Perspectives on Clinical Translation of Smart Nanotherapeutics

Keywords::

• cancer therapy
• clinical translation
• nanotechnology
• nanotherapeutics

Smart nanocarriers have shown great promise in the delivery of various therapeutic payloads to primary and metastatic tumor sites via the enhanced permeability retention (EPR) effect [1–4]. To be efficacious, cancer nanotherapeutics require the following characteristics:

▪	Nanocarriers need to be biodegradable and nontoxic;
▪	Scale-up synthesis need to be feasible and formulation protocol in the clinic by the pharmacist convenient;
▪	The nanocarriers need to be highly stable in blood circulation, with minimal premature drug release;
▪	Minimize uptake by the reticuloendothelial system in the liver, spleen, lung and bone marrow, the nanoparticle drugs need to be small (<100 nm in diameter) and with a ‘stealthy‘ surface;
▪	Delivery of the nanoparticle drug into the tumor sites needs to be high and sustained;
▪	Efficient uptake of the drug-loaded nanoparticles into the tumor cells enhances efficacy and overcomes drug resistance;
▪	Efficient drug release from the nanocarriers at the tumor site or inside the tumor cells;
▪	Drug release from the nanocarriers triggered on demand allows the oncologists to tune the therapeutic index to the patient‘s advantage.

Biodegradable & nontoxic

Since most hard nanoparticles are not biodegradable, acute and long-term toxicity of these agents are of concern. It is unlikely that quantum dots comprised of toxic material such as CdSe, even if encapsulated with inert polymer, will ever be approved for in vivo clinical applications. Gold, carbon, iron and possibly silica are probably less toxic, but they are not easily biodegradable. ‘Soft‘ nanoparticles such as polymer-based micelles or dendrimers are generally biodegradable and are then metabolized and removed by the liver and kidneys. Depending on chemical composition, some soft nanoparticles can be very toxic [5–7]. It is, therefore, very important to understand the biodistribution of the nanocarriers and their components to ensure that they are biodegradable and nontoxic to the patient.

Minimal premature drug release into the circulation

Many nanocarriers may be stable in saline but after intravenous administration, these nanocarriers will encounter many different blood components, some of which could be detrimental to their stability, leading to premature drug release. For example, drug-loaded micelles, unless crosslinked covalently, will dissociate upon interaction with lipoprotein particles (HDL, LDL, VLDL and chylomicron) ubiquitously present in the blood. We have recently used electron paramagnetic resonance studies with spin-labeled telodendrimers to demonstrate that covalent crosslinking of the micellar components avoids dissociation of the nanocarrier and premature drug release. It is clear from the few in vivo studies reported in the literature that reversibly crosslinked micelles can alleviate the problem of premature drug release and therefore significantly improve their therapeutic efficacies [8–13].

Minimal reticuloendothelial uptake

To minimize uptake into the reticuloendothelial system of liver, spleen, lungs and bone marrow, the nanocarriers need to be smaller than 100 nm in diameter. Large nanoparticles tend to cumulate in the liver and lungs. Systematic study with micellar nanoparticles, decorated with a varying number of acidic and basic amino acids on the surface, suggests that uptake by the liver and lungs can be minimized by incorporating slightly negatively charged residues to the nanoparticle surface [14,15].

Tumor targeting

Although nanoparticle drugs less than 100 nm in diameter, even without targeting ligands, can be preferentially taken up by tumors via EPR effects, it has been demonstrated in xenograft models that the addition of tumor-targeting ligands to the nanoparticle drug can enhance anti-tumor efficacies [9,16,17]. Some ligands can induce receptor-mediated endocytosis, leading to the delivery of a high concentration of drugs inside the tumor cells, and perhaps escape drug efflux caused by the P-glycoprotein, thus, overcoming, drug resistance.

Most of the published work on reversible crosslinked nanocarriers relies on the reductive microenvironment inside cancer cells or the acidic pH within the endosomes of the target cells [8–12]. Therefore, to be fully efficacious, the nanoparticle drugs need to be delivered inside the target cancer cells. This can be achieved with nanocarriers decorated with cell surface-targeting ligands or antibody molecules that facilitate endocytic uptake [18–20].

On-demand drug release

We reported on the use of on-demand cleavable linkers to crosslink the nanocarriers. This not only minimizes premature drug release in the circulation, but also allows one to administer exogenous cleavage agents at the desired moment to trigger drug release at the target tissue for enhancing therapeutic effects [9,10]. In the murine ovarian cancer xenograft model, administration of N-acetyl cysteine (a reducing agent) 24 h after each dose of paclitaxel-loaded micelle is superior to paclitaxel-loaded micelle alone [9]. For human applications, the optimal time to give the cleaving agent will need to be worked out.

Scale-up synthesis & convenient formulation protocol in the clinic

Preparation of nanocarriers at the laboratory scale has been reported by many investigators. Once they have been proven to be efficacious in xenograft models and nontoxic in two mammalian species, preparation of these nanoparticle drugs needs to be scaled up for Phase I clinical trials. For some nanoformulations, scale-up synthesis may be very difficult if not impossible. For successful clinical translation, scale-up production of the nanoparticle drugs needs to be economical, reproducible and robust. Shelf-life of the nanoparticle drug needs to be measured in months, not days or hours, and the range of particle size needs to be narrow. Importantly, reconstitution protocols in the clinic needs to be very simple and reproducible. Ideally, the drug-loaded nanoparticles are prepared in lyophilized form with a shelf life of at least 1 year, and what the pharmacist needs to do in the clinic is simply to reconstitute it with water or normal saline and gentle mixing prior to administration to the patients.

Route of administration

The most common route of administration of nanoparticle drugs is the intravenous route [21–23]. This allows systemic delivery of the nanoparticle drug throughout the blood stream, but preferential uptake by the primary and metastatic tumors via EPR effects. The EPR mechanism, however, may not be applicable to small metastatic lesions with few blood vessels. Additional therapies that can reach these small metastatic lesions will be needed. These include free drugs, ligand–drug conjugates or antibody–drug conjugates.

For tumors that are confined regionally, for example, stage III ovarian cancer, where the tumor is present within the abdominal cavity, one may give the nanoparticle drug both intravenously and intraperitoneally. In fact, combination intravenous and intraperitoneal administration of paclitaxel is a common practice for this disease [101]. Intratumoral injection of nanoparticle drug under computed tomography guidance is feasible, and the nanoparticles are expected to be retained at the tumor sites longer than free drugs. This may be useful for the treatment of a regional disease but not for patients with multiple metastatic lesions. Intratumoral injections of a nanoparticle drug may also be useful for the treatment of primary brain cancer since treatment options for this disease are very limited. Pulmonary inhalation of nanoparticle drugs is certainly useful for benign pulmonary diseases such as asthma. It may also be an alternative route for the treatment of lung cancer or tumor metastasis to the lung, but this will need further study.

Intrathecal chemotherapy is routinely administered to patients with acute lymphocytic leukemia and selected patients with lymphoma. This is given for prophylaxis against tumor metastasis to the CNS. Intrathecal chemotherapy is also given to patients with meningeal carcinomatosis. The only two cancer drugs safe to be used in such clinical settings are cytosine arabinoside and methotrexate. There has been no change in this clinical practice in the last few decades. However, with the advances in smart nanocarriers, it is likely that in the future, potent chemotherapeutic drugs or target-specific drugs encapsulated in nanocarriers can be safely given to the patients intrathecally.

Oral administration of nanotherapeutics is possible, but the nanoparticles need to be protected from gastric acidity, perhaps with enteric-coated gel-caps. In addition, to simulate viral entrance through the gut epithelium, the nanoparticles need to be decorated with ligands that bind to endothelial receptors and delivered intracellularly via receptor-mediated endocytosis.

Therapeutic payload & theranostic agents

The majority of the reported therapeutic payloads used in nanoparticle drugs are small-molecule chemotherapeutic agents such as paclitaxel, doxorubicin, vincristine and cisplatin. Small-molecule target-specific drugs such as tyrosine kinase inhibitors, proteasome inhibitors or m-TOR inhibitors are also feasible. Many investigators have also employed nanocarriers to deliver siRNAs and protein drugs to target tissues, but with limited success. Progress in clinical translation of cancer vaccine has been slow. Multifunctional smart nanocarriers may offer a relatively new platform for the development of cancer vaccine.

Many investigators have incorporated radiotracers into nanoparticles for whole body radioimaging [24,25]. In conjunction with loaded conventional chemotherapy or other target-specific drugs, these radiolabeled nanoparticles can serve as theranostic agents. There is evidence that nanoparticles can be preferentially delivered to, and retained at, the tumor site. Conceivably, nanocarriers loaded with chemotherapeutic agents and therapeutic radionuclide, such as ⁹⁰Yt, will allow effective treatment of metastatic cancer with combination chemoradiotherapy [26].

Conclusion

There have been rapid advances in the nanotherapeutic field in the past decade. Many smart nanocarriers have been developed, some of which have great therapeutic potential. However, there remain many challenges in translating these smart nanoparticle drugs into the clinics. Nonetheless, we believe the future of nanotherapeutics is bright, especially for reversibly crosslinked nanocarriers decorated with cancer-targeting ligands that can promote endocytic uptake into tumor cells. Such an approach has the potential of overcoming drug resistance that is often seen in cancers refractory to standard chemotherapies. In addition, many drugs that had previously failed because of formulation or toxicity issues, may possibly be resurrected by employing smart nanocarriers for effective delivery.

Acknowledgements

The authors of this article would like to thank J Kugelmass for editorial assistance.

Financial & competing interests disclosure

KS Lam is the founding scientist of LamnoTherapeutics, a start-up on cancer drug development. KS Lam acknowledges financial support from NIH/NCI R01CA115483, NIH/NIBB R01EB012569 and the Prostate Cancer Foundation Creative Award. J Lee acknowledges financial support from Hyundai Hope on Wheel Young Scholar Award. The authors have no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed.

No writing assistance was utilized in the production of this manuscript.