Overcoming the LER Challenge: Mechanistic Insights and Mitigation Strategies for LER

1. Introduction

Endotoxins (lipopolysaccharides, LPS) are major components of the outer membrane of Gram-negative bacteria, capable of inducing strong pyrogenic responses and systemic inflammation in humans. The BET, based on recombinant Factor C (rFC) reagents or Limulus amebocyte lysate (LAL), is the gold standard for endotoxin detection in parenteral drugs, biologics, and medical devices, as specified in pharmacopeias (USP <85>, USP<86>, EP 2.6.14, JP 4.01). However, the Low Endotoxin Recovery (LER) phenomenon has challenged the reliability of BET, particularly in complex formulations.

LER was first systematically reported by Chen and Vinther in 2013, describing the time-dependent loss of detectable spiked endotoxin activity in certain biologic products. Unlike traditional BET interference (inhibition/enhancement), which can be overcome by dilution, LER is dilution-independent and time-dependent, making it difficult to identify and mitigate. Regulatory authorities, including the U.S. FDA, have emphasized the need for LER hold-time studies to ensure endotoxin detectability in drug products during Biologics License Applications (BLA).

The occurrence of LER poses significant risks: undetected endotoxin contamination may lead to pyrogenic reactions, sepsis, or other adverse events in patients. Previous studies have linked LER to specific formulation components, but the consistency of mitigation strategies remains limited. In this study, we aimed to: (1) summarize the core mechanisms and key inducing factors of LER based on existing literature; (2) verify LER occurrence in aqueous systems; (3) evaluate the mitigation effect of LER buffer; and (4) provide a practical solution for reliable endotoxin detection in LER-prone matrices.

2. Background and Mechanisms of LER

2.1 Definition of LER

LER is formally defined as the failure to recover ≥50% of the spiked endotoxin activity in undiluted drug products or formulations over a specified hold time, even when the BET method is validated to be free of interference. Two consecutive time points with recovery <50% are considered definitive evidence of LER. Unlike BET interference, which affects the LAL-endotoxin reaction directly, LER involves structural alterations of LPS, rendering it undetectable by LAL/rFC reagents.

2.2 Key Inducing Factors of LER

Based on comprehensive literature review (PDA Technical Report No.82, case studies, and pharmacopeial data), the main factors inducing LER are as follows:

• Chelating Agents

Chelating agents are widely used in pharmaceutical formulations as stabilizers or pH adjusters, including citrate, EDTA, and phosphate. These compounds form complexes with divalent cations (Mg²⁺, Ca²⁺) that are critical for maintaining the stability of LPS aggregates. Among them, citrate and EDTA are the most potent LER inducers: citrate concentrations of 5–25 mM and EDTA at 0.1 mM have been shown to trigger significant LER. Phosphate buffers also induce LER but to a lesser extent than citrate, as they compete with LPS for divalent cations rather than forming stable chelates.

• Nonionic Surfactants

Nonionic surfactants such as polysorbate 20 (Tween 20) and polysorbate 80 (Tween 80) are commonly used to prevent protein aggregation in biologic formulations. These surfactants exacerbate LER by intercalating into LPS aggregates, disrupting their supramolecular structure and forming mixed micelles. Tween 20 is more potent than Tween 80 due to its higher diffusivity and shorter fatty acid chain length, which better matches the acyl chains of E. coli LPS lipid A. Concentrations of Tween 20 as low as 0.006% (w/v) can induce LER in the presence of monoclonal antibodies.

2.3 Mechanism of LER

The widely accepted mechanism of LER involves a two-step reaction model for formulations containing chelators and surfactants:

• Destabilization of LPS aggregate: Chelating agents (e.g., citrate) bind to divalent cations, breaking the salt bridges that stabilize LPS aggregates. This reduces the rigidity of LPS supramolecular structures, making them susceptible to further modification.

• Alteration of aggregation state: Surfactants (e.g., Tween 20) intercalate into destabilized LPS aggregates, forming mixed micelles or lamellar structures. This structural rearrangement masks the lipid A moiety of LPS, preventing its interaction with LAL/rFC reagents and reducing detectable endotoxin activity.

3. Case study

(1) Effective Elimination of Interference from EDTA, Tween 20, and Tween 80

When the sample matrix contains EDTA at a concentration of ≥ 0.1 mM, Tween 20 at a concentration of ≥ 0.01 %, and Tween 20 at a concentration of ≥ 0.05 %, it will cause interference with the assay. Dilution with endotoxin-free assay water fails to eliminate this interference, resulting in spiked recoveries falling outside the range of 50–200%.In contrast, the use of LER Sample Dilution Buffer can effectively eliminate such interference and ensure that the spiked recoveries comply with the requirements of the Pharmacopoeia.

Effect of EDTA (A), Tween 20 (B) and Tween 80 (C) Concentration on Spiked Recovery Rates Using Endotoxin-Free Water vs. LER Sample Dilution Buffer

(2) Effective Mitigation of Time-Dependent Effects in LER-Affected Samples

Previous studies have indicated that interference from LER samples increases over time. To simulate this real-world application scenario, matrices containing EDTA, Tween 20, and Tween 80 at varying concentrations were pre-treated with LER Sample Dilution Buffer and 5 EU/mL endotoxin standard 24 hours prior to testing, while another group was treated with the same reagents at the time of testing. The results showed that the detected values were consistent with the theoretical concentrations, and the spiked recoveries remained within the normal range.

The LER sample dilution buffer maintains stable spike recovery in samples containing EDTA (A), Tween 20 (B), and Tween 80 (C) even after overnight incubation.

4. Conclusion

5. Experience rFC Endotoxin Testing in Practice

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6. Endotoxin Detection Resource Series

A Comprehensive Multi-Sample Comparability Study of LAL and rFC Endotoxin Testing Methods

Standards and Regulations for Recombinant Factor C Endotoxin Testing from a Pharmacopoeial Perspective

Interfering Factors in Endotoxin Testing and an In-Depth Analysis of rFC Technology