LTF4 is a cysteinyl-leukotriene produced in vitro, but not reported to date in vivo. It is formed by the incubation of LTE4 with γ-glutamyl transpeptidase and glutathione. LTF4 is a weak agonist in its ability to contract vascular smooth muscle.{547} The rank order of potency of the cysteinyl-leukotrienes to contract vascular smooth muscle is LTD4 > LTC4 > LTE4 >> LTF4.{547,548}