Latanoprost (Item No. 16812) is an F-series prostaglandin (PG) analog which has been approved for use as an ocular hypotensive drug.1 Latanoprost is an isopropyl ester, a prodrug form which is converted to latanoprost (free acid) by endogenous esterase enzymes. The free acid form is 200 times more potent than latanoprost as a ligand for the human recombinant FP receptor.2 5-trans Latanoprost (free acid) is an isomer of latanoprost (free acid) wherein the double bond between carbons 5 and 6 has been changed from cis (Z) to trans (E). The trans isomer of latanoprost occurs as an impurity in commercial preparations of the bulk drug product. The present compound was prepared primarily as an analytical standard for detection and quantitation of this impurity. From what can be inferred from the study of other trans isomers of F-type prostaglandins, the biological activity of this isomer is likely to be similar to that of the cis isomer. However, there are no specific published reports on the biological activity, and on reducing intraocular pressure in particular, of 5-trans latanoprost.WARNING This product is not for human or veterinary use.