Pharmacokinetics
Also known as: PK, Drug kinetics, ADME
Pharmacokinetics is the study of how the body processes a drug over time, including absorption, distribution, metabolism, and excretion (ADME). Pharmacokinetics answers 'what the body does to the drug' and is essential for determining proper peptide dosing, frequency, and route of administration.
Last updated: February 1, 2026
The ADME Framework
Pharmacokinetics is organized around four processes:
Absorption
- How the drug enters the bloodstream
- Affected by route of administration
- Bioavailability is a key measurement
Distribution
- How the drug spreads through body tissues
- Affected by protein binding
- Volume of distribution (Vd) is a key parameter
Metabolism
- How the body chemically modifies the drug
- Primarily in liver (but also other tissues)
- Can activate, deactivate, or create metabolites
Excretion
- How the drug is eliminated
- Primarily through kidneys (urine) and liver (bile)
- Clearance rate determines dosing frequency
Key Pharmacokinetic Parameters
| Parameter | Meaning | Significance |
|---|---|---|
| Cmax | Peak concentration | Maximum drug level reached |
| Tmax | Time to peak | How fast drug reaches max level |
| t½ | Half-life | Time for concentration to halve |
| AUC | Area under curve | Total drug exposure |
| F | Bioavailability | Fraction reaching circulation |
| Vd | Volume of distribution | Extent of tissue distribution |
| CL | Clearance | Rate of drug removal |
Pharmacokinetics of Peptides
Peptides have unique PK considerations:
Challenges
- Poor oral bioavailability (digestive breakdown)
- Short half-lives (enzymatic degradation)
- Limited membrane permeability
- Potential immunogenicity
Solutions in Drug Design
- Amino acid modifications (e.g., semaglutide’s Aib)
- Lipid conjugation (albumin binding)
- PEGylation (increased size)
- Cyclization (stability)
PK Comparison: Natural vs Modified GLP-1
| Parameter | Natural GLP-1 | Semaglutide |
|---|---|---|
| Half-life | 1-2 minutes | ~7 days |
| Dosing frequency | Continuous (impractical) | Once weekly |
| Route | N/A (endogenous) | Subcutaneous or oral |
| DPP-4 resistance | None | High |
Pharmacokinetic Studies
Phase I Trials
- First-in-human studies
- Establish PK parameters
- Determine safe dose ranges
PK Parameters Measured
- Blood concentration over time
- Time-concentration curves
- Individual variation assessment
Factors Affecting Peptide PK
| Factor | Effect on PK |
|---|---|
| Age | May slow metabolism/excretion |
| Kidney function | Affects clearance |
| Body weight | May affect distribution |
| Injection site | Can affect absorption |
| Fed/fasting state | Matters for oral peptides |
Frequently Asked Questions
What’s the difference between pharmacokinetics and pharmacodynamics?
Pharmacokinetics (PK) is “what the body does to the drug” (absorption, distribution, metabolism, excretion). Pharmacodynamics (PD) is “what the drug does to the body” (mechanism of action, effects). Both are needed to understand drug behavior.
Why does half-life matter for dosing?
Half-life determines how long a drug stays active. Short half-life means frequent dosing needed. Long half-life allows less frequent dosing but also means side effects persist longer and the drug takes longer to reach steady state.
How do researchers improve peptide pharmacokinetics?
Common strategies include: modifying amino acids to resist enzymatic breakdown, attaching fatty acid chains for albumin binding, PEGylation to increase molecular size, and creating cyclic structures to increase stability.
Related Peptides
Related Terms
Disclaimer: This glossary entry is for educational purposes only and does not constitute medical advice. Always consult a qualified healthcare provider for medical questions.