FDA Regenerative Medicine: 351/361 HCT/P Determination and RMAT Designation

Executive Summary & Direct Decision Brief

Sponsors developing human cells, tissues, and cellular and tissue-based products (HCT/Ps) face a foundational regulatory fork before clinical entry: determining whether the candidate product is regulated solely as a 361 HCT/P under 21 CFR Part 1271 or as a 351 biological product (or combination product) requiring a Biologics License Application (BLA). Making the wrong classification decision introduces severe regulatory risk, including warning letters, clinical holds, and unapproved product enforcement actions from the FDA's Center for Biologics Evaluation and Research (CBER).

For 351 biological products that meet the definition of a regenerative medicine therapy, Section 3033 of the 21st Century Cures Act established the Regenerative Medicine Advanced Therapy (RMAT) designation. RMAT provides cell and gene therapy sponsors with an expedited development framework that combines all features of the Breakthrough Therapy program with early, structured CBER interactions on manufacturing, nonclinical evidence, and accelerated approval endpoints.

Direct Decision Brief

For regulatory strategy teams, clinical directors, and business development leads, the core operational decision rules are:

1. 351 vs 361 Statutory Determination: A human cell or tissue product qualifies for sole regulation under Section 361 of the Public Health Service (PHS) Act (21 CFR 1271.10(a)) only if it satisfies all four statutory criteria: minimal manipulation, homologous use, no combination with another drug or device (with narrow exceptions), and either lack of systemic effect or reliance on autologous/allogeneic reproductive/first-degree relative use. Failure on any single criterion automatically classifies the product under Section 351, requiring an Investigational New Drug (IND) application and premarket BLA approval.
2. Gene Therapies and Cultured Cells are Always 351: Gene therapy products, genetically modified cellular therapies (such as CAR-T cells), and cell products expanded in culture fail the minimal manipulation threshold as a matter of law. They must be submitted under an IND and approved via a BLA.
3. RMAT Eligibility Requirements: To qualify for RMAT designation under Section 506(g) of the Federal Food, Drug, and Cosmetic Act (FD&C Act), the candidate must:
   • Be a regenerative medicine therapy (cell therapy, tissue engineering product, human cell and tissue product, or combination product incorporating such therapies).
   • Be intended to treat, modify, reverse, or cure a serious or life-threatening disease or condition.
   • Possess preliminary clinical evidence indicating that the product has the potential to address unmet medical needs for such a disease or condition.
4. RMAT Expedited Features vs Breakthrough: RMAT designation grants all features of Breakthrough Therapy designation—including intensive FDA guidance on an efficient drug development program, organizational commitment involving senior managers, and rolling review—plus specific statutory provisions for early interaction on manufacturing (CMC) topics and novel surrogate or intermediate clinical endpoints.
5. No Independent Exemption for 361 Products: RMAT designation is available only to 351 products under IND. A 361-only HCT/P is not subject to premarket approval and cannot receive RMAT designation.
6. Interaction with Accelerated Approval: RMAT designation facilitates, but does not guarantee, accelerated approval under 21 CFR 314 Subpart H or 21 CFR 601 Subpart E. Accelerated approval relies on surrogate endpoints or intermediate clinical endpoints reasonably likely to predict clinical benefit, requiring post-approval confirmatory trials.

351 vs 361 HCT/P Determination: The Four 21 CFR 1271.10(a) Criteria

Under the PHS Act, FDA categorizes human cells, tissues, and cellular and tissue-based products into two distinct regulatory tiers:

• Section 361 HCT/Ps: Regulated solely under Section 361 of the PHS Act and 21 CFR Part 1271 to prevent the introduction, transmission, or spread of communicable diseases. These products do not require premarket review, INDs, 510(k) clearances, or BLAs.
• Section 351 HCT/Ps: Regulated as biological products, drugs, or medical devices under Section 351 of the PHS Act and the FD&C Act. These products require premarket clearance or approval (typically a BLA for cell and gene therapies) and full compliance with IND regulations (21 CFR Part 312), Current Good Manufacturing Practice (CGMP under 21 CFR Parts 210/211 or 820/QMSR), and clinical trial requirements.

To qualify for sole regulation under Section 361, an HCT/P must meet all four criteria set forth in 21 CFR 1271.10(a). If an HCT/P fails even one criterion, it is regulated as a 351 drug, device, or biological product.

Operational Deep Dive into Minimal Manipulation

The minimal manipulation threshold is the single most frequent point of failure for cell therapy developers. FDA's guidance (Regulatory Considerations for Human Cells, Tissues, and Cellular and Tissue-Based Products: Minimal Manipulation and Homologous Use, November 2017, updated July 2020) establishes strict definitions based on whether the tissue is classified as structural or nonstructural:

• Structural Tissues: Includes bone, skin, amniotic tissue, cartilage, tendons, and ligaments. For structural tissue, minimal manipulation means processing that does not alter the original macro- and micro-anatomical structure, extracellular matrix architecture, or mechanical properties relevant to reconstruction, repair, or replacement.
  • Processing that exceeds minimal manipulation: Micronization of amniotic membrane into injectable powders, enzymatic digestion of adipose tissue to harvest stromal vascular fraction (SVF), or severe chemical cross-linking that alters elasticity.
• Nonstructural Cells/Tissues: Includes hematopoietic stem cells, lymph nodes, umbilical cord blood, and peripheral blood. For nonstructural tissue, minimal manipulation means processing that does not alter the cell's inherent biological characteristics (e.g., differentiation potential, proliferation capacity, or metabolic activity).
  • Processing that exceeds minimal manipulation: Ex vivo cell expansion, passage through cell culture, exposure to growth factors or cytokines that induce lineage differentiation, or genetic modification (transduction with lentiviral/AAV vectors or CRISPR editing).

Why Gene Therapies and Advanced Cell Therapies Are Always Section 351

All gene therapy products (in vivo AAV, in vivo mRNA, ex vivo transduced HSCs, ex vivo edited CAR-T/CAR-NK cells) involve genetic material modification or extensive cell processing. As a consequence:

1. Ex vivo edited cell therapies fail Criterion 1 (minimal manipulation) due to vector integration or enzymatic cleavage, and fail Criterion 4 when distributed for allogeneic use in unrelated recipients.
2. In vivo gene therapies (e.g., AAV vectors delivering therapeutic transgenes) do not meet the definition of an HCT/P at all under 21 CFR 1271.3(d), as they are non-cellular genetic vector constructs. They are regulated directly as biological products under PHS Act Section 351.

Sponsors must establish their regulatory pathway early. If there is ambiguity regarding whether a cell product meets Section 361, sponsors should submit a Request for Designation (RFD) or a Pre-RFD to the FDA Office of Combination Products (OCP), or consult CBER's Tissue Reference Group (TRG) prior to filing an IND. (See our Pre-RFD playbook for the request process.)

What Is RMAT Designation and Which Products Qualify?

To accelerate the development and review of promising cell and gene therapies, Congress established the RMAT designation under Section 3033 of the 21st Century Cures Act (enacted December 13, 2016), codified at Section 506(g) of the FD&C Act (21 U.S.C. 356(g)).

                  +-------------------------------------------------------+
                  |               Candidate Cell/Gene Product             |
                  +-------------------------------------------------------+
                                              |
                                              v
                  +-------------------------------------------------------+
                  |        Is the product a 351 Biological Product?       |
                  +-------------------------------------------------------+
                                     /                 \
                                    /                   \
                           NO      /                     \     YES
                                  v                       v
            +---------------------------+   +---------------------------+
            |      361-Only HCT/P       |   |       351 Product         |
            | (21 CFR Part 1271)        |   |    (IND / BLA Required)   |
            |   No Premarket Review     |   +---------------------------+
            |   Ineligible for RMAT     |                 |
            +---------------------------+                 v
                                            +---------------------------+
                                            | Does it meet RMAT statutory|
                                            | criteria under 506(g)?    |
                                            +---------------------------+
                                               - Regenerative Medicine
                                               - Serious/Life-Threatening
                                               - Preliminary Clinical
                                                 Evidence of Potential
                                            +---------------------------+
                                                          |
                                                          v
                                            +---------------------------+
                                            |   Submit RMAT Request to  |
                                            |     CBER (60-Day Clock)   |
                                            +---------------------------+

Statutory Eligibility Criteria

An HCT/P or cell/gene therapy product is eligible for RMAT designation if three conditions are satisfied:

1. Regenerative Medicine Therapy Definition: The product must meet the statutory definition of a regenerative medicine therapy under Section 506(g)(8):
   • Human cell and tissue products (cell therapies, stem cell-derived therapeutics).
   • Tissue engineering products.
   • Human cell and tissue-based combination products (e.g., cell-seeded scaffolds).
   • Other cell and genetically modified cell therapies (including CAR-T, CAR-NK, gene-edited somatic cells).
   • Xenogeneic cell products.
   • Note: Human gene therapies that produce durable modification of somatic cells or tissues (including in vivo gene therapy vectors) qualify as regenerative medicine therapies under FDA guidance.
2. Serious or Life-Threatening Condition: The product must be intended to treat, modify, reverse, or cure a serious or life-threatening disease or condition. FDA interprets "serious" in accordance with 21 CFR 312.300, looking at factors such as impact on survival, day-to-day functioning, or likelihood that the disease will progress if left untreated.
3. Preliminary Clinical Evidence: The sponsor must possess preliminary clinical evidence demonstrating that the product has the potential to address unmet medical needs for the serious disease or condition.
   • Critical Distinction: Unlike Fast Track designation (which can be granted based on cell culture or animal model data), RMAT designation requires human clinical data. This clinical evidence typically comes from an ongoing Phase 1 or Phase 1/2 clinical trial under an active IND.
   • Data Scope: The preliminary clinical evidence must show a positive signal of efficacy or disease modification (e.g., biomarker response, histological repair, clinical functional improvement) in a human patient cohort. Preclinical animal data alone is insufficient.

Submission Timeline and FDA Review Clock

Sponsors submit an RMAT designation request either concurrently with an initial IND submission or as an amendment to an existing IND. FDA's CBER Office of Tissues and Advanced Therapies (OTAT, now reorganized within the Office of Therapeutic Products / OTP) evaluates the request.

• Review Timeline: FDA is required by statute to grant or deny the RMAT designation request no later than 60 calendar days after receiving the request.
• IND Status Gate: FDA will not grant an RMAT designation if the IND is on clinical hold or is placed on clinical hold during the 60-day designation review window.
• Re-submission: If FDA denies an RMAT request due to insufficient preliminary clinical evidence, the sponsor may collect additional clinical data from the ongoing trial and submit a new RMAT request at a later date.

RMAT vs Breakthrough Device & Breakthrough Therapy Designations

Regenerative medicine sponsors often struggle to navigate the overlapping expedited programs offered by FDA: Fast Track, Breakthrough Therapy, RMAT, and Breakthrough Device designation (for device-led combination products).

Understanding the precise differences between RMAT and related programs is essential for structuring FDA interactions:

The Unique Value of RMAT for Cell and Gene Therapies

While RMAT incorporates all the benefits of Breakthrough Therapy, it offers two specific advantages tailored to the cell and gene therapy sector:

1. Early Manufacturing (CMC) Escalation: Cell and gene therapy development is notoriously bottlenecked by CMC challenges: vector potency assays, cell viability, comparability protocols after scale-up, and sterility validation. Section 506(g)(5)(A) explicitly directs FDA to meet with RMAT sponsors to discuss early manufacturing development plans, comparability strategies, and assay validation well ahead of Phase 3.
2. Flexible Post-Approval Pathways: Under Section 506(g)(5)(C), if an RMAT product achieves accelerated approval, FDA has explicit statutory flexibility regarding how post-approval confirmatory requirements can be satisfied. These include:
   • Submission of clinical evidence from ongoing trials.
   • Patient registry data collection (e.g., long-term follow-up registries for gene therapy integration safety).
   • Observational post-market studies or real-world evidence (RWE).
   • Expanded monitoring of clinical outcomes across participating clinical centers.

Interaction with Accelerated Approval (Subpart H / Subpart E)

A common misconception among sponsors is that obtaining RMAT designation automatically grants accelerated approval. RMAT designation is an expedited development process, whereas accelerated approval is a statutory approval pathway. RMAT facilitates early agreement on endpoints that can support accelerated approval, but the sponsor must still prove clinical efficacy and safety.

+-----------------------------------------------------------------------------+
|                            RMAT DESIGNATION                                 |
|   - Granted during clinical development based on preliminary clinical data  |
|   - Early interaction on surrogate / intermediate clinical endpoints         |
+-----------------------------------------------------------------------------+
                                      |
                                      v
+-----------------------------------------------------------------------------+
|                            BLA SUBMISSION                                   |
|   Sponsor proposes approval based on an unvalidated surrogate endpoint      |
|   or an intermediate clinical endpoint (21 CFR 601 Subpart E / 314 Subpart H)|
+-----------------------------------------------------------------------------+
                                      |
                                      v
+-----------------------------------------------------------------------------+
|                          ACCELERATED APPROVAL                               |
|   FDA grants BLA approval with mandatory post-approval confirmatory trials  |
+-----------------------------------------------------------------------------+
                                      |
                                      v
+-----------------------------------------------------------------------------+
|                    POST-APPROVAL CONFIRMATORY COMMITMENT                    |
|   Sponsor completes confirmatory study, registry, or real-world study to    |
|   verify clinical benefit and convert to full BLA approval                   |
+-----------------------------------------------------------------------------+

Statutory Basis for Accelerated Approval in Biologics

Accelerated approval for 351 biological products is governed by Section 506(c) of the FD&C Act and FDA regulations under 21 CFR Part 601, Subpart E (and 21 CFR Part 314, Subpart H for drugs). It allows FDA to approve a BLA based on:

1. A surrogate endpoint that is "reasonably likely to predict clinical benefit" (e.g., expression of a missing enzyme or structural protein following gene transfer, such as micro-dystrophin in Duchenne muscular dystrophy).
2. An intermediate clinical endpoint that can be measured earlier than irreversible morbidity or mortality (IMM) and is reasonably likely to predict clinical benefit.

Post-Approval Requirements and Withdrawal Risks

When an RMAT product receives accelerated approval based on a surrogate endpoint:

• Mandatory Confirmatory Trials: The sponsor is contractually and legally bound to conduct post-approval clinical studies to verify and describe the actual clinical benefit (e.g., functional mobility improvement or survival benefit).
• Accelerated Withdrawal Authority: Under the Food and Drug Omnibus Reform Act of 2022 (FDORA), FDA possesses streamlined procedures to withdraw accelerated approval if:
  • The sponsor fails to conduct required confirmatory trials with due diligence.
  • The confirmatory trial fails to verify clinical benefit.
  • Other evidence demonstrates that the product is not safe or effective under conditions of use.

FDA RMAT Track Record & Program Statistics (As of 2025/2026)

Since the enactment of the 21st Century Cures Act in December 2016, CBER's RMAT program has seen substantial adoption across academic medical centers, biotechnology firms, and global pharmaceutical sponsors.

Analyzing the cumulative program statistics from CBER reporting and industry tracking yields key benchmarks for sponsors evaluating request timing:

RMAT Program Tally (FDA CBER, cumulative through September 30, 2025):
======================================================================
Requests Received:                    ~388
Designations Granted:                 193   (Overall Grant Rate: ~50%)
RMAT-Designated Products Approved:    ~13   (as of mid-2025)
======================================================================

Key Statistical Insights for Sponsors

1. Selective ~50% Grant Rate: CBER grants RMAT designation to roughly half of submitted requests. The primary reason for denial is insufficient preliminary clinical evidence—either because the trial cohort was too small, the follow-up duration was too short to show a durable effect, or the clinical endpoint lacked clear objective measurement.
2. CBER Jurisdiction and Volume Growth: All RMAT designations are managed within CBER's Office of Therapeutic Products (OTP, formerly the Office of Tissues and Advanced Therapies / OTAT). Activity has accelerated sharply: after roughly 9-18 designations per year from FY2017-FY2023, grants jumped to 43 in FY2024 and 50 in FY2025, as in vivo gene therapies and engineered cell therapies (including CAR-T) came to dominate the program.
3. Transition to Commercial Approval: As of mid-2025, about 13 RMAT-designated products had transitioned from designation to full BLA approval (for example, Zevaskyn and Symvess). RMAT accelerates development timelines, but the modest approval count underscores that designation is a development aid, not a guarantee of approval.

Step-by-Step RMAT Application & Interaction Workflow

Sponsors planning to request RMAT designation should execute the following operational sequence:

[Phase 1: Pre-IND / IND Filing]
   │
   ├── Establish 351 BLA jurisdiction (confirm failure of 361 criteria via TRG or Pre-RFD).
   ├── File IND under 21 CFR Part 312 with initial Phase 1/2 clinical protocol.
   └── Define objective clinical endpoints to measure preliminary clinical effect.
   │
[Phase 2: Clinical Signal Capture & RMAT Package Preparation]
   │
   ├── Treat initial patient cohort under IND protocol.
   ├── Gather human clinical data showing positive efficacy signal / unmet need resolution.
   └── Draft RMAT Request Package:
          - Cover Letter explicitly requesting 506(g) designation.
          - Product Description & Regenerative Medicine Classification.
          - Unmet Medical Need & Serious Condition Justification.
          - Preliminary Clinical Evidence Teardown (individual patient data, response rates).
          - Proposed Early Interaction Plan (CMC, surrogate endpoints).
   │
[Phase 3: Formal Submission & 60-Day Clock]
   │
   ├── Submit RMAT Request as IND Amendment via eCTD.
   ├── Monitor IND status (ensure zero clinical hold issues).
   └── Receive FDA Decision Letter within 60 Calendar Days.
   │
[Phase 4: Post-Designation Early Interactions]
   │
   ├── Request Type B (RMAT) Multidisciplinary Meeting.
   ├── Align with CBER on CMC scale-up, potency assays, and comparability.
   └── Agree on surrogate endpoints for potential Accelerated Approval BLA.

Frequently Asked Questions (FAQs)

Does RMAT designation guarantee accelerated approval?

No. RMAT designation provides an expedited development process, early CBER access, and statutory frameworks to discuss surrogate endpoints. It does not alter the scientific or legal standards for BLA approval. The sponsor must still submit adequate and well-controlled clinical trial data demonstrating safety and efficacy before BLA approval or accelerated approval can be granted.

Can a 361-only HCT/P receive RMAT designation?

No. RMAT designation is established under Section 506(g) of the FD&C Act and is applicable only to products subject to premarket approval requirements (351 biological products under BLA or IND). A 361-only HCT/P is regulated solely under 21 CFR Part 1271, does not require an IND or premarket approval, and is therefore ineligible for RMAT designation.

How long does FDA have to decide an RMAT designation request?

FDA (specifically CBER) is required by statute to make a determination on an RMAT designation request no later than 60 calendar days after receipt of the submission.

What happens if an IND is on clinical hold when RMAT is requested?

If an IND is on clinical hold or is placed on clinical hold during the 60-day review period, FDA will deny the RMAT designation request. Sponsors must resolve all clinical hold issues before submitting or re-submitting an RMAT request.

Regulatory & Strategic Checklist for Sponsors

Before submitting an RMAT designation request, complete the following audit checklist:

• 351/361 Classification Clear: Formally documented that the product fails at least one criterion of 21 CFR 1271.10(a) (or is a gene therapy) and is appropriately filed under an IND.
• Regenerative Medicine Status Verified: Confirmed that the product meets the statutory definition of cell therapy, gene therapy, tissue engineering, or cell-based combination product.
• Human Clinical Data Available: Assembled human clinical data from an active IND showing positive safety/efficacy signals. (Preclinical data alone will result in denial).
• Unmet Need Documented: Established that the target indication is a serious or life-threatening condition with limited or inadequate existing therapies.
• IND Free of Clinical Hold: Confirmed active, un-encumbered IND status.
• eCTD Package Prepared: Structured cover letter, clinical evidence summary, and proposed early interaction agenda for CBER review.

Related Guides & Resources

• FDA Prior Knowledge Framework for Cell and Gene Therapy Products — How to leverage platform CMC and clinical data across CGT programs.
• FDA Breakthrough Device Designation Guide — Expedited pathways for innovative medical devices and device-led combination products.
• Combination Products: Drug-Device & Biologic-Device Regulatory Guide — Navigating primary mode of action (PMOA) and inter-center reviews.