A New Class of Chemotherapy Drugs for Treating Cancer in Dogs

Gentler on the body but still packs a wallop.


Last month we talked about the fact that most chemotherapy drugs have to be given by injection under the care of a veterinarian whereas targeted molecular therapies that attack cancer cells can be given in pill form by owners at home. But there is a new type of chemotherapy that can be administered at home — and works differently from conventional chemotherapy to destroy cancerous tumors, proving less hard on the body.

Conventional chemotherapy, given intravenously, works by killing rapidly dividing cells, including not just cancer cells but also rapidly dividing cells in other parts of the body, including the bone marrow, the lining of the intestine, and the cells inside hair follicles. That’s why it needs to be given at two- to three-week intervals. It takes about that long for healthy tissue to recover before undergoing another chemo “assault.”

But the recently introduced kind of chemotherapy, called low-dose, or metronomic, chemotherapy, has a completely different mechanism of action. It’s believed to work not by killing cancer cells directly but by preventing blood vessels from growing into a cancerous tumor and thereby starving it of its nourishment that comes through the blood supply. The process is called anti-angiogenesis. Angiogenesis is the formation of new blood cells; this is the anti-effect.

The two metronomic chemotherapeutic agents being used in cancer-ridden dogs right now are cyclophosphamide (Cytoxan is one of the brand names) and leukeran (Chlorambucil). Because they can be given in very low doses to achieve their goal, they can be given everyday by owners in pill form. People don’t have to come to the vet’s office every two to three weeks and stay for part of the day while the dog receives chemo intravenously. This form of chemo is, not surprisingly, much less toxic than the conventional kind and also relatively inexpensive.

“We don’t have a lot of information yet about how well it works compared to conventional chemotherapy,” says John Berg, DVM, a soft tissue surgeon at Tufts who works to remove cancerous tumors via operations. “We have far more experience with conventional. But a couple of papers suggest that it works well against soft tissue sarcomas — cancers in tissues on the surface of the body just below the skin rather than in the chest, abdomen, or head. The regrowth rate has been seen to be lower after surgical removal of the tumor if the dog receives low-dose chemotherapy following the operation,” he says.

Preliminary evidence suggests that low-dose chemotherapy might also be helpful to dogs with hemangiosarcoma of the spleen. Hemangiosarcoma is a cancer of the cells that form blood vessels, so it would make sense that low-dose chemotherapy, which doesn’t allow blood vessels to form, would work against that type of tumor. (The spleen is the organ that has been studied in this instance because that is the most common location of hemangiosarcoma in the dog.)

Research notwithstanding, this is “probably an area where clinical usage has gotten ahead of the data,” Dr. Berg says. “This is a very new area in veterinary medicine.” Still, because low-dose chemo is so easy to administer, is not terribly expensive, and comes with little to no risk for a dog with a deadly disease, it’s coming into more common use.

“It can be used in combination with conventional chemo,” Dr. Berg says, which means a cancer can be attacked on two different mechanistic fronts. It can also be used in conjunction with targeted molecular therapies. The more lines of attack on a cancer, the better the chances that it can be vanquished.


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