The process of generating PDX models in mice from fresh primary or metastatic human cancer is extensively described in the literature (10 , 13 (link)). While individual groups have developed specific methodological approaches, the fundamentals are common. Table 1 provides a summary of approaches used to generate the most comprehensive PDX collections currently available. Briefly, pieces of primary or metastatic solid tumors maintained as tissue structures are collected by surgery or biopsy procedures. Some studies have also used fluid drained from malignant ascites or pleural effusions. Tumors are implanted as pieces or single cell suspensions, either alone or in some studies coated with matrigel or mixed with human fibroblasts or mesenchymal stem cells. The most common site of implantation is on the dorsal region of mice (subcutaneous implantation), although implantation in the same organ as the original tumor may be an option (orthotopic implantation, i.e. pancreas, oral cavity, ovary, mammary fat pad, brain, etc.). In addition, independently of the tumor origin, several approaches have implanted primary tumors in the renal capsule in an effort to increase engraftment success rates. A variety of mouse strains having different degrees of immunosuppression have been used in these studies. Supplementary Table 1 lists the principal characteristics of the most commonly used mouse strains including their level of immune suppression as well as advantages or disadvantages. For hormone sensitive tumors, some studies have used hormone supplementation with the intent of increasing engraftment rates.
Some approaches may have theoretical advantages with regard to higher and faster engraftment rates and generation of models that better recapitulate human tumors and are, therefore, more predictive. However, it is important to mention that very few studies have properly addressed comparative implantation methods for these endpoints. Studies in which PDX models have been generated simultaneously from primary tumors and metastatic lesions suggest that metastases have a higher engraftment rate (14 (link), 15 ). Defining the most appropriate host mouse strains to generate PDX models is an important consideration. It is assumed that more severely immunosuppressed models such as non-obese diabetic/severe combined immunodeficiency disorder (NOD/SCID) or NOD/SCID/IL2λ-receptor null (NSG) models are better suited for PDX generation due to higher engraftment rates. Indeed, these are the preferred rodent strains for many groups. However, in human breast cancer (HBC) where this question has been robustly interrogated, implantation in NOD/SCID versus NSG mice yielded similar take rates (16 (link)). In addition, host supplementation with estradiol pellets increased engraftment rates from 2.6 to 21.4 % while, for reasons that are unclear, co-implantation with immortalized human fibroblasts decreased engraftment rate (16 (link)). In contrast, in another study, a mixture of irradiated and non-irradiated human fibroblasts provided improved results (17 (link)). Likewise orthotopic tumor implantation (“orthoxenografts”, (18 (link))) may also confer a translational advantage, as the tumor develops in the same anatomical microenvironment. Generation of orthoxenografts is more labor-intensive, requires complex surgery, is more expensive and often requires imaging methods to monitor tumor growth. However, for several tumor types (e.g. ovarian cancer or lung cancer), this approach substantially increases tumor take rates (19 (link)). In this vein, orthotopic implantation in the testis is essential for the growth of testicular germ cell tumors. As for tumor implantation in the renal capsule, it yielded an impressive 90 % engraftment rate in non-small cell lung cancer (NSCLC) as compared to 25% following subcutaneous implantation, although these results were not obtained from a single comparative study (20 (link), 21 (link)). Furthermore, renal cell capsule implantation shortens time to engraftment, which is one of the most important variables for studies seeking to implement real time PDX data for personalized cancer treatment (20 (link)).
Some approaches may have theoretical advantages with regard to higher and faster engraftment rates and generation of models that better recapitulate human tumors and are, therefore, more predictive. However, it is important to mention that very few studies have properly addressed comparative implantation methods for these endpoints. Studies in which PDX models have been generated simultaneously from primary tumors and metastatic lesions suggest that metastases have a higher engraftment rate (14 (link), 15 ). Defining the most appropriate host mouse strains to generate PDX models is an important consideration. It is assumed that more severely immunosuppressed models such as non-obese diabetic/severe combined immunodeficiency disorder (NOD/SCID) or NOD/SCID/IL2λ-receptor null (NSG) models are better suited for PDX generation due to higher engraftment rates. Indeed, these are the preferred rodent strains for many groups. However, in human breast cancer (HBC) where this question has been robustly interrogated, implantation in NOD/SCID versus NSG mice yielded similar take rates (16 (link)). In addition, host supplementation with estradiol pellets increased engraftment rates from 2.6 to 21.4 % while, for reasons that are unclear, co-implantation with immortalized human fibroblasts decreased engraftment rate (16 (link)). In contrast, in another study, a mixture of irradiated and non-irradiated human fibroblasts provided improved results (17 (link)). Likewise orthotopic tumor implantation (“orthoxenografts”, (18 (link))) may also confer a translational advantage, as the tumor develops in the same anatomical microenvironment. Generation of orthoxenografts is more labor-intensive, requires complex surgery, is more expensive and often requires imaging methods to monitor tumor growth. However, for several tumor types (e.g. ovarian cancer or lung cancer), this approach substantially increases tumor take rates (19 (link)). In this vein, orthotopic implantation in the testis is essential for the growth of testicular germ cell tumors. As for tumor implantation in the renal capsule, it yielded an impressive 90 % engraftment rate in non-small cell lung cancer (NSCLC) as compared to 25% following subcutaneous implantation, although these results were not obtained from a single comparative study (20 (link), 21 (link)). Furthermore, renal cell capsule implantation shortens time to engraftment, which is one of the most important variables for studies seeking to implement real time PDX data for personalized cancer treatment (20 (link)).
