Cancer is the second leading cause of death in the United States. The National Cancer Institute estimates that more than 1,500 Americans die of cancer every day, and approximately 600,000 patients with cancer will die in 2011.1
Given that this considerable mortality is primarily due to cancer metastasis to other organs, early detection is crucial for the effective management of cancer. For instance, the survival rate of patients with lung and bronchus cancer, which is the single leading cause of cancer-related death, is approximately 53% if the cancer is detected at an early stage before metastasis to distant tissues, organs or lymph nodes occurs. However, the survival rate significantly decreases to 4% when the cancer is detected in the late stage, i.e.
after it metastasizes to distant tissues.2
Although huge advances have been made in diagnostic technologies, a considerable portion of cancer patients are still diagnosed with metastases due to the poor selectivity and sensitivity of conventional diagnostic techniques.
Current treatment options applicable to metastatic cancers are still confined to chemotherapeutics with combinational regimens. Over the past several decades, considerable efforts have been directed towards the development of potent therapeutic agents. Yet, current anticancer therapeutics is limited in safety and efficacy. Most conventional anticancer agents show a narrow therapeutic window because they are randomly distributed in the whole body following administration. Non-specific biodistribution may cause cytotoxicity to normal and cancer cells alike, which causes severe side effects to achieve sufficient anti-cancer efficacy. The non-specific toxicity of anticancer drugs also limits an injectable dose and thus lessens the therapeutic efficacy.
In an attempt to overcome these major hurdles in the treatment of cancer, various nanoparticle platforms have been extensively developed for cancer diagnostics and therapeutics. Nanoplatforms hold great potential in cancer diagnostics and therapeutics. Given sophisticated nano-structures and huge surface area to volume ratios, nanoparticles can accomodate a variety of diagnostic or therapeutic agents via
chemical conjugation or physical encapsulation.3
Moreover, nanoparticles are known to target tumors via
passive accumulation and/or active-targeting approaches. Combining their capabilties to carry various cargos and to target tumors, they can be employed as targeted cancer diagnostics and therapeutics to get high-quality images indicating the tumor site and to achieve the enhanced therapeutic efficacy without severe cytotoxicity to normal cells. For these reasons, theranostic nanoparticles, a platform for both diagnostic and therapeutic functions, have been explosively investigated as a next-generation nanocarrier system.4–8
Theranostics is a term originally coined to define an approach that combines diagnostics with therapeutics.9
It embraces multiple techniques to arrive at comprehensive diagnosis, molecular images and an individualized treatment regimen.10–13
Recently, there is an effort to tangle the emerging approach with nanotechnologies, in an attempt to develop theranostic nanoplatforms and methodologies.14
Given that cancer is a highly heterogeneous and adaptable disease, diverse types of treatment options need to be chosen depending on patient characteristics and disease progression. Cancer researchers hope that theranostic nanoparticles provide patients with various treatment options that are suitable for individuals, and thereby result in improved prognoses. Furthermore, theranostic nanoparticles can monitor therapeutic efficacy following treatments which can expedite clinician’s individualized therapeutic decisions.
In this article, we will review various types of theranostic nanoplatforms including magnetic nanoparticles, carbon nanotubes, gold nanostructures, polymeric nanoparticles, and silica nanoparticles and discuss their applications in cancer theranostics. A number of review articles describe nanoparticle-based diagnostic or therapeutic systems for the treatment of cancer;3,15,16
however, few articles have focused on theranostic nanoparticles capable of simultaneously imaging and treating cancer. Thus, this review will focus on nanoparticle systems that integrate tumor imaging and therapy into a single system. In detail, we will discuss theranostic applications of diverse nanoplatforms categorized by different nanomaterials such as polymeric, gold, carbon, magnetic and silica nanomaterials. These applications include: 1) diagnostics for the assessment of intracellular localization and in vivo
biodistribution, 2) therapeutics for the treatment, and 3) theranostics for monitoring biological responses and therapeutic efficacy following treatment. Finally, we will address the limitations and future challenges of current theranostic systems based on diverse nanoplatforms.