Lesson 33 of 36 · 15 min
PK, PD, half-life & exposure
Pharmacokinetics asks what the body does to a compound over time. Pharmacodynamics asks what the compound does to the body. Keeping those two ideas separate explains why half-life, exposure and biological response are related but not identical.
PK vs PD
Concentration and effect move through time on different curves.
PK / PD
Concentration and effect do not have to peak together.
A conceptual PK/PD map: pharmacokinetics tracks exposure over time; pharmacodynamics tracks the biological response.
How quickly does exposure rise, peak and decline?
When does the biological effect begin, peak and fade?
Conceptual curves only. They are not scaled to any specific peptide, formulation or dose.
Active metabolites, receptor signalling, downstream biology and repeated exposure can all change the relationship between concentration and response.
PK
Absorption, distribution, metabolism and elimination determine concentration over time.
PD
Receptor engagement, hormone change or another biological response describes what the exposure does.
Half-life
The time associated with a 50% decline under the relevant elimination model. It helps describe persistence, not the full dosing schedule by itself.
Exposure
Metrics such as peak concentration and area under the curve summarize different aspects of how much drug exposure occurred.
Worked comparison
CJC-1295 and ipamorelin show why similar goals do not imply similar exposure.
The classic long-acting CJC-1295 human study reported a multi-day terminal half-life and prolonged GH/IGF-1 effects. The early ipamorelin human PK/PD study reported a much shorter terminal half-life and an acute GH pulse.
Both can affect the GH axis, but the exposure patterns are fundamentally different. Copying frequency or timing from one molecule to the other would ignore the pharmacology.
Formulation matters
The molecule name alone does not define PK.
Depot formulations, albumin binding, chemical modification and route can change absorption and persistence even when the biological target is similar.
That is why CJC-1295 with DAC and market-labelled CJC no DAC should not inherit one another's half-life simply because the names look related.
PK rule
Half-life informs a schedule; it does not manufacture one.
The tested product, route, exposure-response relationship, tolerability and study design all matter.
Key takeaways
- ✓PK describes exposure over time; PD describes biological response.
- ✓Half-life is one property of exposure, not a complete treatment schedule.
- ✓Route and formulation can materially change pharmacokinetics.
- ✓Two compounds acting on the same biological axis can have very different exposure profiles.
- ✓Product-specific PK evidence should not be transferred to a differently formulated product by name alone.
Further reading
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