Beyond the Rabbit Pyrogen Test: A Science-Led Transition Toward the Monocyte Activation Test

A NanoTox position paper on the risk-based, animal-free future of pyrogenicity testing

Executive Summary

Pyrogen testing sits at the intersection of patient safety, scientific rigour and animal welfare. For more than a century the Rabbit Pyrogen Test (RPT) anchored this discipline. That era has now closed in Europe: the European Pharmacopoeia deleted the RPT from its texts (implementation 1 July 2025) and suppressed the dedicated chapter 2.6.8 Pyrogens with effect from 1 January 2026, replacing prescriptive testing with a risk-based framework under new general chapter 5.1.13 Pyrogenicity.

NanoTox’s position is deliberate and unambiguous: routine RPT is not encouraged. Where scientifically appropriate, technically feasible and regulatorily acceptable, the Monocyte Activation Test (MAT, Ph. Eur. 2.6.30) is the preferred approach. NanoTox is accordingly planning the structured implementation of MAT capability as part of the continued development of its testing portfolio — under a quality framework at least as robust as the animal method it replaces.

This paper sets out the scientific and regulatory basis for that direction, the correct decision hierarchy, and the caveats that responsible adoption demands.

1. Why Pyrogen Testing Matters

Pyrogens are substances capable of triggering fever and systemic inflammatory responses following parenteral administration or contact with certain medical devices. They are chemically heterogeneous and derive from more than one source:

The critical scientific point is that not all pyrogens are endotoxins. A conventional Bacterial Endotoxins Test (BET) — whether LAL-based (Ph. Eur. 2.6.14) or recombinant Factor C (rFC, 2.6.32) — principally addresses endotoxin-associated risk and does not necessarily capture the full spectrum of NEPs.

Regulatory implication: selection of a pyrogen-testing strategy must follow a scientifically justified risk assessment, not the assumption that any single method is universally sufficient.

2. Why the Rabbit Pyrogen Test Is Being Retired

The RPT contributed materially to pharmaceutical safety for decades. It is, however, an animal-based biological assay, subject to the inherent variability of using another species as the test system, and its sensitivity to endotoxin is markedly lower than that of the BET or MAT.

Modern regulatory science increasingly prioritises methods that are:

• mechanistically relevant to humans;
• reproducible and adequately validated;
• capable of detecting the relevant hazard;
• appropriate for the specific product matrix;
• compatible with contemporary quality systems; and
• consistent with the 3Rs — Replacement, Reduction and Refinement.

The European regulatory shift (validated timeline)

Note on scope: EDQM describes deletion of the RPT from 57 revised texts adopted at the 179th session (a broader exercise touching ~60 texts including general chapters, dosage-form and individual monographs). NanoTox uses the 57-text figure as the primary, EDQM-verifiable reference.

The governing question has therefore changed. It is no longer “Should the rabbit test be performed?” but rather “What is the scientifically appropriate in-vitro strategy for controlling the pyrogenic risk of this particular product?”

3. MAT: A Human-Relevant Approach

The Monocyte Activation Test exploits human monocytes or monocytic cell systems to detect substances capable of activating the human innate immune response. On exposure to pyrogens, monocytes release inflammatory mediators (e.g. IL-1β, IL-6, TNF-α); their quantification provides an in-vitro readout of pyrogenic activity.

Principal scientific advantages:

• Human relevance — the response is generated by the human innate immune system rather than inferred from a surrogate species.
• Broad detection — MAT can, in principle, detect both endotoxin and non-endotoxin pyrogens, distinguishing it from BET.
• Reproducibility — higher sensitivity and lower inter-assay variability than the RPT.

MAT is codified in Ph. Eur. general chapter 2.6.30, with a dedicated chapter for inherently pyrogenic vaccines (2.6.40), and EDQM identifies it as a recommended approach for broad pyrogen detection.

Emerging vs. validated evidence

The scientific base continues to strengthen. Two recent peer-reviewed contributions are relevant to a GMP-oriented laboratory:

• Daniels R, Van der Elst W, So CK, et al. Fit for purpose testing and independent GMP validation of the monocyte activation test. Curr Res Toxicol. 2025;8:100206. doi:10.1016/j.crtox.2024.100206 — demonstrates product-specific, GMP-grade validation within a contemporary quality environment.
• Burgmaier L, van den Berg J, Gajewi M, et al. Validation of the Monocyte Activation Test Demonstrating Equivalence to the Rabbit Pyrogen Test. Int J Mol Sci. 2025;26(22):11136. doi:10.3390/ijms262211136 — reports validation evidence of MAT–RPT equivalence.

Validated vs. emerging: MAT’s status as a compendial method (2.6.30) is validated regulatory knowledge. Claims of universal RPT-equivalence across all matrices remain product- and context-specific and require case-by-case demonstration — they are not yet a generalisable regulatory assertion.

4. MAT Is Not “One Method Fits Every Product”

Transitioning to MAT must not mean substituting one prescriptive test for another without evaluation. Before MAT is applied to a given product, the laboratory must address:

• product-specific interference and cytotoxicity;
• assay sensitivity and applicable detection limits;
• relative endotoxin vs. NEP risk;
• suitability of the selected cellular system (pooled donor cells, cell line, whole blood);
• adequacy of positive and negative controls;
• assay precision and reproducibility;
• product-specific validation/verification (interference testing, spike recovery, and — for devices — direct monocyte contact);
• applicable pharmacopoeial and regulatory expectations.

MAT implementation must itself be scientifically rigorous and appropriately validated. For a GLP/GMP-oriented laboratory the principle is non-negotiable: replacing an animal test must never reduce scientific control; it must introduce a scientifically justified method under an equally robust quality framework.

5. The Correct Decision Hierarchy Is Risk-Based

Under the 5.1.13 framework, MAT serves broad pyrogen detection, while BET may be appropriate where the risk assessment supports bacterial endotoxins as the relevant concern. Where BET alone is selected to address pyrogenicity, that decision must be justified through the risk-based approach in Ph. Eur. 5.1.13.

The scientifically appropriate hierarchy is therefore not simply RPT → MAT. It is:

Pyrogen Risk Assessment

        ↓

Appropriate In-Vitro Method Selection (MAT and/or BET)

        ↓

Product-Specific Suitability / Validation

        ↓

Regulatory Justification & Dossier Alignment

For EU marketing authorisations, method replacement carries specific variation consequences: a move to a pharmacopoeial BET method (2.6.14/2.6.32) is generally handled as a Type IB variation, whereas adoption of pharmacopoeial MAT (2.6.30) or a non-pharmacopoeial in-vitro method is generally a Type II variation (per EMA Q&A). Marketing authorisation holders should remove RPT from dossiers and assess replacement need accordingly.

6. The International Picture Is Not Fully Harmonised

Europe has moved decisively; the global landscape has not converged.

• United States (FDA): the March 2026, Edition 2 of Pyrogen and Endotoxins Testing: Questions and Answers (issued 18 March 2026) updates FDA’s current thinking by reference to revised USP <85>, <161> and AAMI ST72, withdraws the obsolete 1987 LAL guidance, and confirms that alternative approaches may be used where they satisfy applicable statutory and regulatory requirements. It does not, however, prohibit RPT, nor does it (per contemporaneous commentary) explicitly incorporate USP <86> recombinant methods into FDA-recognised standards.
• Medical devices (US): FDA consensus-standard listings continue to reference legacy pyrogen/endotoxin approaches (e.g. USP <151> Pyrogen Test) alongside modern methods.
• WHO: 2025 guidance on the replacement or removal of animal tests for quality control of biological products includes a dedicated pyrogen chapter supporting non-animal methods — reinforcing the global direction of travel.

Precise regulatory framing: it would be inaccurate to state that the RPT has been universally prohibited worldwide. The defensible position is:

The RPT should no longer be regarded as the routine, preferred approach wherever an adequately validated, scientifically suitable and regulatorily acceptable non-animal method can address the relevant pyrogen risk.

7. NanoTox’s Planned Direction

NanoTox supports the progressive adoption of scientifically justified alternatives to animal testing. Our forward approach to pyrogenicity is built on:

1. MAT as the preferred broad pyrogen-testing approach, where appropriate;
2. a structured implementation programme covering method selection, laboratory capability, personnel training, SOP development, qualification/validation, product-specific suitability assessment and quality-system integration;
3. BET (including rFC) deployed where risk assessment identifies endotoxin as the governing concern; and
4. RPT reserved only where a specific scientific, product-related, pharmacopoeial or regulatory requirement demands it — with documented justification.

This reflects both scientific progress and the responsible application of the 3Rs.

8. Moving Forward

The future of pyrogenicity assessment is not merely the removal of an animal test — it is a move toward better, more human-relevant science. A successful transition requires validated methods, appropriate controls, a scientifically justified risk assessment, current regulatory understanding, and careful attention to the characteristics of each product.

The growing regulatory acceptance of MAT shows how human-relevant in-vitro approaches are entering the mainstream of pharmaceutical and medical-product safety assessment.

At NanoTox, the objective is not simply to replace the RPT, but to establish a scientifically robust, validated and regulatory-aligned approach to pyrogenicity testing — with MAT positioned as the preferred direction wherever it is appropriate.

References

1. EDQM. Ph. Eur. bids adieu to the rabbit pyrogen test in its monographs. 2024. (57 revised texts + chapter 5.1.13, adopted 179th EPC session, June 2024; Supplement 11.8, applicable 1 July 2025.)
2. EDQM. Swan song for general animal safety tests / Pyrogen testing 2.0. 2025–2026. (Chapter 2.6.8 suppressed 1 January 2026.)
3. European Pharmacopoeia. General Chapter 5.1.13 — Pyrogenicity.
4. European Pharmacopoeia. General Chapter 2.6.30 — Monocyte-Activation Test (and 2.6.40 for inherently pyrogenic vaccines).
5. EMA. Quality of medicines: Questions and Answers — Part 1 (RPT deletion; variation categories for method replacement). 2026.
6. WHO. Guidelines on the replacement or removal of animal tests for the quality control of biological products. WHO ECBS, 2025.
7. Daniels R, Van der Elst W, So CK, et al. Fit for purpose testing and independent GMP validation of the monocyte activation test. Curr Res Toxicol. 2025;8:100206. doi:10.1016/j.crtox.2024.100206.
8. Burgmaier L, van den Berg J, Gajewi M, et al. Validation of the Monocyte Activation Test Demonstrating Equivalence to the Rabbit Pyrogen Test. Int J Mol Sci. 2025;26(22):11136. doi:10.3390/ijms262211136.
9. U.S. FDA. Pyrogen and Endotoxins Testing: Questions and Answers — Edition 2. Issued 18 March 2026. (References USP <85>, <161>; AAMI ST72; withdraws 1987 LAL guidance.)
10. ISO/TR 21582:2021. Pyrogenicity — Principle and method for pyrogen testing of medical devices.