The relationship between the enantiomers of separate enantiomorphic pairs is called diastereoisomerism. So, structure (R,R) is the diastereomer to isommers (R,S) and (S,R).Similarly, (R,S) is the diastereomer to (R,R) and (S,S).
Enantiomers have opposite configurations at all chiral carbons, while diastereomers have the same configuration at one or more chiral centers but opposite configurations at others.
The order of potency of these formoterol isomers are: (R,R) > (R,S)=(S,R) > (S,S).So, (R,R) isomer is about 1000 fold potent than the (S,S) and 2 fold more potent than the racemix (R,R / S,S).
The R-enantiomer and the S-enantiomer show different biological activity in the racemic drug.
For example, studies show that formoterol (R,R / S,S) may have proinflammatory properties since it contain (S,S) isomer. The (S,S) isomer is not biologically inert and has an inflammatory effect on T cells.
Thus, the bronchodilating and antiinflammatory effect of formoterol can be attributed entirely to the (R,R) enantiomer.
Since single enantiomer has advantages over the racemic, scientists has finally purified out the (R,R) isomer from the racemicformoterol and named the compound as arformoteroltartrate.