alone.
‘You can see that targeted delivery can make a difference and make the drug more effective,’ Zhang says. ‘You can achieve better therapeutic efficacy with either the same amount of the drug, or less.’
Also in September 2015, Zhen Gu, a biomedical engineer, and coworkers at the University of North Carolina, US, reported loading platelet-coated nanoparticles with two anticancer drugs – TNF-related apoptosis inducing ligand (TRAIL) and doxorubicin – and administered them to mice with breast tumours.2 Platelets are known to promote tumour growth in some cancers and assist tumour metastasis, by aggregating around circulating tumour cells in the bloodstream, protecting them from the immune system as they spread to new tissue.
The platelet-coated nanoparticles survived in the bloodstream for 30 hours, 24 hours longer than uncoated nanoparticles. They also showed greater therapeutic efficacy. ‘They not only control and inhibit the growth of the primary tumour, but they also prevent metastasis,’ explains Gu.
Gu made the nanoparticles using platelets from each mouse in the research, creating personalised nanoparticles for every animal. If this becomes a clinical treatment he says it will be personalised, with platelets taken from the patient or a close genetic relative. This would reduce the immune response – as the nanoparticles would be disguised as the patient’s own cells – increasing circulation time and, therefore, efficacy.
Zhang, however, believes personalisation is unnecessary. To demonstrate the ‘wide applicability of the technology,’ his team used human platelet cells from blood banks in their research. Although non-personalised platelet-coated nanoparticles might not have the really long circulation times reported by Gu, Zhang says their work shows they are ‘long enough for the nanoparticles to find the target’.
Donor platelets could be used as long as the blood types match, like in blood transfusions, Zhang adds. As O-negative blood is a universal donor that can be transfused to any blood group, it suggests a path to universal treatment. ‘We want to make a drug that can be used for all patients, rather than collecting the blood from one patient and making the drug for them’, Zhang explains. ‘That is doable, but you can imagine the cost.’
Harnessing blood cells for targeted drug delivery is not new. In 2012, an international team of researchers led by Ennio Tasciotti at the Houston Methodist Research Institute, US, reported cloaking silicon nanoparticles with cell membranes from leukocytes, or white blood cells, creating cell-like nanoparticles they named LeukoLike Vectors (LLVs).3
LLVs have the potential to target a wide-range of diseases, Tasciotti says. ‘The beauty of this system is that the target tissue is inflamed endothelium [the tissue that lines the interior of blood and lymphatic vessels] and inflamed vessels are the common denominator of a lot of pathologies and diseases. This is why I identified the leukocytes as the source of the cell membranes, because they target inflammation no matter what the reason: injury, infection, cancer, CVD, metabolic disease of the liver.’
In tests, LLVs were cleared more slowly by immune cells, in vitro, compared with uncoated nanoparticles, and in mice models they circulated for longer and accumulated in greater numbers at tumour sites.3,4
These hybrid nanoparticles were also found to interact with endothelium in the same way as leukocytes. When leukocytes bind to inflamed endothelium they activate a cascade of molecular pathways that cause the cells to separate slightly, increasing the endothelium’s permeability. This allows leukocytes to penetrate into tissue from blood vessels. Tasciotti has demonstrated that LLVs also increase the permeability of the endothelium, and can transport and release a therapeutic payload across an inflamed endothelium.3,5
Tasciotti is currently studying the efficacy of LLVs loaded with doxorubicin in mice cancer models. So far, he says, the results look promising and he hopes to publish them later in 2016.
Yet another approach to biologically targeted delivery, meanwhile, concerns the use of stem cells. Stem cells are attractive delivery vehicles for cancer therapies, because they are naturally attracted to cancer cells, have immunosuppresive properties and can migrate across various tissue types.
Researchers at Northwestern University in Chicago, US, aim to start clinical trials of neural stem cells loaded with oncolyotic virus (OV) for the targeted treatment of glioblastoma multiforme – the most common and aggressive form of brain cancer – later in 2016. Neural stem cells have a natural affinity for brain tumours.
‘An oncolyotic virus is an engineered virus that targets cancer cells’, explains Atique Ahmed, assistant professor of neurological surgery at Northwestern. ‘They are genetically engineered to distinguish cancer cells from normal cells and selectively infect and destroy them.’
Despite their potential, OVs can provoke severe immune response and need to be actively transported to the tumour and shielded from the immune system to achieve an effective dose at the tumour site; early results show that stem cells could be the answer.
Glioblastoma multiforme is difficult to treat because it is hard for surgeons to remove the entire tumour, and cells left behind can form new tumours. Stem cells loaded with OV should target any tumour cells left behind after surgery, Ahmed says. Used in combination with radio and chemotherapy, the hope is that this will improve patient survival.
Early results are encouraging. In 2013, the group reported that NSCs loaded with OV injected close to the brain tumour site increased median survival time in mice with glioblastoma multiforme by 46%, compared with naked OV, in combination with ionising radiation and chemotherapy.6
The researchers are currently applying to the US Food and Drug Administration for permission to trial NSCs loaded with OV in patients with newly diagnosed glioblastoma multiforme, following surgical resection. ‘Our goal is to recruit patients by September 2016,’ says Ahmed.
However, this won’t be the first time that NSCs have been involved in clinical trials as a targeted therapy. In 2013, neuroscientist Karen Aboody and her team, at the City of Hope hospital in California, US, assessed the safety of treating glioblastoma multiforme with NSCs genetically modified to express the enzyme cytosine deaminase, which converts the inactive prodrug 5-flurocytosine to the chemotherapeutic agent 5-flurouracil. In theory, due to their attraction to cancer cells, the NSCs should only activate the orally administered prodrug at brain tumour sites, resulting in localised chemotherapy with reduced toxicity to normal tissue. Before the clinical trial, this combination therapy was found to be effective, safe and non-toxic in mice.7
An intracerebral dose of CD-NSCs was administered to 15 patients with recurrent gliomas, followed four days later by a seven-day course of oral 5-flurocytosine. No dose limiting toxicity or significant immune response due to the NSCs was seen, demonstrating the safety of the technique. The researchers also showed that 5-flurouracil was produced at brain tumour sites, providing proof-of-concept for this targeted chemotherapy technique. Aboody hopes to publish the full results later in 2016.
Following the success of the first trial, a Phase 1 dose escalation study with repeat treatment rounds started in 2014. Aboody also plans to start a first-in-human trial of NSCs modified to secrete carboxyl esterase (CE), which converts the prodrug irinotecan to the chemotherapy drug SN-38, in patients with recurrent gliomas during 2016.
Along with the NSC enzyme and prodrug trials, the City of Hope hospital will be a secondary clinical site for the NSC-OV trial once it is up and running at the primary site, Northwestern University.
‘Although we are only at the very initial stages of studying this novel treatment in patients, there is tremendous potential for treatment of primary and secondary brain cancers, as well as metastatic solid tumours outside the brain,’ Aboody says. ‘In preclinical studies, the NSCs have demonstrated tumour tropism to brain cancers whether injected intracerebrally or intravenously – as they also have the ability to cross the blood-brain barrier. NSCs injected intravenously have also demonstrated the ability to target metastatic solid tumour sites in multiple organs.’
1 Che-Ming J. Hu et al, Nature, doi:10.1038/nature15373
2 Quanyin Hu et al, Advanced Mater., doi:10.1002/adma.201503323
3 Alessandro Parodi et al, Nature Nanotechnol., doi:10.1038/nnano.2012.212
4 Michael Evangelopoulos et al, Biomaterials, doi:10.1016/j. biomaterials.2015.11. 054
5 Claudia Corbo et al, Current Drug Targets, doi:10.2174/1389450115666141109211413
6 Alex L. Tobias et al, Stem Cells Transl Med., doi:10.5966/sctm.2013-0039
7 Karen S. Aboody et al, Sci Transl Med, doi:10.1126/scitranslmed.3005365
Michael Allen is a freelance science writer based in Somerset