My cat has been in the feline equivalent (Solensia/frunevetmab) for about two years, and although it had a huge positive impact for the first 12 to 18 months, it seems to have suddenly completely stopped working in the last little while.
One theory is that the immune system eventually starts attacking the monoclonal antibody med itself, another is that as in humans, the OA progression might accelerate under the drug.
I also have mine on Pentosan which did have a noticeable difference for him so that's great. You could possibly talk to your vet about this, it's significantly cheaper than Beransa (110AUD/month) and for my dog it's biannual with a booster shot in between
I actually (anecdotally) heard a similar issue with Pentosan that it stops working after 12-24 months due to the immune system so you have to "time it" at an appropriate time in a dog's life. To be blunt, by appropriate I mean you'd want to start using it 12-24 before the dog is expected to pass and not at the first sign of osteoarthritis.
One theory is that the immune system eventually starts attacking the monoclonal antibody med itself, another is that as in humans, the OA progression might accelerate under the drug.