Bladder cancer is one of the most common malignancies of the urinary system. Research into its mechanisms of invasion and metastasis, as well as the development of novel therapeutics, relies heavily on animal models that can faithfully recapitulate the human tumor microenvironment.

However, conventional orthotopic bladder cancer models have long been challenged by two major limitations:

• Low tumor establishment rates: Even in immunocompetent C57BL/6J mice, innate immune clearance often prevents tumor cells from successfully engrafting in the bladder.
• Poor reproducibility: Commercially available cell lines are prone to phenotypic drift during serial passaging, resulting in reduced invasiveness and substantial variability across experimental batches.

To address these challenges, InnoStar developed two novel highly invasive bladder cancer cell lines, MB49-Luc-1 and MB49-Luc-2, and established an optimized orthotopic bladder cancer model based on these cell lines. This platform achieved a tumor establishment rate of up to 96%, providing a robust and reliable tool for bladder cancer research and preclinical drug development.

Highly Invasive Cell Lines Selected Through In Vivo Screening

The foundation of this model lies in two cell lines generated through a combined in vivo–in vitro selection strategy:MB49-Luc-1 and MB49-Luc-2.

Unlike commercially available MB49-Luc cells, these two cell lines were not obtained through simple Transwell-based selection. Instead, the parental cells first underwent a rigorous in vivo selection process within the bladders of C57BL/6J mice. This process enriched for tumor cells capable of evading immune clearance, successfully engrafting in the bladder, and maintaining stable tumor growth under physiological conditions.

Cells were subsequently isolated from the resulting tumors and further characterized in vitro to identify clones with the strongest invasive potential. Selected clones were continuously passaged for more than ten generations. Throughout this process, comprehensive phenotypic stability assessments—including luciferase expression analysis, cell proliferation evaluation, and STR authentication—were performed every 3–5 passages to ensure long-term stability and minimize phenotypic drift.

Both MB49-Luc-1 and MB49-Luc-2 exhibited markedly enhanced migratory and invasive capabilities. As demonstrated by the Transwell invasion assays (Figures 1 and 2), the number of cells invading through the membrane was significantly higher than that observed for the parental MB49-Luc cell line.

Figure 1. Representative images from Transwell invasion assays of different MB49-Luc cell lines (crystal violet staining, 20× magnification).

Figure 2. Quantification of invaded cells in the Transwell invasion assay.

Enhanced Model Performance

Following Co60 irradiation at doses ranging from 1–4 Gy, mice were inoculated with MB49-Luc-1 cells. For MB49-Luc-2 studies, mice received 2 Gy Co60 irradiation prior to tumor cell inoculation. Compared with the parental MB49-Luc cell line, both optimized cell lines demonstrated substantially improved tumor-forming performance across multiple evaluation parameters.

• Tumor Growth Kinetics: In vivo bioluminescence imaging (Figures 3 and 4) revealed rapid and sustained increases in luminescence signals in both the MB49-Luc-1 and MB49-Luc-2 groups beginning on Day 6 post-inoculation. The corresponding growth curves exhibited substantially steeper slopes than those observed in parental MB49-Luc groups receiving the same irradiation regimen, indicating enhanced tumor growth capacity.

Figure 3. Tumor growth kinetics of three MB49-Luc cell lines in mice receiving different irradiation doses.

Figure 4. Representative in vivo bioluminescence images of mice following inoculation with different MB49-Luc cell lines.

• Final tumor burden: As shown in Figure 5, the mean terminal tumor weights in the MB49-Luc-1 and MB49-Luc-2 groups reached 275 mg and 260 mg, respectively, compared with only 35 mg in the parental MB49-Luc group. The differences were highly significant (P < 0.01).

Figure 5. Comparison of bladder weights (including tumor tissue) among different experimental groups.

• Tumor establishment rate: A marked improvement in tumor establishment efficiency was observed. Using the same evaluation criteria, inoculation with the parental MB49-Luc cell line resulted in successful tumor establishment in only 2 of 23 surviving mice, corresponding to a tumor establishment rate of 8.7%.

Optimized Model Development Strategy

In addition to the use of highly invasive cell lines, the model development workflow was systematically optimized to further improve tumor establishment efficiency and reproducibility. 

1. Mild Immunosuppression 

Instead of employing high-dose irradiation, which may cause excessive toxicity and compromise animal health, a low-dose Co60 irradiation regimen (1–4 Gy) was administered 48 hours prior to tumor inoculation. This approach creates a transient immunological window that facilitates tumor cell engraftment while preserving key features of the native immune microenvironment.

2. Precise Bladder Pretreatment

Prior to tumor cell implantation, the bladder was briefly instilled with dilute hydrochloric acid for approximately one minute to gently disrupt the urothelial lining and expose the underlying basement membrane, thereby promoting tumor cell adhesion. PBS washes were performed both before and after treatment to ensure a controlled and reproducible procedure. 

3. Delayed Needle Withdrawal Technique

Following surgical exposure of the bladder, the tumor cell suspension was injected along the bladder wall. The needle was maintained in place for 30–60 minutes after injection to allow sufficient time for cell attachment before withdrawal, thereby minimizing cell leakage and maximizing tumor establishment efficiency. 

4. Optional Two-Step Implantation Strategy

For studies requiring larger cohorts, the protocol also supports a two-step implantation approach. After primary tumor establishment, tumor tissues can be harvested and expanded ex vivo before being implanted into a second cohort of mice. This secondary implantation procedure does not require additional whole-body irradiation, significantly simplifying the workflow while reducing animal use.

Applications: A Robust Platform for Mechanistic Research and Drug Discovery

Combining the highly invasive MB49-Luc-1 and MB49-Luc-2 cell lines with an optimized model development strategy, this orthotopic bladder cancer model platform offers several key advantages:

• High clinical relevance: Tumors grow orthotopically in the bladders of immunocompetent mice, enabling a more physiologically relevant representation of tumor–stroma interactions and the immune microenvironment.
• Excellent stability and reproducibility: The rigorously selected cell lines exhibit stable phenotypic characteristics, ensuring consistent and comparable results across studies.
• Real-time monitoring: Luciferase labeling enables noninvasive in vivo bioluminescence imaging, allowing dynamic assessment of tumor growth and progression without the need for serial animal sacrifice.

This model provides an efficient and reliable platform for investigating the mechanisms of bladder cancer invasion and metastasis, evaluating novel anticancer therapies, and assessing responses to immunotherapies.

For drug discovery applications, the model is particularly well suited for evaluating murine immune checkpoint inhibitors, as it preserves an intact immune system and can faithfully reflect antitumor immune responses in an immunocompetent setting. The platform can also be used to assess the efficacy of chemotherapeutic agents such as gemcitabine and cisplatin, as well as oncolytic viruses. In addition, antibody–drug conjugates (ADCs) may be evaluated when the target demonstrates cross-reactivity with the corresponding murine antigen or when suitable murine surrogate molecules are available.

Given its preserved immune microenvironment, this model is especially valuable for assessing combination therapy strategies that require an immunocompetent setting.