Stimuli-sensitive nanopreparations for combination cancer therapy

Nanocarriers have revolutionized drug delivery practices over the past couple of decades, primarily due to the advances in materials chemistry, nanotechnology and nanomedicine. This in turn, has spurred the development of a number of novel nanocarrier-based platforms and treatment strategies for cancer. It is now clear that to manage a disease as complex as cancer, a single or stand-alone treatment strategy may not suffice. Present day drug delivery strategies progressively lean towards "multi-pronged" combination approaches to make cancer treatments more effective. To that end, nanocarriers which simultaneously incorporate multiple drugs that affect different pathways and act through different mechanisms, or combinations of drugs with biological therapeutics like genes, antibodies, proteins or siRNAs have been the focus of recent active research. Furthermore, nanocarriers which respond to a variety of intrinsic cues afforded by the tumor microenvironment like low pH, elevated redox potential, over-expressed enzymes and hyperthermia as well as to externally applied stimuli such as magnetic field, ultrasound or light have been developed to trigger site-specific drug release. In this review, we focus specifically on nanocarriers that simultaneously exhibit stimuli-sensitivity and incorporate various combinations of conventional small molecule chemotherapeutic agents and biologics. We provide an overview of the different internal and external stimuli most relevant to cancer, and discuss selected examples of stimuli-sensitive combination nanopreparations from the recent literature with respect to each stimulus. Finally, we discuss multifunctional stimuli-sensitive nanopreparations which incorporate various combinations of drugs, biologics and targeting ligands within a single carrier that form so-called "smart" nanopreparations.