Hi,
Prednisone is a corticosteroid drug. It is converted into
prednisolone in our body by the liver's specific enzymes; thus, it can be active.
Usually,
corticosteroids mimic the
cortisol, the natural corticosteroid produced in our body. Therefore, prednisone has many effects in our body.
In cases of cancer (as your husband), prednisone is used to lower the body's immune response and as a potent inflammation reducer.
With regards to being fatigued, all the factors you mentioned (Xalkori, brain
radiation, being inactive for months) and especially cancer (with cancerous cells taking all the nutrients to your husband), all of them make him fell fatigued.
To suppress his immune system (which is currently fighting against cancer with the help of Xalkori and radiation), your doctors have prescribed prednisone.
Hope it helped! All the best to your husband and all of you!