Hello, I'm Dr Ralston, thank you for your question.
Fortikor is also known as benazepril. It has been used in renal disease in cats to increase blood flow to the kidney and to help with filtration of wastes from the blood by increasing the filtration rate of these wastes. It may also help reduce protein that is entering into the urine. It is thought that protein can contribute to urinary tract inflammation in cats and cause some cystitis. But, again, it is called idiopathic cystitits for a reason. Idiopathic means, we don't know what causes it. So there are many potential causes.
Fortikor is probably not the most important medication in the treatment of your cat. But, I do think it helps somewhat. Even if just to reduce any amount of protein in the urine. Remember, your vet might not register protein in the urine on tests, but it doesn't mean the protein isn't there. The smallest proteins can be found in the urine only by specific tests. Microalbuminuria is detected at VERY low levels only by specific tests that are usually sent out of the office and must be requested. These can tell us when there is a VERY low amount of protein that is starting to show up in the urine. But, because they can detect low amounts, it can tell us that a problem is just starting or is very early.
Cystophan is actually a pretty cool medication, and one of the only medications available to us for treating feline interstitial cystitis. The thought behind this is bascially that the bladder wall and urinary tract is inflammed or irritated. It might have something to do with the protein as I mentioned, but some theorize that it is because of faulty mucus layers in the bladder (basically). Others think that it is due to stress or anxiety mostly. So, this medication combines elements that help to repair and protect the mucus lining of the urinary tract, and also provide some relief from anxiety in the cat. These together can help treat the disease, so that medication is also useful.
K/D is a diet for kidneys. That is true. But, it also helps to promote urination and healthy urinary tract systems in general. The cost isn't much more than regular diet. And keep in mind your cat's values might be normal in the blood now BECAUSE of the food and medications you are giving. If you stopped it could get worse. It wouldn't hurt any normal cat to be on k/d for life.
So, I do recommend staying with the Fortikor. This disease is complex and nobody understands all of it, but each little thing you can do to help it will make the pet better.
I'd like to share two very good references with you. They are VERY dense though, so set aside some time to read through them.(Click here for a link to a feline behavior symposium article regarding treating idiopathic cystitis. This is actually a very interesting read, but you will see how complicated it is)(Click here for this link about cystophan, from the makers of the drug. It really explains the science and theory behind the product with full illustrations. Again, a lot of information about the disease itself, might answer some other questions you have)
And one last one (link to information on FIC)
In the end there is so much to this disease - environmental, food/diet, stress, other secondary problems, enrichment, medications. So many different choices. And this is typical of a disease with no cure. Lots of ideas mean lots of people trying to solve the problem. However, lots of different solutions usually mean that none of them really work that great or all are about equal. Or else, there would only be one solution used, right?
Thank you for your question, I hope I have provided you with enough information to make a quality decision regarding the health of your pet.