GLP Studies: Total vs Free PK Assay Selection
For GLP (Good Laboratory Practice) studies, the choice between total and free PK assays depends on the drug modality and presence of soluble targets, but when measuring therapeutic antibodies or biologics with significant soluble ligand/shed receptor, free drug assays are essential for accurate pharmacokinetic characterization and dose projections. 1
Key Decision Framework
For Small Molecules and RNA Therapeutics
- GLP-compliant toxicology studies are mandatory for final authorization applications, though preliminary pharmacology and biodistribution studies do not require GLP compliance 2
- When toxicology data is obtained alongside PK information, GLP-compliance must be respected in all procedures yielding toxicological data 2
- Standard PK assays measuring total drug are typically sufficient for small molecules unless significant protein binding affects the pharmacologically active fraction 1
For Therapeutic Antibodies and Biologics
- Free circulating antibodies represent the pharmacologically active form needed to determine PK/PD relationships, safety margin calculations, and dose projections from animals to humans 1
- In the absence of or with low amounts of soluble ligand/shed receptor, total and free drug species are often equivalent, making assay format less critical 1
- In the presence of significant soluble ligand, assay design becomes critical as different formats can produce markedly different PK profiles and affect data interpretation 1
Critical Assay Considerations
When Free Drug Measurement is Essential
- Therapeutic antibodies with soluble targets require free drug assays to accurately characterize exposure and establish PK/PD relationships 1
- Free drug measurements are necessary for accurate dose projections and safety margin calculations in regulatory submissions 1
- Reagent characterization is critical to understanding which drug species are being measured 1
When Total Drug Measurement is Important
- Total drug may be important for evaluating the dynamic interaction between the drug and target 1
- Total drug exposure assessment helps understand overall drug disposition 1
Common Pitfalls to Avoid
- Assay format selection without considering target biology: Failure to account for soluble ligand/shed receptor presence can lead to inaccurate PK parameter estimation 1
- Inadequate reagent characterization: Without proper characterization, you cannot determine which drug species (free vs total) your assay actually measures 1
- Assuming equivalence: Do not assume free and total drug are equivalent without evidence, particularly for biologics with soluble targets 1
Regulatory Context
- GLP compliance is mandatory for toxicology studies supporting final drug authorization, though early-phase PK studies may be conducted under non-GLP conditions 2
- The choice of PK assay (total vs free) should be justified based on the drug's mechanism of action and target characteristics 1
- For regulatory toxicity studies, PK and toxicokinetic (TK) studies must determine plasma PK and elimination with validated analysis methods in relevant species 2