Interchangeable Biosimilar Designation: Requirements and Switching Studies
An interchangeable biosimilar is a biosimilar that also meets the interchangeability standard in 42 U.S.C. 262(k)(4): it must be expected to produce the same clinical result as the reference product in any given patient, and for products given more than once, switching risk must not be greater than continued use of the reference product. FDA decides what evidence is needed case by case. Interchangeable products may be substituted at the pharmacy level under state law, and current product status should be checked in the FDA Purple Book.
Key Takeaways
Key Takeaways
- Interchangeable biosimilars must produce the same clinical result in any given patient (individual-level standard) and demonstrate that switching risk does not exceed continuous reference product use
- FDA evaluates interchangeability using a totality-of-the-evidence approach; its 2024 draft update says a switching study or other additional clinical study may not generally be needed in every case
- Interchangeable products may be substituted at the pharmacy without prescriber involvement, but substitution rules vary by state law
- The first approved interchangeable biosimilar receives 1 year of market exclusivity under BPCIA Section 262(k)(6)
- Interchangeable biosimilar designation represents the highest tier of similarity determination under the Biologics Price Competition and Innovation Act (BPCIA). While a biosimilar must demonstrate it is "highly similar" with "no clinically meaningful differences," an interchangeable biosimilar must additionally demonstrate that it can be expected to produce the "same clinical result" in any given patient and that the risk of alternating or switching is no greater than continuous use of the reference product.
- This higher standard exists because interchangeable products can be substituted at the pharmacy level, which means a patient might be switched from reference to interchangeable (or vice versa) without the prescriber's explicit involvement. FDA's current approach is case specific and focuses on whether the available evidence supports the statutory switching standard.
- In this guide, you will learn:
- The statutory definition and requirements for interchangeability
- How FDA's current guidance approaches switching evidence
- Additional data considerations beyond biosimilarity
- Pharmacy-level substitution rules and state law landscape
- Strategic considerations for pursuing interchangeability
- Where to verify current interchangeability status
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Interchangeable Biosimilar: Statutory Definition and Standard
Under 42 U.S.C. 262(k)(4), an interchangeable biological product is a biosimilar that is expected to produce the same clinical result as the reference product and, for products administered more than once, does not create greater risk from switching than continued reference-product use.
Statutory Framework
| Provision | Citation | Requirement |
|---|---|---|
| Interchangeability definition | 42 U.S.C. 262(k)(4) | Same clinical result + no additional switching risk |
| Data requirements | 42 U.S.C. 262(k)(4) | Sufficient information to support determination |
| First interchangeable exclusivity | 42 U.S.C. 262(k)(6) | First interchangeable gets 1 year market exclusivity |
| Pharmacy substitution | State-level laws | Varies by state |
Interchangeability vs. Biosimilarity: The Higher Standard
| Standard | Biosimilar | Interchangeable |
|---|---|---|
| Similarity to reference | Highly similar, no clinically meaningful differences | Same as biosimilar |
| Same clinical result | Implied but not individually demonstrated | Must produce same result in ANY given patient |
| Switching risk | Not assessed | Alternating/switching risk must not exceed reference-only risk |
| Data requirements | Analytical + clinical PK/PD + comparative clinical | All biosimilar data plus any additional interchangeability evidence FDA determines is needed |
| Pharmacy substitution | No (prescriber must specify) | Yes (state-law dependent) |
| Prescriber involvement | Required for prescribing | Not required for substitution |
The distinction between "any given patient" and the population-level biosimilarity standard is critical. Biosimilarity addresses no clinically meaningful differences, while interchangeability adds the statutory same-clinical-result and switching-risk standards in 42 U.S.C. 262(k)(4).
Biosimilar Interchangeability: FDA Guidance Framework
FDA issued final interchangeability guidance in 2019 and then issued a draft update in June 2024. The 2024 draft is important because FDA states that its scientific approach has evolved and that a switching study or studies may not be needed in every application.
Key Principles from FDA Guidance
| Principle | Description |
|---|---|
| Integrated approach | Interchangeability data should be evaluated in the context of the totality of the biosimilar development data |
| Case-specific evidence | FDA may evaluate analytical, PK/PD, immunogenicity, clinical, and switching information based on the product and application |
| Evolving switching-study expectations | FDA's 2024 draft update says applicants may instead explain why the comparative analytical and clinical data already submitted support the statutory switching standard |
| Presentation matters | Device, container closure, and instructions for use should not undermine safe substitution |
| Purple Book controls current status | Current interchangeability status should be confirmed in the Purple Book rather than static article tables |
When a Switching Study Is Required
| Product Characteristics | FDA approach |
|---|---|
| Product administered more than once | The statute requires a showing that switching risk is not greater than continued reference use |
| Product administered as a single dose | Switching concerns may be different; FDA evaluates case by case |
| Any product with presentation or use differences | FDA may consider whether device or usability differences affect safe substitution |
| Need for a dedicated switching study | Not automatic in every case under FDA's 2024 draft update |
Interchangeable Biosimilar Requirements: Evidence FDA May Evaluate
When FDA Requests Clinical Switching Data
If FDA concludes that clinical switching data are needed for a specific product, the design historically discussed in FDA guidance has centered on comparative PK and immunogenicity. The exact design is product-specific and should not be treated as a universal template.
Illustrative Study Design Elements
| Element | Standard Approach |
|---|---|
| Design | Randomized, double-blind, multiple-switch crossover |
| Population | Sensitive population consistent with biosimilar clinical study |
| Arms | Arm 1: Switch between reference and proposed interchangeable (alternating); Arm 2: Reference product only (continuous) |
| Number of switches | Sufficient alternations to assess the proposed switching question |
| Switch intervals | Based on dosing interval of the product; sufficient washout between periods |
| Primary endpoint | PK parameters (AUC, Cmax) comparing switched vs. continuous arms |
| Key secondary | Immunogenicity (ADA incidence, titers, neutralizing antibodies) |
| Other secondary | Safety, tolerability, efficacy markers |
Example of a Historically Discussed Switching Schema
Three-Period Switching Design Example (for subcutaneous product dosed every 2 weeks):
| Period | Arm 1 (Switching) | Arm 2 (Continuous Reference) |
|---|---|---|
| Period 1 (Weeks 0-6) | Reference (R) x 3 doses | Reference (R) x 3 doses |
| Period 2 (Weeks 6-12) | Interchangeable (I) x 3 doses | Reference (R) x 3 doses |
| Period 3 (Weeks 12-18) | Reference (R) x 3 doses | Reference (R) x 3 doses |
| Period 4 (Weeks 18-24) | Interchangeable (I) x 3 doses | Reference (R) x 3 doses |
PK Criteria When Comparative PK Is Used
| PK Parameter | Analysis | Equivalence Margin |
|---|---|---|
| AUC (within a dosing interval) | 90% CI of geometric mean ratio | 80-125% |
| Cmax | 90% CI of geometric mean ratio | 80-125% |
| Ctrough | Supportive analysis | 80-125% (if pre-specified) |
If a comparative PK switching study is used, the PK analysis generally compares the switched arm to the continuous reference arm.
Immunogenicity Assessment in Switching Studies
| Assessment | Purpose | Timing |
|---|---|---|
| ADA incidence | Compare ADA rates between switching and continuous arms | Pre-each period + post-period |
| ADA titers | Compare titer distributions | With each ADA-positive sample |
| Neutralizing antibodies | Compare NAb incidence and titers | With each ADA-positive sample |
| Clinical impact of ADA | Assess effect on PK, efficacy, safety | Integrated analysis |
| Treatment-emergent ADA | Compare new ADA development | Pre-switch vs. post-switch samples |
| Treatment-boosted ADA | Compare boosting of existing ADA | In previously ADA-positive subjects |
FDA's 2024 draft update makes this section more nuanced than older summaries suggested: the central question is whether the evidence in the application supports the statutory switching standard, not whether every sponsor follows one fixed study design.
Additional Data Beyond Biosimilarity
What FDA Requires Beyond the Biosimilar Data Package
| Data Category | Biosimilar | Additional for Interchangeability |
|---|---|---|
| Analytical similarity | Extensive head-to-head comparison | Same (no additional) |
| Animal studies | If needed | Same (no additional) |
| Clinical PK/PD | Comparative PK/PD as appropriate for biosimilarity | Additional switching-focused PK/PD may be needed in some cases |
| Comparative clinical | Product-specific and may be reduced depending on the biosimilar data package | Additional clinical switching data may or may not be needed |
| Immunogenicity | Comparative immunogenicity as appropriate | FDA may focus on whether available data adequately address switching risk |
| Formulation/presentation | Same as reference or scientifically justified | Differences in delivery device must not affect user ability |
Presentation and Device Considerations
For interchangeable products, the administration device and instructions for use must not present barriers to safe substitution.
| Consideration | Regulatory question |
|---|---|
| Device or presentation differences | Could the difference affect safe substitution or user performance? |
| Instructions for use | Are the instructions adequate for the proposed presentation and use conditions? |
| Human factors or usability | Does the presentation support safe real-world use where substitution could occur? |
FDA's 2019 interchangeability guidance makes presentation a real review issue: if the proposed interchangeable uses a different delivery system or presentation, FDA evaluates whether that difference could affect safety, effectiveness, or practical substitutability.
Pharmacy-Level Substitution: State Law Landscape
Federal Framework
The BPCIA establishes interchangeability as a federal designation but does not preempt state pharmacy practice laws. Substitution rules are governed at the state level.
| Federal Action | Effect |
|---|---|
| FDA determines interchangeability | Product is designated as interchangeable in the Purple Book |
| State law governs substitution | Each state determines whether and how substitution occurs |
| No federal substitution mandate | FDA designation enables, but does not require, state substitution |
Common State-Law Features
| State-Law Feature | Description |
|---|---|
| Substitution authority | Many states tie pharmacy-level substitution to FDA interchangeability status |
| Notification or recordkeeping | Some states require notice to the prescriber, the patient, or both, and some require records to be maintained |
| Prescriber opt-out | State law typically preserves a mechanism for the prescriber to block substitution |
| Operational details | Timing, notice method, and record retention rules vary and should be checked state by state |
Key State Law Provisions
| Provision | Common Approach |
|---|---|
| Notification to prescriber | Depends on state law |
| Notification method | Often electronic or through routine pharmacy records, but it varies |
| Patient communication | Depends on state law and pharmacy practice requirements |
| Recordkeeping | State-specific |
| Dispense as written | Prescriber opt-out remains important in practice |
When developing a commercialization strategy for an interchangeable biosimilar, conduct a state-by-state analysis of substitution laws before launch. The rules vary significantly, and your field force must understand local substitution dynamics. Some states have enacted biosimilar substitution laws that specifically reference FDA's interchangeability determination as the trigger for pharmacy substitution authority.
First Interchangeable Exclusivity
BPCIA provides the first approved interchangeable biosimilar product with a period of market exclusivity during which no other 351(k) applicant may receive interchangeability designation for the same reference product.
Exclusivity Period
| Exclusivity Trigger | Duration |
|---|---|
| First interchangeable is commercially marketed | 1 year from first commercial marketing |
| Alternatively: first interchangeable approved + litigation | 18 months from approval if patent litigation resolved |
| Alternatively: 42 months from approval | If no patent litigation resolution |
Practical Impact
| Impact | Description |
|---|---|
| Interchangeability exclusivity effect | A subsequent 351(k) applicant cannot be determined interchangeable for the same reference product during the applicable exclusivity period |
| No impact on biosimilar-only products | Other biosimilars without interchangeability can still be approved and marketed |
Current Interchangeability Status
The number and identity of FDA-licensed interchangeable products can change as new supplements and BLAs are approved. Rather than rely on a static list in an article, use the Purple Book as the authoritative source for:
- Whether a product is licensed as a biosimilar
- Whether FDA has also determined it to be interchangeable
- The reference product and licensure history
Strategic Considerations: When to Pursue Interchangeability
Whether interchangeability is worth pursuing is product specific. Sponsors typically weigh:
- the statutory evidence burden for the product
- whether the presentation or device raises additional substitution questions
- the practical relevance of state substitution laws for the product's channel of use
- the product's competitive landscape and lifecycle strategy
References
A biosimilar is highly similar to the reference product with no clinically meaningful differences. An interchangeable biosimilar meets the additional statutory standard in 42 U.S.C. 262(k)(4): it must be expected to produce the same clinical result in any given patient and, for products administered more than once, the risk of switching must not be greater than continued use of the reference product. Interchangeable products may be substituted at the pharmacy under state law.