My little RB Quasar lost interest in eating, too. Finally resorted to Winstrol, a type of steroid. The vet initially gave a shot of some type of steroid and then I put the Winstrol in her food for the rest of her life, I think about six years, until I lost her to kidney failure. These days, the vets have better ideas about why cats don't feel like eating, but back then they didn't. They just treated the symptoms. But it did work.

Good luck to you.