Rifampicin significantly increased the mean area under the plasma concentration-time curve ( AUC ) of ( R ) - and ( S ) - fexofenadine ( @-fold and @-fold , respectively ) and decreased the renal clearance of ( R ) - and ( S ) - fexofenadine ( @-fold and @-fold , respectively ) , causing marked differences in the mean amounts of these enantiomers excreted into the urine in the rifampicin phase ( P < @ ) .