Targeted mesothelioma nanotherapy

Department of Biomedical Engineering, Emory University and Georgia Institute of Technology

Significant progress has been made in the development of new agents against cancer and new delivery technologies. Proteomics and genomics continue to uncover molecular signatures that are unique to cancer. Yet, the major challenge remains in targeting and selectively killing cancer cells while affecting as few healthy cells as possible. Nanometer-sized particles have novel optical, electronic, and structural properties that are not available from either individual molecules or bulk solids. When linked with tumor-targeting moieties, such as tumor-specific ligands or monoclonal antibodies, these nanoparticles can be used to target cancer-specific receptors, tumor antigens (biomarkers), and tumor vasculatures with high affinity and precision.

Conventional cancer therapy and diagnostics involves the application of catheters, surgery, biopsy, chemotherapy, and radiation. Most current anticancer agents do not greatly differentiate between cancerous and normal cells. This leads to systemic toxicity and adverse effects. Consequently, the systemic application of these drugs often causes severe sideeffects in other tissues (e.g. bone marrow suppression, cardiomyopathy, and neurotoxicity), which greatly limits the maximal allowable dose of the drug. In addition, rapid elimination and widespread distribution into nontargeted organs and tissues requires the administration of a drug in large quantities, which is uneconomical and is often complicated because of nonspecific toxicity.

Nanotechnology could offer a less invasive alternative, enhancing the life expectancy and quality of life of the patient. The diameter of human cells spans 10-20 µm. The size of cell organelles ranges from a few nanometers to a few hundred nanometers. Nanoscale devices can readily interact with biomolecules on the cell surface and within the cells in a noninvasive manner, leaving the behavior and biochemical properties of those molecules intact. In their ‘mesoscopic’ size range of 10-100 nm in diameter, nanoparticles have more surface areas and functional groups that can be linked to multiple optical, radioisotopic, or magnetic diagnostic and therapeutic agents. When linked with tumor-targeting ligands such as monoclonal antibodies, these nanoparticles can be used to target tumor antigens (biomarkers), as well as tumor vasculatures with high affinity and specificity. In this article, we discuss different targeting strategies for nanoscale drug delivery systems (see Scheme 1), and offer a perspective on cancer nanotherapy.

Scheme. 1. Targeting strategies for nanoscale drug delivery systems.

Passive targeting of mesothelioma tumors

Solid tumors have a diffusion-limited maximal size1 and 2 of about 2 mm 3 and will remain at this size until angiogenesis occurs, thus granting them access to the circulation3. Rapid vascularization to serve fast-growing cancerous tissues inevitably leads to a leaky, defective architecture and impaired lymphatic drainage. This structure allows an enhanced permeation and retention (EPR) effect4 and 5 (first described by Matsumura et al.6), as a result of which nanoparticles accumulate at the tumor site. For such a passive targeting mechanism to work, the size and surface properties of drug delivery nanoparticles must be controlled to avoid uptake by the reticuloendothelial system (RES)7. To maximize circulation times and targeting ability, the optimal size should be less than 100 nm in diameter and the surface should be hydrophilic to circumvent clearance by macrophages (large phagocytic cells of the RES). A hydrophilic nanoparticle surface safeguards against plasma protein adsorption, and can be achieved through hydrophilic polymer coating (e.g. by polyethylene glycol (PEG), poloxamines, poloxamers, and polysaccharides) or the use of branched or block copolymers8 and 9. The covalent linkage of amphiphilic copolymers (polylactic acid, polycaprolactone, and polycyanonacrylate) chemically coupled to PEG9, 10 and 11 is generally preferred, as it avoids aggregation and ligand desorption when in contact with blood components.

An alternative passive targeting strategy is to use the unique tumor environment in a scheme called tumor-activated prodrug therapy. The drug is conjugated to a tumor-specific molecule and remains inactive until it reaches the target12 (Fig. 1). Overexpression of the matrix metalloproteinase (MMP), MMP-2, in melanoma has been shown in a number of preclinical as well as clinical investigations. Mansour et al.13 reported a water-soluble maleimide derivative of doxorubicin (DOX) incorporating an MMP-2-specific peptide sequence (Gly-Pro-Leu-Gly-Ile-Ala-Gly-Gln) that binds rapidly and selectively to the cysteine-34 position of circulating albumin. The albumin-doxorubicin conjugate is cleaved efficiently and specifically by MMP-2, releasing a doxorubicin tetrapeptide (Ile-Ala-Gly-Gln-DOX) and subsequently doxorubicin. pH and redox potential have also been explored as drug-release triggers at the tumor site14.

Fig. 1. Tumor-activated prodrug delivery and targeting. The anticancer agent is conjugated to a biocompatible polymer via an ester bond. The linkage is hydrolyzed by cancer-specific enzymes, or by high or low pH, at the tumor site, at which time the nanoparticle releases the drug.

Yet another passive targeting method is the direct local delivery of anticancer agents to tumors. This approach has the obvious advantage of excluding the drug from the systemic circulation. However, administration can be highly invasive, as it involves injections or surgical procedures. For some tumors that are difficult to access, such as lung cancers, the technique is nearly impossible to use.

Active targeting of mesothelioma tumors

Active targeting is usually achieved by conjugating to the nanoparticle a targeting component that provides preferential accumulation of nanoparticles in the tumor-bearing organ, in the tumor itself, individual cancer cells, intracellular organelles, or specific molecules in cancer cells. This approach is based on specific interactions such as lectin-carbohydrate, ligand-receptor, and antibody-antigen.

Lectin-carbohydrate is one of the classic examples for targeted drug delivery. Lectins are proteins of nonimmunological origin that are capable of recognizing and binding to glycoproteins expressed on cell surfaces. Lectin interactions with certain carbohydrates are very specific. Carbohydrate moieties can be used to target drug delivery systems to lectins (direct lectin targeting), and lectins can be used as targeting moieties to target cell surface carbohydrates (reverse lectin targeting). However, drug delivery systems based on lectin-carbohydrate have been developed mainly to target whole organs, which can harm normal cells. Therefore, in most cases, the targeting moiety is directed toward specific receptors or antigens expressed on the plasma membrane or elsewhere at the tumor site.

Multiple drug resistance (MDR), which is a major challenge in chemotherapy, often stems from the overexpression of the plasma membrane P-glycoprotein (Pgp). In general, Pgp acts as an efflux pump to extrude positively charged xenobiotics – including some anticancer drugs – out of the cell. Many tumor cells are resistant to doxorubicin, which is a Pgp substrate. To overcome the resistance, poly(cyanocarylate) nanoparticles have been developed. Adsorption of the nanoparticles onto the plasma membrane and the subsequent release of doxorubicin leads to saturation of Pgp. Furthermore, the negatively charged degradation products of the polymer form an ion pair and neutralize the positive charge of doxorubicin19, enhancing the diffusion of the drug across the plasma membrane. Blagosklonny proposed an approach to selectively kill resistant cancer cells that is based on a temporary increase in the resistance of sensitive cells against certain drugs by specific protectors, such as pharmacological inhibitors of apoptosis. These protectors are pumped out by MDR cells, while increasing the resistance in sensitive cells that do not have active drug efflux pumps. After applying a cytotoxic drug, sensitive cells are protected and survive the exposure, while unprotected MDR counterparts are killed. By abolishing dose-limiting side-effects of chemotherapy, this strategy might provide a means to selectively treat aggressive and resistant cancers. Tsuruo suggested that antibodies to P-glycoprotein overexpressed on multidrug resistant (MDR) cells could make an attractive targeting moiety.

The overexpression of receptors or antigens in many human cancers lends itself to efficient drug uptake via receptor-mediated endocytosis (cellular ingestion) – see Fig. 2. Since glycoproteins cannot remove polymer-drug conjugates that have entered the cells via endocytosis, this active targeting mechanism provides an alternative route for overcoming MDR.