February 15, 2009 (Vol. 29, No. 4)

StrataGraft, ExpressGraft, and StrataTest Skin Tissues Are All Derived from NIKS Line

While performing basic research on keratinocytes at the University of Wisconsin, Madison, Lynn Allen-Hoffmann, Ph.D., routinely obtained cells discarded from infant circumcision tissue and grew them in monolayer cultures.

In an experiment to look at cell longevity, a small group of cells from one infant stood out because they survived an extraordinarily long time. Later, Dr. Allen-Hoffmann discovered that the cells also grew into three-dimensional, healthy skin. More than a decade later, that same cell line serves as the basis for the technology platform at Stratatech, the tissue-engineering company founded by Dr. Allen-Hoffmann in Madison, WI, eight years ago. 

Dr. Allen-Hoffmann was inspired to pursue tissue engineering while watching a patient with third-degree burns covering 95% of his body, receive painful grafts of his own skin. “That was my epiphany day,” she says, “when I refocused my efforts to develop a cell line that could be clinically useful.”

Stratatech’s mission is to create universal skin grafts that can be used on all patients to heal a variety of wounds. The serendipitous discovery of the immortal skin cell line, which led to the flagship product StrataGraft® skin tissue, gives the company a distinct advantage in the tissue-engineering arena,” Dr. Allen-Hoffmann says.

Last fall, Stratatech presented the results of its first Phase I/II clinical trial of StrataGraft skin tissue. In the study, which included 15 patients with severe skin injuries ranging from burns to flesh-eating bacteria, StrataGraft caused no safety problems and performed as well as cadaver skin, the current standard therapy.

The keystone of Stratatech’s approach is a human skin cell line that forms a fully stratified multilayered human tissue with the physical strength and biological characteristics of intact human skin, according to the company.

First-Generation Therapeutic Product

StrataGraft skin tissue is the first product of its type to be assessed by the Center for Biologics Evaluation and Research at the FDA. Like cadaver skin or donated tissue, StrataGraft skin tissue is applied to cover a wound for one to two weeks until a patient’s own skin can be prepared for a transplant. An alternative to cadaver skin is sorely needed because it is hard to obtain and carries a risk of infection.

Dr. Allen-Hoffmann named the continuous cell line NIKS®, which stands for “near-diploid immortalized keratinocytes that form skin.” A rare mutation makes the cells immortal, and they serve as the source of a perpetual supply of skin tissue. Studies of NIKS cells show that they grow, differentiate, and develop into skin tissue as well as normal human keratinocytes do. This progenitor cell line exhibits the same physical, chemical, and histological properties of skin.

StrataGraft skin tissue is made by plating a mixture of collagen and fibroblasts to form a layer using standard tissue-culture techniques. “We do not use bovine collagen,” Dr. Allen-Hoffmann stresses, thereby eliminating the risk of prion contamination.

Once the collagen gels, the NIKS cells are plated onto the collagen in a proprietary growth medium. As the cells grow, they naturally differentiate into all the normal layers of human skin, including the basal, spinous, granular, and stratum corneum barrier layers. It takes about 15 days to generate biologically active skin, which is then packaged as a 44-cm2 piece of tissue that can be cut to fit wounds.

“The tissue is amazingly strong, and it can be sutured or stapled using traditional surgical techniques,” comments Dr. Allen-Hoffmann.

The application of StrataGraft skin tissue to a wound causes the sustained delivery of natural growth factors, cytokines, and other beneficial substances to promote tissue regeneration and wound healing. StrataGraft skin tissue gives wound-care physicians a durable, easy-to-use product, according to Dr. Allen-Hoffman. The results of the first clinical trial showed that when StrataGraft skin tissue is put on a patient, it maintains its physical integrity and does not disintegrate into the wound as do competing products, she adds.

The continuous NIKS cell line has been thoroughly characterized as virus-free and nontumorigenic, and it offers high batch-to-batch consistency. As a result, the use of StrataGraft skin tissue eliminates the risk of infections from contaminated donor tissue or cadaver skin, Dr. Allen-Hoffman says. StrataGraft skin tissue also should reduce pain, discomfort, and infection, since it closes wounds quickly, she adds.

“We think that this will make significant changes in the outcomes of skin-injury patients,” says Dr. Allen-Hoffmann, who predicts that these benefits should translate into shorter hospital stays and lower costs.

Second-Generation Pipeline

The company’s second-generation product line, called ExpressGraft™, consists of skin tissues that are genetically engineered to express and secrete factors to fight bacterial infections, promote blood flow, or reduce scarring during healing. The first product will target chronic diabetic skin ulcers, which become infected and lead to impaired wound healing and amputation.

Chronic diabetic skin ulcers are typically treated with antibiotics, but the strains of bacteria involved often grow resistant. Stratatech is tackling the problem with human tissues that secrete biologically active antimicrobial peptides designed to stimulate the body’s innate immune system. The Department of Defense is also funding research at Stratatech on a field-ready antimicrobial wound dressing. A ready-to-use antimicrobial skin substitute is needed to close wounds and reduce infections on battlefields.

Another type of engineered skin is designed to heal chronic skin wounds by promoting angiogenesis. Factors that activate the network of genes that promote vascularization have been programmed into NIKS cells with nonviral vectors. The neuropathy associated with diabetes is addressed in a different engineered cell line that stimulates nerve function in the extremities. Patients with skin ulcers often experience tissue breakdown because they lose nerve function and cannot feel damage occurring to the skin.

Stratatech scientists have developed another tissue line that expresses a proteinase inhibitor. Proteolytic activity naturally increases as skin ages, contributing to tissue breakdown and poor wound healing in the elderly. No biological therapies exist that target the underlying proteolytic nature of chronic wounds. Stratatech plans to start clinical trials of some of these new products in 2009. 

In addition to treating patients, StrataTech’s technology platform can be used to screen consumer products. The company sells StrataTest™ skin tissue, a miniaturized version of StrataGraft, to help researchers test potential drug compounds or consumer products like cosmetics. “It provides a biologically relevant, in vitro assay,” explains Dr. Allen-Hoffmann.

Due to a shortage of cultured human cells for pharmaceutical and cosmetic testing, researchers must rely on animal testing or cell lines that poorly duplicate actual human-skin performance. This raises concerns about ethics, accuracy, cost, and reproducibility. “There’s a real need for reliable alternatives to animal testing,” she adds.

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