Selective Tumor Cell Targeting Using Low-Affinity, Multivalent Interactions

293

479

A key challenge in nano-science is to design ligand-coated nanoparticles that can bind selectively to surfaces that display the cognate receptors above a threshold (surface) concentration. Nanoparticles that bind monovalently to a target surface do not discriminate sharply between surfaces with high and low receptor coverage. In contrast, "multivalent" nano-particles that can bind to a larger number of ligands simultaneously, display regimes of "super selectivity" where the fraction of bound particles varies sharply with the receptor concentration. We present numerical simulations that show that multivalent nano-particles can be designed such that they approach the "on-off" binding behavior ideal for receptor-concentration selective targeting. We propose a simple analytical model that accounts for the super selective behavior of multivalent nano-particles. The model shows that the super selectivity is due to the fact that the number of distinct ligand-receptor binding arrangements increases in a highly nonlinear way with receptor coverage. Somewhat counterintuitively, our study shows that selectivity can be improved by making the individual ligand-receptor bonds weaker. We propose a simple rule of thumb to predict the conditions under which super selectivity can be achieved. We validate our model predictions against the Monte Carlo simulations.ne of the key challenges in nano-medicine is to acquire the ability to design supramolecular constructs that can target surfaces that display a motif or receptor above a threshold concentration while leaving surfaces with lower coverage of such receptors unaffected (1-4). Experiments indicate that such selective behavior can be obtained using multivalency (5-8). During multivalent interactions a type of particle (henceforth referred to as the "guest") uses multiple ligands to bind simultaneously to several of the receptors displayed by another type of particle or surface (the "host") (9, 10). Mammen et al. (9) recognized the importance of this type of system more than ten years ago. Since then, the concept of multivalency has found numerous applications in cell biology (7, 11), supramolecular chemistry (10), nano-medicine (4, 6), immunology (12, 13), and cancer treatment (2,3,5,14), to name but a few examples. The work of Davis et al. There is a substantial body of theoretical work that aims to explain the pronounced enhancement in binding strength that certain multivalent systems can present in comparison with their monovalent counterparts (15-17). In particular, Kitov and Bundle (18) have pointed out that the strength of multivalent binding can be enhanced if there are many possible permutations in the binding pattern of receptors and ligands. The role of steric repulsion and conformational entropy in multivalent systems have been studied using molecular theories (19) and Monte Carlo (MC) simulations (20, 21). However, a unified picture that explains why high selectivity is observed in some experimental realizations of multivalent systems but not in others is still lacki...

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confidence: 99%

“…Experiments indicate that such selective behavior can be obtained using multivalency (5)(6)(7)(8). During multivalent interactions a type of particle (henceforth referred to as the "guest") uses multiple ligands to bind simultaneously to several of the receptors displayed by another type of particle or surface (the "host") (9, 10).…”

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confidence: 99%

Designing super selectivity in multivalent nano-particle binding

Martínez-Veracoechea

Frenkel

2011

293

479

“…Such selectivity can be achieved by exploiting multivalency. A series of recent experimental and theoretical papers have shown that multivalent carriers (nanoparticles or polymers) can distinguish target surfaces (cells) on the basis of their receptor concentration, rather than just on the basis of the presence of a suitable receptor (5)(6)(7)(8)(9).…”

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confidence: 99%

Optimal multivalent targeting of membranes with many distinct receptors

Curk

Dobnikar

Frenkel

2017

Cells can often be recognized by the concentrations of receptors expressed on their surface. For better (targeted drug treatment) or worse (targeted infection by pathogens), it is clearly important to be able to target cells selectively. A good targeting strategy would result in strong binding to cells with the desired receptor profile and barely binding to other cells. Using a simple model, we formulate optimal design rules for multivalent particles that allow them to distinguish target cells based on their receptor profile. We find the following: (i) It is not a good idea to aim for very strong binding between the individual ligands on the guest (delivery vehicle) and the receptors on the host (cell). Rather, one should exploit multivalency: High sensitivity to the receptor density on the host can be achieved by coating the guest with many ligands that bind only weakly to the receptors on the cell surface.(ii) The concentration profile of the ligands on the guest should closely match the composition of the cognate membrane receptors on the target surface. And (iii) irrespective of all details, the effective strength of the ligand-receptor interaction should be of the order of the thermal energy k B T, where T is the absolute temperature and k B is Boltzmann's constant. We present simulations that support the theoretical predictions. We speculate that, using the above design rules, it should be possible to achieve targeted drug delivery with a greatly reduced incidence of side effects.T he fact that most cells can be recognized from the outside is advantageous for the normal functioning of an organism, but it can be a disadvantage when specific cells are being targeted by pathogens. Cells betray their identity (and state of health) by the composition profile of molecules that are exposed on their outer surface. In what follows, we call these molecules "receptors," irrespective of whether they are receptors in the biological sense (they are receptors for the ligands that will be used to recognize them). It would clearly be advantageous if diseased cells could be selectively targeted by a drug-delivery vehicle on the basis of their receptor profile. Here, the crucial word is "selective": We wish to target only those cells that have the correct receptor profile, as binding of drug-delivery vehicles to other cells may lead to undesired side effects.Targeted drug delivery is based on identifying a specific marker (peptide, sugar) that is unique to the targeted group of cells. Binding to a single marker type can be effective if this molecule is presented in sufficient quantities on the outer surface of the targeted cell. However, in many cases of practical importance (e.g., many types of cancer), the markers that are known are not unique to cancer cells, but just overexpressed. Over the past 20 y many nanoparticle-based targeting methods have been developed. However, thus far, effective tumor drug delivery is hampered by the lack of reliable, unique markers (1, 2).To recognize the simultaneous presence of a mixture o...

“…Here we have shown the functional advantage of covalent protein polymerization for capture of cells. Multivalency is a powerful way to enhance biological function and with the right ligand organization gives dramatic improvements in affinity, kinetics and specificity (42,43). The multivalency of the SpyLigase-assembled affibody polymers enhanced the capture of cancerous cells expressing low levels of tumor antigen, without loss of specificity.…”

Section: Discussionmentioning

confidence: 99%

SpyLigase peptide–peptide ligation polymerizes affibodies to enhance magnetic cancer cell capture

Fierer

Veggiani

Howarth

2014

155

162

Individual proteins can now often be modified with atomic precision, but there are still major obstacles to connecting proteins into larger assemblies. To direct protein assembly, ideally, peptide tags would be used, providing the minimal perturbation to protein function. However, binding to peptides is generally weak, so assemblies are unstable over time and disassemble with force or harsh conditions. We have recently developed an irreversible protein-peptide interaction (SpyTag/SpyCatcher), based on a protein domain from Streptococcus pyogenes, that locks itself together via spontaneous isopeptide bond formation. Here we develop irreversible peptide-peptide interaction, through redesign of this domain and genetic dissection into three parts: a protein domain termed SpyLigase, which now ligates two peptide tags to each other. All components expressed efficiently in Escherichia coli and peptide tags were reactive at the N terminus, at the C terminus, or at internal sites. Peptide-peptide ligation enabled covalent and site-specific polymerization of affibodies or antibodies against the tumor markers epidermal growth factor receptor (EGFR) and HER2. Magnetic capture of circulating tumor cells (CTCs) is one of the most promising approaches to improve cancer prognosis and management, but CTC capture is limited by inefficient recovery of cells expressing low levels of tumor antigen. SpyLigase-assembled protein polymers made possible the isolation of cancerous cells expressing lower levels of tumor antigen and should have general application in enhancing molecular capture.bionanotechnology | synthetic biology | metastasis | antibody | nanobiotechnology