By the late 1980s, NIDCR scientist Dr. Bruce Baum was frustrated. He had been searching for new drugs and other treatments that might help restore adequate salivary flow in people whose salivary glands had been damaged by radiation treatment for cancer. Yet, despite all of his hard work, Baum said he had not come close to solving the problem. That's when he decided to turn to gene transfer, sometimes called gene therapy. If a fluid-transporting gene could be transferred into the damaged glands, he could potentially restore some degree of salivary flow and secretion into the mouth. According to Baum, the big question was: How would he deliver a gene into a salivary gland? The answer came to him several weeks later when he realized it might be possible to put a gene and its viral vector into a syringe and infuse them directly into the gland through its opening in the mouth. After more than a decade of systematically working out the science of gene transfer to the salivary glands, Baum and colleagues are ready to move the science into the clinic. The Inside Scoop recently met with Dr. Baum and talked to him about dry mouth in recovering cancer patients and the design of his upcoming clinical trial.
Nearly all patients with head and neck cancer have their tumors irradiated for several weeks. As I understand it, the problem is radiation doesn't discriminate between tumor cells and healthy salivary glands. Is that correct?
That’s essentially right. The radiation kills the tumor cells but unfortunately also affects the nearby salivary glands. The damaged glands become less permeable to the water that naturally flows through them and either will yield less saliva or stop working altogether. If the damage is extensive, cancer patients will notice a persistent and irritating dryness in their mouths called radiation-induced xerostomia, or, more colloquially, dry mouth. It’s a very common problem. A survey of head and neck cancer patients comes to mind that was published a few years ago. It found that 65 percent of long-term survivors - defined as living three years post radiation therapy - had moderate to severe xerostomia. 1
Why is this parched sensation so irritating?
There’s an old Blues song titled, “You don’t miss your water til the well runs dry.” That’s certainly true with saliva. Most people don’t think of it until the moisture in their mouths runs dry, and that’s what makes xerostomia so irritating. Recovering cancer patients wonder, “Why is my mouth so dry?” But the dryness is more than an irritation. It can be a significant problem that impairs a person’s ability to chew, swallow, and even speak. In addition, dry mouth can lead to oral infections, such as tooth decay and Candidiasis.
Can anything be done to increase salivary flow?
For some people, yes. If tissue remains that is water permeable and capable of secretion, compounds such as pilocarpine can stimulate the glands to produce more saliva. The problem is most patients have salivary glands that have stopped working.
The radiation leaves the glands water impermeable. Let me explain. Salivary glands kind of look like a bunch of grapes attached to a stem. The grapes are called acinar cells. In people with working salivary glands, water enters the acinar cells, where myriad proteins are added, and the mixture then flows through the stems for further processing and ultimately exit into the mouth as saliva. In most head and neck cancer patients, the radiation has wiped out the acinar cells. They’re left with a network of water-impermeable stems that have no salivary flow. There is nothing left to stimulate.
And that's where salivary gene transfer is different than pilocarpine. It will for the first time help those with impermeable salivary glands.
That’s our hope. The gene transfer that we have developed builds on the fundamental fact that the stems, or ducts, are water impermeable in the irradiated glands. We think that with gene transfer, the ducts will have the potential to move fluid and secrete it into the mouth.
The answer, as I've heard you say, is osmosis.
Correct, it’s that concept everybody learns in junior high school. Water naturally follows an osmotic energy gradient, flowing from areas of low to high salt concentration. We believe the irradiated duct cells can generate such an osmotic gradient. Over the last decade or so, I’ve worked with scientists in our laboratory to determine whether transferring a gene into the cells that line the ducts can take advantage of the ability of these cells to generate an osmotic gradient and secrete saliva into the mouth. In August 2006, we received approval from the Food and Drug Administration to conduct the first gene-transfer study in people with radiation-induced xerostomia.
Which gene will be transferred?
The gene is Aquaporin-1. It encodes a large protein that transports fluid by forming pores, or water channels, in the cell membrane. Our hope is the transferred gene will produce reasonable levels of the aquaporin-1 protein in duct cells, setting in motion a therapeutic domino effect. That is, the aquaporin-1 protein will open up water channels in the duct cells. That will allow the very rapid movement of water through the duct in response to the osmotic gradient that we believe can be generated.
The gene is contained in a vector, which serves almost like a trojan horse to deliver aquaporin-1 to its target. Which vector will you use?
An adenovirus. It’s a common respiratory virus that has been used in gene transfer studies for many years. The virus has been modified and won’t make a person sick.
Will the first study be a Phase I investigation, meaning it will evaluate the safety and tolerability of the gene transfer in the participants?
It will be simultaneously a Phase I and II study, which is quite commonly done today. As part of the Phase II component, we will collect some efficacy data. By that, I mean we will measure whether salivary fluid output improves in the patients and, in turn, if they notice that their mouths feel moister and less dry.
How large will the study be?
The study will enroll 15 to 21 patients.
All local participants?
Well, in theory, they could live anywhere. But the patients will stay in the hospital here in Bethesda at least for the first three days. They must be back here on Day 7 and 14. If people were to come from the West Coast, they would probably have to stay in the area for an extended period. That could be a logistical problem. We will track the patients on Day 28, Day 42, and so on for a full year. If the FDA decides that they should be followed longer, we’ll do so.
Are there conceptual advantages to performing gene transfer in salivary glands as opposed to an internal organ?
Absolutely. First, unlike most internal organs, a salivary gland is not critical for life. Should a problem arise, people will continue to survive with low levels of saliva. Second, we have direct access to the gland. Its opening is right there in the mouth, and we can infuse the Aquaporin-1 gene directly into a major salivary gland. There is no need for anaesthesia or surgery.
Into which salivary gland will you deliver the vector?
We’ll use a single parotid gland. The parotid gland is the largest of the three major salivary glands. If you touch the side of your face, the parotid gland is located under the skin between the jawbone and the lobe of the ear.
A concern with gene transfer studies is the vector might accidently integrate into the DNA of cells elsewhere in the body and cause tumors and other problems. Is that a concern?
In theory, yes. Practically speaking, no. Adenoviruses are non-integrating viruses. Secondly, because a fibrous capsule surrounds the salivary gland, the vector is essentially walled off from the rest of the body. One of the things we did in our animal studies is look for the vector throughout the body. In rats, the glands aren’t as well encapsulated as they are in humans or primates. The animals eat hard chow, so you tend to see the virus in their mouth but it doesn’t spread throughout their body.
The rats had no tumors? Even salivary tumors?
That is correct. They had no tumors anywhere.
With the gene transfer, how much saliva might a patient produce?
We hope to restore 70 to 80 percent of normal salivary flow. Based on our earlier studies with miniature pigs, we saw significant increases in salivary flow on days 3 and 7. By day 14, it was, on average, a little above background. We had expected it to decrease by days 14 to 21. Then comes the obvious question: What next? You can’t give the virus again because the patients will have raised antibodies against it.
So, what next?
We have another vector called an adeno-associated virus, or AAV. In contrast to adenovirus, AAV lasts a very long time and provides quite stable gene transfer. In mice, it can last essentially as long as the animals are alive, which was up to two years.
Why not launch the study with the longer lasting vector?
To err on the side of caution. This study marks the first time that a viral vector will be delivered into a human salivary gland. Secondly, because our research to date has been conducted in animals, we have not yet demonstrated that radiation-damaged salivary duct cells in people can generate an osmotic gradient and support fluid flow. The adenovirus is essentially self limiting and will be removed from each patient by their immune system. If, by chance, the strategy doesn’t work, I didn’t want to have patients with a useless gene-transfer vector in their bodies indefinitely.
So, the goal would be to build on this study and move to Phase III. Is that right?
Well, no. It’s actually an interesting point. If this works, we could say that the Aquaporin-1 strategy works in humans. That is, the physiology of duct cells is such that they could generate an osmotic gradient. The strategy would be to go to an adeno-associated virus vector for long term expression as a Phase I/II study. If that works, the goal would be to advance to a Phase III clinical trial.
Good luck with the study.
Thanks. Let me just say, this study represents a start. It’s a very good start, though, based on solid science. I hope other research groups will take note of salivary gene transfer and continue to develop it even further. The salivary glands offer unique therapeutic possibilities that should be more intensively explored, not only by dental scientists but throughout medical research. More importantly, for all of the reasons that I’ve mentioned, it would be a big win for recovering cancer patients who are forced to battle radiation-induced xerostomia.
1Dirix P et al. "Radiation-Induced Xerostomia in Patients with Head and Neck Cancer," Cancer 107: 2525-2534, December 1, 2006.