| Summary: | Abstract Background The herbal medicinal product Bronchipret® TP film-coated tablets contains a fixed combination of thyme and primula dry extracts (BRO) and has long and successfully been used for the treatment of acute bronchitis. However, the underlying pharmacological mechanisms of action have not been determined so far. We report a tiered approach applying in vivo and in vitro studies to investigate the pharmacodynamic activity and underlying mechanisms of action, and to identify possible active ingredients contributing to the product’s pharmacodynamic activity. Results In an LPS-induced rat model of bronchoalveolitis oral administration of BRO effectively ameliorated the influx of leukocytes into the lung. This was accompanied by reduced levels of leukotriene (LT) B4, cysteinyl-LTs (cysLT), and prostaglandin (PG) E2. We also found that BRO potently reduced the production of LTB4 in vitro in rat whole blood stimulated with the Ca2+-ionophore A23187 whereas the effect on PGE2 was much less pronounced. The transferability of these findings to human cells was assessed by measuring the effects of BRO on A23187-stimulated human monocytes and neutrophils. BRO and thyme extract were potent inhibitors of LTB4 and cysLT production. We further investigated the effects of BRO, thyme extract as well as single compounds of thyme extract on enzymatic activity of 5-lipoxygenase (5-LO). 5-LO activity was potently reduced by BRO and thyme extract. Of the single ingredients, thymoquinone and rosmarinic acid showed most potent inhibitory activity against 5-LO, with lower potency for thymol and carvacrol. Conclusion BRO attenuates inflammation-induced leukotriene formation in vivo and affects leukotriene biosynthesis in vitro via 5-LO inhibition. This inhibitory activity appears to be primarily related to the thyme extract component. However, none of the single ingredients tested seems to fully account for the activity of thyme extract alone. Rather, the interaction of different compounds seems to be required for its overall inhibitory effect on leukotriene production.
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