Gastric acid blocking agents can lead to a b12 deficiency, which can lead to anemia and neurological disorders. The steroids irritate the gastric lining, hence the need for the antacids, which lower the b12 levels.
The steroids will have the biggest effect with helping the arthritis, while the antacids are likely to treat the steroid side effects, and the B12 is to treat the side effects of the meds treating the first side effects. Just my pediatric hypothesis projected onto cats.