Rifampicin significantly increased the mean area under the plasma concentration-time curve ( AUC ) of ( R ) - and ( S ) - fexofenadine ( 3.10-fold and 3.48-fold , respectively ) and decreased the renal clearance of ( R ) - and ( S ) - fexofenadine ( 0.40-fold and 0.47-fold , respectively ) , causing marked differences in the mean amounts of these enantiomers excreted into the urine in the rifampicin phase ( P < 0.001 ) .