No shunting towards LTB4 when blocking LTC4sin vitro

Cysteinyl leukotrienes (LTC4, LTD4 and LTE4) are lipid mediators inducing inflammation and bronchoconstriction in allergic asthma and inhibition of cysteinyl leukotriene production is an effective treatment for some asthma patients. Leukotriene A4 (LTA4) is the precursor for both LTC4 and the neutrophil chemoattractant leukotriene B4 (LTB4). The enzyme LTC4 synthase (LTC4s) converts LTA4 into LTC4, while LTA4 hydrolase converts LTA4 into LTB4. A previous study on LTC4s-/- mice showed that loss of LTC4s does not cause shunting towards increased LTB4 production 1 . If using inhibition of LTC4s as a way to pharmacologically inhibit cysteinyl leukotriene production it is important that no shunting towards LTB4 occurs. In this study we tested whether this lack of shunting holds true also in humans using an in vitro model based on human whole blood. Heparinized whole blood was added to 96 well plate and incubated with different concentrations of an LTC4s inhibitor. Calcium ionophore was added and the plate was incubated for 15 minutes before the reaction was stopped using acetonitrile. LTC4 and LTB4 were measured using mass spectrometry. The production of LTC4 decreased in a dose response manner in response to the LTC4s inhibitor while the LTB4 production was stable. No shunting towards LTB4 production was observed when inhibiting LTC4 production by blocking LTC4s in a human whole blood assay. 1. Kanaoka et al 2001, J Biol. Chem. Vol 276 pp 22608-22613.