The ICH E5 guideline, “Ethnic Factors in the Acceptability of Foreign Clinical Data,” is the international standard that defines when and how bridging studies may be required. It addresses:

• Intrinsic ethnic factors (e.g., genetics, metabolism, diet, age)
• Extrinsic factors (e.g., healthcare infrastructure, clinical practice)
• Criteria for acceptability of foreign data
• Design and analysis of bridging studies

Bridging studies are especially relevant in submissions to regulatory authorities like Japan’s PMDA and in EMA procedures involving extrapolation from non-European populations. In contrast, the FDA often accepts global clinical data if trial diversity and data integrity are strong, though bridging may still be required for certain populations.

Types of Bridging Studies: PK, PD, and Clinical Confirmation

Bridging studies vary based on the information gap identified by regulators. Common types include:

• Pharmacokinetic (PK) Bridging: Compares drug absorption, distribution, metabolism, and elimination between populations.
• Pharmacodynamic (PD) or Biomarker-Based Bridging: Compares mechanism-of-action or biological markers.
• Clinical Bridging Studies: Shorter efficacy/safety studies in the local population to validate extrapolation.

Example: A U.S.-based clinical trial for a monoclonal antibody may be supported by a Japanese PK bridging study comparing exposure levels in 30 healthy Japanese volunteers to 30 non-Asian subjects, demonstrating comparable AUC and Cmax values.

Regulatory Expectations, Case Studies, and Best Practices in Bridging Strategy

EMA Expectations for Bridging Studies

The EMA generally accepts global clinical data submitted in MAAs, provided the study population includes adequate European representation. Bridging studies are usually not required if:

• The drug shows consistent PK/PD across regions
• Ethnic sensitivity is minimal (e.g., in monoclonal antibodies)
• Multiregional Clinical Trials (MRCTs) already include EU participants

However, if pivotal studies are conducted entirely outside the EU—particularly in Asia or Latin America—EMA may request:

• Additional PK studies in European patients
• Real-world evidence from EU practice settings
• Bridging justifications in Module 2.5 (Clinical Overview)

Bridging Study Design: Sample Sizes and Statistical Considerations

Most PK bridging studies use small sample sizes (n=20–50 per arm), randomized 1:1 to compare ethnic groups. Key design elements include:

• Primary endpoints: AUC, Cmax, Tmax
• Statistical analysis: Geometric mean ratio (GMR), 90% confidence intervals (CI)
• Acceptance range: GMR within 80–125% (bioequivalence criteria)

Example dummy table:

Population	AUC (ng·hr/mL)	Cmax (ng/mL)	GMR (CI)
Asian	3200 ± 480	210 ± 35	0.97 (0.92–1.03)
Non-Asian	3250 ± 500	215 ± 30	—

Japan: A Region with Stringent Bridging Requirements

Japan’s PMDA often mandates region-specific data. Bridging strategies are essential for MAAs filed in Japan, especially when the development program originates in the U.S. or EU. Strategies include:

• Dedicated Japanese PK studies
• Use of local Phase 1 and Phase 3 bridging arms
• Pre-submission consultations with PMDA

Sponsors frequently conduct parallel development, integrating Japanese subjects early in the program to avoid standalone bridging studies.

Bridging Data in Biosimilar and Vaccine Submissions

Bridging is critical in biosimilar submissions where minor PK differences may lead to significant changes in efficacy or immunogenicity across populations. Similarly, vaccine MAAs require region-specific immune response and safety data due to potential differences in baseline immunity and pathogen exposure.

For example, a dengue vaccine developed in Latin America may require bridging data from Southeast Asian populations to support submission in Thailand or Indonesia.

Regulatory Submission Strategy and Justification

The bridging strategy should be:

• Explained in Module 2.5 and 2.7 (Clinical Overview and Summaries)
• Supported by ethnic sensitivity analysis in Module 5.3
• Aligned with prior scientific advice or protocol assistance
• Included in the overall risk–benefit evaluation

Sponsors should also include bridging study protocols and reports in Module 5.3.1 (Clinical Study Reports – Pharmacokinetics).

Case Study: Bridging Study in Global Oncology Submission

A global oncology sponsor filed an MAA based on a U.S.-based Phase 3 study. EMA requested additional exposure and safety data in Europeans due to:

• Lack of EU enrollment in pivotal trial
• Suspected PK variability in CYP2D6 metabolizers

The sponsor rapidly conducted a Phase 1 bridging study in 36 EU cancer patients, demonstrating consistent exposure. EMA accepted the data, and the MAA was approved 14 months post-submission.

Conclusion: Bridging Science and Regulation

Bridging studies are not mere technicalities—they are critical enablers of global regulatory alignment and public health. By scientifically addressing ethnic differences, sponsors can streamline global development and reduce the need for full-scale duplication across regions.

Whether filing with the EMA, PMDA, or other national agencies, an effective bridging strategy should be rooted in ICH E5 guidance, tailored to the drug’s characteristics, and justified clearly in the MAA. With proper planning, sponsors can reduce timelines, lower development costs, and bring innovation to patients worldwide.