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 microM and oral ED50 of 18 mg/kg, respectively), it did not inhibit LTB4 production in calcium ionophore stimulated human whole blood at concentrations to 200 microM. To reduce binding of WY-50295T to serum albumin, 250 microM of a naphthalene sulfonic acid (> 99.9% binding to albumin primarily at the carboxylic site) and 250 microM sulfanilamide (binding to nonspecific sites) separately or in combination 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 unchanged. When purified human neutrophils in the presence of fatty acid saturated albumin (fraction V) was employed, the 5-LO inhibitory activity of WY-50295T was prevented. Zileuton (5 microM) inhibited LTB4 production by 99% in the presence of these albumins. Also, rat albumin presented WY-50295T to purified 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 reduction of drug uptake (passive diffusion) using purified human vs rat neutrophils may account for the inactivity of WY-50295T in the human whole blood assay.