Current Approaches to Skin Sensitization Assessment: OECD TG 406, TG 429, and ISO 10993-10:2021
Allergic contact dermatitis is an immune-mediated delayed-type hypersensitivity reaction that develops in response to chemicals and medical devices. Determining a substance’s potential to cause skin sensitization (allergy) is is a critical step in the safety assessment of chemicals, cosmetic ingredients, pesticides, biocidal products, and medical devices. Sensitization differs from other types of local toxicity due to dose-response relationships and immune memory; an individual who has become sensitized may develop a severe reaction even upon re-exposure to very low concentrations. Therefore, the selection of the appropriate test method is of vital importance for both regulatory compliance and user safety. Our laboratory adheres to the three fundamental international standards in this field — OECD TG 406 (GPMT/Buehler), OECD TG 429 (LLNA), and ISO 10993-10:2021 — in accordance with GLP and GMP principles, using the most up-to-date scientific approaches.
OECD TG 406: Traditional In Vivo Sensitization Tests – GPMT and Buehler Methods
OECD TG 406 is a classic in vivo test guideline used to evaluate the skin sensitization potential of chemical substances. Two different methods are defined within this guideline: Guinea Pig Maximization Test (GPMT) and Buehler Test. Both methods use the guinea pig model, and the basic principle involves evaluating the allergic response through a challenge (provocation) application after a specific period following the induction (sensitization) phase with the test substance.
The GPMT is a highly sensitive method that uses an adjuvant (Freund’s Complete Adjuvant – FCA) to boost the immune system, thereby enabling the detection of even substances with low sensitization potential. Induction is achieved through a combination of intradermal injection and topical application. The Buehler Test, on the other hand, does not use an adjuvant; induction is performed solely through topical application. Therefore, the Buehler Test models human exposure scenarios more realistically but has lower sensitivity compared to GPMT. While the Buehler Test is preferred for topical drugs and cosmetic ingredients, the GPMT is more suitable for industrial chemicals and pesticides. In both methods, evaluation is performed by grading skin reactions (erythema and edema) at 24 and 48 hours after the challenge.
OECD TG 429: Local Lymph Node Assay (LLNA) – A Modern and Quantitative Approach
OECD TG 429 is a modern alternative using a mouse model that offers significant advantages over guinea pig tests in the assessment of skin sensitization. The Local Lymph Node Assay (LLNA) is the first validated in vivo method that measures the induction phase of sensitization and expresses the results quantitatively. The basic principle of the test involves the topical application of the test substance to the mouse ear skin, followed by the measurement of lymphocyte proliferation in regional (auricular) lymph nodes using radioactively labeled thymidine (³H-methyl thymidine).
The most significant advantages of the LLNA compared to the GPMT and Buehler methods are as follows:
- Quantitative result: The sensitization potential is expressed numerically by calculating the Stimulation Index (SI).
- EC3 value: The effective concentration of the test substance that causes 3% stimulation can be calculated; this value allows for a comparative assessment of sensitization potential.
- Fewer animals and shorter duration: While the GPMT requires 20–30 guinea pigs and 4–6 weeks, the LLNA requires only 16–20 mice and 1–2 weeks.
- 3R compliance: The LLNA better aligns with the principles of Reduction (fewer animals) and Refinement (less invasive procedures).
- Objective evaluation: Since it is based on radioactivity measurement rather than visual scoring, subjective judgments are minimized.
The OECD currently recommends the LLNA (TG 429) as the primary method for assessing skin sensitization to chemical substances; it positions guinea pig tests (TG 406) as a backup method only for specific cases where the LLNA cannot be applied.
ISO 10993-10:2021: Evaluation of Skin Sensitization for Medical Devices
ISO 10993-10:2021 is the international standard defining skin sensitization tests within the scope of biocompatibility assessment for medical devices. This standard includes test methods that use extracts obtained from the device or the device material itself to evaluate the potential for allergic contact dermatitis posed by medical devices and biomaterials. ISO 10993-10 is a comprehensive standard that covers not only sensitization but also skin irritation testing; with the 2021 revision, these two areas of evaluation have been addressed in detail.
Three primary methods are recommended for skin sensitization assessment under ISO 10993-10:2021:
- Guinea Pig Maximization Test (GPMT): Preferred in situations requiring high sensitivity; particularly suitable for permanent implants and devices with prolonged skin contact.
- Buehler Test (Closed Patch Test): An adjuvant-free method using topical application for induction; particularly suitable for surface devices and products with short-term contact.
- Local Lymph Node Assay (LLNA): A quantitative alternative using a mouse model; officially accepted for medical devices in accordance with ISO 10993-10:2021.
The most significant difference between ISO 10993-10:2021 and previous versions is its adoption of a risk-based approach to test method selection and its reference to in vitro/ in chemico alternative methods (e.g., methods covered by OECD TG 442C, TG 442D, and TG 442E). The standard states that, in certain cases, a sensitization assessment can be completed without animal testing, based on chemical characterization data and existing literature.
The table below presents a comparative analysis of the three main guidelines:
| Feature | OECD TG 406 – GPMT | OECD TG 406 – Buehler | OECD TG 429 – LLNA | ISO 10993-10:2021 |
|---|---|---|---|---|
| Test System | Guinea pig (Hartley or Dunkin-Hartley) | Guinea pig (Hartley or Dunkin-Hartley) | Mouse (CBA/Ca or C57BL/6) | Guinea pig or mouse (with device extract/material) |
| Use of Adjuvant | Yes (Freund’s Complete Adjuvant – FCA) | No | No (no adjuvant required) | Method-dependent (yes in GPMT, no in Buehler, no in LLNA) |
| Induction Route | Intradermal injection + topical application | Topical application only (occlusive patch) | Topical application (ear skin) | Application of the device extract or material via an appropriate route |
| Test Duration | 4–6 weeks | 4–6 weeks | 1–2 weeks | Depends on the method (1–6 weeks) |
| Number of Animals | 20–30 (test + control) | 20–30 (test + control) | 16–20 (test + control) | Depends on the method (16–30) |
| Result Type | Qualitative (positive/negative), skin reaction scoring | Qualitative (positive/negative), skin reaction scoring | Quantitative (SI – Stimulation Index, EC3 value) | Qualitative or quantitative (method-dependent) |
| Evaluation Time | 24 and 48 hours after challenge | 24 and 48 hours after challenge | 5 days after the last application (lymph node analysis) | Method-dependent |
| 3R Compliance | Low-Medium | Medium | High (Reduction and Refinement) | Medium-High (with LLNA option) |
| Regulatory Use | REACH, CLP, EPA FIFRA, pesticides and biocidal products | Topical drugs, cosmetic ingredients | REACH, CLP, EPA – recommended as the primary method | EU MDR (2017/745), ISO 10993-1, FDA 510(k), medical device biocompatibility |
| Sensitivity | High (detects even weak sensitizers) | Moderate (better models human exposure) | High (comparable to GPMT) | Method-dependent |
Test Method Selection Guide
The table below will help you select the appropriate skin sensitization test based on your product type and regulatory objective:
| Product Type / Requirement | Recommended Method | Rationale |
|---|---|---|
| Industrial chemicals, pesticides, biocidal products (under REACH/CLP) | OECD TG 429 (LLNA) | OECD primary method; quantitative data (EC3), fewer animals, shorter duration, ideal for REACH. |
| Topical drug formulation, cosmetic ingredient (non-EU regulations) | OECD TG 406 (Buehler) | Topical application without an adjuvant more realistically models human exposure scenarios. |
| Substances suspected of being weak sensitizers, when maximum sensitivity is required | OECD TG 406 (GPMT) | Highest detection capacity thanks to the use of an adjuvant; capable of detecting low-potential sensitizers. |
| Situations where LLNA cannot be applied (e.g., if radioactive labeling is not possible) | OECD TG 406 (GPMT or Buehler) | In specific situations where LLNA alternatives are not suitable, guinea pig tests serve as a backup method. |
| Medical device – permanent implant, long-term skin contact | ISO 10993-10:2021 (GPMT) | High sensitivity provides the best assessment of the potential sensitization risk posed by permanent devices. |
| Medical device – surface device, short-term contact | ISO 10993-10:2021 (Buehler or LLNA) | The topical application or quantitative LLNA option is sufficient for devices with short-term contact and is 3R-compliant. |
| Medical device – mandatory biocompatibility under the EU MDR (2017/745) | ISO 10993-10:2021 | The MDR references the ISO 10993 series for the biocompatibility assessment of medical devices; the sensitization test is mandatory for all devices that come into contact with the skin. |
Modern Alternatives: In Vitro and In Chemico Methods
Today, there are in vitro and in chemico methods—developed as alternatives to animal testing for skin sensitization assessment and validated by the OECD—available. Our laboratory offers these alternative methods as part of its testing portfolio:
| OECD Guidance | Method | Stage of Sensitization Measured | Test System |
|---|---|---|---|
| TG 442C | Direct Peptide Reactivity Assay (DPRA) | Key step 1: Protein binding (haptenation) | Chemical (in chemico) – synthetic peptides |
| TG 442D | KeratinoSens™ / ARE-Nrf2 Luciferase Assay | Key step 2: Keratinocyte activation | In vitro – human keratinocyte cell line |
| TG 442E | h-CLAT / U-SENS™ / IL-8 Luc Assay | Key Step 3: Dendritic cell activation | In vitro – human monocyte/dendritic cell-like cell lines |
Since these alternative methods can only assess a single stage of sensitization on their own, they provide the highest accuracy when used in combination within the framework of the OECD’s Defined Approach (DA). Since animal testing is prohibited in the EU, particularly for cosmetic products, these in vitro/ in chemico methods are the only mandatory option. By applying these modern alternatives, our laboratory is able to provide sensitization assessments for cosmetic and personal care products entirely without the use of animals.
Conducting Skin Sensitization Tests in Our Laboratory
In our GLP- and GMP-certified laboratory, we conduct skin sensitization tests in full compliance with international standards. Our experienced team of toxicologists and veterinary pathologists evaluates your product’s physicochemical properties, chemical structure, and regulatory requirements before each study to recommend the most appropriate testing strategy. We have the infrastructure for radioactive labeling for LLNA and fully implement GPMT and Buehler tests. For medical devices, in accordance with ISO 10993-10:2021, tests are conducted using extracts prepared from your device in compliance with ISO 10993-12, and the results are integrated into your biocompatibility assessment dossier. Furthermore, by offering in vitro/in chemico alternatives that do not require the use of animals, we demonstrate the utmost commitment to the 3R principles.
Frequently Asked Questions
| Question | Answer |
|---|---|
| Can the LLNA always replace the GPMT and the Buehler test? | In most cases, yes. The OECD recommends the LLNA as the primary method and positions guinea pig tests only as a backup for situations where the LLNA cannot be applied. However, some regulatory authorities may require the GPMT for specific product types. |
| Which sensitization test is mandatory for cosmetic products? | Under the EU Cosmetics Regulation (EC 1223/2009), animal testing for cosmetic products and their ingredients is completely prohibited. Therefore, the use of in vitro/in chemico methods covered by OECD TG 442C, 442D, and 442E, in conjunction with the Defined Approach (DA), is mandatory. |
| Under what circumstances is a sensitization test required for medical devices? | In accordance with ISO 10993-1, a sensitization assessment is required for all medical devices that come into contact with the skin. Under the EU MDR (2017/745), this data is required in the biocompatibility dossier depending on the device’s class and duration of contact. |
| Can LLNA be used under ISO 10993-10:2021? | Yes. ISO 10993-10:2021 officially recognizes GPMT, Buehler, and LLNA for skin sensitization assessment. LLNA is preferred, especially in cases where compliance with the 3R principles is a priority. |
| How long does it take to receive test results? | LLNA: 3–4 weeks (including reporting). GPMT/Buehler: 6–8 weeks (including reporting). In vitro alternatives: 2–3 weeks. The total duration may vary depending on the workload and quality assurance review. |
| If a substance is to be evaluated both as a chemical and as a medical device component, which test should be performed? | Since the regulatory contexts differ, both evaluations may be required separately. Testing in accordance with OECD TG 429 or TG 406 is appropriate for chemical classification; testing under ISO 10993-10:2021 is appropriate for medical device biocompatibility. Our laboratory offers integrated solutions to meet both of these needs. |
Skin sensitization is a toxicological endpoint that directly affects user safety and cannot be overlooked. Accurately determining the risk of allergic reactions using scientific methods is a critical factor in establishing your product’s reliability and market acceptance. Our laboratory conducts all skin sensitization tests covered by OECD TG 406, OECD TG 429, and ISO 10993-10:2021 in accordance with GLP standards and with the utmost adherence to the 3R principles. To work with us on planning the most appropriate sensitization assessment strategy for your chemical substance or medical device, contact us.
