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Sensor Issues
Life Under the Microscope: Quantifying Live Cell Interactions to Improve Nanoscale Drug Delivery Angus P. R. Johnston ACS Sens., Just Accepted Manuscript • DOI: 10.1021/acssensors.6b00725 • Publication Date (Web): 20 Dec 2016 Downloaded from http://pubs.acs.org on December 22, 2016
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ACS Sensors
Life Under the Microscope: Quantifying Live Cell Interactions to Improve Nanoscale Drug Delivery Angus P.R. Johnston* Drug Delivery, Disposition and Dynamics, Monash Institute of Pharmaceutical Sciences, Monash University, Parkville, Victoria 3052, Australia ARC Centre of Excellence in Convergent Bio-Nano Science and Technology, Monash University, Parkville, Australia KEYWORDS: drug delivery, nanoparticles, internalization, trafficking, fluorescence. ABSTRACT: The behavior of cells and how they react to stimuli is critically important for drug development, drug delivery and understanding the molecular basis of many diseases. However, we still lack a comprehensive understanding of these interactions, particularly in relation to drug delivery from nanoparticles. This Sensors Issues article discusses the importance of quantifying these interactions and highlights some key areas were advances in sensor technology has the potential to transform our understanding of drug delivery and cell biology.
Transporting drugs to the specific subcellular locations where they are therapeutically active is fundamental to effective drug delivery.1 siRNA for gene silencing, or peptides for therapeutic vaccination require delivery to the cytoplasm, whereas DNA for gene therapy requires delivery to the nucleus. Rather than relying on passive transport of the drug through the cell, there has been a focus on engineering nanoparticle delivery systems to target drug release to specific cellular organelles. To this end, there has been an explosion of interest in nanoparticle systems that can deliver a wide range of therapeutics.24 However, while nanoparticle technology holds promise, the therapeutic outcomes have largely been hit and miss.5 To achieve effective therapeutic delivery, a good deal of knowledge relating to how cells interact with nanoparticles and the drugs they carry is required. There are three key parameters that must be understood when engineering a nanoparticle delivery system. 1) Does the nanoparticle enter the cell? 2) Where do the nanoparticle and drug go inside the cell, and how do they get there? and 3) What is the local environment that the nanoparticle is exposed to, so a release mechanism can be engineered to deliver the drug when it reaches the required location. There are significant deficiencies in our knowledge of all three areas. Our lack of understanding largely comes down to the lack of quantitative tools to accurately measure these interactions. This means there is a significant opportunity to develop new sensors to probe these fundamental cellular processes. This article focuses on the issues associated with developing quantitative sensors in live cells and highlights areas where there is scope for significant advances in cellular understanding and drug delivery.
Association and Internalization The first barrier to drug delivery is crossing the plasma membrane. There is little point in establishing sophisticated methods to determine the cellular fate of a nanoparticle if it remains anchored to the surface of the cell. Therefore, the internalization kinetics of nanoparticles should be the first point of analysis for drug delivery systems. Quantifying association can readily be achieved by fluorescently labeling nanoparticles and using flow cytometry to measure the fluorescence intensity of thousands of individual cells. A common misconception in the literature is that the intensity measured is indicative of nanoparticle uptake. Flow cytometry measures the total fluorescence intensity of a cell, regardless of whether it is on the surface or if it has been internalized. The interchangeable use of the term internalization when the experimental evidence only shows cellular association is misleading and leads to incorrect conclusions around the susceptibility of nanoparticles to be internalized. The importance of differentiating between association and internalization is exemplified when the two are compared in different cell lines (Figure 1a & b). Polymer nanoparticles incubated with 3T3 mouse embryonic fibroblasts and C1R human B-lymphocytes show rapid association with cells.6 Greater than 90% of the cells had at least one particle after 15 min, and both cell lines appeared to behave similarly. However, internalization in the two cell lines differs significantly. 3T3 cells showed rapid internalization and after 2 hours >80% of the cells had internalized nanoparticles. In contrast, C1R cells showed
