Last Updated: July 2017
Measuring treatment response in Patient Derived Xenograft (PDX) models at The
Experimental Design Frequently Asked Questions (FAQ)
How are cohorts of tumor bearing mice established for dosing studies?
For cohort expansion donor mice are created using viable, cryopreserved low-passage tumor fragments
(P2-P5 unless otherwise noted). With the exception of prostate tumors, all tumors are implanted in female
NSGTM mice. Mice are between 6-8 weeks of age at the time of implantation. Tumors are typically
implanted subcutaneously into the right flank using a trocar. Breast cancer tumors are sometimes
implanted in the mouse mammary fat pad. Implantation site is subcutaneous unless otherwise noted.
Tumors used to establish donor mice are subjected to standard Quality Control procedures to ensure
tissue provenance and to validate key tumor markers.
Tumor bearing mice are grouped in cohorts of 8-10 mice using single or rolling enrollment for dosing
studies. Mice with tumor sizes of 70-300 mm3 are randomized between treatment groups. Baseline tumor
volumes are established and dosing initiation begins on Day 0 or Day 1.
Typical routes of drug administration are as follows:
Oral gavage (PO)
What is the protocol for dosing and measuring drug response in PDX models?
Once tumors become palpable, tumor volumes are measured up to 3X a week with digital calipers. Mice
are monitored until their individual tumor volume reaches the approved protocol volume limit.
The in vivo response to treatment is assessed using the percentage of tumor volume change (ΔVol) at the
final study day (i.e., seven days after the last treatment) compared with the baseline tumor volume at Day 0
or Day 1. The criteria for response classification classification is based on the combination of the best
response and average response (see below) adapted from Gao et al., 2015 (Nature Medicine 21, 1318-1325).
Step 1: Calculate percent change in tumor volume for each animal as V: ((end_volume - start_volume)/start_volume) * 100)
Step 2: Within each group find the minimum V as Vm
Step 3: Within each group find the mean (average) V as Va
Step 4: Determine RECIST category as shown in the table below
|RECIST Category||Best Response: Vm ||Average Response: Va|
|Complete Response (CR)|| < -95% ||< -40% |
|Partial Response (PR)|| < -50%|| <-20% |
|Stable Disease (SD)|| < 35%|| < 30% |
|Progressive Disease (PD)|| Anything else |
How is animal welfare monitored during dosing studies?
Body weight and clinical observations, including body condition score, grooming and tumor integrity are
monitored up to 3X a week for the study duration. Additionally, daily cage side observations are conducted
to monitor animal welfare. Veterinary staff is contacted if animals show any signs of distress and veterinary
recommendations are followed.
What is the procedure for measuring tumor volume?
Tumor volume measurements are taken up to 3X weekly throughout the duration of each study to assess
tumor response to therapy. It is possible for tumor responses to initially exhibit regression (shrinkage) in
response to therapy and then resume progressive growth, therefore tumors must be continually monitored
throughout the study or risk missing this response. Tumor volumes are calculated from digital caliper raw
data by using the formula:
Volume (mm3) = (l x w2) / 2
The value w (width) is the smaller of two perpendicular tumor axes and the value l (length) is the larger of two
perpendicular axes. Mean tumor volume growth curves and means are calculated for each treatment group.
How are the results of dosing studies represented graphically?
The results of dosing studies are presented using 4 types of plots: Waterfall, Treatment Group, Spider, and
Tumor Growth Inhibition. Each plot type is described briefly below.
Treatment Group plot. A Treatment Group plot is a line plot showing the average change in tumor volume for
all of the tumor bearing mice in a particular treatment arm (one line per treatment arm). Standard error bars are
typically included for each time point measured.
Spider plot. A Spider plot is a line plot showing the change in tumor volume from baseline for each tumor
bearing mouse in a study.
Waterfall plot. A Waterfall plot presents the response to a treatment for each individual animal in a study as
indicated by change in tumor volume. The data displayed on the graphic are limited to those mice that remain on study for the full duration of the project.
The plot reflects the difference in tumor volume at the end of the study
compared to initial tumor volume calculated on Day 0 of therapeutic dosing initiation. The horizontal (x) axis is
baseline; vertical bars are drawn for each tumor bearing mouse, either above or below the baseline. The
vertical (y) axis represents the % tumor volume change. Vertical lines above the x axis represent non-
responders; vertical bars below the x axis represent response to treatment. Waterfall plots are typically
displayed left to right from worst response to best.
Tumor Growth Inhibition (TGI) plot. The bars in the TGI plot summarize the % of tumor volume change in
treatment arms relative to the control. The % TGI is defined as (1 - (mean volume of treated tumors)/(mean
volume of control tumors)) x 100%.