1: What to Measure and When

Introduction

One of the most important goals of Phase 1 clinical trials is to understand how a new drug behaves in the human body. This is achieved by studying pharmacokinetics (PK) and pharmacodynamics (PD). An effective PK/PD sampling strategy ensures robust data generation to support safety, dose escalation, and future trial design. In this tutorial, we explore what to measure, when to collect samples, and how to align with regulatory and scientific expectations.

Understanding PK and PD

What Is Pharmacokinetics (PK)?

Pharmacokinetics is the study of how the body absorbs, distributes, metabolizes, and excretes a drug. PK parameters help determine the optimal dose and frequency of administration.

Key PK parameters include:

• Cmax: Maximum concentration of drug in plasma
• Tmax: Time to reach Cmax
• AUC: Area under the concentration-time curve (total exposure)
• t½: Elimination half-life
• CL: Clearance
• Vd: Volume of distribution

What Is Pharmacodynamics (PD)?

Pharmacodynamics assesses the biological effect of the drug on the body. PD markers can be direct (e.g., blood pressure) or indirect (e.g., biomarker expression).

Common PD measurements include:

• Receptor occupancy
• Enzyme inhibition levels
• Biomarker concentration (e.g., cytokines, hormones)
• Clinical endpoints (e.g., clotting time, glucose reduction)

Designing a PK Sampling Strategy

Sampling Time Points

Time points should cover the entire PK profile—from absorption to elimination. For single-dose studies, a typical schedule includes:

• Pre-dose (baseline)
• 0.25–0.5 hours (early absorption)
• 1–2 hours (approaching Cmax)
• 4–6 hours (distribution phase)
• 8–12 hours (early elimination)
• 24–48 hours (later elimination)
• 72+ hours if half-life is long

For multiple ascending dose (MAD) studies, both peak and trough samples are collected on multiple days to evaluate steady-state kinetics.

Number of Samples

• Minimum 8–12 samples for full PK profile (non-compartmental analysis)
• 3–4 samples for limited PK (bioequivalence or sparse designs)

Sample Types

• Plasma: Most common and preferred for small molecules
• Serum: Used when plasma proteins may interfere with analysis
• Whole blood: For intracellular or RBC-bound drugs
• Urine/Feces: For elimination and metabolite profiling

Sample Handling and Storage

Ensure all samples are collected using validated procedures:

• Anticoagulant selection (e.g., K2EDTA)
• Immediate centrifugation and plasma separation
• Storage at -20°C or -80°C as per analyte stability
• Labeling with subject ID, timepoint, date

Designing a PD Sampling Strategy

Selection of PD Biomarkers

PD markers must be:

• Relevant to mechanism of action
• Validated and sensitive for detecting change
• Correlated with drug exposure if possible

Sampling Timeline

• Align with PK time points (if PD response is fast)
• Delayed sampling if PD effect lags behind PK
• Long-term monitoring for sustained biological effects

Common PD Sampling Matrices

• Blood (plasma, serum, PBMCs)
• Urine (e.g., electrolyte changes)
• Saliva (for non-invasive hormone levels)
• Tissue or biopsy (for receptor studies, oncology)

Analytical Considerations

• Assay validation (sensitivity, precision, selectivity)
• Time-stamped sample collection
• Analysis performed under GLP or GCLP standards

PK/PD Integration

Integrated PK/PD analysis helps define exposure–response relationships. This is crucial in selecting the optimal dose and frequency for Phase 2 studies.

Approaches include:

• Non-compartmental analysis (NCA): Direct parameter estimates
• Compartmental modeling: Complex simulations using software like Phoenix WinNonlin
• PK/PD modeling: Simulates drug effect over time

Example: Sampling Schedule for a Phase 1 Study

Time Point	Sample Type	Purpose
Pre-dose (0h)	Plasma	Baseline concentration
0.5h	Plasma + Biomarker	Absorption phase
2h	Plasma + PD (IL-6)	Peak drug and response
6h	Plasma	Distribution phase
24h	Plasma + Urine	Elimination and excretion
48h	Plasma	Late elimination

Best Practices

• Coordinate with bioanalytical labs early in trial planning
• Ensure all sampling tubes and labels are pre-prepared
• Use standardized PK/PD worksheets and chain-of-custody forms
• Train clinical staff on exact sampling windows
• Predefine backup sampling plan in case of missed timepoints

Regulatory Expectations

• FDA: Requires PK data in the IND application to support dose selection
• EMA: Emphasizes exposure-response modeling for biologics and biosimilars
• CDSCO: Demands validated methods and justification of sample size and timepoints

Conclusion

In Phase 1 trials, designing an intelligent PK and PD sampling strategy can mean the difference between a successful development program and avoidable delays. The goal is not just to collect blood samples—but to collect the right samples at the right time, using validated methods, aligned with your clinical objectives and regulatory roadmap. When done correctly, PK/PD data becomes the scientific foundation for clinical advancement and precision dosing in future phases.