WY-50295 tromethamine: a 5-lipoxygenase inhibitor without activity in human whole blood

The 5-LO inhibitor, WY-50295 tromethamine (T) prevented leukotriene release (LTB4 production) in calcium ionophore stimulated, purified human and rat neutrophils. However, whereas WY-50295T inhibited both in vitro and ex vivo rat whole blood leukocyte LTB4 formation (IC50 = 40 mu M and oral ED50 of 18 mg/kg, respectively), it did not inhibit LTB4 production in calcium iono phore stimulated human whole blood at concentrations to 200 mu M. To reduce binding of WY-50295T to serum albumin, 250 mu M of a naphthalene sulfonic acid (>99.9% binding to albumin primarily at the carboxylic site) and 250 m u M sulfanilamide (binding to nonspecific sites) separately or in combinati on were preincubated in whole blood prior to addition of WY-50295T; however , WY-50295T still did not inhibit 5-LO and free drug blood levels were unch anged. When purified human neutrophils in the presence of fatty acid satura ted albumin (fraction V) was employed, the 5-LO inhibitory activity of WY-5 0295T was prevented. Zileuton (5 mu M) inhibited LTB4 production by 99% in the presence of these albumins. Also, rat albumin presented WY-50295T to pu rified rat neutrophils more effectively than human albumin (i.e. WY-50295T was more active in the presence of rat albumin). These results suggest that the high affinity binding of WY-50295T to human albumin and possibly the r eduction of drug uptake (passive diffusion) using purified human vs rat neu trophils may account for the inactivity of WY-50295T in the human whole blo od assay.