PDX Model: an Advanced In Vivo Model for Tumor Research and Personalized Treatment

PDX model, full name Patient-Derived Tumor Xenograft (PDX) model, is a tumor model constructed by transplanting tumor tissue from tumor patients into immunodeficient mice and allowing the tumor tissue to grow in the mice. This model has not undergone any artificial culture, so its biological characteristics are more complete and more similar to clinical practice. It is considered to be the best tumor animal model at this moment.

PDX Model Establishment Process

 

The establishment of PDX. a Showing the establishment process of PDX. b Showing the factors affecting the engraftment rate and the categories of PDX mice[1]

The steps to build a PDX (Patient-Derived Xenograft) model usually include the following:

1. Prepare tumor tissue specimens. First, tumor tissue needs to be obtained from the patient. These tissues can be cut into 2 to 3 mm cubic fragments or made into single cell suspensions. Tumor specimens need to be washed three times with a cold solution containing fetal bovine serum (FBS) and penicillin/streptomycin before transplantation.

2. Transplantation. A small incision is created on the lower back of 4 to 8 week-old immunodeficient mice for subcutaneous transplantation, and the tumor tissue sample is transplanted into the surgical area. Tumor tissue can also be transplanted to the subrenal capsule, subcutaneous, or in situ. Subcutaneous transplantation is the most commonly used method because it provides heterogeneity of tumor cells, rarely metastasizes, and better simulates the initial tumor microenvironment.

3. Observation and passage. After successful transplantation, tumor tissue will grow in the mouse. When the tumor size reaches 1 to 2 cm cubic, it usually takes 2 to 4 months. The tumor can be dissected and re-transplanted into other mice. The first generation is F1, and the second and third generations are F2 and F3. The third generation (F3) and above mice can be used for drug treatment research.

4. Model verification. To judge whether the construction of the PDX model is successful, several conditions must be met, such as the transplanted tumor can be stably transmitted to the second or third generation, the growth curve is highly consistent, the incubation period tends to be stable, the tumor formation time is generally not more than 12 weeks, and the tumor can grow stably after resuscitation and re-transplantation in the mouse body.

5. Drug screening and research. The PDX model retains the basic characteristics of the microenvironment and cells of the primary tumor, and can be used for high-throughput drug efficacy evaluation, individualized precision medication plan formulation and other studies.

Advantages and Disadvantages of PDX Models

 

Jin, Jiankang & Yoshimura, Katsuhiro & Sewastjanow-Silva, Matheus & Song, Shumei & Ajani, Jaffer. (2023). Challenges and Prospects of Patient-Derived Xenografts for Cancer Research. Cancers. 15. 4352. 10.3390/cancers15174352.

High intratumor heterogeneity and molecular diversity make this model the closest to the actual human body; the predictability of clinical results greatly improves the application of this model in clinical and even drug development; even the problem that the tumor tissue stroma will change from human to mouse during the culture process is a great improvement compared with other existing models-at least a part of the human stroma is retained in the PDX model.

Of course, the PDX model has inherent disadvantage-the low transplantation success rate is still a key factor restricting its widespread application. After all, human tissue cultured in mice is inevitably rejected. Researchers are making various efforts to solve this problem. The common practice is to use mice with higher immunodeficiency to reduce the occurrence of immune rejection; at the same time, some researchers are trying to develop transgenic mouse models that can simulate the complete human immune system.

The Role of PDX in Studying Chemotherapy, Targeted Therapy, Immunotherapy and Other New Cancer Treatment Methods.

 

PDX in the new era of cancer treatment. This figure shows the current conundrums of cancer treatment including restricted beneficiaries, tumor heterogeneity, drug resistance as well as tumor metastasis and recurrence, and shows the versatile functions of PDX in developing therapeutics against cancer[2]

The PDX model retains most of the characteristics of the primary tumor at the histopathological, molecular biological and genetic levels, so it has good predictive power for clinical efficacy. This makes it widely used in new drug development, especially in the screening of patients for clinical trials of targeted drugs and the study of predictive biomarkers. In addition, PDX models can also be used to evaluate the efficacy of personalized chemotherapy for clinical tumors by administering drugs in parallel to patients and PDX model mice.

When constructing PDX models, it is usually necessary to transplant the collected tumor samples into experimental animals. Commonly used experimental animals include mice and nude mice, which have a high transplantation success rate. However, this process usually requires strict ethical approval to ensure that the way the samples are obtained does not cause additional harm to the patient.

The field of tumors is experiencing a period of rapid development. We need to have a more detailed understanding of tumor tissues in order to choose the best treatment plan. Because each patient's tumor is unique, cancer treatment should also choose personalized treatment. It is extremely necessary to select the most effective, least toxic and predictive drug efficacy tests for patients in personalized tumor treatment. It is extremely necessary to select xenograft tissues that are very similar to the patient's cancer. This transplant can quickly evaluate the patient's responsiveness to various treatment options and help to select the best treatment plan for subsequent treatment.

To achieve this goal, the first generation or early PDX models have been used in personalized tumor treatment. This transplant functionally retains the molecular heterogeneity and histological complexity of the patient's original tumor, including the matrix of the original tumor in the tumor microenvironment and simulated cell-cell interactions. After a large number of studies, it has been shown that the first-generation PDX model is suitable for predicting drug responsiveness.

Some studies have found that the response of PDX model drugs is significantly correlated with clinical drug response. For example, in non-small cell lung cancer, PDX models were used to test the efficacy of the three most commonly used first-line chemotherapy drugs. The results showed that about two-thirds of non-small cell lung cancer patients were sensitive to first-line chemotherapy drugs, while the other one-third of patients showed resistance. Interestingly, patients had different sensitivities to all chemotherapy regimens, with some patients being sensitive to one chemotherapy regimen but not to another, indicating the potential for personalized regimen selection. The potential use of personalized chemotherapy PDX models has also been recently discovered in the treatment of prostate cancer.

Illumina genome sequencing of a patient's neuroendocrine prostate tumor revealed a homozygous deletion on chromosome 9p21 that affects the survival of the methylthioadenosine phosphorylase (MTAP) and CDKN2-ARF genes, which is common in many cancers. Tumor-derived xenografts of LTL352 were tested by array comparative genomic hybridization and found to have the same gene deletion. Mice treated with MTAP-deficient LTL352 xenografts were given high doses of 6-thioguanine (6-TG) combined with methylthioadenosine (to protect normal cells from 6-TG toxicity), and the tumor tissue was attenuated without significant effects on the host.

This study shows that PDX models, when used appropriately and in conjunction with advanced genomic profiling techniques, can be used to identify important therapeutic targets and potential personalized cancer treatments. In another study, researchers used PDX models derived from patients with advanced cancer to monitor a large number of anticancer drugs and screen the most effective drugs to treat the PDX donor patient. The response was highly correlated between the PDX model and the patient.

Limitations and Future Directions of PDX Models

A key limitation of PDX models is that they must use immunodeficient hosts for tumor cell implantation and transfer. Mice with a deficient immune system can avoid rejection of human cancer tissue after injection and allow tumor tissue growth. For this reason, PDX models are limited in their use for monitoring immune-mediated drugs (including vaccines and immunomodulators such as anti-PD1) or drugs that require activation of immunogenic components for their efficacy (such as anti-CD40 antibodies).

“Personalized immunity” mice, which reconstitute a complete immune system by extracting hematopoietic stem cells from the bone marrow of cancer patients, may provide new models to observe the role of autoimmune responses. These models may allow the testing of drugs that target immune or stromal components.

In order to obtain reproducible and reliable results using PDX models, it is critical to maintain cell identity and structure. Although after In Vivo cell expansion, only a few cells undergo changes in whole chromosome copy number, cell morphology, growth rate, and gene expression profile compared to early xenografts, it is prudent to establish a permanent stock of xenografts to ensure that cell identity and composition are preserved and to avoid damage caused by serial passaging and unnecessary use.

Some studies have shown that PDX models can be stored in 10% DMSO and placed in liquid nitrogen for long-term storage. Xenografts using SRC transplant sites have a 95% resuscitation rate after storage using this method. These stored xenografts can therefore serve as a source of primary tumor tissue lines, and reproducible, reliable results can be obtained.

Cancer genetics is a profound revolution that allows the collection of large amounts of genomic information for the first time, including the evaluation of a complete cancer genome, to aid clinical decision making. PDX models may improve the effectiveness of personalized cancer treatment by combining more extensive personalized medicine measures such as personalized cancer genetics or whole genome analysis. In addition, the combination of tumor genetics informatics by observing responses in mouse experiments may discover new biomarkers for monitoring drug efficacy.

 

Reference:

1. Liu Y, Wu W, Cai C, Zhang H, Shen H, Han Y. Patient-derived xenograft models in cancer therapy: technologies and applications. Signal Transduct Target Ther. 2023 Apr 12;8(1):160. doi: 10.1038/s41392-023-01419-2IF: 39.3 Q1 . PMID: 37045827; PMCID: PMC10097874.

 

2. Liu, Yihan & Wu, Wantao & Cai, Changjing & Zhang, Hao & Shen, Hong & Han, Ying. (2023). Patient-derived xenograft models in cancer therapy: technologies and applications. Signal Transduction and Targeted Therapy. 8. 10.1038/s41392-023-01419-2.