Mimicking Tumors: Toward More Predictive In Vitro Models for Peptide- and Protein-Conjugated Drugs

Bioprinted Multi-Composition Array Mimicking Tumor Microenvironments to Evaluate Drug Efficacy with Multivariable Analysis

Current organ-on-a-chip technologies confront limitations in effectively recapitulating the intricate in vivo microenvironments and accommodating diverse experimental conditions on a single device. Here, a novel approach for constructing a multi-composition tumor array on a single microfluidic device, mimicking complex transport phenomena within tumor microenvironments (TMEs) and allowing for simultaneous evaluation of drug efficacy across 12 distinct conditions is presented. The TME array formed by bioprinting on a microfluidic substrate consists of 36 individual TME models, each characterized by one of three different compositions and tested under four varying drug concentrations. Notably, the TME model exhibits precise compartmentalization, fostering the development of self-organized vascular endothelial barriers surrounding breast cancer spheroids affecting substance transport. Multivariable screening and analysis of diverse conditions, including model complexity, replicates, and drug concentrations, within a single microfluidic platform, highlight the synergistic potential of integrating bioprinting with microfluidics to evaluate drug responses across diverse TME conditions comprehensively.

Biodistribution and toxicity assessment of methoxyphenyl phosphonium carbosilane dendrimers in 2D and 3D cell cultures of human cancer cells and zebrafish embryos

The consideration of human and environmental exposure to dendrimers, including cytotoxicity, acute toxicity, and cell and tissue accumulation, is essential due to their significant potential for various biomedical applications. This study aimed to evaluate the biodistribution and toxicity of a novel methoxyphenyl phosphonium carbosilane dendrimer, a potential mitochondria-targeting vector for cancer therapeutics, in 2D and 3D cancer cell cultures and zebrafish embryos. We assessed its cytotoxicity (via MTT, ATP, and Spheroid growth inhibition assays) and cellular biodistribution. The dendrimer cytotoxicity was higher in cancer cells, likely due to its specific targeting to the mitochondrial compartment. In vivo studies using zebrafish demonstrated dendrimer distribution within the vascular and gastrointestinal systems, indicating a biodistribution profile that may be beneficial for systemic therapeutic delivery strategies. The methoxyphenyl phosphonium carbosilane dendrimer shows promise for applications in cancer cell delivery, but additional studies are required to confirm these findings using alternative labelling methods and more physiologically relevant models. Our results contribute to the growing body of evidence supporting the potential of carbosilane dendrimers as vectors for cancer therapeutics.

Capturing the spatial and temporal dynamics of tumor stroma for on-chip optimization of microenvironmental targeting nanomedicine

Malignant cells grow in a complex microenvironment that plays a key role in cancer progression. The "dynamic reciprocity" existing between cancer cells and their microenvironment is involved in cancer differentiation, proliferation, invasion, metastasis, and drug response. Therefore, understanding the molecular mechanisms underlying the crosstalk between cancer cells and their surrounding tissue (i.e., tumor stroma) and how this interplay affects the disease progression is fundamental to design and validate novel nanotherapeutic approaches. As an important regulator of tumor progression, metastasis and therapy resistance, the extracellular matrix of tumors, the acellular component of the tumor microenvironment, has been identified as very promising target of anticancer treatment, revolutionizing the traditional therapeutic paradigm that sees the neoplastic cells as the preferential objective to fight cancer. To design and to validate such a target therapy, advanced 3D preclinical models are necessary to correctly mimic the complex, dynamic and heterogeneous tumor microenvironment. In addition, the recent advancement in microfluidic technology allows fine-tuning and controlling microenvironmental parameters in tissue-on-chip devices in order to emulate the in vivo conditions. In this review, after a brief description of the origin of tumor microenvironment heterogeneity, some examples of nanomedicine approaches targeting the tumor microenvironment have been reported. Further, how advanced 3D bioengineered tumor models coupled with a microfluidic device can improve the design and testing of anti-cancer nanomedicine targeting the tumor microenvironment has been discussed. We highlight that the presence of a dynamic extracellular matrix, able to capture the spatiotemporal heterogeneity of tumor stroma, is an indispensable requisite for tumor-on-chip model and nanomedicine testing.

Mimicking the Biology of Engineered Protein and mRNA Nanoparticle Delivery Using a Versatile Microfluidic Platform

To investigate the delivery of next-generation macromolecular drugs, such as engineered proteins and mRNA-containing nanoparticles, there is an increasing push towards the use of physiologically relevant disease models that incorporate human cells and do not face ethical dilemmas associated with animal use. Here, we illustrate the versatility and ease of use of a microfluidic platform for studying drug delivery using high-resolution microscopy in 3D. Using this microfluidic platform, we successfully demonstrate the specific targeting of carbonic anhydrase IX (CAIX) on cells overexpressing the protein in a tumor-mimicking chip system using affibodies, with CAIX-negative cells and non-binding affibodies as controls. Furthermore, we demonstrate this system’s feasibility for testing mRNA-containing biomaterials designed to regenerate bone defects. To this end, peptide- and lipid-based mRNA formulations were successfully mixed with colloidal gelatin in microfluidic devices, while translational activity was studied by the expression of a green fluorescent protein. This microfluidic platform enables the testing of mRNA delivery from colloidal biomaterials of relatively high densities, which represents a first important step towards a bone-on-a-chip platform. Collectively, by illustrating the ease of adaptation of our microfluidic platform towards use in distinct applications, we show that our microfluidic chip represents a powerful and flexible way to investigate drug delivery in 3D disease-mimicking culture systems that recapitulate key parameters associated with in vivo drug application.

Establishment of a 3D model of tumor-driven angiogenesis to study the effects of anti-angiogenic drugs on pericyte recruitment

Hepatocellular carcinoma (HCC), as a well-vascularized tumor, has attracted increasing attention in antiangiogenic therapies. Notably, emerging studies reveal that the long-term administration of antiangiogenic drugs induces hypoxia in tumors. Pericytes, which play a vital role in vascular stabilization and maturation, have been documented to be associated with antiangiogenic drug-induced tumor hypoxia. However, the role of antiangiogenic agents in regulating pericyte behavior still remains elusive. In this study, by using immunostaining analysis, we first demonstrated that tumors obtained from HCC patients were highly angiogenic, in which vessels were irregularly covered by pericytes. Therefore, we established a new 3D model of tumor-driven angiogenesis by culturing endothelial cells, pericytes, cancer stem cells (CSCs) and mesenchymal stem cells (MSCs) with microcarriers in order to investigate the effects and mechanisms exerted by antiangiogenic agents on pericyte recruitment during tumor angiogenesis. Interestingly, microcarriers, as supporting matrices, enhanced the interactions between tumor cells and the extracellular matrix (ECM), promoted malignancy of tumor cells and increased tumor angiogenesis within the 3D model, as determined by qRT-PCR and immunostaining. More importantly, we showed that zoledronic acid (ZA) reversed the inhibited pericyte recruitment, which was induced by sorafenib (Sora) treatment, through fostering the expression and activation of ErbB1/ErbB2 and PDGFR-β in pericytes, in both an in vitro 3D model and an in vivo xenograft HCC mouse model. Hence, our model provides a more pathophysiologically relevant platform for the assessment of therapeutic effects of antiangiogenic compounds and identification of novel pharmacological targets, which might efficiently improve the benefits of antiangiogenic treatment for HCC patients.

A Hybrid In Silico and Tumor-on-a-Chip Approach to Model Targeted Protein Behavior in 3D Microenvironments

Simple Summary

Engineered proteins possess a great therapeutic potential, but the development of such therapies is impeded during preclinical studies by the lack of in vitro models that accurately simulate the human physiology. Animal models, on the other hand, also have difficulties predicting human responses, and are ethically concerning. In this study, we employed a hybrid approach where we combined mathematical modeling with 3D in vitro models that mimic aspects of the tumor microenvironment, in order to simulate the delivery of therapeutic proteins targeting cancer cells and to predict the biological activity. By cross-comparing simulated and experimental data from 3D models, we were able to correctly predict the best dose needed to deliver toxic proteins specifically to tumor cells, while leaving the surrounding non-tumor cells untouched. This study shows the potential of combining computational approaches with novel in vitro models to advance the development of protein therapeutics.

Abstract

To rationally improve targeted drug delivery to tumor cells, new methods combining in silico and physiologically relevant in vitro models are needed. This study combines mathematical modeling with 3D in vitro co-culture models to study the delivery of engineered proteins, called designed ankyrin repeat proteins (DARPins), in biomimetic tumor microenvironments containing fibroblasts and tumor cells overexpressing epithelial cell adhesion molecule (EpCAM) or human epithelial growth factor receptor (HER2). In multicellular tumor spheroids, we observed strong binding-site barriers in combination with low apparent diffusion coefficients of 1 µm²·s⁻¹ and 2 µm² ·s⁻¹ for EpCAM- and HER2-binding DARPin, respectively. Contrasting this, in a tumor-on-a-chip model for investigating delivery in real-time, transport was characterized by hindered diffusion as a consequence of the lower local tumor cell density. Finally, simulations of the diffusion of an EpCAM-targeting DARPin fused to a fragment of Pseudomonas aeruginosa exotoxin A, which specifically kills tumor cells while leaving fibroblasts untouched, correctly predicted the need for concentrations of 10 nM or higher for extensive tumor cell killing on-chip, whereas in 2D models picomolar concentrations were sufficient. These results illustrate the power of combining in vitro models with mathematical modeling to study and predict the protein activity in complex 3D models.

Factors to consider when interrogating 3D culture models with plate readers or automated microscopes

Along with the increased use of more physiologically relevant three-dimensional cell culture models comes the responsibility of researchers to validate new assay methods that measure events in structures that are physically larger and more complex compared to monolayers of cells. It should not be assumed that assays designed using monolayers of cells will work for cells cultured as larger three-dimensional masses. The size and barriers for penetration of molecules through the layers of cells result in a different microenvironment for the cells in the outer layer compared to the center of three-dimensional structures. Diffusion rates for nutrients and oxygen may limit metabolic activity which is often measured as a marker for cell viability. For assays that lyse cells, the penetration of reagents to achieve uniform cell lysis must be considered. For live cell fluorescent imaging assays, the diffusion of fluorescent probes and penetration of photons of light for probe excitation and fluorescent emission must be considered. This review will provide an overview of factors to consider when implementing assays to interrogate three dimensional cell culture models.

Microengineered 3D Tumor Models for Anti-Cancer Drug Discovery in Female-Related Cancers

The burden of cancer continues to increase in society and negatively impacts the lives of numerous patients. Due to the high cost of current treatment strategies, there is a crucial unmet need to develop inexpensive preclinical platforms to accelerate the process of anti-cancer drug discovery to improve outcomes in cancer patients, most especially in female patients. Many current methods employ expensive animal models which not only present ethical concerns but also do not often accurately predict human physiology and the outcomes of anti-cancer drug responsiveness. Conventional treatment approaches for cancer generally include systemic therapy after a surgical procedure. Although this treatment technique is effective, the outcome is not always positive due to various complex factors such as intratumor heterogeneity and confounding factors within the tumor microenvironment (TME). Patients who develop metastatic disease still have poor prognosis. To that end, recent efforts have attempted to use 3D microengineered platforms to enhance the predictive power and efficacy of anti-cancer drug screening, ultimately to develop personalized therapies. Fascinating features of microengineered assays, such as microfluidics, have led to the advancement in the development of the tumor-on-chip technology platforms, which have shown tremendous potential for meaningful and physiologically relevant anti-cancer drug discovery and screening. Three dimensional microscale models provide unprecedented ability to unveil the biological complexities of cancer and shed light into the mechanism of anti-cancer drug resistance in a timely and resource efficient manner. In this review, we discuss recent advances in the development of microengineered tumor models for anti-cancer drug discovery and screening in female-related cancers. We specifically focus on female-related cancers to draw attention to the various approaches being taken to improve the survival rate of women diagnosed with cancers caused by sex disparities. We also briefly discuss other cancer types like colon adenocarcinomas and glioblastoma due to their high rate of occurrence in females, as well as the high likelihood of sex-biased mutations which complicate current treatment strategies for women. We highlight recent advances in the development of 3D microscale platforms including 3D tumor spheroids, microfluidic platforms as well as bioprinted models, and discuss how they have been utilized to address major challenges in the process of drug discovery, such as chemoresistance, intratumor heterogeneity, drug toxicity, etc. We also present the potential of these platform technologies for use in high-throughput drug screening approaches as a replacements of conventional assays. Within each section, we will provide our perspectives on advantages of the discussed platform technologies.

3D Tumor Spheroid Models for In Vitro Therapeutic Screening of Nanoparticles

The anticancer activity of compounds and nanoparticles is most often determined in the cell monolayer. However, three-dimensional (3D) systems, such as tumor spheroids, are more representing the natural tumor microenvironment. They have been shown to have higher invasiveness and resistance to cytotoxic agents and radiotherapy compared to cells growing in 2D monolayer. Furthermore, to improve the prediction of clinical efficacy of drugs, in the past decades, even more sophisticated systems, such as multicellular 3D cultures, closely representing natural tumor microenvironment have been developed. Those cultures are formed from either cell lines or patient-derived tumor cells. Such models are very attractive and could improve the selection of tested materials for clinical trials avoiding unnecessary expensive tests in vivo. The microenvironment in tumor spheroids is different, and those differences or the interaction between several cell populations may contribute to different tumor response to the treatment. Also, different types of nanoparticles may have different behavior in 3D models, depending on their nature, physicochemical properties, the presence of targeting ligands on the surface, etc. Therefore, it is very important to understand in which cases which type of tumor spheroid is more suitable for testing specific types of nanoparticles, which conditions should be used, and which analytical method should be applied.

Influence of ClearT and ClearT2 Agitation Conditions in the Fluorescence Imaging of 3D Spheroids

3D tumor spheroids have arisen in the last years as potent tools for the in vitro screening of novel anticancer therapeutics. Nevertheless, to increase the reproducibility and predictability of the data originated from the spheroids it is still necessary to develop or optimize the techniques used for spheroids’ physical and biomolecular characterization. Fluorescence microscopy, such as confocal laser scanning microscopy (CLSM), is a tool commonly used by researchers to characterize spheroids structure and the antitumoral effect of novel therapeutics. However, its application in spheroids’ analysis is hindered by the limited light penetration in thick samples. For this purpose, optical clearing solutions have been explored to increase the spheroids’ transparency by reducing the light scattering. In this study, the influence of agitation conditions (i.e., static, horizontal agitation, and rotatory agitation) on the Clear^T and Clear^T2 methods’ clearing efficacy and tumor spheroids’ imaging by CLSM was characterized. The obtained results demonstrate that the Clear^T method results in the improved imaging of the spheroids interior, whereas the Clear^T2 resulted in an increased propidium iodide mean fluorescence intensity as well as a higher signal depth in the Z-axis. Additionally, for both methods, the best clearing results were obtained for the spheroids treated under the rotatory agitation. In general, this work provides new insights on the Clear^T and Clear^T2 clearing methodologies and their utilization for improving the reproducibility of the data obtained through the CLSM, such as the analysis of the cell death in response to therapeutics administration.

Ultrastructural Features of Gold Nanoparticles Interaction with HepG2 and HEK293 Cells in Monolayer and Spheroids

Use of multicellular spheroids in studies of nanoparticles (NPs) has increased in the last decade, however details of NPs interaction with spheroids are poorly known. We synthesized AuNPs (12.0 ± 0.1 nm in diameter, transmission electron microscopy (TEM data) and covered them with bovine serum albumin (BSA) and polyethyleneimine (PEI). Values of hydrodynamic diameter were 17.4 ± 0.4; 35.9 ± 0.5 and ±125.9 ± 2.8 nm for AuNPs, AuBSA-NPs and AuPEI-NPs, and Z-potential (net charge) values were −33.6 ± 2.0; −35.7 ± 1.8 and 39.9 ± 1.3 mV, respectively. Spheroids of human hepatocarcinoma (HepG2) and human embryo kidney (HEK293) cells (Corning ® spheroid microplates CLS4515-5EA), and monolayers of these cell lines were incubated with all NPs for 15 min–4 h, and fixed in 4% paraformaldehyde solution. Samples were examined using transmission and scanning electron microscopy. HepG2 and HEK2893 spheroids showed tissue-specific features and contacted with culture medium by basal plasma membrane of the cells. HepG2 cells both in monolayer and spheroids did not uptake of the AuNPs, while AuBSA-NPs and AuPEI-NPs readily penetrated these cells. All studied NPs penetrated HEK293 cells in both monolayer and spheroids. Thus, two different cell cultures maintained a type of the interaction with NPs in monolayer and spheroid forms, which not depended on NPs Z-potential and size.

Patient-Derived In Vitro Models for Drug Discovery in Colorectal Carcinoma

Lack of relevant preclinical models that reliably recapitulate the complexity and heterogeneity of human cancer has slowed down the development and approval of new anti-cancer therapies. Even though two-dimensional in vitro culture models remain widely used, they allow only partial cell-to-cell and cell-to-matrix interactions and therefore do not represent the complex nature of the tumor microenvironment. Therefore, better models reflecting intra-tumor heterogeneity need to be incorporated in the drug screening process to more reliably predict the efficacy of drug candidates. Classic methods of modelling colorectal carcinoma (CRC), while useful for many applications, carry numerous limitations. In this review, we address the recent advances in in vitro CRC model systems, ranging from conventional CRC patient-derived models, such as conditional reprogramming-based cell cultures, to more experimental and state-of-the-art models, such as cancer-on-chip platforms or liquid biopsy.

The importance of scoring recognition fitness in spheroid morphological analysis for robust label-free quality evaluation

Because of the growing demand for human cell spheroids as functional cellular components for both drug development and regenerative therapy, the technology to non-invasively evaluate their quality has emerged. Image-based morphology analysis of spheroids enables high-throughput screening of their quality. However, since spheroids are three-dimensional, their images can have poor contrast in their surface area, and therefore the total spheroid recognition by image processing is greatly dependent on human who design the filter-set to fit for their own definition of spheroid outline. As a result, the reproducibility of morphology measurement is critically affected by the performance of filter-set, and its fluctuation can disrupt the subsequent morphology-based analysis. Although the unexpected failure derived from the inconsistency of image processing result is a critical issue for analyzing large image data for quality screening, it has been tackled rarely. To achieve robust analysis performances using morphological features, we investigated the influence of filter-set's reproducibility for various types of spheroid data. We propose a new scoring index, the "recognition fitness deviation (RFD)," as a measure to quantitatively and comprehensively evaluate how reproductively a designed filter-set can work with data variations, such as the variations in replicate samples, in time-course samples, and in different types of cells (a total of six normal or cancer cell types). Our result shows that RFD scoring from 5000 images can automatically rank the best robust filter-set for obtaining the best 6-cell type classification model (94% accuracy). Moreover, the RFD score reflected the differences between the worst and the best classification models for morphologically similar spheroids, 60% and 89% accuracy respectively. In addition to RFD scoring, we found that using the time-course of morphological features can augment the fluctuations in spheroid recognitions leading to robust morphological analysis.

3D cultures for modeling nanomaterial-based photothermal therapy

Photothermal therapy (PTT) is one of the most promising techniques for cancer tumor ablation. Nanoparticles are increasingly being investigated for use with PTT and can serve as theranostic agents. Based on the ability of near-infrared nano-photo-absorbers to generate heat under laser irradiation, PTT could prove advantageous in certain situations over more classical cancer therapies. To analyze the efficacy of nanoparticle-based PTT, preclinical in vitro studies typically use 2D cultures, but this method cannot completely mimic the complex tumor organization, bioactivity, and physiology that all control the complex penetration depth, biodistribution, and tissue diffusion parameters of nanomaterials in vivo. To fill this knowledge gap, 3D culture systems have been explored for PTT analysis. These models provide more realistic microenvironments that allow spatiotemporal oxygen gradients and cancer cell adaptations to be considered. This review highlights the work that has been done to advance 3D models for cancer microenvironment modeling, specifically in the context of advanced, functionalized nanoparticle-directed PTT.

Unravelling Receptor and RGD Motif Dependence of Retargeted Adenoviral Vectors using Advanced Tumor Model Systems

Recent advances in engineering adenoviruses are paving the way for new therapeutic gene delivery approaches in cancer. However, there is limited knowledge regarding the impact of adenoviral retargeting on transduction efficiency in more complex tumor architectures, and the role of the RGD loop at the penton base in retargeting is unclear. To address this gap, we used tumor models of increasing complexity to study the role of the receptor and the RGD motif. Employing tumor-fibroblast co-culture models, we demonstrate the importance of the RGD motif for efficient transduction in 2D through the epithelial cell adhesion molecule (EpCAM), but not the epidermal growth factor receptor (EGFR). Via optical clearing of co-culture spheroids, we show that the RGD motif is required for transduction via both receptors in 3D tumor architectures. We subsequently employed a custom-designed microfluidic model containing collagen-embedded tumor spheroids, mimicking the interplay between interstitial flow, extracellular matrix and adenoviral transduction. Image analysis of on-chip cleared spheroids indicated the importance of the RGD motif for on-chip adenoviral transduction. Together, our results show the interrelationship between receptor characteristics, the RGD motif, the 3D tumor architecture and retargeted adenoviral transduction efficiency. The findings are important for the rational design of next-generation therapeutic adenoviruses.

Cytocompatibility of Titanium, Zirconia and Modified PEEK after Surface Treatment Using UV Light or Non-Thermal Plasma

A number of modifications have been developed in order to enhance surface cytocompatibility for prosthetic support of dental implants. Among them, ultraviolet (UV) light and non-thermal plasma (NTP) treatment are promising methods. The objective of this study was to compare the effects of UV light and NTP on machined titanium, zirconia and modified polyetheretherketone (PEEK, BioHPP) surfaces in vitro. Machined samples of titanium, zirconia and BioHPP were treated by UV light and NTP of argon or oxygen for 12 min each. Non-treated disks were set as controls. A mouse fibroblast and a human gingival fibroblast cell line were used for in vitro experiments. After 2, 24 and 48 h of incubation, the attachment, viability and cytotoxicity of cells on surfaces were assessed. Results: Titanium, zirconia and BioHPP surfaces treated by UV light and oxygen plasma were more favorable to the early attachment of soft-tissue cells than non-treated surfaces, and the number of cells on those treated surfaces was significantly increased after 2, 24 and 48 h of incubation (p < 0.05). However, the effects of argon plasma treatment on the cytocompatibility of soft tissue cells varied with the type of cells and the treated material. UV light and oxygen plasma treatments may improve the attachment of fibroblast cells on machined titanium, zirconia and PEEK surfaces, that are materials for prosthetic support of dental implants.

Insights into Active Targeting of Nanoparticles in Drug Delivery: Advances in Clinical Studies and Design Considerations for Cancer Nanomedicine

Nanomedicine is a promising strategy for improving clinical outcomes for cancer therapies, by improving drug efficacy through enhanced delivery to disease sites. It is of importance for ultimate clinical success to consider the contributing factors to achieving this goal, such as size, chemistry, and functionality of nanoparticle delivery systems, and how these parameters influence tumor localization and uptake. This Topical Review will first discuss the evolution and progress of nanoparticles for cancer drug delivery and the current challenges that remain to be addressed. Strategies for overcoming the limitations of passive targeting through active targeting approaches, and the current state of such nanomedicines in the clinic will be highlighted. Finally, novel approaches toward the design of active targeted nanoparticles building on our growing understanding of nanobio interactions are considered, in order to shed light on future design considerations for accelerating clinical translation of nanomedicines.

Tumor Drug Penetration Measurements Could Be the Neglected Piece of the Personalized Cancer Treatment Puzzle

Precision medicine aims to use patient genomic, epigenomic, specific drug dose, and other data to define disease patterns that may potentially lead to an improved treatment outcome. Personalized dosing regimens based on tumor drug penetration can play a critical role in this approach. State-of-the-art techniques to measure tumor drug penetration focus on systemic exposure, tissue penetration, cellular or molecular engagement, and expression of pharmacological activity. Using in silico methods, this information can be integrated to bridge the gap between the therapeutic regimen and the pharmacological link with clinical outcome. These methodologies are described, and challenges ahead are discussed. Supported by many examples, this review shows how the combination of these techniques provides enhanced patient-specific information on drug accessibility at the tumor tissue level, target binding, and downstream pharmacology. Our vision of how to apply tumor drug penetration measurements offers a roadmap for the clinical implementation of precision dosing.

Cell-Based Assays on Microfluidics for Drug Screening

Microfluidics is an appealing platform for drug screening and discovery. Compared with the conventional drug screening methods based on Petri dishes and experimental animals, microfluidic devices have many advantages including miniaturized size, ease-to-use, high sensitivity, and high throughput. More importantly, bioassays on microfluidics can avoid ethical issues which can be a big obstacle hindering the performance of the experiments on animals or human being. Furthermore, three-dimensional (3D) microchips can recapitulate various biochemical and biophysical conditions in vivo and mimic the natural microenvironment of the tissues/organs, providing versatile in vitro models for biomedical applications. In this Perspective, we will focus on the cell-based microfluidic assays for drug screening. Meanwhile, we also propose potential solutions for the difficulties in this field and discuss the prospects of microfluidics-based technologies for drug screening.

On-chip anticancer drug screening - Recent progress in microfluidic platforms to address challenges in chemotherapy

There is an increasing need for advanced and inexpensive preclinical models to accelerate the development of anticancer drugs. While costly animal models fail to predict human clinical outcomes, in vitro models such as microfluidic chips ('tumor-on-chip') are showing tremendous promise at predicting and providing meaningful preclinical drug screening outcomes. Research on 'tumor-on-chips' has grown enormously worldwide and is being widely accepted by pharmaceutical companies as a drug development tool. In light of this shift in philosophy, it is important to review the recent literature on microfluidic devices to determine how rapidly the technology has progressed as a promising model for drug screening and aiding cancer therapy. We review the past five years of successful developments and capabilities in microdevice technology (cancer models) for use in anticancer drug screening. Microfluidic devices that are being designed to address current challenges in chemotherapy, such as drug resistance, combinatorial drug therapy, personalized medicine, and cancer metastasis are also reviewed in detail. We provide a perspective on how personalized 'tumor-on-chip', as well as high-throughput microfluidic platforms based on patient-specific tumor cells, can potentially replace the more expensive and 'non-human' animal models in preclinical anticancer drug development.

3D Cell-Based Assays for Drug Screens: Challenges in Imaging, Image Analysis, and High-Content Analysis

The introduction of more relevant cell models in early preclinical drug discovery, combined with high-content imaging and automated analysis, is expected to increase the quality of compounds progressing to preclinical stages in the drug development pipeline. In this review we discuss the current switch to more relevant 3D cell culture models and associated challenges for high-throughput screening and high-content analysis. We propose that overcoming these challenges will enable front-loading the drug discovery pipeline with better biology, extracting the most from that biology, and, in general, improving translation between in vitro and in vivo models. This is expected to reduce the proportion of compounds that fail in vivo testing due to a lack of efficacy or to toxicity.

Octa-arginine boosts the penetration of elastin-like polypeptide nanoparticles in 3D cancer models

Elastin-like polypeptide (ELP) nanoparticles are a versatile platform for targeted drug delivery. As for any type of nanocarrier system, an important challenge remains the ability of deep (tumor) tissue penetration. In this study, ELP particles with controlled surface density of the cell-penetrating peptide (CPP) octa-arginine (R8) were created by temperature-induced co-assembly. ELPs formed micellar nanoparticles with a diameter of around 60 nm. Cellular uptake in human skin fibroblasts was directly dependent on the surface density of R8 as confirmed by flow cytometry and confocal laser scanning microscopy. Remarkably, next to promoting cellular uptake, the presence of the CPP also enhanced penetration into spheroids generated from human glioblastoma U-87 cells. After 24 h, uptake into cells was observed in multiple layers towards the spheroid core. ELP particles not carrying any CPP did not penetrate. Clearly, a high CPP density exerted a dual benefit on cellular uptake and tissue penetration. At low nanoparticle concentration, there was evidence of a binding site barrier as observed for the penetration of molecules binding with high affinity to cell surface receptors. In conclusion, R8-functionalized ELP nanoparticles form an excellent delivery vehicle that combines tunability of surface characteristics with small and well-defined size.

Anticancer polymers designed for killing dormant prostate cancer cells

The discovery of anticancer therapeutics effective in eliminating dormant cells is a significant challenge in cancer biology. Here, we describe new synthetic polymer-based anticancer agents that mimic the mode of action of anticancer peptides. These anticancer polymers developed here are designed to capture the cationic, amphiphilic traits of anticancer peptides. The anticancer polymers are designed to target anionic lipids exposed on the cancer cell surfaces and act by disrupting the cancer cell membranes. Because the polymer mechanism is not dependent on cell proliferation, we hypothesized that the polymers were active against dormant cancer cells. The polymers exhibited cytotoxicity to proliferating prostate cancer. Importantly, the polymer killed dormant prostate cancer cells that were resistant to docetaxel. This study demonstrates a new approach to discover novel anticancer therapeutics.

Biotin functionalized PEGylated poly(amidoamine) dendrimer conjugate for active targeting of paclitaxel in cancer

In the current study, we employed poly(amidoamine) (PAMAM) dendrimers of generation 4 (G4) to deliver paclitaxel (PTX), a poorly soluble anti-cancer agent precisely to cancer cells via its conjugation on dendrimer surface. Further, G4 PAMAM has been PEGylated (PEG) and tagged with Biotin, an essential micronutrient for cellular functions, receptors of which are overexpressed in certain cancers. The synthesized multifunctional conjugates were characterized by ¹H NMR and zeta potential analysis techniques. In addition, the conjugates were evaluated in vitro in cell monolayers and 3D spheroids of biotin receptor over-expressed A549 cell line (human non-small cell lung cancer). G4 PTX PEG-Biotin conjugate penetrated at significantly higher extent in monolayers as well as spheroids as studied by flow cytometry and confocal microscopy by visualizing the cells at varied depth. The G4 PTX PEG-Biotin conjugate demonstrated higher cytotoxicity compared to free PTX and G4 PTX PEG conjugate as assessed by MTT assay in monolayers and Presto Blue assay in detached spheroidal cells. G4 PTX PEG-Biotin demonstrated significant inhibition of growth of tumor spheroids. Therefore, the newly synthesized biotin anchored PTX-conjugated dendrimer system is promising and could be further explored for efficiently delivering PTX to biotin receptor overexpressed cancers.

Establishment of 2D Cell Cultures Derived From 3D MCF-7 Spheroids Displaying a Doxorubicin Resistant Profile

In vitro 3D cancer spheroids generally exhibit a drug resistance profile similar to that found in solid tumors. Due to this property, these models are an appealing for anticancer compounds screening. Nevertheless, the techniques and methods aimed for drug discovery are mostly standardized for cells cultured in 2D. The development of 2D cell culture models displaying a drug resistant profile is required to mimic the in vivo tumors, while the equipment, techniques, and methodologies established for conventional 2D cell cultures can continue to be employed in compound screening. In this work, the response of 3D-derived MCF-7 cells subsequently cultured in 2D in medium supplemented with glutathione (GSH) (antioxidant agent found in high levels in breast cancer tissues and a promoter of cancer cells resistance) to Doxorubicin (DOX) is evaluated. These cells demonstrated a resistance toward DOX closer to that displayed by 3D spheroids, which is higher than that exhibited by standard 2D cell cultures. In fact, the 50% inhibitory concentration (IC₅₀ ) of DOX in 3D-derived MCF-7 cell cultures supplemented with GSH is about eight-times higher than that obtained for conventional 2D cell cultures (cultured without GSH), and is only about two-times lower than that attained for 3D MCF-7 spheroids (cultured without GSH). Further investigation revealed that this improved resistance of 3D-derived MCF-7 cells may result from their increased P-glycoprotein (P-gp) activity and reduced production of intracellular reactive oxygen species (ROS).

Customizable biomaterials as tools for advanced anti-angiogenic drug discovery

The inhibition of angiogenesis is a critical element of cancer therapy, as cancer vasculature contributes to tumor expansion. While numerous drugs have proven to be effective at disrupting cancer vasculature, patient survival has not significantly improved as a result of anti-angiogenic drug treatment. Emerging evidence suggests that this is due to a combination of unintended side effects resulting from the application of anti-angiogenic compounds, including angiogenic rebound after treatment and the activation of metastasis in the tumor. There is currently a need to better understand the far-reaching effects of anti-angiogenic drug treatments in the context of cancer. Numerous innovations and discoveries in biomaterials design and tissue engineering techniques are providing investigators with tools to develop physiologically relevant vascular models and gain insights into the holistic impact of drug treatments on tumors. This review examines recent advances in the design of pro-angiogenic biomaterials, specifically in controlling integrin-mediated cell adhesion, growth factor signaling, mechanical properties and oxygen tension, as well as the implementation of pro-angiogenic materials into sophisticated co-culture models of cancer vasculature.

Octa-arginine modified poly(amidoamine) dendrimers for improved delivery and cytotoxic effect of paclitaxel in cancer

Cell penetrating peptides (CPP) have the ability to penetrate the cell membrane and have been associated with various cargos for their facile intracellular translocation. The current study involves the synthesis of a CPP, octa-arginine (R8)-modified poly(amidoamine) dendrimer of generation 4 (G4), which has additionally been PEGylated and conjugated to the poorly soluble anticancer drug, paclitaxel (PTX). The synthesized dendrimer conjugates were characterized by proton nuclear magnetic resonance (1H-NMR) Spectroscopy and zeta potential measurements and evaluated in vitro in cell monolayers and 3D spheroids. Cellular uptake study in human cervical cancer cell line (HeLa) revealed that R8 modification significantly improved the cell association of conjugates. G4-PTX- polyethylene glycol (PEG)-R8 conjugate demonstrated enhanced cytotoxic potential and higher induction of apoptosis compared to free PTX and G4-PTX-PEG. Further, the penetrability of fluorescently labeled F-G4-PTX-PEG-R8 was evaluated in 3D spheroids of HeLa at various depths by using confocal microscopy. G4-PTX-PEG-R8 induced cell death and inhibited the growth in 3D spheroids as competently as in monolayers. The enhanced intracellular translocation of R8-modified dendrimers resulted in improved anticancer efficacy of PTX. Therefore, the newly developed dendrimer system is efficient for the intracellular delivery of PTX in cancer cells and has a strong potential to be utilized as an effective chemotherapeutic agent for cancer.

Penetration in 3D tumor spheroids and explants: Adding a further dimension to the structure-activity relationship of cell-penetrating peptides

Drug delivery into tumors and metastases is a major challenge in the eradication of cancers such as epithelial ovarian carcinoma. Cationic cell-penetrating peptides (CPPs) are a promising group of delivery vehicles to mediate cellular entry of molecules that otherwise poorly enter cells. However, little is known about their penetration behavior in tissues. Here, we investigated penetration of cationic CPPs in 3D ovarian cancer spheroids and patient-derived 3D tumor explants. Penetration kinetics and distribution after long-term incubation were imaged by confocal microscopy. In addition, spheroids and tumor explants were dissociated and cell-associated fluorescence determined by flow cytometry. CPPs with high uptake activity showed enhanced sequestration in the periphery of the spheroid, whereas less active CPPs were able to penetrate deeper into the tissue. CPPs consisting of d-amino acids were advantageous over l-amino acid CPPs as they showed less but long lasting cellular uptake activity, which benefitted penetration and retention over time. In primary tumor cultures, in contrast to nonaarginine, the amphipathic CPP penetratin was strongly sequestered by cell debris and matrix components pointing towards arginine-rich CPPs as a preferred choice. Overall, the data show that testing in 3D models leads to a different choice of the preferred peptide in comparison to a standard 2D cell culture.

Application of 3D cultured multicellular spheroid tumor models in tumor-targeted drug delivery system research

Tumor-targeted drug delivery systems are promising for their advantages in enhanced tumor accumulation and reduced toxicity towards normal organs. However, few nanomedicines have been successfully translated into clinical application. One reason is the gap between current pre-clinical and clinical studies. The prevalent in vitro models utilized in pre-clinical phase are mainly based on the two-dimensional (2D) cell culture and are limited by the difficulty of simulating three-dimensional physiological conditions in human body, such as three-dimensional (3D) architecture, cell heterogeneity, nutrient gradients and the interaction between cells and the extracellular matrix (ECM). In addition, traditional animal models have drawbacks such as high-cost, long periods and physiological differences between animal and human. On the other hand, the employment of 3D tumor cell culture models, especially multicellular tumor spheroids (MCTS), has increased significantly in recent decades. These models have been shown to simulate 3D structures of tumors in vitro with relatively low cost and simple protocols. Currently, MCTS have also been widely exploited in drug delivery system research for comprehensive study of drug efficacy, drug penetration, receptor targeting, and cell recruitment abilities. This review summarizes the delivery barriers for nano-carriers presented in tumor microenvironment, the characteristics and formation methods for applicable multicellular tumor spheroid culture models and recent studies related to their applications in tumor-targeted drug delivery system research.

Sepia ink oligopeptide induces apoptosis and growth inhibition in human lung cancer cells

Sepia ink oligopeptide (SIO), as a tripeptide extracted from Sepia ink, could be used as an inducer of apoptosis in human prostate cancer cells. We designed a cyclo-mimetic peptide of SIO by introducing a disulfide bond to stabilize the native peptide into beta turn structure, and produced a peptide with higher cell permeability and stability. Through labeling an FITC to the N-terminus of the peptide, the cell permeability was examined. Stabilized peptide showed enhanced cellular uptake than linear tripeptide as indicated by flow cytometry and cell fluorescent imaging. The high intracellular delivery of stable SIO could more efficiently inhibit cell proliferation and induce apoptosis. Furthermore, the expression of the anti-apoptotic protein Bcl-2 was down-regulated, whereas pro-apoptotic proteins P53 and caspase-3 were up-regulated by stable SIO. In conclusion, our study is the first to use stable SIO to induce apoptosis in two lung cancer cells A549 and H1299.

Bridging Bio-Nano Science and Cancer Nanomedicine

The interface of bio-nano science and cancer medicine is an area experiencing much progress but also beset with controversy. Core concepts of the field-e.g., the enhanced permeability and retention (EPR) effect, tumor targeting and accumulation, and even the purpose of "nano" in cancer medicine-are hotly debated. In parallel, considerable advances in neighboring fields are occurring rapidly, including the recent progress of "immuno-oncology" and the fundamental impact it is having on our understanding and the clinical treatment of the group of diseases collectively known as cancer. Herein, we (i) revisit how cancer is commonly treated in the clinic and how this relates to nanomedicine; (ii) examine the ongoing debate on the relevance of the EPR effect and tumor targeting; (iii) highlight ways to improve the next-generation of nanomedicines; and (iv) discuss the emerging concept of working with (and not against) biology. While discussing these controversies, challenges, emerging concepts, and opportunities, we explore new directions for the field of cancer nanomedicine.