As described above, engineered skin tissue and cartilage are becoming available for certain medical applications, and strategies to engineer bony tissues are close to receiving FDA approval. We foresee dental applications of these engineered tissues within the next few years. However, reconstruction of complex tissue defects made up of multiple cell types has not yet been attempted in the craniofacial complex, even in preclinical trials. Such a goal (for example, engineering a complete and functional salivary gland) will likely take about 10 to 15 years.
Mineralized tissue defects. Tissue engineering is already being applied to the repair of periodontal defects, with the use of BMPs: the future is now. Indeed, considerable research activity is focused on applying tissue engineering principles to dental and craniofacial structures, probably because of the ease of access to these sites and the extent and nature of the clinical problems. Many problems managed by general dentists or specialists are prime candidates for tissue-engineering solutions, including fractures of bones and teeth, craniofacial skeletal defects, destruction of the pulp-dentin complex and periodontal disease.41
BMPs and other growth-factor–rich preparations are being applied with a variety of natural and synthetic scaffolds. The latter are particularly important considerations for many dental and craniofacial applications. Not only are biologically appropriate scaffolds required for the cells and inductive factors, but the scaffolds should not adversely affect patient appearance. In that regard, an advantage may be gained from polymers that are allowed to flow into a defined site, rather than those that are fixed or implanted. Such polymers are currently being developed by a number of research groups.42,43
Gene therapy. There are several craniofacial examples of using gene therapy (see "Engineering Salivary Gland Function" below). The most substantial body of work uses gene-transfer techniques as either primary or adjunctive therapies for head and neck cancers.44–46 Already several early-stage clinical studies have been conducted. Most of the focus has been on squamous-cell carcinoma, and some incremental progress has been achieved. In general, the cancer gene therapy effort is enormous, representing a large proportion of all gene therapy research. In the next decade, clinicians will likely be able to use gene-transfer technologies as part of their standard treatment of all neoplasias. Gene therapy also may offer a potentially novel approach to the treatment of severe chronic pain. Many studies have shown that genes can be readily transferred to cells in the central nervous system of animal models.47,48 Finegold and colleagues49 recently showed that viral-mediated transfer of the ß-endorphin gene leads to effective analgesia in a rat pain model.
Engineering salivary gland function. Although most work in engineering new organ growth has focused on tissues whose loss or failure will lead to the patient’s death (for example, the liver or endocrine pancreas), there are many circumstances involving tissue loss that are non–life-threatening, yet that markedly affect quality of life. Included in the latter group is the loss of salivary gland parenchyma and, thus, the ability to make saliva. For example, patients who receive ionizing radiation, or IR (> 52 gray) as part of their treatment for head and neck cancer experience irreversible salivary gland damage. In addition, patients with the autoimmune exocrinopathy Sjögren’s syndrome, or SS, also suffer the loss of salivary secretory tissue. Without saliva, these patients experience dysphagia, rampant caries, mucosal infections (for example, candidiasis), dysgeusia and considerable oral discomfort. Many patients receiving IR or those with SS experience complete gland destruction.
About two years ago, we initiated a pilot program to develop an artificial salivary gland for patients with little to no remaining secretory tissue. We have made substantial initial progress, proceeding from fairly rudimentary studies using a natural substratum (denuded trachea) to the use of engineered polymer scaffolds.50,51 These efforts have focused on creating a rather simple device—a "blind-end" tube—suitable to engraft in the buccal mucosa of patients whose salivary parenchyma has been destroyed.52 The lumen of these tubes would be lined with compatible epithelial cells and be physiologically capable of unidirectional water movement (Figure 3; see page 314). We believe that there is a realistic opportunity to develop a first-generation artificial salivary gland suitable for initial clinical testing relatively soon (within about 10 years).
The principal research focus of one of us (B.B.) involves the transfer of genes to salivary glands. Major salivary glands are inviting targets for gene transfer for many reasons, but most important is the ease of access to parenchymal cells. Through cannulation of the orifices of the main excretory duct, the clinician has direct access to almost every epithelial cell in the gland. We began to use gene-transfer methods primarily to develop new treatments for patients undergoing IR and those with SS who had remaining nonsecretory, ductal epithelial cells. Our initial aim was rather simplistic: to make these surviving ductal cells secretory in nature, and thus capable of fluid movement.
We hypothesized that the major impediment to fluid flow from nonsecreting ductal cells was the absence of a pathway for water in their luminal membranes. Our strategy was to transfer the gene for a protein functioning as such a pathway—the water channel aquaporin-1—into radiation-surviving cells via a recombinant adenovirus. The virus, AdhAQP1, was tested in an irradiated rat model. After four months, rats exposed to 21 Gy had an approximate 65 percent reduction in salivary flow (Figure 453). Three days after being given AdhAQP1, these rats experienced an increase in fluid production to near-normal levels. Conversely, when irradiated rats were given a control virus, they exhibited no increase in flow.54
We also have hypothesized that salivary glands may be well-suited for gene therapeutics (using transferred genes as drugs). An obvious application for this concept is to augment saliva with gene products for upper-gastrointestinal, or GI, tract disorders. Salivary secretions bathe the upper-GI tract mucosa continuously, and we envision both prophylactic and therapeutic applications. An alternative strategy is to direct needed therapeutic proteins into the bloodstream (that is, in an endocrine direction for systemic use).
Using rodent models, we showed that both applications are possible in principle. For example, we examined the potentially life-threatening condition of azole-resistant mucosal candidiasis in immunosuppressed patients as a possible target disorder. After performing adenoviral-mediated gene transfer, we were able to express the human anticandidal peptide histatin 3 in rat salivary glands and show that the recombinant histatin 3 could kill fluconazole-resistant Candida albicans.55 We also showed that after adenoviral-mediated gene transfer, both human alpha-1-antitrypsin and human growth hormone could be secreted into the bloodstream from rat salivary glands.56,57