In this short article, we give our readers an overview on tumor models available in cancer research and drug discovery and explain how our product development and research efforts fit in the big picture. We will address some basic questions such as what is a tumor model, and why does this matter? What are the latest trends in tumor models, and what is Ectica’s contribution?
Tumor Models in Drug Discovery: A Basic Overview
Whether you meet Ectica’s team at conferences or simply browse our product pages, you quickly understand that pretty much everything has to do with tumor cells, stromal cells, where these are sourced, how these are cultured, imaged and used in the laboratory. But why does it matter at all, and perhaps how is this different from what already exists?
Ectica is in the business of developing tools and enabling technologies to advance cancer research and drug discovery, and in particular in creating innovative human tumor models. We could not resist to add already the term “human” because this is a key aspect of our activities. But let’s take a step back: What is a tumor model, and why would this not be human?
A tumor model is used by pre-clinical researchers to study the development and the progression of cancer, and also to test medicines before they are given to patients. Medicines need to provide a benefit to the patient by slowing down the growth or by eliminating cancer cells. How would drug developers know if that is the case prior to administer this to the patients? They use a pre-clinical tumor model, work hard to obtain good results on it and hope that these results translate to the patients, who will be carefully monitored during the administration of the medicine in clinical trials.
You now appreciate the risk related to the selection of the pre-clinical model and the importance of investing in always improving pre-clinical tumor models. A tumor model can be made of cells, tissues, animals or computational programs which can describe how certain things will occur in the human body when a medicine is taken. Models vary in complexity, and it is wrong to assume that the more complex a model is, the better it will serve.
The right attitude is to use a model that is good for the specific aspect of the patient’s tumor to be studied and extrapolate conclusions related to this aspect only, with intellectual rigor. Models vary in applicability, some enable large-scale studies leading to big data acquisition in a short period of time, some require long establishment times and slow data acquisition. The variability of the data acquired is depending on the model selected. Let’s have an overview on the available tumor models and some important terminology, which is sometimes complicated and difficult to agree upon even among experts. At least this is the terminology that we find useful.
Cancer cell lines, patient-derived tumor xenografts, primary patient-derived cell cultures
Human cancer cell lines originate from human cancerous tissues and are maintained by passaging for long time, even decades, in vitro typically on tissue culture plastic surfaces (in sort of Petri dishes). These are established and commercially available models. The culture conditions under which these cells lines have been established are typically minimalistic in nature and do not represent the complexity of a human organ, we therefore say that these are not organotypic cultures.
These cells underwent a strong selection process to survive under these in vitro conditions and are quite homogenous, in the sense that they do not model the cellular heterogeneity of a human tumor. We could see these cells as a subpopulation of the initial tumor cells, which the literature reports to have certainly deviated from their original state. How much of the original (primary) human biology is modelled by these cancer cells lines depends on the genetic, epigenetic, transcriptomic and proteomic differences which must be considered.
Certainly, cancer cell lines are established to be easily put in culture and maintained by various laboratories in parallel with low cost and minimal variability among researchers. Because every laboratory can expand and maintain these cells, differences can be introduced (on purpose or not). Thanks to the large amounts available, they are ideal for genetic manipulations and for large-scale projects, for instance in pharmacogenomics and other screenings.
Interestingly, cancer cell lines can be subsequently moved to more complex environments (organotypic cultures) like in three-dimensional (3D) co-cultures with other cell types. These other cells are not necessarily cancerous, but are part of the tumor microenvironment (TME) and therefore influence tumor progression and response to therapies. These cells are for instance stromal cells (fibroblasts and mesenchymal cells of various type) holding the tumor and healthy tissues together via the connective stroma, but also endothelial cells constituting the vascular and lymphatic vessel structure, and finally the immune cells that keep us safe from infections but also from abnormally dividing tumor cells.
Scientists demonstrated in various occasions that the complex cell-cell interactions which can be mimicked in advanced 3D co-cultures/organ-on-a-chip systems confer superiority in terms of drug sensitivity compared to non-organotypic/standard cultures.
Cancer cell lines can be engrafted in mice to grow tumors in living organisms (in vivo pre-clinical models). Because of the limitations mentioned above, many researchers say that engrafting homogenous cell lines (not representative of the tumor heterogeneity and which are lacking the tumor stroma) translate poorly to the clinic. Pre-clinical researchers have therefore been focusing on the engraftment of heterogeneous primary human tumors (patient-derived tumor xenograft or PDX models).
Typically, after surgical resection tumors are dissociated and injected into host animals as single-cell suspensions or encapsulated inside a hydrogel material (murine basement membrane extracts) to support the engraftment’s efficiency, but the procedure depends on the type of tumor.
There are two important key valuable aspects in these models: Primary cancer cells here grow in a 3D - obviously organotypic - microenvironment and mostly retain the original molecular and phenotypic profiles even after passaging during the establishment of a PDX line. However, compared to in vitro cell cultures the establishment and the maintenance of PDX models is time-consuming and costly, primarily because of the animal care and surgery infrastructure.
The applicability of compounds is lower compared to in vitro cell cultures reducing the size of the study and there are important ethical concerns in the use of animals as laboratory models and as source of laboratory reagents which is prompting regulatory changes supporting alternatives (discussed in another article here). A final and important aspect driving our development activities is the role of the human stroma, which in PDX models is often replaced by the murine stroma, making it difficult to conduct tumor-stroma interaction studies (link).
Advanced organotypic cell culture models of primary cancer patient-derived cells (sometimes abbreviated PDCs) have the potential of maximizing the advantages of the models described above and positively impacting both the drug discovery field and the way therapeutic decisions in the clinic will be made (precision medicine). We prefer to refer to these models with the term ex vivo primary patient-derived cell cultures, but the term patient-derived organoids (PDOs) or patient’s tumor avatars are very popular today.
Technically speaking it is totally correct to call these in vitro models, however we think that the use of non-expanded primary patient cells represents a paradigm shift justifying the differentiating term ex vivo. Primary patient-derived cells are typically cultured in 3D hydrogels (mainly made of murine basement membrane extracts) and when possible expanded and bio-banked. Researchers recently reported the establishment of more than thousands tumor avatars displaying high genomic and transcriptomic fidelity with the primary tumors, presenting a platform for personalized medicine and drug discovery.
The limitations attributed to primary patient-derived cells/organoids often reported in the literature comprise: the lack of standardized methods to develop tumor-stroma co-cultures, which are necessary to study the role of the stroma in tumor progression and sensitivity to therapies. The second is the limited control over the culture conditions resulting from the use of animal-derived culture medium components and murine basement membrane extracts matrices, which we covered in the previous blog article here. Ectica’s experts brought innovative solutions to the market addressing these limitations to make primary patient-derived cell co-cultures the new standard.
Synthetic hydrogel matrices for primary patient-derived cells and defined human patient-derived stromal models for advanced co-cultures
Ectica recognized the importance of continuously improving the technologies enabling primary cancer patient-derived cells and today it is not only at the forefront of the development of synthetic hydrogel-based screening tools (3DProSeed hydrogel plate) but also a leading developer of standardized and well-characterized primary stromal cultures to which researchers can easily add primary tumor cells for tumor-stroma interaction studies (3DProSeed StromaLine).
The 3DProSeed hydrogel well plate is a 96-well imaging plate containing pre-assembled synthetic (animal-free) and optical transparent hydrogels ideal for the culture of patient-derived cells and for high-content analysis. Ectica promotes the innovative concept of sequentially adding primary stromal cells in a first step (e.g. cancer associated fibroblasts, bone marrow mesenchymal and endothelial cells and others) to create an artificial and controlled model of the tumor stroma and in a second step add tumor cells. This is enabled by the patented hydrogel surface technology that promotes repeated cell infiltration into the hydrogel via simple pipetting.
If you found this short blog article interesting and wish to discuss what is the best tumor model for your studies and adopt our technology, please contact us.
Figure 1: Bright-field images of patient-derived lung adenocarcinoma cells growing in a 3DProSeed synthetic hydrogel pre-populated with primary lung cancer associated fibroblasts. Tumor cells grow in association with the fibroblastic network and not in hydrogel regions devoid of fibroblasts.