Stelfonta for Dogs with Mast Cell Tumors: New Injectable Drug!

Molly Jacobson is a writer and also the editor of The Dog Cancer Survival Guide, published by Maui Media. A lifelong dog lover and self-professed dog health nerd, she is all too familiar with dog cancer. She has been supporting readers of this blog since the beginning. Molly earned a BA from Tufts University, and after a career in bookselling and book publishing attended The Swedish Institute to become a licensed massage therapist in New York State, licensed by the medical board. Her fascination with health is both personal and global, and she is most proud of how this site and the associated publications have revolutionized not only our approach to dog health, but our own health.

Updated: May 7th, 2021

Summary

Dr. Sue Ettinger, AKA Dr. Sue Cancer Vet, is very happy about this mast cell tumor treatment. Stelfonta is an injectable that may help your dog.

Dr. Sue Ettinger AKA Dr. Sue Cancer Vet, the veterinary oncologist and co-author of The Dog Cancer Survival Guide, vlogs regularly on her YouTube channel about all things dog cancer. And what she’s most excited about lately is Stelfonta®, a promising injectable for mast cell tumors.

So … will it help your dog with mast cell tumors?

Let’s find out!

What Is Stelfonta?

Stelfonta is a new injectable drug that veterinarians use to kill mast cell tumors. It’s injected directly into the tumor itself, your dog doesn’t need anesthesia, and most don’t even need sedatives (!).

Once injected, the drug essentially kills the mast cell tumor directly. Now the next part, and the next couple of weeks, gets graphic.

The mast cell tumor sloughs off as it dies, leaving a wound that heals over and fills in. Think the same kind of effect that you may have seen if you used Neoplasene.

Gross, right? Well, mast cell tumors are pretty gross themselves.

Here’s the thing, and this is what Dr. Sue is so happy about … 75% of dogs heal completely within 28 days. That’s a VERY high success rate!

Stelfonta is the name brand of tiglianol tiglate, which is extracted from the seed of the blushwood plant.

But, because there is an open wound and other potential problems (see below), you cannot use this on just any mast cell tumor.

These Mast Cell Tumors Could Possibly Be Treated with Stelfonta …

Stelfonta is approved by the FDA for the treatment of mast cell tumors in dogs, but not for EVERY mast cell tumor. Here is the official language from the Stelfonta website:

STELFONTA injection is indicated for the treatment of all non-metastatic cutaneous mast cell tumors and non-metastatic subcutaneous mast cell tumors located at or distal to the elbow or the hock in dogs.

Got that? Maybe not. OK, so here’s what that means:

Tumors have to be non-metastatic … which means they haven’t spread yet.
If the mast cell tumors are subcutaneous … which means they are located in the deeper layers of the skin … they must be located at or below the elbow or the hock …. which means on the lower arm or paw or on the lower leg or paw.
If the mast cell tumors are cutaneous … which means they are located in the skin or in the deeper layers of the skin … they can be treated anywhere they are accessible for injection.

… As Long as They Aren’t Too Large!

Here’s another thing: tumors must be smaller than 10 cubic centimeters. What does that mean?

The formula Dr. Sue gives is this: length in centimeters X width in centimeters X height in centimeters … and then divide that number in half.

L X W X H / 2 < 10

Here’s a real-world example: a wound that is 5 cm wide, 3 cm long, and 2 cm high would calculate out in cubic centimeters to …

5cm X 3cm X 2cm = 30 cubic cm
Now divide that number in half …
30 cubic cm / 2 = 15 cubic cm

That tumor would be too large for this treatment.

Here’s another example: a wound that is 3cm wide, 3cm long, and 2cm high.

3cm X 3cm X 2cm = 18 cubic cm
Now divide that number in half …
18 cubic cm / 2 = 9 cubic cm

That tumor would be eligible!

The size of the tumor is limited because of dosing.

You don’t want to give too much Stelfonta at a time — it wouldn’t be safe. The maximum dose happens to be what you would need for a 10 cubic centimeter tumor … and so that is why that size is the biggest tumor that can be treated.

Note: Some mast cell tumors can be shrunk a little with steroids. If a tumor is a little bigger than 10 cubic centimeters, your veterinarian may want to try steroids to shrink it and treat with Stelfonta.

Bottom Line on Which Mast Cell Tumors Might Be Eligible for Stelfonta

You might be able to use Stelfonta if:

The tumor is on the outside of the body, in the skin.
It’s relatively small, no more than 10 cubic centimeters.
It’s not otherwise a good candidate for surgery … for example, on the forearms, wrists, or paws, or on the lower leg, ankle, or paws. Some mast cell tumors on the face, ear, muzzle, and other hard-to-operate areas may also be eligible.
The mast cell tumors haven’t metastasized.

So, you can see how early detection is a good thing — if you’ve caught the tumor when it’s small, this might help.

How Stelfonta Is Given and Why Veterinarians Are Excited About It

Every dog lover fighting cancer I know has at some point thought “isn’t there just something they can shoot right into the tumor to just kill it off??”

Well, this may be an agent that could do that. (Even someday, in other tumors, the company says.)

Stelfonta is essentially an alternative to surgery, which is still the preferred method of tumor removal. But the wide margins veterinarians want in a surgery aren’t always available in certain locations.

Surgery is still the preferred method of mast cell tumor removal. Stelfonta is an alternative to surgery for certain tumors that are hard to treat.

For example, there just isn’t a lot of tissue in the paw. Look at your own hand. There are a lot of bones and cartilage and joints in there! If you needed a surgery, would they be able to get 2-3 centimeter margins? Not in most places.

That’s why Stelfonta is being used in these hard-to-operate areas.

By injecting the tumor directly, and killing it off directly, you’re hopefully “getting it all out” without having to remove healthy tissues to get wide margins.

If your dog isn’t a candidate for surgery due to old age, or anesthesia worries, this is also nice.

Stelfonta is just a needle — no big deal. Most dogs don’t even need a sedative to take it. It’s like any other shot — just a little needle, and you’re on your way.

But after that treatment, that’s when the healing process starts. And it can be kind of graphic, so bear with me.

What To Expect After Treatment

As simple as treatment may be, post-treatment things get pretty dramatic.

Once the injection is in, the drug goes to work killing tumor cells directly AND stimulating an immune response.

First Stage: Tumor Dying

To be blunt, the tumor starts to break down as it dies. Normally, we just remove tumors. We don’t see them sort of … dissolve.

At the same time, inflammation sent by the immune system causes the tumor to open up as a wound.

There is an open wound where the tumor used to be. That wound sloughs off and breaks apart until the tumor is gone, at which point, there will just be a … hole.

That’s the first stage of healing.

Second Stage: Wound Healing

That hole will close up over the next few weeks, which is the second stage of healing. And interestingly, they found that it does not require standard wound care.

These wounds heal best if they are left uncovered.

They don’t need antibiotics.

And your dog can even lick it.

Most wounds will be healed by day 28.

Surgical wounds usually close up after 14 days … so 28 for complete response (tumor gone, wound healed) isn’t actually that long. (That’s my opinion, anyway!)

Prepare Yourself

If you are thinking about using Stelfonta, it is REALLY important that you prepare yourself for what you might see. A big hole in your dog is something you need to prepare for.

Dr. Sue recommends going to the Virbac website to see images of wounds during and after treatment so you understand what you will likely encounter.

Medications Given Along with Stelfonta to Manage Degranulation

Because mast cell tumors are prone to degranulation, certain medications are given before, during, and after the Stelfonta injection.

What’s degranulation?

Mast cells make and store all sorts of chemicals, including histamine, heparin, and several other substances. Degranulation is the sudden release of those chemicals.

When a whole bunch of histamine hits the system, you will see vomiting, diarrhea, itching, and swelling.

When a whole bunch of heparin hits the system, you might see bleeding. (Heparin is a blood thinner.)

Let’s just say that it’s pretty messy when a mast cell tumor degranulates.

Steroids

So to treat for that possibility, dogs are given steroids two days before treatment and for eight days afterward — ten days total.

Benadryl and Pepcid or Similar H1 and H2 Blockers

On the day of treatment, and continuing for 7 days after, dogs are given H1 and H2 blockers.

Benadryl is a common H1 blocker, and Pepcid AC is a common H2 blocker. If your dog has mast cell tumors, he or she may even already be on these over-the-counter drugs.

Stelfonta Side Effects

So, here’s the thing: side effects of Stelfonta, also called adverse events, are super common. But not because it’s a really dangerous drug.

Side Effects from the Tumor Dying

The treatment itself is sort of like a slow-moving surgery.

The tumor is dying, which means you’ll see things like sloughing.

This also causes things like wound formation, localized swelling, redness, bruising, profound swelling, possibly pain, possibly lameness.

Side Effects from the Immune System Stimulation

Stelfonta stimulates the immune response directly.

When you stimulate the immune system, it reacts with inflammation, for example. Local lymph nodes might get swollen, because it’s working so hard.

But keep in mind that the studies behind this drug found that 92% of dogs experienced “rapid” wound healing. So even though it’s dramatic, and yucky, it heals quickly.

And with 75% of dogs having a wound completely healed … and the tumor gone … within 28 days … that’s a really high success rates.

Side Effects from Steroids

Other side effects don’t come from the tumor dying and the resulting wound healing … they come from the steroids used.

Steroids like prednisone can cause all sorts of short-term side effects like thirst, more urination, increased appetite, panting.

Side Effects from Degranulation

And finally, some side effects come from the degranulation of mast cell tumors. When these cancer cells get killed, they release all those histamines, heparin, and other cytokines.

As a result, you can see vomiting, diarrhea, lethargy, appetite loss. You can see panting, bruising, swelling. Heparin, the blood thinner, can cause big bruising. You might also see increased heart rate or low blood pressure.

These May Be Familiar Side Effects

All of these risks may be present for every dog with mast cell tumors, however. Mast cell tumors can degranulate at any time. A massive degranulation is one of the things dog lovers dealing with this disease fear most.

This possibility is one reason Stelfonta can only be used with smaller tumors.

So, the bottom line is this: between the sloughing wound and resulting inflammation, the side effects from steroids and the possibility of degranulation, you might see some side effects.

That’s why it’s so important to read all the stories on the Virbac website — to make sure you understand.

How Effective Is Stelfonta?

So, that’s a lot, right? I mean, I know I have a hard time with watching my dog’s wounds heal. But if it’s worth it … so is it?

Keep in mind that:

surgery for mast cell tumors requires really wide, clean margins to make sure it doesn’t recur
the areas you use Stelfonta on are hard-to-operate areas

Here are the numbers Dr. Sue gives us in her video (below).

75% of dogs had a complete response (meaning the tumor is totally gone) after just ONE treatment. And they were healed by day 28.

So what happened to the rest? Well, they could get a second treatment on day 28.

And that often did the trick. After 1 or 2 treatments, 87% of dogs in the study had a complete response.

But mast cell tumors tend to recur … so how long did that response last?

After twelve months, 89% of dogs were still disease-free.

That’s a great result. If your dog has a tumor that is eligible, and is not a good candidate for surgery, that’s an EXCELLENT alternative.

So Should You Consider Stelfonta for Your Dog?

Not every dog should use Stelfonta. Big tumors are out, as are tumors located deeper in the body or on the trunk (where you can’t tell just how deep they really go).

But just-a-little-bigger-than-10-cubic-centimeter tumors might be shrunk with steroids … and become eligible.

And if your dog has a mast cell tumor in a challenging location, like the paw or the lower limbs, where you might not be able to get a good margin … it’s worth considering.

Dr. Sue also considers Stelfonta for the muzzle, tail, tail base, face, ear, vulva … anywhere that it would be hard to get a complete margin.

It’s also a possibility if your dog can’t go under anesthesia for regular surgery due to heart disease or similar.

Your oncologist and/or general practitioner will be able to determine if this is a good choice for your dog’s specific case.

What To Do If You Think Stelfonta Could Help Your Dog

First, explore the relevant links and Dr. Sue’s recent videos about Stelfonta and mast cell tumors below. I recommend you watch all of them and absorb this material if you think it may apply to your own dog.

After all, this is a brand-new drug with a brand-new FDA approval, so you may be the first in your veterinarian’s practice to ask about it. 🙂

Also make sure you visit Stelfonta’s website so you understand the drug and the healing process.

Then, talk to your veterinary oncologist or your general practitioner about it if it feels right to do so.

It’s a dramatic drug, but cancer is a dramatic disease … and mast cell tumors are particularly dramatic!

Be Well, and Stay Safe …

Warm Aloha,

Molly Jacobson
Editor, The Dog Cancer Survival Guide