Solensia® A New Approach To Arthritis Pain In Cats

Ron Hines DVM PhD

 The Special Needs Of Older Cats

   What To Do When Your Cat Is In Pain

Before 2013, when Zoetis  was still the animal medications division of Pfizer Drug Company , they purchased three patents for monoclonal antibodies that targeted a particular messenger protein involved in nerve growth and nerve signal transmission, nerve growth factor (NGF). One of them, tanezumab had been bioengineered to be used as a treatment to relieve arthritis pain in humans. Another, bedinvetmab (Librela®) was bioengineered to be accepted by dogs and a third one, frunevetmab  (Solensia®) had been bioengineered to be acceptable for cats.

You see – all mAb medications, when they are scrutinized by your or your pet’s immune system, must be recognized as not being something foreign to the body. If the immune system’s decision is that they are foreign it will attempt to destroy them as quickly as possible. You can think of it as being required to be fluently in the language of the country you are visiting in order not to be immediately deported. All the current monoclonal anybody drugs of today, for humans or animals begin there genesis in a mouse. Then every protein related specifically to mice is removed and replaced by proteins only found in the species for which the drug is intended to be used. The product is then called a “chimeric antibody.

What Happened To The One Pfizer Designed To Treat Arthritic Pain In Humans?

Pfizer and their shareholders had great hopes that tanezumab (as a human arthritis pain reliever) would be a blockbuster money earner for the Company. First off, as best we know, mAb s are not addictive – which is the major drawback of narcotics. Second, mAb s would be extremely hard (or impossible) to overdose on in the way narcotic pain killers are.

However, a worrisome problem occurred in the later stages of human clinical trials. (read here)  Significantly more people who received the periodic drug injection needed knee or hip replacements than those that did not receive the drug. So the FDA refused to grant Pfizer approval to sell the drug. The EMA did likewise.

The incidence of those unwanted events increased as the dose got higher and in the human patients that received the drug for more than 90 days. Those side effects were worse in the people who continued to take NSAIDs during the clinical trial. Local itching at the injection site, dermatitis and hair loss though not nearly as worrisome were also commonly reported side effects.

Although those government agencies refused to approve the sale of the “humanized” product, both the FDA and the EMA approved Zoetis’ request to market their “felinized” product, Solensia®. Their “caninized” product, Librela® (bedinvetmab) was also approved by the EMA for sale in the European Union – but as of writing this article in March of 2022  I do not believe that Librela®  has been approved yet in North America.

Are The Cat Products Safe For Long Term Use In My Cat?

The short answer to that is no one knows.

I am not aware of any studies that followed cats receiving Solensia® for more than a few months. If Zoetis ran any studies, they were not published.

 Solensia® reduces the amount of nerve growth factor (NGF)  in your cat’s body. Nerve growth factor in mature cats is involved in pain transmission and nerve health. During a cats growth from conception to maturity NGF helps guide nerves from the brain to their appropriate final locations and assists in removing unnecessary cells as they occur (apoptosis). Figuring that out earned one lady a Nobel Prize. (read here)

 Do Cat’s Need Better Pain Control Options?

Yes

Dogs handle oral non-steroidal anti inflammatory drugs (NSAID) like carprofen  (Rimadyl®),  firocoxib  (Previcox®) and meloxicam  (Metacam®)  relatively well. When side effects do occur in dogs, they are usually due to gastrointestinal bleeding. But the toxic effects of NSAIDs in cats often relate to kidney damage. (read here) Cats can only be given certain NSAIDs (eg meloxicam) – and when attempted, in very low doses or for very short periods. How even those very low doses,when given long-term affect your cat’s kidneys is unknown. Cats in short term acute pain appear to respond well to fentanyl patches. But the DEA discourages their use in veterinary medicine.

Corticosteroid drugs like prednisolone also decrease the pain and  inflammation of arthritis. But they too have significant side effects when given long-term to cats as well as to people. (read here)  The most common one in cats is the generation of diabetes. (read here) The effectiveness of tramadol and gabapentin  in cats, dogs and humans in combating arthritis pain is debatable.

Where Can I Find Unbiased Information Regarding Solensia®?

Try the Cat Owner Solensia Feedback Page

I will begin with a warning that people whose animals experience unexpected side effects from a medication tend to report them to me more frequently than those that were satisfied with their experience using a drug. That is the case with human drugs as well. You can see that in my webpage that concerns another Zoetis mAb product, Cytopoint. However the Solensia® Cat Owner Feedback Page should be a good place to start.

Other unbiased online sources will be harder for you to find. Perhaps social media – although I myself have no trust in posts and blogs. That is because the majority of the information you find online and in other media will be generated by the pharmaceutical companies themselves and their downstream sellers intent on selling their products to you. You might even find them at the bottom of this webpage. Your clicking on those ads and your donations are what keep my website online. 

Promotional material will always emphasize the positive and de-emphasize the negative. The only pet medications that have zero potential to cause side effects are the ones that don’t have the potential to have positive health benefits either.

Zoetis is quite generous in funding veterinary college research studies that find their products beneficial. Long ago, veterinary colleges themselves funded their medication research. However the product manufacturers themselves fund most new medication studies these days. There is no doubt that purse string control influences study outcomes. (read here ,   here  &  here)

What About The Cat Owner Surveys That Zoetis Conducted?

Zoetis reported that 77 % of cat owners saw a reduction in signs of pain when their cats were treated monthly with Solensia® (in a 3 month long study). And that 12.9 % of the cats that received Solensia® were more active during the first three weeks after their first injection. That difference between the medicated and non-medicated group declined over time. The evaluation was said to have been based on   “subjective” (owner’s opinion?) as well as “objective” data (activity monitors?).

Owner opinion has been found to be an unreliable way to measure a drug’s effectiveness ( In the same experiment 67% of cat owners whose cats received the placebo injection thought their cat had improved too). It’s just human nature to try to maintain a positive attitude and hope for the best. (read here  &  here) Activity monitors like the one the cat at the top of this page is wearing around its neck are really the only valid way to judge the effectiveness of any pain-control medication given to cats. (read here)

Doubly so because your cat can’t talk. The EMA website reports that in a study of Solensia® in 182 client-owned cats who received the drug and 93 cats that received a placebo injection instead, after several months of injections, 66.7% of the cat owners whose cats received Solensia® thought their cat was more active and sociable than before. However 52.06% of the owners of cats that received the placebo injection also reported that their cat was more active and sociable than before. By the third shot, there was no statistical difference in the opinions of the owners of cats that received Solensia® and those that received the placebo..

Another problem is that veterinarians have no scientific way to measure arthritic pain level in your cat. Just like you, your cat can become reluctant to move for a plethora (=a very large number) of reasons that have nothing to do with its joints.

Joint x-rays are unreliable indicators of pain. Studies have shown that only ~50% of people over the age of 65 that have x-ray evidence of arthritic knee joint changes experience any pain. And those that have joint pain do not necessarily have x-ray evidence of arthritis.  As for hip pain, only 9.1 – 15.6% of people that experienced hip pain had any x-ray evidence of hip arthritis. And of those that did have evidence of hip arthritis, only 20% reported frequent hip pain.

I believe that it will only be after Solensia® has been in wide spread use in cats for a year or two that pet owners and veterinarians alike will be able to decide how effective and safe this medication actually is.  I do hope with all my heart that it will be both effective and safe in the cats that really need it. The acute pain that accompanies an injury, a torn ligament, muscle strains, car accident or surgery is real. But pain in those cases has positives as well as negative effects. Pain decreases motion, and that reluctance to move and inactivity gives your cat’s body a chance to heal.    

 More About The Product

Please read the entire Solensia® label online because there are more warnings I did not mention and because the label directions may have changed by the time you read this article.

 Solensia® comes in 1 ml vials. Each contains 7 mg of frunevetmab solution. It is to be given by subcutaneous injection. Monoclonal antibody drug molecules like Solensia® are too large to be absorbed through the intestine intact.

Not mentioned by the Company, but on my mind is that injections given to cats are best given in the lower leg or tail because cats have been known to develop injection site tumors. (read here). Reading from the EMA website, the suggested dose is 1 – 2.8 mg/kg of body weight given once a month. So at the time I am writing this article, cats 7 kg (15.4 lbs) or less would get one vial and cats 7 – 14 kg (< 30.8 lbs) would get two vials – once a month. It took 6.2 days for the level of the drug in their blood to peak after the cat’s first injection.  That might mean that you need to be patient before making a judgement. But with repeated monthly injections the cats frunevetmab blood levels became constant. The Company suggests that Solensia® not be given to cats that are still growing (less than 12 month old) or that weigh less than 2.5 kg (5.5 lbs). I mentioned earlier that nerve growth hormone (NGF) which Solensia® targets was necessary for proper nerve development. 

Old cats often have kidney health issues. (read here) The EMA suggests that your veterinarian consider that and dispense the drug on a risk-to-benefit basis. We do not know how or if lowering your cat’s NGF level affects its kidneys. But there is an interplay. (read here)

There are other questions to be answered regarding the long-term benefit of Solensia® in reliving chronic arthritis pain in cats. When laboratory rats were given injections of mouse nerve growth factor in an attempt to produce antibodies against NGF similar to what Solensia® does, the rats did produce anti-NGF antibodies. However that had deleterious effects throughout their nervous system.  (read here) That study was performed long ago. Perhaps it was accurate; perhaps it wasn’t. Perhaps Zoetis keeps its recommended monthly dose low enough to avoid the problem.

Another unknown is if cats receiving an mAb such as Solensia® eventually, after multiple monthly injections, recognize that it is foreign to their body and destroy it. If so, will the drug loose its effectiveness over time? There are instances of that occurring, it is called loss of response or LOR. (read about that here)

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