Can animal studies be waived for a 351(k) application?

Yes. Section 351(k)(2)(A) states that the Secretary may determine that animal studies are "not necessary" based on the available data. FDA has increasingly waived animal toxicity studies for well-characterized product classes (particularly monoclonal antibodies) when analytical similarity is robust and there are no novel formulation or route concerns. The decision should be discussed at a BPD meeting.

How long does it take to prepare a 351(k) application?

Preparation time depends on the maturity of the analytical package, the volume of module-ready content, manufacturing readiness, and how much of the dossier is assembled in parallel with development. Rather than relying on a generic duration benchmark, sponsors should build their plan from the specific data, writing, QC, and publishing workstreams required for the application.

Can a 351(k) biosimilar get orphan drug exclusivity?

No. [Orphan drug exclusivity](/blog/orphan-drug-designation) blocks approval of the "same drug" for the same orphan indication. However, FDA has stated that a biosimilar cannot be considered the "same drug" as the reference product for orphan exclusivity purposes under certain circumstances. The interaction between orphan exclusivity and biosimilar approval has been the subject of litigation and evolving FDA policy. ---

351(k) Biosimilar Application: Data Requirements 2026

351(k) Biosimilar Application: Requirements and Data Packages

Quick Answer

A 351(k) biosimilar application is a BLA submitted under Section 351(k) of the PHS Act requesting approval of a biological product shown to be biosimilar to an FDA-licensed reference product. The application requires a totality-of-evidence data package including: (1) extensive analytical similarity studies comparing structural, functional, and purity attributes across multiple reference product lots; (2) animal studies if residual uncertainty exists; (3) clinical pharmacology (PK/PD) equivalence study; (4) at least one comparative clinical efficacy/safety study with immunogenicity assessment; and (5) full CMC data for the proposed biosimilar. The application follows the standard eCTD format with biosimilar-specific content in each module.

Key Takeaways

The 351(k) application uses a stepwise development approach: analytical similarity, animal studies, clinical pharmacology (PK/PD), and potentially clinical efficacy trials
Analytical similarity is the foundation — extensive physicochemical and functional characterization against the reference product drives the scope of clinical studies
Clinical pharmacology studies (PK/PD equivalence) are required; clinical efficacy trials may be waived if analytical and PK/PD data are sufficiently robust
Indication extrapolation to all reference product indications is possible when supported by totality of evidence and mechanism of action analysis
The 351(k) biosimilar application is the regulatory vehicle established by the Biologics Price Competition and Innovation Act (BPCIA) for abbreviated licensure of biosimilar biological products. Unlike a full 351(a) BLA, which requires independent demonstration of safety, purity, and potency through a complete clinical development program, the 351(k) application leverages FDA's finding of safety, purity, and potency for the reference product and demonstrates that the proposed biosimilar is highly similar to the reference product with no clinically meaningful differences.
This guide provides a detailed breakdown of what goes into each section of the 351(k) application, the data requirements at each step of the stepwise development approach, and practical guidance on study design and extrapolation strategy.
In this guide, you will learn:
The statutory framework for 351(k) applications
Module-by-module eCTD content requirements
Analytical similarity study design and data expectations
When animal studies can be waived
Clinical pharmacology study design for biosimilars
Comparative clinical study requirements
Extrapolation strategy and scientific justification
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351(k) Application Requirements: Statutory Framework

Definition

A 351(k) application is a BLA submitted under Section 351(k) of the Public Health Service Act (42 U.S.C. 262(k)), requesting licensure of a biological product on the basis of its biosimilarity to a reference product that has been licensed under Section 351(a). The application must contain information demonstrating biosimilarity based on analytical studies, animal studies (including toxicity), and a clinical study or studies (including immunogenicity and PK/PD), unless any such study is deemed unnecessary by the Secretary.

Statutory Requirements Under 42 U.S.C. 262(k)

Requirement	Statutory Provision	Notes
Reference product	Must be licensed under 351(a)	Must be US-licensed; identify by proper name and BLA number
Biosimilarity data	Analytical, animal, and clinical studies	Secretary may determine any component unnecessary
Same route, dosage form, strength	Required to match reference product	Or provide scientific justification for differences
Manufacturing facilities	Adequate for biologics	21 CFR 600-680 compliance
Labeling	Must include biosimilar-specific labeling	Per FDA biosimilar labeling guidance (2018)
Exclusivity check	Cannot be approved until 12 years after reference product first licensure	Filing permitted after 4 years

351(k) vs 351(a) Comparison

Application Element	351(a) Full BLA	351(k) Biosimilar BLA
Clinical data	Full Phase 1-3 program	Abbreviated: PK/PD + 1 comparative study (typically)
Nonclinical data	Full pharmacology/toxicology program	Abbreviated: may be waived with robust analytics
CMC data	Full (no abbreviation)	Full (no abbreviation)
Analytical data	Standard characterization	Extensive head-to-head comparison with reference
Review fee (PDUFA)	Full application fee	Biosimilar application fee (lower)
User fee category	BLA-1	BLA-2 (biosimilar)

Key Statistic

The PDUFA application fee for a 351(k) biosimilar BLA is lower than for a 351(a) BLA. For FY 2026, the biosimilar BLA application fee is approximately half the full BLA fee. Additionally, first biosimilar applicants qualifying as small businesses may request a fee waiver.

Biosimilar Data Package: eCTD Module Structure

The 351(k) application is submitted in eCTD format following the same modular structure as a 351(a) BLA, but with biosimilar-specific content.

Module 1: Administrative Information

Section	Content	Biosimilar-Specific Notes
1.1	Cover letter	State this is a 351(k) application; identify reference product
1.2	Form FDA 356h	Check BLA type as 351(k); identify reference product BLA number
1.3.1	Patent information	Not listed in Orange Book; BPCIA patent provisions apply
1.3.3	Exclusivity	No independent data exclusivity for biosimilar
1.5	Labeling	Biosimilar-specific labeling per FDA guidance; nonproprietary name with suffix
1.6	Environmental assessment	Categorical exclusion typically applicable
1.12	Pre-submission correspondence	BPD meeting minutes, FDA feedback
1.14	Debarment certification	Required
1.15	Financial disclosure	For all investigators in clinical studies

Module 2: Summaries and Overviews

Section	Content	Biosimilar Emphasis
2.3	Quality Overall Summary	Comparative analytical data summary, head-to-head comparison with reference
2.4	Nonclinical Overview	Animal study results (if conducted); justification if waived
2.5	Clinical Overview	Biosimilarity rationale, totality-of-evidence assessment, benefit-risk
2.7	Clinical Summary	PK/PD study results, comparative clinical study results, immunogenicity

Module 3: Chemistry, Manufacturing, and Controls

Module 3 for a 351(k) application is not abbreviated. Full CMC data is required, equivalent to what would be submitted in a 351(a) BLA.

Section	Content	Key Biosimilar Requirements
3.2.S	Drug substance	Complete characterization, manufacturing process, specifications
3.2.P	Drug product	Formulation, manufacturing, specifications, stability
3.2.R	Regional information	Comparative analytical data reports (head-to-head with reference product)
3.2.A	Appendices	Analytical method validation, reference standard documentation

Critical Module 3 content for biosimilars:

Side-by-side analytical comparison data for every quality attribute
Statistical analysis of similarity (equivalence testing, quality range)
Forced degradation comparison (stress testing under identical conditions)
Functional/biological activity comparison using multiple assays

Module 4: Nonclinical Study Reports

Section	Content	When Required
4.2.1	Pharmacology	If residual uncertainty from analytical comparison
4.2.3	Toxicology	At least one comparative toxicity study (or scientific justification for waiver)
4.3	Literature references	Published nonclinical data supporting biosimilarity

Module 5: Clinical Study Reports

Section	Content	Typical Requirements
5.3.1	Biopharmaceutic studies	Comparative PK studies
5.3.3	Clinical efficacy	Comparative clinical study report
5.3.4	Clinical safety	Integrated safety from all clinical studies
5.3.5.3	Immunogenicity reports	Comparative immunogenicity data

Totality of Evidence: The FDA Review Framework

FDA reviews the 351(k) application using a totality-of-evidence approach, as described in the 2015 guidance "Scientific Considerations in Demonstrating Biosimilarity to a Reference Product."

Evidence Hierarchy

Evidence Level	Data Type	Role in Assessment
Primary	Analytical similarity (structure, function, purity)	Foundation; most sensitive for detecting differences
Supportive	Animal studies (toxicity, PK)	Addresses residual uncertainty from analytics
Confirmatory	Clinical PK/PD	Confirms similar human exposure and response
Clinical	Comparative efficacy/safety/immunogenicity	Confirms no clinically meaningful differences

How Totality of Evidence Works in Practice

The principle is that the strength of the analytical evidence determines the extent of clinical evidence needed:

Analytical Similarity Result	Clinical Program Implications
Highly similar across all attributes	Reduced clinical program (smaller study, PD endpoint) may be acceptable
Similar with minor differences in some attributes	Standard clinical program required
Differences in quality attributes	Extensive clinical program; may not be approvable as biosimilar

Pro Tip

Present the totality-of-evidence assessment in Module 2.5 (Clinical Overview) as a structured argument. Start with the analytical foundation, explain what each analytical finding means for clinical performance, identify any residual uncertainties, and show how the clinical data resolves those uncertainties. FDA reviewers appreciate a clearly articulated biosimilarity argument rather than a simple data dump.

Analytical Similarity Studies: Detailed Requirements

Study Design Principles

Principle	Implementation
Head-to-head comparison	Same assay, same conditions, same day for biosimilar and reference
Multiple reference lots	Minimum 10 lots to capture lot-to-lot variability
Lot selection	Include lots spanning manufacturing dates and remaining shelf life
Multiple biosimilar lots	Include lots from process development and commercial-scale
Blinded testing	Analysts should not know which samples are biosimilar vs. reference

Comprehensive Analytical Assessment

Category	Tests	Acceptance Criteria Approach
Primary structure	Intact mass, peptide mapping (LC-MS), amino acid analysis, N/C-terminal sequencing	Identical sequence; modifications within reference range
Higher-order structure	Far-UV CD, near-UV CD, FTIR, DSC, HDX-MS	Overlay comparison; statistically similar profiles
Post-translational modifications	Glycan mapping (HILIC, CE), sialylation, oxidation, deamidation, C-terminal lysine	Within reference product quality range
Charge variants	cIEF, IEX-HPLC	Acidic, main, basic peak percentages within reference range
Size variants	SEC-HPLC, CE-SDS (reduced/non-reduced), AUC, DLS	Monomer, aggregate, fragment percentages within reference range
Biological activity	Cell-based potency, receptor binding (SPR/ELISA), ADCC, CDC, apoptosis	Relative potency within equivalence margins
Process-related impurities	HCP (ELISA), residual DNA, protein A (if applicable)	Below specification limits
Forced degradation	Thermal, oxidative, pH, light, agitation stress	Similar degradation pathways and rates

Statistical Approaches

Method	Application	Description
Equivalence test	Quantitative attributes with sufficient data	Two one-sided test (TOST) with margins based on reference variability
Quality range	Attributes with well-characterized reference	Biosimilar results fall within mean +/- 3SD (or similar) of reference
Graphical comparison	Complex profiles (glycan maps, spectra)	Overlay plots with quantitative similarity metrics
Multivariate analysis	Overall similarity assessment	PCA, Mahalanobis distance across multiple attributes

Key Statistic

A typical analytical similarity package for a monoclonal antibody biosimilar includes 50-70 individual tests across 10+ reference product lots and multiple biosimilar lots. The data package for the analytical section alone can exceed 1,000 pages in the eCTD submission.

Biosimilar Animal Studies: When and What

FDA Position on Animal Studies

FDA's 2015 guidance states that animal studies, including animal toxicity studies, may not be needed in all cases. The decision depends on the residual uncertainty remaining after analytical comparison.

Scenario	Animal Studies Expected?	Rationale
Robust analytical similarity, well-characterized product class	May be waived	Analytics sufficiently address uncertainty
Minor analytical differences of uncertain clinical relevance	Likely required	Need functional assessment in vivo
Novel formulation or excipients	Likely required	Need local tolerance assessment
New route of administration	Required	No clinical experience with this route
Complex product (e.g., ADC, PEGylated)	Typically required	Greater uncertainty in analytical prediction

If Animal Studies Are Conducted

Study Type	Design	Endpoints
Comparative PK	Single-dose, parallel group in relevant species	AUC, Cmax, Tmax comparison
Repeat-dose toxicity	Comparative design, same dose levels	Clinical observations, clinical pathology, histopathology, ADA
Local tolerance	If new formulation or route	Injection site reactions, histopathology

Pro Tip

Request FDA's position on animal study necessity at the BPD Type 1 or Type 2 meeting. Submit your analytical similarity data (even if preliminary) in the briefing document and ask specifically whether animal toxicity studies are needed. FDA has increasingly waived animal studies for well-characterized product classes (e.g., monoclonal antibodies) when analytical similarity is robust, saving 1-2 years of development time.

Clinical Pharmacology: PK/PD Equivalence Studies

PK Study Design

Design Element	Typical Approach
Study design	Randomized, double-blind, single-dose, crossover (or parallel if crossover not feasible)
Population	Healthy volunteers (if safe) or patients (if safety concerns in healthy volunteers)
Dose	Single dose at a level sensitive to detect PK differences
PK parameters	AUC0-inf, AUC0-t, Cmax (primary); Tmax, t1/2 (secondary)
Equivalence margins	80-125% for AUC and Cmax (standard bioequivalence criteria)
Sample size	Powered to demonstrate equivalence; typically 50-200 per arm
ADA assessment	Pre-dose and at multiple post-dose time points

PD Study (When Applicable)

A PD study may be conducted alone or as a PK/PD study when a validated PD biomarker exists.

PD Study Element	Details
Biomarker	Must be clinically relevant and sensitive to detect differences
Examples	Absolute neutrophil count (filgrastim), INR (anticoagulants)
Design	Comparative, equivalence design
Endpoints	PD parameter AUC, Emax, time to response

Comparative Clinical Studies: Efficacy, Safety, and Immunogenicity

Efficacy Study Design

Design Element	Requirement
Design	Randomized, double-blind, parallel-group, active-controlled (reference product is comparator)
Indication	Most sensitive indication for detecting differences (not necessarily largest market)
Population	Sensitive and homogeneous population
Primary endpoint	Clinical endpoint or validated PD endpoint sensitive to detect product differences
Equivalence margins	Pre-specified, clinically justified; typically based on treatment effect of reference vs. placebo
Duration	Sufficient to assess primary endpoint (varies by indication)
Statistical analysis	Equivalence or non-inferiority with two-sided 95% CI

Indication Selection for Clinical Study

Selection Criterion	Rationale
Scientific sensitivity	Choose indication where differences between products would be most detectable
Mechanism of action relevance	Ensure the studied mechanism is relevant to all proposed indications
Patient population homogeneity	Reduce variability to increase statistical sensitivity
Endpoint sensitivity	Use endpoints with the narrowest equivalence margins
Practical feasibility	Patient recruitment, study duration, cost

Immunogenicity Assessment

Comparative immunogenicity assessment is mandatory in every 351(k) application.

Element	Requirement
Assay approach	Tiered: screening, confirmatory, titer, neutralizing antibody
Assay format	Same validated assay for both biosimilar and reference
Sampling schedule	Pre-dose, multiple post-dose time points, end of study
Analysis	Compare ADA incidence, titers, NAb rates between arms
Clinical correlation	Assess impact of ADA on PK, efficacy, and safety
Duration	Sufficient to capture ADA development (typically 6-12 months)

Key Statistic

FDA's 2019 guidance "Immunogenicity Testing of Therapeutic Protein Products" requires that the ADA assay used in biosimilar clinical studies must be capable of detecting antibodies to both the biosimilar and the reference product. A single assay platform should be used to avoid assay-dependent differences in ADA detection rates. Drug tolerance of the assay must be characterized and reported.

Extrapolation of Indications

Extrapolation allows a biosimilar approved based on clinical data in one indication to receive licensure for additional indications of the reference product without conducting separate clinical studies in each indication.

Extrapolation Criteria

Criterion	Assessment
Mechanism of action	Is the MOA the same across proposed indications?
Target/receptor	Is the same target engaged in each indication?
PK/PD	Is the PK/PD relationship similar across indications?
Immunogenicity	Are there indication-specific immunogenicity concerns?
Safety	Are there unique safety considerations in any indication?
Formulation/route	Is the same formulation and route used across indications?

Extrapolation Decision Framework

Scenario	Extrapolation Likely?	Justification Needed
Same MOA, same target, all indications	Yes	Standard scientific justification
Same MOA but different target engagement in some indications	Case-by-case	May need indication-specific functional data (e.g., ADCC for oncology)
Different MOA in different indications	No	Separate clinical data likely required
Indication-specific safety concerns	May limit extrapolation	Must address specific safety signals

Practical Example: Adalimumab Biosimilar

Reference Product Indication	MOA	Extrapolation Basis
Rheumatoid arthritis (studied)	TNF-alpha inhibition	Direct clinical data
Psoriatic arthritis	TNF-alpha inhibition	Same MOA, same target
Ankylosing spondylitis	TNF-alpha inhibition	Same MOA, same target
Crohn's disease	TNF-alpha inhibition	Same MOA, same target
Ulcerative colitis	TNF-alpha inhibition	Same MOA, same target
Plaque psoriasis	TNF-alpha inhibition	Same MOA, same target
Hidradenitis suppurativa	TNF-alpha inhibition	Same MOA, same target
Uveitis	TNF-alpha inhibition	Same MOA, same target

Pro Tip

Submit your extrapolation strategy and scientific justification as part of the BPD Type 2 meeting briefing document. FDA will provide written feedback on whether extrapolation is acceptable for each proposed indication. This avoids the risk of submitting a 351(k) BLA with extrapolation claims that FDA will reject, requiring additional clinical studies and delaying approval.

Post-Submission: Review and Approval

Review Timeline

Event	Timeline
351(k) submission	Day 0
60-day filing review	Days 0-60
Filing decision	Day 60
PDUFA review period	10 months (standard) or 6 months (priority) from filing
Information requests	Throughout review
Pre-approval inspection	During review period
Advisory committee	If convened (less common for biosimilars with robust data)
PDUFA action date	Month 10 or Month 6 from filing

Post-Approval Requirements

Requirement	Details
Pharmacovigilance	Standard adverse event reporting (21 CFR 600.80)
Annual BLA reports	Required under 21 CFR 601.12
Manufacturing changes	Reported per 21 CFR 601.12 (PAS, CBE-30, or annual report)
Post-marketing studies	May be required if any residual biosimilarity questions
Labeling updates	Must maintain consistency with reference product labeling updates

A 351(a) BLA is a full application requiring independent demonstration of safety, purity, and potency through a complete development program. A 351(k) BLA is an abbreviated application that relies on FDA's previous finding of safety, purity, and potency for the reference product and demonstrates biosimilarity through a stepwise, totality-of-evidence approach. The 351(k) pathway results in an abbreviated (not eliminated) clinical program.

Key Regulatory References

Reference	Citation
PHS Act Section 351(k)	42 U.S.C. 262(k)
BPCIA	P.L. 111-148, Section 7002 (2010)
Scientific Considerations in Demonstrating Biosimilarity	FDA Guidance (2015)
Quality Considerations in Demonstrating Biosimilarity	FDA Guidance (2015)
Clinical Pharmacology Data to Support Biosimilarity	FDA Guidance (2014)
Biosimilar Product Labeling	FDA Guidance (2018)
Immunogenicity Testing of Therapeutic Protein Products	FDA Guidance (2019)
Formal Meetings Between FDA and Biosimilar Sponsors	FDA Guidance (2015)
Nonproprietary Naming of Biological Products	FDA Guidance (2017)
Considerations in Demonstrating Interchangeability	FDA Guidance (2019)
ICH Q5E	Comparability of Biotechnological Products
ICH Q6B	Specifications for Biotechnological Products
FDA Purple Book	purplebooksearch.fda.gov