Ezatiostat HCl Liposomes for Injection (TLK199) Pharmacokinetics. Formation of metabolites is assumed to be unidirectional. Ezatiostat undergoes de-esterification to both TLK235 and TLK236; however, because the quantity of TLK235 measured in this study is consistently less than the level of quantification, this pathway is ignored (dashed lines). TLK236 undergoes further de-esterification to TLK117. Each of ezatiostat and TLK236 can be eliminated via more than one pathway. However, this study provides no insight into the fraction of each entity eliminated by each pathway.