A Step-by-Step Guide to Designing a Microdosing Study for Phase 0 Trials

What is a Microdosing Study?

A microdosing study involves the administration of extremely low, sub-therapeutic doses of a drug to human subjects to evaluate early pharmacokinetic (PK) and pharmacodynamic (PD) behavior. These studies fall under the category of Phase 0 clinical trials and are designed to provide rapid, human-specific data with minimal risk.

Typically, the administered dose is not expected to have any therapeutic or toxic effect but is high enough to allow measurement of drug concentration using ultra-sensitive analytical methods.

Step 1: Define the Study Objectives

Before protocol development, clearly define what you want to learn from the study:

• Does the drug reach systemic circulation?
• What is the absorption, half-life, and clearance profile?
• Can we confirm target engagement via biomarkers?
• Which compound among candidates should be advanced to Phase 1?

Clear objectives guide every design decision—from sampling schedules to bioanalytical assay selection.

Step 2: Determine the Microdose

According to regulatory definitions:

• A microdose is less than 1/100th of the pharmacologically active dose
• It should not exceed 100 micrograms for small molecules
• For biologics, the dose should not exceed 1/100th of the No Observed Adverse Effect Level (NOAEL)

Use available preclinical PK/PD data, allometric scaling, and in silico models to estimate a safe and detectable microdose.

Step 3: Choose the Route of Administration

The route should match the intended clinical use to ensure relevance. Common options include:

• Oral (tablets, capsules)
• Intravenous (IV)
• Subcutaneous or inhalation (for biologics or novel delivery systems)

Ensure formulation development is optimized for the low dose being administered.

Step 4: Select Analytical Methods for Detection

Due to the ultra-low drug concentrations, microdosing studies require highly sensitive and validated analytical techniques, such as:

• LC-MS/MS (Liquid Chromatography-Mass Spectrometry)
• Accelerator Mass Spectrometry (AMS) – capable of detecting 1 part per trillion
• PET Imaging – for tissue distribution using radiolabeled drugs

Validation should meet regulatory requirements for limit of detection (LOD), accuracy, and reproducibility.

Step 5: Design the Study Protocol

Components of a microdosing study protocol include:

Sample Size

• Typically 6 to 15 subjects per study
• May include crossover or parallel group design

Inclusion Criteria

• Healthy volunteers for general PK studies
• Patients in case of tissue-targeted oncology trials

Dosing Schedule

• Single-dose administration
• Sample collection at multiple time points: pre-dose and post-dose (e.g., 0.5h, 1h, 2h, 4h, 8h, 24h)

Endpoints

• Cmax, Tmax, AUC, half-life
• Target engagement using biomarkers or imaging

Monitoring

Although the doses are sub-therapeutic, monitor for unexpected adverse events and report them as per protocol.

Step 6: Ensure Regulatory and Ethical Compliance

Microdosing studies fall under Exploratory INDs (U.S.), Scientific Advice (EMA), or Phase 0 Guidance (India, Japan).

Submission must include:

• Preclinical single-dose tox data (1 species, GLP compliant)
• Assay validation reports
• Investigator Brochure (IB) and informed consent forms
• Ethics Committee / IRB approvals

Ensure compliance with ICH M3(R2), OECD GLP, and respective national health authorities.

Step 7: Data Collection and Analysis

Use validated tools and software for non-compartmental analysis (NCA) or model-based PK/PD analysis. Important parameters include:

• AUC – extent of absorption
• Cmax and Tmax – peak concentration and time
• Half-life (t½) – duration in circulation
• Bioavailability (if IV and oral data are available)

Biomarker or imaging endpoints should be analyzed for evidence of biological activity or distribution.

Case Example: Microdosing in CNS Drug Development

A CNS-targeted compound was microdosed (100 μg) in 10 healthy volunteers. Serial blood sampling and PET imaging confirmed blood-brain barrier penetration and receptor binding within 4 hours of administration. Based on this, the compound advanced confidently into Phase 1 trials.

Common Pitfalls to Avoid

• Assuming microdose PK is always predictive of full-dose PK (may not apply for saturable kinetics)
• Poor assay sensitivity leading to non-detectable results
• Neglecting formulation stability at low doses
• Inadequate documentation for regulatory approval

Summary for Clinical Research Students

Designing a microdosing study requires interdisciplinary expertise—from formulation science and bioanalytics to clinical protocol development and regulatory strategy. As a student or aspiring professional in clinical research, regulatory affairs, or pharmacology, understanding these design elements will help you contribute effectively to early-phase drug development.

Microdosing studies are a powerful tool in modern clinical science—if designed right, they can save time, money, and guide smarter development decisions.