IWP-L6 (Porcn Inhibitor III) is stable in human plasma over 24 h, it was rapidly metabolized in rat plasma (t1/2 = 190 min), murine plasma (t1/2 = 2 min), and the murine liver S9 fractions (t1/2 = 26 min). The major metabolites are the amide cleavage products. Similar species-dependent metabolitic profiles due to the involvement of carboxylesterase (CES) have been reported with other drug candidates. Despite its modest metabolic stability in mouse-derived plasma, IWP-L6 was highly active in zebrafish. IWP-L6 exhibited more potent activity.