Boswellic acids are pharmacologically active ingredients of frankincense with anti-inflammatory properties. It was shown that in vitro 11-keto-boswellic acids inhibit 5-lipoxygenase (5-LO, EC 126.96.36.199), the key enzyme in leukotriene biosynthesis, which may account for their anti-inflammatory effectiveness. However, whether 11-keto-boswellic acids interfere with 5-LO under physiologically relevant conditions (i.e., in whole blood assays) and whether they inhibit 5-LO in vivo is unknown. Inhibition of human 5-LO by the major naturally occurring boswellic acids was analyzed in cell-free and cell-based activity assays. Moreover, interference of boswellic acids with 5-LO in neutrophil incubations in the presence of albumin and in human whole blood was assessed, and plasma leukotriene B(4) of frankincense-treated healthy volunteers was determined. Factors influencing 5-LO activity (i.e., Ca(2+), phospholipids, substrate concentration) significantly modulate the potency of 11-keto-boswellic acids to inhibit 5-LO. Moreover, 11-keto-boswellic acids efficiently suppressed 5-LO product formation in isolated neutrophils (IC(50)=2.8 to 8.8 muM) but failed to inhibit 5-LO product formation in human whole blood. In the presence of albumin (10 mg/ml), 5-LO inhibition by 11-keto-boswellic acids (up to 30 muM) in neutrophils was abolished, apparently due to strong albumin-binding (>95%) of 11-keto-boswellic acids. Finally, single dose (800 mg) oral administration of frankincense extracts to human healthy volunteers failed to suppress leukotriene B(4) plasma levels. Our data show that boswellic acids are direct 5-LO inhibitors that efficiently suppress 5-LO product synthesis in common in vitro test models, however, the pharmacological relevance of such interference in vivo seems questionable.