text: In contrast to << diethylcarbamazine >>, piriprost (U-60,257; 6,9-deepoxy-6,9-(phenylimino)-delta 6,8-prostaglandin I1), which inhibits the formation of sulfidopeptide leuktrienes in RBL cells at the 5-lipoxygenase step (EC50 5 microM), did not inhibit the [[ leukotriene synthetase ]] of these cells.