Hydroxychloroquine (Plaquenil) and chloroquine (Aralen) are slightly different variations of the same medication. Plaquenil is a disease inhibitor
for Lupus... reducing the progression of Lupus. It takes 2-3 months to build up in your system, and requires an annual eye exam due to rarely but possibly affecting the eyes. It seldom has any effect on EXISTING symptoms, but may keep new symptoms from appearing.
Here is some more information. /www.hopkinslupus.org/lupus-treatment/lupus-medications/antimalarial-drugs/
As a disease inhibitor, Plaquenil works much differently than Prednisone, which reduces inflammation thus reducing symptoms. These two are often used together, but are not interchangeable as your question somewhat suggests.
If Prednisone is not effective, or if it's needed for more than a few months, most rheumatologists phase out the Prednisone and try one of these three: Imuran, Cellcept, or MTX. Many Lupus patients will respond to one of these - but it's trial and error as to which medication will work for any individual patient. It's not known how or why these work.
For me, Imuran did nothing - nothing good, nothing bad. Cellcept, however, was my miracle and put my Lupus into a "quiet" state. (They don't say remission with Lupus, as it's easily re-triggered by even the smallest stress.)
Let us know if you have more questions,