Jane C. Atkinson, D.D.S., and Bruce J. Baum:
Saliva provides the principal protective milieu for the teeth, and patients with significantly decreased salivary output have an increased prevalence of dental caries.
Therefore, therapies that increase the overall fluid output of these individuals are believed to have the potential of reversing early carious lesions. Although many systemic diseases are associated with alterations in salivary output, the most pronounced salivary dysfunction occurs in patients with Sj�gren�s syndrome, patients who have received therapeutic radiation to the head and neck, and patients taking medications that interfere with salivary secretory processes.
Salivary hypofunction secondary to medication is by far the most common cause of salivary dysfunction. Medications often inhibit cholinergic signaling pathways in salivary tissues, and thereby decrease the fluid output of the gland. Interference in other peripheral and central signaling pathways can also reduce salivary output and alter salivary composition. While 300 to 400 medications are believed to interfere with salivary secretion, the specific inhibitory mechanisms are defined for only small subsets of drugs. The impact of prolonged anticholinergic medication on salivary tissues still requires definition. The most practical and common method for treatment is to work with the patient�s primary care physician to either alter the medication to a less xerogenic type or reduce the dose while maintaining the required therapeutic effect.
Salivary hypofunction after gland irradiation is very difficult to treat because salivary parenchyma within the radiation field are permanently damaged. Similarly, clusters of infiltrating lymphocytes replace the salivary parenchyma of patients with advanced Sj�gren�s syndrome. Both conditions are reasonably common in the United States. Head and neck cancer affects 30,000 to 40,000 new patients each year, most of whom are treated with therapeutic irradiation. These patients are typically middle-aged males, and often are individuals from economically disadvantaged backgrounds. Sj�gren�s syndrome affects about 1 million persons in the United States, currently estimated to reflect a 9:1, female:male ratio. In most studies, the mean age at diagnosis is between 40 and 50.
Both irradiation and Sj�gren�s syndrome lead to the loss of salivary acinar cells, the only cell type in the glands that is capable of fluid movement. Both conditions exhibit considerable heterogeneity. Some patients experience minimal parenchymal cell loss, while others may have no epithelial tissue surviving, with glands entirely replaced by nonsecretory tissue (e.g., connective tissue, inflammatory cells). Patients with remaining functional acinar tissue can be treated pharmacologically, using a parasypathomimetic secretogogue.
The first such drug approved in the United States was pilocarpine, marketed as Salagen. Pilocarpine possesses both modest, relatively nonspecific muscarinic agonist activity as well as weak b-adrenergic agonist activity. Its effectiveness in increasing salivary output has been demonstrated in several clinical studies of patients with radiation-induced salivary hypofunction or Sj�gren�s syndrome. Recently, a second secretogogue for such patients, Cevimeline, was approved for use by the U.S. Food and Drug Administration. Cevimeline is a more specific drug, with a preference for activation of the primary muscarinic receptor subtype responsible for fluid flow from salivary glands, the so-called M3 receptor. However, this medication has not been tested in clinical trials as extensively as pilocarpine.
Radiation damage to salivary glands can be limited by preradiation planning (conformal and static multisegmental intensity modulated technique) that spares as much salivary tissue as possible. Use of the oxygen radical scavenger amifostine during radiation treatment may also decrease damage to glands. Other investigators are surgically repositioning submandibular salivary glands to the submental space before radiation to maintain gland function. While several anti-inflammatory medications have been tested for the treatment of Sj�gren�s syndrome, only alpha interferon treatment has been shown to increase salivary output.
For patients with more extensive gland damage there is currently no conventional therapy to enhance salivary secretion. This circumstance provided the impetus ~10 years ago for the application of gene transfer technology to repair irradiation- or autoimmune-damaged salivary glands. The initial goal of these studies was to re-engineer the function of the surviving nonfluid secreting ductal cells in damaged glands to a secretory phenotype.
The first peer-reviewed publication on gene transfer to salivary glands was published in 1994. Since then, several laboratories have reported that gene transfer to salivary glands can readily be accomplished. Most of these studies have utilized viral vectors to mediate gene transfer. Viral vectors can be extremely efficient in transferring genes, but can pose a safety risk. An alternative means of gene transfer is to use nonviral methods. Perhaps the most successful form of nonviral gene transfer involves the use of cationic liposomes. This method is much less efficient than preferred viral vectors, but poses relatively little safety risk.
In 1997, a study reported by Delporte and colleagues described the "correction" of irradiation-induced salivary hypofunction in rats through transfer of the cDNA encoding aquaporin 1, a mammalian water channel (permeability pathway). Gene transfer was accomplished using a replication-deficient, first generation, recombinant adenovirus. Irradiated rats administered a control adenovirus exhibited salivary flow rates ~65 percent lower than sham-irradiated animals. Conversely, when animals were administered the aquaporin 1-encoding adenovirus 4 months after irradiation, salivary flow rates were indistinguishable from control levels at 3 days postadministration. This approach is currently being tested in large animal studies.
Thus, the specific value of aquaporin 1 gene transfer for irradiated salivary glands must be considered speculative and not ready for clinical testing. It is not known whether insertion of a water channel into the surviving ductal cells will lead to correction of glandular hypofunction. However, gene transfer without question can be readily accomplished in vivo in salivary glands and is potentially of considerable clinical value to enhance salivary secretions. If aquaporin 1 cannot be used as a transgene for repair of damaged glands, physiological studies will doubtless lead to a better choice.
Gene transfer can also be utilized to augment salivary secretions, such as the transfer of a the gene for a secretory protein that will be secreted in an exocrine manner. The proof of concept for this possibility has been shown in animal studies through transfer of the human histatin 3 cDNA in rat submandibular glands. Histatin 3, which normally is not secreted in rodent saliva, was secreted at high levels (up to 1 mg/ml) after gene transfer. DNA vaccination is another potential clinical use for salivary glands as a gene transfer target site to enhance saliva. For example, Kawabata and colleagues (1999) showed that delivery of the cDNA for the P. gingivalis fimbrial protein into murine salivary glands led to the production of secretory immunoglobulin A directed at this microbial protein.
Gene transfer to repair damaged glands can only be an option if epithelial tissue survives either irradiation or autoimmune damage. If the gland is fully replaced by fibrotic tissue, gene transfer cannot lead to an enhancement of saliva production, since no system exists to produce and transport fluid into the mouth. To address this circumstance, we recently began to develop an artificial salivary gland using well-established principles of tissue engineering in combination with genetic engineering. The prototype design includes a biodegradable substratum shaped as a blind end tube (i.e., like a test tube) coated with a layer of purified extracellular matrix proteins involved in cellular organization, followed by a monolayer lining of polarized epithelial cells capable of unidirectional fluid secretion. Initial feasibility studies have been reported. Given the success of other groups in developing functional, fluid-secreting bioartificial organs, notably the bladder, it is reasonable to expect that an artificial salivary gland suitable for clinical testing will be developed within the next decade.
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