Alternatively, the ethyl esters could function as pro-drugs that release the corresponding acid in the parasite. To investigate compound permeability, adult worms were treated with 20 μM of the acid 15 or the ethyl ester 22 for 24 h, and then the media exchanged three times. The worms were then lysed and the levels of the acid 15 or the ethyl ester 22 were determined using LCMS (Figure ).