NANOPARTICLE DELIVERY - Ultra-Small Particles Offer Big Hope in Cancer Theranostics

INTRODUCTION

Ultra-small sub-10 nm particles hold unique properties and represent an emerging area of investigation for biomedical applications. These ultra-small particles are able to fill the gap between single molecules and conventional larger-size nanoparticles in terms of their spatial dimensions, as well as their physicochemical and pharmacokinetic (PK) properties. In contrast to regular nanoparticles used in medical applications, such as liposomes or block copolymer micelles with sizes larger than 10 nm, ultra-small particles with sizes below the glomerular filtration cut-off are able to clear the body efficiently via the urinary system, reducing the risk of adverse events. Additionally, as a result of the faster diffusion of small objects relative to larger ones, ultra-small particles can effectively penetrate solid tumors and the surrounding tumor microenvironment, both of great importance for efficient drug delivery. At the same time, ultra-small particles exhibit increased surface area-to-volume ratios as compared with those for larger-size particles, leading to relatively high drug-loading capacities and ligand numbers.

A FIRST-IN-CLASS ULTRA-SMALL PARTICLE PLATFORM FOR FIGHTING CANCER

Elucida Oncology^TM is focused on transforming outcomes for patients harboring primary solid tumors or metastatic disease. The company was founded in 2016 with the goal of personalized cancer care using an ultra-small hybrid silica particle, C-Dots (Figure 1), for image-guided surgical treatment or for delivery of a variety of therapies to improve patient outcomes. The C-Dot structure serves as a treatment transport unit, and is uniquely designed to accurately deliver diagnostic, imaging, and therapeutic agents to cancer cells, or to safely clear the body through the kidneys. C-Dots share many similarities with proteins, e.g., with respect to size and surface properties, but contain amorphous silica as a core scaffold (Figure 1). In fact, silica is one of the major biogenic materials, and it widely exists in plants and animals, such as grasses and marine microbes. While natural proteins are constructed from linear chains of amino acids (Figure 1), C-Dots are assembled on the basis of a three-dimensional (3D) crosslinked silica network as the core, onto which biocompatible polymers and functional peptides, drugs, and contrast labels are covalently attached (Figure 1). Based on the rich library of silica forming alkoxy silane compounds, silica chemistry can be adopted to precisely manipulate particle architecture and properties, rendering CDots a powerful platform with highly engineered functions for biomedical and clinical applications, and constituting organic-inorganic hybrid protein mimics.¹

UNIQUE PARTICLE DESIGN EVOLUTION

This silica-based biomaterial platform was originally invented by Prof. Ulrich Wiesner at Cornell University in 2005.² The first-generation larger-size (~30 nm hydrodynamic diameter) fluorescent core-shell silica particles synthesized in alcohol solutions covalently encapsulated fluorescent dyes within its core. The rigid silica shell surrounding these dyes led to a significant increase in per dye fluorescence brightness. Since several dye molecules were simultaneously encapsulated inside a single particle without significant detectable energy transfer between them, the overall fluorescence brightness of the particles was very high when compared to a regular fluorescent dye.³ In 2009, the Wiesner group reported on the development of the first silica particles with sizes below 10 nm, and sterically stabilized with a surface layer of poly(ethylene glycol) (PEG).^4,5 As part of a first collaborative effort with Prof. Michelle Bradbury at Memorial Sloan Kettering Cancer Center (MSK), these sub-10 nm PEGylated silica particles were demonstrated to rapidly clear through the kidneys after murine tail vein injection.⁶ The team subsequently attached cyclic arginine-glycine-aspartic acid (cRGD) cancer-targeting peptides to the end of some of the PEG chains on the particle surface to target surface integrin receptors.⁷ In vivo studies in small animal tumor models demonstrated a favorable safety profile and high tumor targeting efficacy of these sub-10 nm integrin-targeted silica particles, leading to Food and Drug Administration (FDA) investigational new drug (IND) approval for a Phase 1 first-inhuman clinical trial using C-Dots in 2011.⁸

Since then, a new generation of CDots has been developed, now synthesized in aqueous instead of alcohol solutions using a highly efficient chemical approach.^9-11 This versatile synthesis allows the silica core of C-Dots to be precisely engineered at the single atomic layer level, with the PEGylation step integrated into a one-pot type batch reaction. C-Dots (Figure 2) can encapsulate a variety of spectrally distinct dyes exhibiting different absorption/emission characteristics in the visible to near-infrared range, yielding an ultrabright particle with exquisitely high detection sensitivity and fluorescence-based multiplexing capabilities for clinical cancer care. Multiple cancer-targeting peptides are attached to the end of some of the PEG chains, endowing C-Dots with multi-valency enhancement to increase potency and cellular binding affinity. Radioisotopes can be attached onto the silica surface, enabling, e.g., positron emission tomography (PET) imaging or radiotherapy. Furthermore, a large number of small molecule drugs can be simultaneously “clicked onto” additional functional groups of the organic C-Dot shell for drug delivery applications.^12,13 Inserting a number of these functional groups does not influence the favorable biodistribution and PK profiles observed for C-Dots. Figure 2 shows an example in which a total of five functions were integrated in a single C-Dot, i.e., fluorescence, cancer-targeting, pH-sensing, radioisotope chelating, and drug delivery, while the overall C-Dot size remained around 7 nm, thereby providing a particle with both diagnostic and therapeutic features, i.e., a particle for theranostics.¹¹

C-DOT DIFFERENTIATORS

There are several advantages of the C-Dot platform that differentiate it from other particles used in cancer theranostics that could have a profound impact on the dose-limiting toxicity associated with current cancer drugs. First, the size of C-Dots is ultra-small and can be precisely tuned to sub-10 nm sizes with less than 1 nm accuracy, important for achieving efficient renal clearance and extended circulation lifetimes. ⁹ Second, the surface properties of C-Dots are engineered by a novel PEGylation mechanism, which facilitates further surface modifications.¹⁰ This specific reaction mechanism increases their colloidal stability in blood serum. As a result, and unlike other nanoparticles that adsorb proteins onto their surface, a protein corona does not form around the C-Dot surface under physiological conditions.^10,14 This prevents C-Dots from being identified by the mononuclear phagocytic system (MPS) and helps achieve desirable biodistribution profiles. Third, a single 7 nm C-Dot can integrate up to five surface functionalities without exceeding the threshold for renal clearance, a characteristic not demonstrated by any other platform.

Interestingly, it has been found that the PK profiles of C-Dots do not significantly change in the presence of functional ligands having different molar masses and chemical properties. These properties have endowed C-Dots with distinctive PK profiles, as well as high cancer-targeting efficiencies. C-Dots are renally cleared after intravenous injection in both preclinical models and human subjects.^8,15 At the same time, C-Dot surface functionalization with a variety of targeting moieties, has shown high targeted delivery and uptake (up to 17% ID/g for anti-HER2 scFv fragments) and target-to-background ratios (>20 at 10 days post-injection).¹⁶ It is substantially higher than that typically found for larger nanoparticle platforms. More importantly, the remaining C-Dots that do not end up in tumors are rapidly cleared from the body without substantial off-target effect. This “target-or-clear” property of CDots highlights the enormous potential of this platform for a variety of nanomedicine applications (Figure 3).

APPLICATIONS IN CANCER DIAGNOSIS & TREATMENT

This versatile platform enables the development of a family of C-Dots that are adapted with different targeting moieties, contrast labels, and/or therapies, some of which are in active clinical trials at MSK for image-guided cancer treatment of primary/metastatic disease. For example, CDots, in which near-infrared fluorescent dyes are encapsulated inside the core and which bear cancer-targeting peptides on the surface, are currently being used for image-guided surgery in a Phase 2b clinical trial.⁹ These ultrabright near-infrared fluorescent C-Dots can light up cancerous lymph nodes, and aid in their surgical removal using real-time fluorescence imaging guidance.¹⁷ In addition, targeted C-Dots can be modified for multi-modality (PET-optical) imaging, enabling them to both detect and treat cancer via PET-optical imaging and radiotherapy.¹⁸ Additionally, it has been recently shown that a single 7 nm C-Dot is able to deliver a large number of small molecule drugs for cancer treatment, while preserving the PK profile and “target-or-clear” properties required for renal clearance. By comparison, a much bigger antibody-drug conjugate can only load single-digit numbers of drug molecules at a time. This significantly improved payload amount, and the desired PK profile of C-Dots offer great potential to reduce side effects of cytotoxic drugs.

Interestingly, C-Dots are also able to kill cancer cells without using a cytotoxic drug.¹⁹ In nutrient-deprived cancer cells, CDots can trigger a special non-apoptotic cell death mechanism, i.e., ferroptosis, which sets itself apart from mechanisms observed for other conventional particle platforms. Importantly, healthy tissues in the same animal are not adversely affected. This constellation of unique interactions that selectively trigger specific cell death pathways in cancer cells and surrounding environment further sets C-Dots apart from the classic nanoparticles, rendering them more like protein mimics.

THE POTENTIAL TO TRANSFORM CLINICAL PRACTICE

Elucida Oncology, together with MSK and Cornell University, are now focusing on the further development and the commercialization of C-Dots for clinical cancer care. We expect C-Dots, by overcoming the suboptimal pharmacokinetic properties typically found with larger particle probes and antibodies, will be able to significantly improve targeted detection and treatment of disease while eliminating dose-limiting toxicity, in turn, transforming clinical practice.

It took the research team more than a decade to bring C-Dots from the original lab reaction batch to the current state with multiple ongoing clinical trials. In addition to the authors, a significant number of individuals have made substantial contributions to the development of this particle platform.

In particular, the authors acknowledge the contributions of Alexander Andrievsky, Prof. Barbara Baird, Dr. Daniel Bonner, Dr. Andrew Burns, Ying Cong, Nikhil Dhawan, Jennifer Drewes, Tom Gardinier, Dr. Erik Herz, Josh Hinckley, Dr. Teresa Kao, Dr. Ferdinand Kohle, Dr. Daniel Larson, Songying Li, Carlie Mendoza, Dr. Hooisweng Ow, Dr. Teeraporn Suteewong, Melik Turker, Prof. Watt Webb, and Duhan Zhang from Cornell University. Contributors on the MSK side included Dr. Cameron Brennan, Dr. Sarah Cheal, Dr. Feng Chen, Dr. Nick Chen, Dr. Elisa DeStanchina, Dr. Hedvig Hricak, Dr. John Humm, Dr. Rupa Juthani, Dr. Daniella Karassawa, Dr. Sung Eun Kim, Dr. Steven Larson, Dr. Jason Lewis, Dr. Serge Lyaschenko, Dr. Brian Madajewski, Dr. Michael McDevitt, Dr. Lee McDonald, Dr. Ingo Mellinghoff, Dr. Larry Norton, Dr. Michael Overholtzer, Dr. Snehal Patel, Dr. Oula Penate-Medina, Dr. Evan Phillips, Dr. Charles Rudin, Dr. Peter Scardino, Dr. Howard Scher, Dr. Sonia Sequeira, Dr. Hilda Stambuk, Dr. Jedd Wolchok, Dr. Barney Yoo, Dr. Robert Young, Dr. Pat B. Zanzonico, and Dr. Li Zhang. We also would like to acknowledge contributions of Dr. Fabio Gallazzi, Prof. Thomas Quinn, and Dr. Xiuli Zhang from the University of Missouri.

REFERENCES

1. Kotov NA. Inorganic nanoparticles as protein mimics. Science. 2010;330:188-189.
2. Ow H, Larson D, Srivastava M, et al. Bright and stable core-shell fluorescent silica nanoparticles. Nano Lett. 2005;5:113-117.
3. Larson DR, Ow H, Vishwasrao HD, et al. Silica nanoparticle architecture determines radiative properties of encapsulated fluorophores. Chem Mater. 2008;20:2677-2684.
4. Herz E, Burns A, Bonner D, et al. Large stokes-shift fluorescent silica nanoparticles with enhanced emission over free dye for single excitation multiplexing. Macromol Rap Commun. 2009;30:1907-1910.
5. Herz E, Ow H, Bonner D, et al. Dye structure – optical property correlations in near-infrared fluorescent core-shell silica nanoparticles. J Mater Chem. 2009;19:6341-6347.
6. Burns A, Vider J, Ow H, et al. Fluorescent silica nanoparticles with efficient urinary excretion for nanomedicine. 2009;9:442-448.
7. Benezra M, Penate-Medina O, Zanzonico PB, et al. Multimodal silica nanoparticles are effective cancer-targeted probes in a model of human melanoma. J Clin Invest. 2011;121:2768-2780.
8. Phillips E, Penate-Medina O, Zanzonico PB, et al. Clinical translation of an ultrasmall inorganic optical-PET imaging nanoparticle probe. Sci Transl Med. 2014;6:260.
9. Ma K, Mendoza C, Hanson M, et al. Control of ultrasmall sub-10 nm ligand-functionalized fluorescent core-shell silica nanoparticle growth in water. Chem Mater. 2015;27:4119-4133.
10. Ma K, Zhang D, Cong Y, et al. Elucidating the mechanism of silica nanoparticle PEGylation processes using fluorescence correlation spectroscopies. Chem Mater. 2016;28:1537-1545.
11. Ma K, Wiesner U. Modular and orthogonal post-PEGylation surface modifications by insertion enabling penta-functional ultrasmall organic-silica hybrid nanoparticles. Chem Mater. 2017;29:6840-6855.
12. Yoo B, Ma K, Zhang L, et al. Ultrasmall dual-modality silica nanoparticle drug conjugates: design, synthesis, and characterization. Bioorg Med Chem. 2015;23:7119-7130.
13. Yoo B, Ma K, Wiesner U, et al. Expanding analytical tools for characterizing ultrasmall silica-based nanoparticles. RSC Adv. 2017;7:16861-16865.
14. Benezra M, Phillips E, Overholtzer M, et al. Ultrasmall integrin-targeted silica nanoparticles modulate signaling events and cellular processes in a concentration-dependent manner. Small. 2015;11:1721-1732.
15. Chen F, Ma K, Benezra M, et al. Cancer-targeting ultrasmall silica nanoparticles for clinical translation: physicochemical structure and biological property correlations. Chem Mater. 2017;29:8766-8779.
16. Chen F, Ma K, Madajewski B, et al. Ultrasmall targeted nanoparticles with engineered antibody fragments for imaging detection of HER2-overexpressing breast cancer. Nat Comm. 2018;9:4141.
17. Bradbury M, Pauliah M, Zanzonico P, et al. Intraoperative mapping of sentinel lymph node metastases using clinically translated ultrasmall silica nanoparticle. Wiley Interdiscip. Rev. Nanomed. Nanobiotechnol. 2016;8:535-553.
18. Chen F, Ma K, Zhang L, et al. Target-or-clear zirconium-89 labeled silica nanoparticles for enhanced cancer-directed uptake in melanoma: a comparison of radiolabeling strategies. Chem Mater. 2017;29:8269-8281.
19. Kim SE, Zhang L, Ma K, et al. Ultrasmall nanoparticles induce ferroptosis in nutrient-deprived cancer cells and suppress tumour growth. Nat Nanotechnol. 2016;11:977-985.

Dr. Ulrich Wiesner is a co-founder of Elucida Oncology and is the original inventor of C-Dots and has continued to invent a family of next-generation C-Dot products for different biomedical applications. He has authored over 250 peer-reviewed articles and has been recognized with numerous awards, including those from the German Chemical Society and the American Chemical Society. Dr. Wiesner earned his PhD from the Johannes-Gutenberg University with work at the Max Planck Institute for Polymer Research, both in Mainz, Germany. He has also studied at the University of California, Irvine, and performed post-doctoral studies at ESPCI, a Grande Ecole, in Paris, France. Dr. Wiesner is currently the Spencer T. Olin Professor of Engineering in the Department of Material Sciences and Engineering at Cornell University and is Co-Director of the MSK-Cornell Center for Translation of Cancer Nanomedicines (MC2TCN), one of six Centers of Cancer Nanotechnology Excellence (CCNE) in the US funded by the National Cancer Institute (NCI).

Dr. Michelle Bradbury is a co-founder of Elucida Oncology who co-developed the C-Dot theranostic platform and led the successful translation of C-Dots into the clinic. She earned her BA in Chemistry from the University of Pennsylvania, her MS in Nuclear Engineering, and her PhD in Nuclear Engineering (Radiological Sciences) from the Massachusetts Institute of Technology. She then earned her MD at George Washington University School of Medicine, followed by an Internship in Surgery, Residency in Diagnostic Radiology, and a Fellowship in Neuroradiology at the Bowman Gray School of Medicine. She pursued a second fellowship in Molecular Imaging at MSKCC. She currently holds dual appointments as a Professor of Radiology at the Gerstner Sloan Kettering Graduate School of Biomedical Sciences at Memorial Sloan Kettering (MSK) and at Weill Medical College of Cornell University. She is also Co-Director of the MSKCC-Cornell Center for Translation of Cancer Nanomedicines (MC2TCN), Director of Intraoperative Imaging, and Co-Chair of the Innovations and Technology Team at MSK. She has been named to several national and international nanomedicine boards.

Dr. Kai Ma is a co-founding scientist of Elucida Oncology. He coinvented the latest generation of C-Dot platform during his PhD studies at Cornell University and developed a series of chemical approaches to produce multifunctional C-Dots for different cancer diagnosis and treatment applications. He is currently the Vice President of Materials Science R&D and Manufacture of Elucida Oncology, leading the company’s development and manufacture of C-Dots. Dr. Ma has authored more than 20 peer-reviewed publications in prestigious journals, including Nature, Nature Nanotechnology, and Nature Communications. He earned a BS in Physics from the University of Science and Technology of China, and his PhD in Materials Science and Engineering from Cornell University.