On this page
How do you write the nonclinical pharmacokinetic summaries in CTD 2.6.4 and 2.6.5?
Describe the analytical basis and the actual absorption, distribution, metabolism, excretion and interaction findings, then explain meaningful patterns across studies. Keep analyte, matrix, species, route and exposure definitions visible. Reconcile the written account with the tabulated summary before using the findings in nonclinical or human-exposure interpretations.
Before you begin
M4S sections 2.6.4 and 2.6.5, including relevant analytical methods and nonclinical kinetic studies. Human clinical pharmacology has a separate summary.
What you will prepare: A method-aware account of disposition and exposure that supports nonclinical interpretation and cross-species assessment.
Sections covered in this guide (2)
Define what the measurements represent
Collect the analytical-method and validation reports alongside the absorption, distribution, metabolism, excretion and interaction reports. The useful starting question is not simply “what was the AUC?” but “which analyte, matrix and experimental conditions produced it?”
Create an editorial data dictionary for analytes, metabolites, matrices, units and important assay limitations. Distinguish parent-drug measurements from total radioactivity, and total concentrations from unbound concentrations. A shared abbreviation is not enough to establish that two measurements are comparable.
Introduce the kinetic package and explain which methods support which results. Missing validation or method-range information should be assigned to an analytical owner; it should not disappear when the study is summarized.
Tell the disposition story with traceable comparisons
Follow the M4S topics for methods, absorption, distribution, metabolism, excretion, interactions and other kinetic studies. Use the narrative to identify meaningful patterns across the actual species, routes and dose regimens, with important limitations retained.
For absorption, distinguish observed exposure from inferred bioavailability. For distribution, preserve the tissue, time and measurement basis. In metabolism and excretion, explain which species or pathways were observed rather than substituting a generic ADME description. Where interactions were investigated, retain the conditions under which an effect was or was not seen.
Build 2.6.5 tables from the applicable M4S examples and the actual report content. Check that each comparison uses compatible definitions and that every reported parameter has a source. Tables should make study differences visible, not hide them behind a single pooled value.
Worked example: total radioactivity becomes parent exposure
Fictional editorial exercise: a distribution report measures total radioactivity. The draft written summary describes the result as a concentration of unchanged parent drug in tissue.
Compare the analytical method and reported analyte with the sentence and table header. Correct the description to the measured quantity. Ask the study scientist which conclusions about parent or metabolites are supported and which remain unresolved. Inspect other tables that copied the same label.
Before using these data in an exposure comparison, verify that the clinical value has a compatible basis. If it does not, the summary should explain the limitation and route the question to the pharmacokinetic reviewer. A numerical ratio with mismatched quantities does not become meaningful because it has more decimal places.
Create a comparison dictionary before calculating ratios
The most useful preparation tool is often a small data dictionary rather than another plot. Assign the following fields to every parameter that will be compared across studies.
| Field | Example of a distinction to retain | What an unexplained mismatch can do |
|---|---|---|
| Measured quantity | Parent drug, named metabolite or total radioactivity | Turn a disposition finding into a false claim about parent exposure |
| Matrix and binding basis | Plasma versus whole blood; total versus unbound | Produce a ratio between biologically different measurements |
| Exposure interval | A defined sampling interval versus extrapolation | Make two values labeled AUC appear directly comparable |
| Conditions | Species, sex, route, regimen and fed state where relevant | Hide the design differences behind an apparent kinetic pattern |
| Method and units | Method version, quantitation limits and reported units | Convert a method limitation into an apparent biological difference |
Use this editorial dictionary to annotate the 2.6.5 tables and to check the narrative comparisons. It does not replace the source report or an expert assessment of whether a comparison is scientifically appropriate.
Worked writing decision: two studies both report “AUC,” but one is parent-drug exposure during a specified dosing interval and the other is total radioactivity over a longer observation period. Keep the results in separately defined rows. A unit conversion alone cannot make them the same quantity. Ask the pharmacokinetic reviewer what conclusion, if any, can be drawn across them.
Carry the corrected definitions into the toxicology summaries when those authors use the values for exposure context. For the clinical pharmacology summary, identify explicitly which nonclinical comparisons are relevant and which lack an appropriate human counterpart. This prevents a well-labeled nonclinical table from becoming an overstated cross-species claim downstream.
Your preparation checklist
0/4 checkedUse this to track your review in this visit. Checks are not saved and do not establish regulatory compliance.
Frequently asked questions
Where does human pharmacokinetic evidence belong?
The nonclinical summaries in 2.6.4 and 2.6.5 concern the nonclinical kinetic package. Human clinical pharmacology is summarized in 2.7.2, with the underlying reports in the applicable Module 5 locations. Cross-reference relevant comparisons without relocating the human program into the nonclinical summary.
Is total radioactivity equivalent to unchanged parent-drug concentration?
No. Total radioactivity can represent parent and drug-related components captured by the measurement. Identify what the analytical method actually measures before naming a result. Do not label total radioactivity as parent exposure unless the source method and scientific interpretation support that specific conclusion.
Can values from different species be averaged in a summary table?
Do not average them merely to simplify presentation. Species, route, dose, method and parameter definitions can explain important differences. Present interpretable study-level results and use the responsible expert’s supported comparison. A single pooled value can conceal the evidence needed to assess relevance.
What should the writer do with a missing method limitation?
Identify the affected result and obtain the analytical owner’s assessment before relying on it in a conclusion. Retain the uncertainty in the working record and, when consequential, the summary. Assuming that an unreported limitation does not exist can overstate the measurement’s fitness for the intended comparison.
Sources and revisions
Requirements, source recommendations and editorial preparation advice have different roles. Review the scope and revision of the source you use.
Guidance
ICH M4S(R2): Nonclinical overview and summaries ↗Step 4, December 20, 2002; sections 2.4 and 2.6 and Appendices A/B. FDA M4S and Safety Appendices remain labeled final, August 2001; ICH R2 includes subsequent editorial changes.
Technical specification · placement only
FDA eCTD v4.0 comprehensive hierarchy ↗Version 2.2, February 2025. Section 2.6.4. A heading identifies placement, not mandatory applicability.