In Vivo Methods to Check Anticancer Activity

1. Transplanted tumor models - tumor cells/tissue are implanted into animals
2. Spontaneous/induced tumor models - tumors develop in situ within the animal

1. Transplanted Tumor Models

A. Xenograft Models (Most Common)

1. Culture the cancer cell line to exponential growth phase
2. Harvest and count cells (viability >90% by trypan blue exclusion)
3. Prepare cell suspension in saline or Matrigel (~1-10 × 10⁶ cells per 100-200 µL)
4. Inject subcutaneously into the flank of immunodeficient mice
5. Allow tumor to establish to 100-200 mm³ before starting treatment
6. Randomize animals into groups (vehicle control, positive control, treatment groups)
7. Administer test compound at set doses and schedule (e.g., daily, every other day)
8. Monitor tumor size and body weight 2-3 times per week
9. Sacrifice at study endpoint; collect tumor, blood, and organs

V = (L × W²) / 2 where L = longest dimension, W = shortest perpendicular dimension

B. Syngeneic Models

• Advantage: Intact immune system - useful for immunotherapy research
• Disadvantage: Uses mouse tumor cells, not human cells

C. Hollow Fiber Assay (HFA)

• Human tumor cells are grown inside semipermeable polyvinylidene fluoride (PVDF) hollow fibers (0.5-1 mm diameter, 2 cm long; MW cutoff 500,000 Da)
• Fibers are implanted subcutaneously and intraperitoneally in the same mouse
• One animal can support up to 6 cancer cell lines simultaneously
• Cell viability is measured by MTT assay at days 3, 7, and 10
• Animals are dosed from days 3 to 7; study ends at day 7-10

• Faster evaluation time
• Smaller quantity of test compound required
• Tests multiple tumor cell lines in two physiological compartments (IP + SC) in one animal
• Bridges gap between in vitro and full xenograft studies

2. Spontaneous / Induced Tumor Models

A. Carcinogen-Induced Models

• Tumor cells injected intraperitoneally
• Treatment started on day 1 post-inoculation
• Endpoints: survival time, ascites volume, viable tumor cell count, hemoglobin, WBC/RBC counts

B. Genetically Engineered Mouse Models (GEMMs)

• Most clinically relevant for studying tumor initiation and progression
• Used for chemoprevention studies

3. Key Parameters and Endpoints Measured

Primary Efficacy Endpoints

Secondary / Toxicity Endpoints

Formula for % Tumor Inhibition:

% Inhibition = [(Mean tumor weight of control - Mean tumor weight of treated) / Mean tumor weight of control] × 100

4. Standard In Vivo Models by Cancer Type

5. Drug Administration Routes in In Vivo Studies

• Oral (p.o.) - gavage; mimics clinical oral dosing
• Intraperitoneal (i.p.) - most common; rapid absorption
• Intravenous (i.v.) - for drugs requiring parenteral delivery
• Subcutaneous (s.c.) - slow release
• Topical - for skin cancer models

6. Criteria for a Compound to be Considered Active In Vivo

• TGI ≥ 50% compared to vehicle control
• Tumor regression (decrease from baseline tumor volume)
• Increased median survival of at least 25% (T/C value ≥ 125%) in survival-based models
• No significant toxicity (body weight loss < 20%)

7. Regulatory / Ethical Considerations

• All in vivo studies must follow the 3Rs principle: Replace, Reduce, Refine
• IACUC (Institutional Animal Care and Use Committee) approval is mandatory
• Study design should follow GLP (Good Laboratory Practice) guidelines for regulatory submissions
• Number of animals per group typically: 6-10 per group for adequate statistical power

Animal Models Used in In Vivo Anticancer Testing (Other Than Rats & Mice)

1. Zebrafish (Danio rerio)

• Key features: Transparent embryos (allow real-time visual observation of tumor growth), lack adaptive immune system (useful for xenotransplantation), small size, high reproductive rate
• Model type: Xenotransplantation - human tumor cells or patient-derived cells implanted into the perivitelline space of zebrafish embryos
• What can be studied: Tumor engraftment, metastasis, angiogenesis, drug efficacy, drug resistance
• Advantages: Low cost, fast (results in days), high-throughput screening possible, transparent body allows imaging without sacrifice
• Cancer types studied: Colorectal, breast, melanoma, leukemia, and more

2. Rabbit (Oryctolagus cuniculus)

• Key features: Easy to handle, non-aggressive, cost-effective compared to larger mammals, rapid reproduction
• Uses: Pharmacokinetic studies, liver cancer interventional studies (hepatic artery embolization), reproductive toxicity testing
• Notable use: Li et al. used rabbit models to develop a temperature-sensitive liquid embolization agent with oxaliplatin for targeted liver tumor treatment
• Cancer types: Liver (VX2 tumor model is a classic rabbit liver cancer model), prostate, colorectal

3. Beagle Dogs (Canis lupus familiaris)

• Key features: Spontaneous cancer occurrence closely resembles human cancers in clinical presentation, histological features, molecular characteristics, and treatment response
• Uses: Pharmacokinetic studies (primary model), toxicity testing, immunotherapy evaluation
• Cancer types: Prostate cancer (anatomically similar prostate to humans), breast cancer, colorectal cancer (CRC), clear cell renal cell carcinoma (ccRCC), pancreatic ductal adenocarcinoma (PDAC)
• Advantage: Drug metabolism in dogs is more similar to humans than rodents

4. Non-Human Primates (NHP) - Monkeys

• Species used: Cynomolgus macaque, rhesus macaque
• Uses: Late-stage safety/toxicity testing, immunotherapy studies, pharmacokinetic studies for biologics and monoclonal antibodies
• Limitation: High cost, strict ethical regulations, limited use under 3Rs principles
• When used: Only when data cannot be obtained from lower species

5. Pigs (Sus scrofa)

• Key features: Skin structure and biological responses to many compounds closely resemble humans
• Uses: Dermatological cancer models (skin cancer), photodynamic therapy studies, drug toxicity testing, surgical procedure development
• Limitation: Large size, high maintenance cost, complex husbandry

6. Tree Shrews (Tupaia spp.)

• Key features: Phylogenetically close to primates, short reproductive cycle, low cost compared to monkeys, large litter size
• Prone to: Spontaneous breast cancers, making them valuable for breast cancer research
• Uses: Breast cancer models, gene editing therapy studies, LNP (lipid nanoparticle) delivery system research
• Advantage: May replace NHPs in many applications while retaining genetic proximity to humans

7. Drosophila melanogaster (Fruit Fly)

• Key features: Shares ~75% of human disease-related genes, well-characterized genetics, short lifecycle, very low cost
• Drug administration: Compounds blended directly into fly food - simple and scalable
• Uses: High-throughput in vivo screening of anticancer compounds, combination therapy testing, target validation
• Advantage: Ethically favorable (invertebrate), fast (results in 1-2 weeks), highly upscaleable
• Limitation: No circulatory or lymphatic system comparable to mammals

8. Caenorhabditis elegans (Nematode Worm)

• Key features: Fully transparent body, invariant cell lineage (every cell is mapped), ~35% of genes have human orthologs, can study up to 10 of the 14 hallmarks of cancer
• Uses: Studying apoptosis, DNA damage response, oncogenic signaling, early drug screening
• Historical note: Discovery of apoptosis-regulating genes in C. elegans led to the 2002 Nobel Prize in Physiology or Medicine
• Drug application: Compounds added directly to culture medium
• Advantage: Extremely low cost, high-throughput, no major ethical constraints

9. Chick Chorioallantoic Membrane (CAM) Model (Avian Embryo)

• Mechanism: Tumor cells are implanted on the highly vascularized CAM of a developing chick embryo
• Uses: Tumor engraftment, angiogenesis assays, metastasis studies, short-term drug efficacy testing
• Advantages: Low cost, no full animal ethics approval required (embryo stage), allows direct visualization of tumor and blood vessels, medium-throughput
• Limitation: Short experimental window (~7-10 days), cannot model systemic immunity
• Best for: Angiogenesis inhibition studies, bridging gap between in vitro and full mouse models

Summary Comparison Table

Standard Articles on Zebrafish for In Vivo Anticancer Activity

1. High-Content Drug Screening in Zebrafish Xenografts (2023)

• Presents a robust workflow for drug screening in zebrafish xenografts using high-content imaging in 96-well format
• Describes automated tumor cell detection and tumor size analysis over consecutive days
• Covers multiple cancer types: pediatric sarcomas, neuroblastoma, glioblastoma, leukemia
• Demonstrates fast, cost-efficient quantification of antitumor efficacy of small compounds in large cohorts in vivo
• Compares injection sites and cell labeling dyes for different cancer entities

"This fast and cost-efficient assay enables the quantification of anti-tumor efficacy of small compounds in large cohorts of a vertebrate model system in vivo."

2. Zebrafish, Nanotechnology and Personalized Medicine (2022)

• Reviews zebrafish as a model for nanomedicine and immunotherapy evaluation
• Discusses "avatar" (zAvatar) models - patient-derived xenografts in zebrafish for personalized medicine
• Covers high genomic homology with humans, large offspring, rapid development
• Evaluates novel anticancer therapies including nanoparticle-based drug delivery systems
• Covers drug screening, toxicity evaluation, and interaction with cancer cells in vivo

3. Zebrafish Xenograft in Glioblastoma Research (2024)

• Comprehensive review of zebrafish transgenic and xenograft models specifically for glioblastoma multiforme (GBM)
• Compares zebrafish to traditional mouse and canine GBM models
• Highlights advantages: small size, genetically identical large offspring, rapid development, transparency, cost-effectiveness
• Covers genetic and anatomical similarities with humans enabling translatable brain cancer research
• Addresses tumor microenvironment (TME) dynamics in GBM using zebrafish

4. Zebrafish in Translational Immuno-Oncology (2025 - Most Recent)

• Reviews zebrafish immune system development and tools for tracking innate and adaptive immune responses
• Covers modeling of immune evasion, checkpoint molecules, and TME dynamics using transgenic and xenograft approaches
• Evaluates "zAvatar" (patient-derived xenograft) platforms for high-throughput drug screening and personalized therapy
• Discusses next-generation immunotherapies: CAR T cells, CAR NK cells, LAG-3, TIM-3, TIGIT checkpoint inhibitors
• Honestly addresses limitations: temperature sensitivity, immature adaptive immunity in larvae, interspecies pharmacokinetic differences

5. Zebrafish Xenografts in Breast Cancer (2025)

• Systematic review of zebrafish breast cancer xenograft models
• Covers brain metastasis, bone metastasis, tumor angiogenesis, and drug screening in BC
• Provides insights into molecular mechanisms, potential drug targets, efficacy and toxicity evaluation
• Discusses zebrafish application in personalized medicine for breast cancer
• Evaluates strengths and limitations of the model with future directions

6. Zebrafish Xenograft Model to Study Human Cancer - Methods (2022)

• Practical step-by-step methodology article - directly useful for setting up a zebrafish anticancer study
• NIH-supported research
• Covers xenograft implantation technique, cancer cell preparation, injection into embryos, imaging protocols, data analysis

Summary Table