The present invention relates a composition for treating rheumatic diseases. The composition comprises a compound having the formula wherein R.sub.a is selected from --OCH.sub.3, --OCH.sub.2CH.sub.3, --CH.sub.3, --CH.sub.2CH.sub.3, --CCCH.sub.3, --CHCH--CH.sub.3, or CH.sub.2--CHCH.sub.2; and one or more anti-rheumatic agents. A method of using the composition for treating rheumatic diseases in humans and animals is also disclosed.