Dermal Permeability and Toxicity Testing

Historically, the skin was considered to be simply as a physical barrier; although one that could sometimes be employed as part of a reformulation strategy for potentially toxic drugs in a topical delivery system or for the treatment of topical diseases such as Eczema, Allergic Contact Dermatitis, Psoriasis, Allergic Dermatitis, Acne, Rosacea, Fungus, Pruritis, and scalp conditions such as dandruff.

More recent research on skin brought the appreciation of the skin as a very complex organ. This also brought growing concerns from occupational health and regulatory authorities about topical skin reactions to chemicals and topical treatments. The 3R's animal protection principles (Refine, Reduce, Replace) are spurring industry to change the approach to testing for skin irritation and transdermal delivery models, bringing emphasis to the use of the in vitro methods in which Apredica specializes.

Contact us to discuss your dermal testing needs.

Dermal Assay Services

Apredica's dermal assays are performed using MatTek's in vitro 3-D human epithelial tissue equivalents equivalents, for which Apredica is a MatTek Qualified Lab. Human skin assays available through Apredica include:

• Skin Irritation Test: ECVAM-validated skin irritation test method predicts skin irritation of test substances.
• Dermal Absorption (percutaneous absorption): Similar in design to a Caco-2 assay, dermal absorption measures the permeability of a test agent across a dermal barrier.
• Phototoxicity (photoirritation): ECVAM-validated in vitro phototoxicity method to determine the acute toxicity of test agents exposed to skin and subsequent light exposure.

Related Services

• Caco-2
• MDCK
• Cytotoxicity
• Phototoxicity with immortalized cells (3T3 or HaCaT)
• Eye irritation
• Nasal/lung absorption (EpiAirway^®)
• Buccal Absorption (EpiOral^®)

Customization

Common issues associated with dermal testing for which customized approaches can be created:

• The test compound is highly insoluble
• Need to test in cell model not commonly available

Ask us how we might adapt skin culture assays to your special situation.

Dermal Assays - Purpose and Place

Human skin cultures are becoming increasing popular replacements for certain types of animal testing, for several reasons:

• Cost: Skin culture assays are typically more cost-effective and higher throughput than animals.
• Regulatory: In the EU, there is a strong initiative to replace animal testing with in vitro human equivalents, where available. This is especially relevant to the cosmetics industry whose testing is now constrained by the EU's REACH requirements.
• Species comparison: Skin cells are metabolically active, and are assumed to be the primary metabolic organ for test compounds administered topically. Like most metabolism, human skin CYP-450 enzymes differ from those of animals.

Principle of Dermal Absorption Assays

Dermal cultures are grown to confluence and differentiated for three weeks on filters. Test agent is added to one side of the monolayer, and permeability is assessed by analyzing the concentration of the test agent on the other side of the monolayer using LC/MS/MS.

Principle of Dermal Irritation Assays

Dermal cultures are grown to confluence and differentiated for three weeks on filters. Test agent is applied to the surface of the culture and incubated for various amounts of time. Irritation is assessed by MTT cell viability. Detection of IL-1 and other cytokines released as an inflammatory response can be performed by ELISA and indicates inflammation.

Principle of Phototoxicty Assays

Epiderm^® tissues, or the immortalized cell lines mouse Balb/c 3T3, or human HaCaT are grown to each one's specifications. The cells or tissues are treated with compound and either exposed or not exposed to UVA and incubated further. The tissues or cells are then treated with MTT and cell viability is measured. Lower viabilities in the UVA treated cells or tissue suggests a test agent is a phototoxin.

Background and Validation of Dermal Assays

MatTek's dermal models have been extensively validated. See MatTek's research.

Dermal Drug Delivery: Challenges and Opportunities

The criteria for selection of topical drug candidates are quite different from those for oral or intravenous therapeutics. While of course potency remains critical, solubility and protein binding are much less important. Understanding metabolism in the skin is very important and should be done as soon as practical. In terms of DMPK, which systemic half-life is unimportant, topical half-life is very important. Contact us to learn about the skin metabolism assays we can design to meet your program's needs.

Understanding non-UV absorption is critical. Safety and toxicology barriers should be observed, like for other compounds. But topical therapeutics have additional safety challenges. They need to be non-skin irritant, non-eye irritant (if applied on the face), non-genotoxic, non-carcinogenic, non-photocarcinogenic, non-skin sensitising, non-skin allergic, and non-phototoxic. Contact us to discuss the issues related to your topical therapeutics program.

This chart summarizes the typical research strategy used for dermal products and topical drugs.