Getting Personal

Despite the great progress we’ve made in understanding cancer biology, the unfortunate reality is that most newly developed anticancer agents still fail clinical trials. Why? Either because of poor drug efficacy, or because of their substantial side effects (1). When this happens, it’s often not just because of an incomplete comprehension of human cancer biology – other factors also play a role, including the use of unsuitable preclinical models for treatment evaluation and a lack of appropriate biomarkers for patient selection. So how can we overcome these barriers? The answer lies in patient-derived xenografts (PDXs), which are fast becoming the new gold standard models for oncological drug development.

For decades now, conventional cell lines have been the standard tool for in vitro drug testing; without a doubt, their ease of use has improved our understanding of tumor biology. Unfortunately, cell lines aren’t particularly good at predicting drug responses in the clinic (2,3) – and it’s not hard to see why. Tumor cell lines are maintained in 2D culture, without a tumor microenvironment. These artificial growing conditions cause significant biological changes and lead to clonal selection, which prevents conventional cell lines from reflecting the overall profile of the original tumors or the complexity of tumor subpopulations. Even implanting the cells into mouse models for in vivo drug testing doesn’t improve their ability to predict the efficacy of anticancer drugs (4).

PDXs circumvent these limitations. They’re based on implanting human tumor cells into immunocompromised mice to develop models that are exclusively passaged from mouse to mouse with no in vitro culture. To obtain PDX models, dissociated cells or tumor fragments are usually implanted subcutaneously. Although human stroma is replaced by its mouse counterpart during the first passages, this direct implantation maintains realistic growth conditions to preserve tumor architecture, intratumoral heterogeneity and the molecular profile of the original patient tumor (2).

Once numerous studies showed that PDXs ably mimicked patient sensitivity to chemotherapies and targeted therapies (3), their use in drug development grew rapidly and large collections were set up (1,5) (Figure 1). In addition to their role in drug development, PDXs offer the opportunity to perform integrative analyses of their histological, molecular and sensitivity profiles, meaning that researchers can use them to identify predictive biomarkers at a preclinical stage. That’s useful for many reasons – not just preclinical drug testing, but also selection and stratification of patients for future clinical trials (Figure 2).

A crucial role for pathology

So how do pathologists fit into this scheme? One of pathologists’ major responsibilities in cancer research is the management of biobanks – vital for the appropriate storage of clinical samples, the traceability of such samples, and the collection of relevant patient information. Pathologists analyze the morphological and histological features of collected tumor samples, as well as examine clinically approved predictive biomarkers to elucidate molecular characteristics. This contributes to a better comprehension of tumor heterogeneity and a clearer classification of the tumors, both of which can influence patients’ therapeutic options. By interacting closely with research scientists, clinical pathologists also contribute to discovering predictive or prognostic molecular signatures and to developing companion diagnostic tools crucial to boosting the success rates of drugs in clinical trials (6).

To be clinically relevant, a PDX model should accurately represent intra- and intertumoral complexity. To that end, pathologists have been cast in the unusual role of analyzing PDXs as they would patient tumors, in order to guarantee their pertinence in oncology research. Together with surgeons and clinicians, pathologists improve the PDX engraftment rate by guaranteeing sufficient tissue withdrawal and decreasing the time between resection and xenotransplantation. After initial engraftment in the mouse, pathologists track the major characteristics (vascularization, stromal content, necrosis, and relevant clinical biomarkers) of the PDX during establishment and over multiple passages to ensure that it remains representative of the patient tumors. They also study the potential changes in murine organs during treatment in order to evaluate the compounds’ safety and efficacy. Clearly, this extensive characterization by pathologists is key to choosing the most appropriate models for testing compounds, interpreting responses obtained in vivo, and identifying biomarkers and potential indications for subsequent clinical trials.

Revving up research

While PDX models are already having a significant impact on the research and development of novel therapies, they’re also now widely used for tumor biology investigation, drug screening and, most recently, preclinical biomarker identification. Patient tumors represent a limited source of material for cellular and molecular analyses, whereas PDXs provide an unlimited source of FFPE, DNA, RNA and protein samples. More material means more potential analysis, which in turn means more potential to improve patient care! And as new targeted therapies and high-throughput technologies emerge, the way we diagnose cancer is changing. Therapies are guided not only by tumor histotypes, but by the expression levels or mutation status of particular genes – molecular classifications that represent significant advances in tumor categorization. These recent technical advances and the ability to study both the mouse stroma and the human tumor cells have enabled scientists to describe new molecular classifications (7) and to study tumor heterogeneity in more detail (8). Ultimately, we hope that knowledge will facilitate the discovery of new treatment options for tricky patients.

Several biomarkers are already approved for testing in specific cancer histotypes and for certain targeted therapies. Most prominently, the ERBB2 (HER2) amplification in breast and gastric cancer serves as an indication for trastuzumab treatment – and other cancer subtypes that overexpress ERBB2 may benefit from the same therapy. But not all ERBB2-amplified cancers respond well to trastuzumab, highlighting the need to develop new compounds and identify better predictive biomarkers (9). That’s where PDXs become valuable – and indeed, recent investigations conducted on ERBB2-amplified PDXs have led to more efficient anti-ERBB2 compounds (10)!

Looking at limitations

For all their benefits, PDX models are not without challenges. Most are established by subcutaneous implantation of tumor cells on the flanks of immunocompromised mice. Although the site allows rapid transplantation during passages and easy tumor growth monitoring during drug testing, it’s an important change in the anatomic microenvironment and can influence the engraftment rate of some histotypes. The replacement of human stromal cells by their mouse counterparts during the first passages of the PDX can also affect tumor growth conditions. Finally, it’s essential for the mouse to have a degree of immunosuppression in order to establish the model system (11) – but the lack of an intact immune system makes it impossible to test immunotherapies (Figure 2).

Depending on the tumor histotypes and degree of cell aggressiveness, the engraftment time varies between two and 12 months (12,13). Although PDXs from gastric, pancreas and colon cancers – not to mention metastases and aggressive tumors in general – are relatively easy to establish, hormone-dependent cancers like breast cancer are much more challenging, and therefore underrepresented in PDX collections (14,15). Biases like these force researchers to test large PDX collections and include rare subtypes to be sure they’re effectively recapitulating cancer variety. Such extensive in vivo drug testing can be expensive and time consuming – but fortunately, it’s not the only way to use these models. Single-mouse trials have recently proven their clinical relevance (1), and ex vivo drug screening of PDX cell suspensions cultured in 3D is another efficient and cost-effective strategy.

Model perfect?

As PDX models become more popular, collections have grown and engraftment techniques have improved. To make full use of these tools for cancer research, though, we need to resolve several issues: the impossibility of testing immune-based therapies; the transfer from drug-oriented to patient-oriented studies with the development of personalized “avatar” mouse models; and the microenvironment approximation of heterotopic PDX.

Major progress has already been made in immuno-oncology. Evaluating immune-based therapies requires relevant models growing on immunocompetent mice, and researchers are currently attempting to develop PDX in humanized mice by transferring human hematopoietic cells into immunodeficient mice (16,17). Furthermore, mouse avatars have emerged as an interesting translational platform for individualized medicine. The avatar concept is based on generating PDX from a particular patient and testing several drugs to identify the most effective treatment for that patient. It has demonstrated its promise in several pilot studies (18,19) – but, like any other method, avatar models have limitations, including engraftment times and rates that can be incompatible with clinical settings.

The absence of an adequate microenvironment in heterotopic PDX is another hurdle we will need to overcome to get the most out of these models. To circumvent it, PDX can be developed from orthotopically implanted tumors (which are placed in the organ of origin to mimic the original anatomical microenvironment as closely as possible). It usually increases the engraftment rate (3) and better reflects the patient’s drug sensitivities (20). However, it’s also labor-intensive, involves complex surgery, and requires efficient imaging techniques to monitor tumor evolution during drug sensitivity evaluation. We need more in-depth studies to evaluate, on a large scale, the feasibility of using orthotopic, humanized or avatar models for drug testing and personalized therapy.

All of these considerations lead us to consider PDX as an attractive preclinical model for studying tumor biology, new therapeutic options and predictive biomarkers. It’s my hope that these models will help us improve stratification into clinical trials and develop personalized treatment approaches for our patients.