Breast Cancer PDX Models: Tumor Acquisition and Implantation

Patient-derived xenograft (PDX) models are a crucial tool in breast cancer research, enabling scientists to study tumor behavior and treatment responses in a biologically relevant setting. By implanting patient tumor samples into immunocompromised mice, these models preserve the genetic and histological characteristics of human tumors more effectively than traditional cell line-based studies.

Developing effective PDX models requires careful tumor handling and precise implantation to maintain fidelity.

Tumor Acquisition

High-quality tumor specimens are essential for successful breast cancer PDX models, as sample integrity directly impacts the model’s ability to replicate the original tumor’s biology. Fresh tumor tissue is typically collected from patients undergoing surgery or biopsy, with close coordination among oncologists, pathologists, and researchers to minimize ischemic time. Delays between excision and implantation can cause cellular degradation and altered gene expression, compromising model fidelity (Cancer Cell, Bruna et al., 2016). Best practices recommend processing tissue within 30 minutes to two hours post-excision to preserve tumor architecture and molecular characteristics.

Sterile handling is crucial to prevent contamination and degradation. Transport media such as RPMI-1640 with antibiotics and antifungals help maintain tissue viability during transit. Some protocols use hypothermic preservation solutions like HypoThermosol to extend the implantation window without significantly affecting viability. Tumors preserved in optimized media for up to 24 hours retained over 90% of their histopathological features (Nature Communications, Byrne et al., 2017), highlighting the importance of proper handling.

Tissue selection is another key factor, as tumors are often heterogeneous. Pathologists select viable, non-necrotic regions with high cellularity, avoiding fibrosis or treatment-induced damage. Tumors from chemotherapy-naïve patients tend to establish more robust PDX models, as prior treatment can induce genetic changes affecting engraftment (Breast Cancer Research, Dobrolecki et al., 2016). However, post-therapy specimens provide insights into treatment resistance and tumor evolution.

Implantation Techniques

Successful PDX model establishment depends on implantation methods that preserve tumor architecture and heterogeneity. The implantation site significantly affects engraftment efficiency and tumor growth. Orthotopic implantation, where tumor fragments are placed into the mammary fat pad of immunocompromised mice, is preferred for mimicking the native tumor microenvironment. This approach facilitates interactions between tumor cells and stromal components, maintaining histopathological fidelity. Subcutaneous implantation, while technically simpler, lacks these interactions and has lower engraftment rates for certain breast cancer subtypes (Breast Cancer Research and Treatment, DeRose et al., 2011).

Orthotopic implantation involves cutting tumor fragments into small, uniform pieces (2–4 mm³) for consistent engraftment. Fragment-based implantation better preserves tumor heterogeneity than single-cell suspensions, which can disrupt tumor-stroma interactions (Molecular Cancer Therapeutics, Tentler et al., 2012). Matrigel, a basement membrane matrix, enhances engraftment by providing extracellular support and promoting angiogenesis. It is particularly beneficial for hormone receptor-positive breast cancers, facilitating adhesion and vascularization (Nature Reviews Clinical Oncology, Hidalgo et al., 2014).

The choice of host mouse strain also influences implantation success. Immunodeficient strains such as NOD/SCID and NSG (NOD/SCID/IL2Rγnull) mice reduce graft rejection and improve tumor take rates. NSG mice have demonstrated higher engraftment success, exceeding 80% in some studies (Breast Cancer Research, Zhang et al., 2013). For hormone receptor-positive tumors, estrogen supplementation supports tumor growth and maintains receptor expression. Implanting an estrogen pellet subcutaneously before tumor engraftment increases take rates and preserves receptor status over multiple passages (Oncotarget, McAuliffe et al., 2015).

Molecular Characterization Steps

After establishing a breast cancer PDX model, molecular characterization ensures the xenograft retains the genetic and histopathological features of the original tumor. Histological validation through hematoxylin and eosin (H&E) staining allows pathologists to compare tissue architecture, cellular morphology, and stromal composition. Immunohistochemistry (IHC) assesses biomarkers such as estrogen receptor (ER), progesterone receptor (PR), and HER2 to confirm receptor status across passages. Since receptor expression can change over time, periodic reassessment is necessary.

Genomic profiling provides deeper insights into tumor fidelity. Whole-exome sequencing (WES) or targeted next-generation sequencing (NGS) identifies mutations, copy number variations, and structural alterations retained between the patient tumor and the xenograft. Over 90% of driver mutations in primary breast tumors persist in PDX models (Cell Reports, Eirew et al., 2015), demonstrating their robustness in preserving tumor genetics. RNA sequencing captures transcriptomic changes influencing tumor behavior and treatment response. Differences in gene expression between patient tumors and xenografts may arise due to the absence of human immune and stromal components, requiring careful interpretation.

Proteomic and metabolomic analyses add another layer of characterization by examining functional changes in tumor biology. Mass spectrometry-based proteomics detects shifts in protein expression and post-translational modifications affecting tumor progression or drug sensitivity. Metabolomic profiling identifies alterations in metabolic pathways, particularly in triple-negative breast cancer (TNBC) PDX models. TNBC xenografts exhibit enhanced glycolytic activity compared to hormone receptor-positive models, highlighting potential metabolic targets for therapy (Nature Metabolism, Zhang et al., 2020). These molecular insights validate PDX models and inform precision oncology applications.

Subtypes Observed In Animal Models

Breast cancer PDX models capture the diversity of tumor subtypes, making them valuable for studying biological behaviors and treatment responses. Hormone receptor-positive (HR+) tumors, including ER+ and PR+ subtypes, often retain receptor expression in xenografts, though hormone dependence can vary over passages. Some ER+ PDX models develop reduced estrogen sensitivity, reflecting endocrine resistance observed in patients. This allows researchers to investigate resistance mechanisms and test novel endocrine therapies.

HER2-positive breast cancers, characterized by HER2 gene amplification, have been successfully propagated in PDX models, though maintaining HER2 overexpression can be challenging. HER2-targeted therapies such as trastuzumab or lapatinib can alter HER2 expression over time, mirroring clinical resistance. PDX models help explore adaptive resistance mechanisms and test combination treatment strategies.

Triple-negative breast cancer (TNBC), which lacks ER, PR, and HER2 expression, exhibits high engraftment rates, likely due to its aggressive nature and tumor-initiating properties. TNBC PDX models have identified subtype-specific vulnerabilities, such as reliance on DNA damage repair pathways in BRCA-mutated tumors or unique metabolic dependencies in mesenchymal-like variants. Given TNBC’s heterogeneity, these models aid in identifying predictive biomarkers and evaluating targeted therapies, including PARP inhibitors and immune checkpoint blockers.