Veterinarians use electrochemotherapy to treat melanoma in horses
Malignant melanoma is a skin cancer in humans and dogs that metastasizes and spreads and is always fatal without early intervention. In horses, however, most of these tumors are benign growths that appear in older gray horses.
There are several ways to treat them, depending on their size and location. These include surgical removal, chemotherapy to shrink the tumors, autogenous vaccines, radiation and electrochemotherapy.
Kelly Farnsworth of Washington State University uses the latter method, also called electroporation, on several types of equine tumors.
Benefits of electroporation
This technology enhances the effects of chemotherapy.
“By passing an electrical current through the tissue at a certain wave length and pulse rate, we cause it to temporarily open up small pores or channels within the cell walls. When those are opened, this allows fluid or substances to be transferred in and out of the cells,” he says.
Thus, it is easier to get chemotherapy drugs into the tumor cells.
“Many of these drugs that we use do not cross the cell membranes very easily,” Farnsworth notes. “They have to be actively transported into the cells or go in by diffusion.”
Mechanism of action
“By doing the electroporation we make little holes that allow the chemotherapy drugs to get inside the cells very rapidly. Once they are inside they can exert their effect,” says Farnsworth.
These drugs disrupt the DNA and kill the cells when they try to multiply.
“Electroporation increases the effectiveness of drugs like cysplatin significantly just because we can get it inside the cells,” he explains.
“We use an electroporator device – also called an electrochemotherapy machine – to do this. These were originally designed in research settings for getting different things inside cells,” Farnsworth adds. “One major use is to help transport viral particles for vaccine development.”
“Electrochemotherapy was first used and described almost 20 years ago and has gained wider use in both human and veterinary medicine in recent years,” he says.
At this point this technique for treating tumors is still relatively new and there aren’t many of these machines available for veterinary use.
“There have only been a couple of published studies showing efficacy of its use,” says Farnsworth.
A veterinarian could refer an equine patient to someone like Farnsworth, however, to be treated with this machine at Washington State University.
“I am sure that in the next few years these machines will be available in other referral centers. Right now there are only a few in this country that I am aware of,” Farnsworth says.
Ken Sullins of Marion DuPont Scott Equine Medical Center at Leesburg, Va. has been working with one machine that is being custom made and an additional machine has been purchased through a commercial research group.
“The first published reports about the use of electrochemotherapy in horses came out in Europe, a couple years ago,” Farnsworth explains. “This technology will become more widely used as people see the results. At this point the tumors it’s been used for are melanomas, sarcoids and squamous cell carcinomas.”
The first paper detailing results demonstrated the efficacy of the treatment in sarcoids, which noted a very high success rate.
“One paper looked at its use in melanomas and showed that it was effective, though it was just a small case report,” says Farnsworth.
This may give horse owners one more treatment option for the future.
“This will be an important tool. The limitation is that we have to get the tumor down to a reasonable size before we use this,” he says.
“We often have to debulk the tumor surgically and then treat the area where the tumor was because we are treating the area in small, bite-size pieces,” Farnsworth notes. “We have to be able to pass the electric current through it, so we are limited by the size of the tumor.”
“We recently treated a cat, however, that had a tumor on its side that was as big as a potato. When it came back for the second treatment in two weeks, the tumor was half that size. After one more treatment it was down to size of a pea. Even our oncologist was amazed,” he says.
With the successes seen, Farnsworth notes that more are likely down the road.
“We are using the same chemotherapy drug that’s ordinarily used but at a very low total dose,” Farnsworth says. “We don’t have to put as much in if we can get it to the right place. This is one of the primary advantages.”
The electrochemotherapy treatment is safer for the animal and the people administering the drug.
“But we are passing electrical current through the tissue and have to use some caution and thought about where we are doing it on the patient,” he comments. “The animal is receiving a shock.”
Farnsworth notes that the treatment only takes one to five minutes to complete and can be performed in a variety of situations.
“We’ve done some of the horses anesthetized and some just standing and sedated, depending on the location,” he says. “We’ve done some distal limb tumors with the horse awake, but if the horse’s temperament dictates we anesthetize the animal with a very short procedure.”