Benefits of Nanotechnology for Cancer
Nanoscale devices are one hundred to ten thousand times smaller than human cells. They are similar in size to large biological molecules ("biomolecules") such as enzymes and receptors. As an example, hemoglobin, the molecule that carries oxygen in red blood cells, is approximately 5 nanometers in diameter. Nanoscale devices smaller than 50 nanometers can easily enter most cells, while those smaller than 20 nanometers can move out of blood vessels as they circulate through the body. Because of their small size, nanoscale devices can readily interact with biomolecules on both the surface and inside cells. By gaining access to so many areas of the body, they have the potential to detect disease and deliver treatment in ways unimagined before now.
Biological processes, including ones necessary for life and those that lead to cancer, occur at the nanoscale. Thus, in fact, we are composed of a multitude of biological nano-machines. Nanotechnology provides researchers with the opportunity to study and manipulate macromolecules in real time and during the earliest stages of cancer progression. Nanotechnology can provide rapid and sensitive detection of cancer-related molecules, enabling scientists to detect molecular changes even when they occur only in a small percentage of cells. Nanotechnology also has the potential to generate entirely novel and highly effective therapeutic agents.
Ultimately and uniquely, the use of nanoscale materials for cancer, comes down to its ability to be readily functionalized and easily tuned; its ability to deliver and / or act as the therapeutic, diagnostic, or both; and its ability to passively accumulate at the tumor site, to be actively targeted to cancer cells, and to be delivered across traditional biological barriers in the body such as dense stromal tissue of the pancreas or the blood-brain barrier that highly regulates delivery of biomolecules to / from, our central nervous system.
Passive Tumor Accumulation
An effective cancer drug delivery should achieve high accumulation in tumor and spare the surrounding healthy tissues. The passive localization of many drugs and drug carriers due to their extravasation through leaky vasculature (named the Enhanced Permeability and Retention [EPR] effect) works very well for tumors. As tumor mass grows rapidly, a network of blood vessels needs to expand quickly to accommodate tumor cells’ need for oxygen and nutrient. This abnormal and poorly regulated vessel generation (i.e. angiogenesis) results in vessel walls with large pores (40 nm to 1 um); these leaky vessels allow relatively large nanoparticles to extravasate into tumor masses. As fast growing tumor mass lacks a functioning lymphatic system, clearance of these nanoparticles is limited and further enhances the accumulation. Through the EPR effect, nanoparticles larger than 8 nm (between 8-100 nm) can passively target tumors by freely pass through large pores and achieve higher intratumoral accumulation. The majority of current nanomedicines for solid tumor treatment rely on EPR effect to ensure high drug accumulation thereby improve treatment efficacy. Without targeting cell types expressing targeting ligand of interest, this drug delivery system is called passive targeting.
Before reaching to the proximity of tumor site for EPR effect to take place, passive targeting requires drug delivery system to be long-circulating to allow sufficient level of drug to the target area. To design nano-drugs that can stay in blood longer, one can “mask” these nano-drugs by modifying the surface with water-soluble polymers such as polyethylene glycol (PEG); PEG is often used to make water-insoluble nanoparticles to be water-soluble in many pre-clinical research laboratories. PEG-coated liposomal doxorubicin (Doxil) is used clinically for breast cancer leveraging passive tumor accumulation. As in vivo surveillance system for macromolecules (i.e., scavenger receptors of the reticuloendothelial system, RES) reportedly showed faster uptake of negatively charged nanoparticles, nano-drugs with a neutral or positive charge are expected to have a longer plasma half-life.
Utilizing EPR effect for passive tumor targeting drug delivery is not without problems. Although EPR effect is a unique phenomenon in solid tumors, the central region of metastatic or larger tumor mass does not exhibit EPR effect, a result of an extreme hypoxic condition. For this reason, there are methods used in the clinics to artificially enhance EPR effect: slow infusion of angiotensin II to increase systolic blood pressure, topical application of NO-releasing agents to expand blood and photodynamic therapy or hyperthermia-mediated vascular permeabilization in solid tumors.
Passive accumulation through EPR effect is the most acceptable drug delivery system for solid tumor treatment. However, size or molecular weight of the nanoparticles is not the sole determinant of the EPR effect, other factors such as surface charge, biocompatibility and in vivo surveillance system for macromolecules should not be ignored in designing the nanomedicine for efficient passive tumor accumulation.
Active Tumor Targeting
EPR effect, which serves as nanoparticle ‘passive tumor targeting’ scheme is responsible for accumulation of particles in the tumor region. However, EPR does not promote uptake of nanoparticles into cells; yet nanoparticle/drug cell internalization is required for some of the treatment modalities relying on drug activation within the cell nucleus or cytosol (1). Similarly, delivery of nucleic acids (DNA, siRNA, miRNA) in genetic therapies requires escape of these molecules from endosome so they can reach desired subcellular compartments. In addition, EPR is heterogenous and its strength vary among different tumors and/or patients. For these reasons, active targeting is considered an essential feature for next generation nanoparticle therapeutics. It will enable certain modalities of therapies not achievable with EPR and improve effectiveness of treatments which can be accomplished using EPR, but with less than satisfactory effect. Active targeting of nanoparticles to tumor cells, microenvironment or vasculature, as well as directed delivery to intracellular compartments, can be attained through nanoparticle surface modification with small molecules, antibodies, affibodies, peptides or aptamers.
Passive targeting (EPR effect) is is the process of nanoparticles extravasating from the circulation through the leaky vasculature to the tumor region. The drug molecules carried by nanoparticle are released in the extracellular matrix and diffuse throughout the tumor tissue. The particles carry surface ligands to facilitate active targeting of particles to receptors present on target cell or tissue. Active targeting is expected to enhance nanoparticle/drug accumulation in tumor and also promote their prospective cell uptake through receptor mediated endocytosis. The particles, which are engineered for vascular targeting, incorporate ligands that bind to endothelial cell-surface receptors. The vascular targeting is expected to provide synergistic strategy utilizing both targeting of vascular tissue and cells within the diseased tissue.
Most of the nanotechnology-based strategies which are approved for clinical use or are in advanced clinical trials rely on EPR effect. It is expected that next generation nanotherapies will use targeting to enable and enhance intracellular uptake, intracellular trafficking, and penetration of physiological barriers which block drug access to some tumors.
Transport across Tissue Barriers
Nanoparticle or nano-drug delivery is hampered by tissue barriers before the drug can reach the tumor site. Tissue barriers for efficient transporting of nano-drugs to tumor sites include tumor stroma (e.g. biological barriers) and tumor endothelium barriers (e.g. functional barriers). Biological barriers are physical constructs or cell formation that restrict the movement of nanoparticles. Functional barriers can affect the transport of intact nanoparticles or nanomedicine into the tumor mass: elevated interstitial fluid pressure and acidic environment for examples. It is important to design nanoparticles and strategies to overcome these barriers to improve cancer treatment efficacy.
Tumor microenvironment (TME) is a dynamic system composed of abnormal vasculature, fibroblasts and immune cells, all embedded in an extracellular matrix (EMC). TME poses both biological and functional barriers to nano-drug delivery in cancer treatment. Increase cell density and abnormal vasculature elevate the interstitial fluid pressure within a tumor mass. Such pressure gradient is unfavorable for free diffusion of the nanoparticles and is often a limiting factor for the enhanced permeability and retention (EPR) effect. When tumor mass reaches 106 cells in number, metabolic strains ensue. Often, cells in the core of this proliferating cluster are distanced by 100-200 um from the source of nutrient: 200um is a limiting distance for oxygen diffusion. As a result, cancer cells in the core live at pO2 levels below 2.5-10mmHg and become hypoxic; anoxic metabolic pathway can kick in and generate lactic acid. Nanoparticles become unstable in an acidic environment and delivery of the drugs to target tumor cells will be unpredictable. EMC of the tumor provides nutrient for cancer cells and stromal cell. It is a collection of fibrous proteins and polysaccharides and expands rapidly in aggressive cancer as the result of stromal cell proliferation. The most notorious biological barrier to cancer treatment is pancreatic stroma in pancreatic ductal adenocarcinoma (PADC). Pancreatic cancer stroma has the characteristics of an abnormal and poorly functioning vasculature, altered extracellular matrix, infiltrating macrophages and proliferation of fibroblasts. Not only tumor-stroma interactions have been shown to promote pancreatic cancer cell invasion and metastasis, but TME and tumor stroma also create an unfavorable environment for drug delivery and other forms of cancer treatments.
Because EPR effect is a clinically relevant phenomenon for nano-carriers’ tumor penetration, strategies have been developed to address the tumor endothelium barrier. Strategies to reduce interstitial fluid pressure to improve tumor penetration include ECM-targeting pharmacological interventions to normalize vasculature within TME; hypertonic solutions to shrink ECM cells; hyperthermia, radiofrequency (RF) or high-intensity focused ultrasound (HIFU) to enhance nano-drug transport and accumulation. These strategies can also alleviate hypoxic conditions in larger tumor mass. Although TME and tumor mass pose a harsh and acidic environment for nano-carrier stability, pH-responsive nano-carrier designs leveraging this unique feature are gaining interest in recent years. Many of the strategies described above are used to address tumor stroma barrier.
Another formidable tissue barrier for drugs and nanoparticle delivery is the blood-brain barrier (BBB). BBB is a physical barrier in the central nervous system to prevent harmful substances from entering the brain. It consists of endothelial cells which are sealed in continuous tight junction around the capillaries. Outside the layer of epithelial cell is covered by astrocytes that further contribute to the selectivity of substance passage. As BBB keeps harmful substances from the brain, it also restricts the delivery of therapeutics for brain diseases, such as brain tumors and other neurological diseases. There have been tremendous efforts in overcoming the BBB for drug delivery in general. The multi-valent feature of nanoparticles makes nano-carriers appealing in designing BBB-crossing delivering strategies. One promising nanoparticle design has transferrin receptor-targeting moiety to facilitate transportation of these nanoparticles across the BBB.