Topical diseases are becoming increasingly attractive to the pharmaceutical industry as an expanding and profitable sector. Once considered a niche therapeutic segment, today skin diseases collectively outnumber conditions such as obesity, hypertension and cancer. The global market for skin therapy reached $17.1 billion in 2015 and with a five-year compound annual growth rate (CAGR) of 3.6%, it is projected to reach $20.4 billion in 2020. In this article, I present the in vitro skin tissue model and related platform technology with relevance to its application in topical drug screening and development.
Skin, as the largest organ in the human body, serves as the first line of defense against harsh environment and microbial invasion. Prevalence of some of the most common skin diseases is on the rise. With an aging population, incidents of geriatric dermal conditions are also on the increase. Heightened concerns about skin health and appearance are causing more people to seek intervention. This presents an urgent need to develop innovative products and expand those existing for other indications to meet the challenge.
The seven leading therapeutic categories of topical diseases include psoriasis, dermatitis, acne, skin cancer, alopecia, skin infection and rosacea. Any new product targeting these indications must necessarily demonstrate clinical safety and efficacy to be approved by FDA. However, clinical studies are inherently costly and time consuming and must, therefore, be backed by sufficient preclinical data. Thus, it is imperative for drug developers to conduct detailed preclinical studies to screen, identify and validate the lead compound prior to proof-of-concept First-in-Human (FIH) study. Later clinical phases involve formulation optimization, penetration enhancement and innovative topical delivery to provide the final product formulation. Once again, it is essential to validate the new formulations in preclinical studies prior to the expensive clinical trials.
Biologically, skin is a perfect candidate for using in vitro tissue models to facilitate drug development for the following reasons:
- Topical drug products are targeted at skin with intended local action. No systemic absorption and very low bioavailability is expected from these products. This simplicity allows for a tissue model to be a good representative of this system which is less complex than most oral or parental drug products.
- Though skin has its own pharmacokinetic properties, namely absorption, distribution, metabolism and excretion, most of these actions are confined to the local area.
- Skin structure is relatively simple and homogeneous compared to many other tissues and organs. Human skin consists of two layers, epidermis and dermis, separated by a basement membrane. Further, the structure of human skin is relatively uniform throughout the body surface. This allows for the development of robust biomimetic in vitro tissue models with valid controls and high throughput screening application.
- The diverse manifestations of skin diseases originate from the unique physiological and immunological processes confined within the organ, making the tissue model to be a convenient self-contained system with accessible endpoint markers.
- Skin tissue can be cultured and used in experiments which represent real life scenarios. Several 2D and 3D models, representing single layer, bilayer or full thickness skin, have been used as human skin equivalents.
Based on these advantages, it is desirable to use an in vitro skin model to mimic an in vivo disease state for drug product screening and pharmacodynamic studies. Fortunately, the research in in vitro skin tissue model has matured considerably in the last two decades. Models of targeted skin disorders have been developed and validated and are currently commercially available for the pharmaceutical industry.
Psoriatic Skin Model
The leading in vitro skin model which is fully developed and validated for drug screening and formulation development is the psoriatic skin model. Etiology of psoriasis, an autoimmune disease affecting 2-3% population around the world, is largely unknown. Both genetic and environmental factors have been implicated. The disease is characterized by hyperproliferation and abnormal differentiation of keratinocytes in the skin’s epidermal layer. Pathophysiological studies have revealed that psoriasis is a T cell-mediated autoimmune disease. A number of growth factors, cytokines and transcription factors are involved in the disease process, which can serve as biomarkers in the in vitro skin models.
The commercially available psoriatic skin model is supplied by MatTek Corporation. The tissue is a combination of normal human epidermal keratinocytes and psoriatic fibroblasts harvested from psoriatic lesions. The cells are cultured on specially prepared cell culture inserts in serum free medium to form a multilayered, highly differentiated structure. The psoriasis tissues maintain their psoriatic phenotype as evidenced by increased basal cell proliferation, expression of psoriasis-specific markers and elevated release of psoriasis-specific cytokines. Morphologically, the tissue model closely mimics lesional psoriatic human tissues. This model provides researchers with a human-relevant, highly controlled, in vitro model to assess the safety and efficacy of lead therapeutic compounds and to study other basic biological phenomena in psoriasis.
The psoriasis tissue model exhibits characteristic psoriatic phenotype as evidenced by increased expression of psoriasis-associated markers including human β-defensin-2 (HBD2), psoriasin, SKALP/elafin, keratinocyte hyperproliferative cells and proinflammatory cytokines/ chemokines such as IL-6, IL-8, GM-CSF, and IP-10.
A typical in vitro experiment involves the following steps:
- Tissue Preparation: Freshly produced skin tissue (within 48 hours of production) is procured for the experiment
- Dose Application: The testing substrate or formulation is applied on to the tissue along with positive and negative controls
- Gene Expression: RNA is isolated from the tissue at various time points and reverse-transcribed to generate cDNA. qPCR is performed on the cDNA to determine the biomarker level
- Histological Evaluation: The tissue is H&E stained and morphology is evaluated under an optical microscope
Reconstructed Human Epidermis Model
Reconstructed Human Epidermis Model is a ready-to-use, highly differentiated 3D tissue model consisting of normal, human-derived epidermal keratinocytes cultured on specially prepared tissue culture inserts. It is commercially available as EpiDerm® tissue model from MatTek Corporation. The EpiDerm 3D human tissue model is used for a diverse range of applications including safety and risk assessment and biological efficacy. These include:
- Skin Irritation
- Skin corrosion
- Skin hydration
- Percutaneous drug delivery
For instance, EpiDerm 3D tissue model can be used in a traditional Franz Diffusion Cell setup similar to cadaver or surgically harvested skin for percutaneous drug delivery studies. EpiDerm tissue model being lab-cultured tissue, is disease-free, eliminating a concern associated with human skin tissue. With more than 30 years in the market, EpiDerm is a proven, a robust tissue model for drug research and development.
Thus, in vitro tissue model offers an excellent tool for preclinical testing of topical drug candidates and formulations. It is conducive for high-throughput screening. By adopting this model, drug companies can both cut the costs accompanying expensive clinical trials and expedite product development. Convenient access to in vitro technology will also help to conduct pharmacokinetic studies on this platform. This dramatically enhances the chances of identifying lead candidates and ideal formulations.