Predicting diffusive transport of cationic liposomes in 3-dimensional tumor spheroids
Graphical abstract
Introduction
Nanotechnology, an important tool in cancer translational research, can be used to deliver diagnostics and therapeutics to tumor interstitium, cell membrane (e.g., antibodies), or intracellular compartments (e.g., RNAi) [2], [3], [4]. As nanoparticles (NP) are made of different types of materials, with different sizes, surface charges and surface modifications, there is a potential to tailor the design of NP for its intended function. Such goals can be greatly facilitated by quantitative tools that predict the NP transport and residence in tumors, including delivery to intratumoral subcompartments where the intended targets are located.
We hypothesize that diffusive transport of NP in 3-dimensional (3D) tumor spheroids can be predicted based on two groups of parameters. The first group that describes the interactions between NP and tumor cells (i.e., NP binding, uptake and retention in cells), as they are specific to the selected NP and selected cells, can be experimentally measured in monolayer cultures. The second group is the parameters that determine the NP diffusivity in tumor interstitium; diffusivity is calculated based on NP size and tumor microenvironment parameters (i.e., tumor cell density, interstitial void volume fraction and extracellular matrix proteins/fibers, which jointly account for the effects of 3D spheroid structure and composition). We tested the above hypothesis in an earlier study and found that this approach successfully predicted the diffusive transport of two NP with different surface charges and different sizes (nearly neutral liposomes at − 10 mV and 130 nm diameter, and negatively charged polystyrene beads at − 49 mV and 20 nm diameter), with > 96% of the predicted data within the 95% confidence intervals (CI) of the experimental results [1]. On the other hand, the model yielded inferior performance (60% predicted data within 95% CI of experimental results) for a cationic liposome that contained a fusogenic lipid and underwent size increase in the presence of tumor cells.
As cationic liposomes are popular carriers of drugs and gene therapeutics, we conducted the present study to determine if the inability to predict their diffusive transport was due to positive surface charge or fusogenic lipid. Cationic liposomes with varying surface charges and varying fractions of fusogenic lipid were studied, including compositions desired for in vivo applications, e.g., 5 mol% pegylated lipids to achieve stealth property, and a mixture of neutral and cationic lipids (50 mol% cholesterol plus 10–30 mol% DOTAP or 1,2-dioleoyl-3-trimethylammonium propane) that has been used in clinical studies [5]. The content of fusogenic lipid DOPE (1,2-dioleoyl-sn-glycero-3-phospho-ethanolamine), which destabilizes the endosomal membrane and promotes release of nucleic acid [6], was varied from 1 to 20 mol%. We further evaluated model modifications to account for the time-dependent changes in extracellular liposome concentrations and liposome sizes.
Section snippets
Overview
We (a) established the governing equations for liposome transport and interactions with cells in spheroids, (b) measured the liposome–cell biointerface parameters in monolayer cultures, (c) calculated the effective liposome diffusivity in spheroid interstitium, (d) used the equations and model parameters to simulate the diffusive transport of cationic liposomes in 3D systems, (e) experimentally measured the liposome concentration–depth profiles in 3D tumor cell spheroids, and (f) evaluated
Liposome properties
We studied eight cationic liposomes that contained 5 mol% PEG-DSPE, 50 mol% cholesterol, varying DOTAP content (10–30 mol%) and varying DOPE content (0–20 mol%); the remaining lipid was DPPC and ranged from 5 to 34 mol%. These liposomes had an initial average size of about 135 nm, about 2-fold range in PDI (0.3–0.5), and about 1.7-fold range in zeta potential (25 to 44 mV). Liposomes were denoted by their DOTAP and DOPE contents, i.e., C10-1 indicates 10 mol% DOTAP plus 1 mol% DOPE whereas C20-20
Discussion
Clinical utility of NP depends on the successful delivery to the target site. Studies in the last two decades have identified multiple barriers to NP delivery and transport in solid tumors [4], [7], [8], [9]. In addition, some NP properties produce opposite outcomes. For example, NP are frequently surface-modified with targeting ligands to enhance selectivity, but ligand binding to cells retards NP transport. Similarly, pegylation increases circulation times but also decreases the endocytosis
Acknowledgments
This research is supported in part by a research grant R01 EB015253 from the National Institute of Biomedical Imaging and Bioengineering, NIH and DHHS.
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