 LXs are rapidly metabolized, mainly by macrophages, to inactive products by being oxidized at carbon 15 to form 15-keto (also termed 15-oxo) LX products by a 15-hydroxyprostaglandin dehydrogenase; 15-oxo-LXA4 may be further metabolized to 13,14-dihydro-LXA4 by an oxidoreductase.15-Epi-LXA4 and 15-epi-LXB4 are more resistant to the dehydrogenation enzyme than their LX epimers.In consequence of the operation of this anabolic pathway, LXs have very short half-lives "in vivo", the epi-LXs have longer "in vivo" half-lives and thereby greater potencies than their LX epimers, and synthetic lipoxins that are metabolically resistant to this pathway have been prepared, used in animal models to study LX activities, and tested as potential therapeutic agents in animals and humans.