Synthesis of a radiolabeled enniatin cyclodepsipeptide [3H‐methyl]JES 1798

For receptor binding studies and the elucidation of the mode of action of the potent anthelmintic compound JES 1798 a tritium labeled compound at very high specific activity was necessary. Tritium was introduced by methylation of the N‐demethyl precursor JES 2314. The identity of [N‐methyl‐ 3 H]JES 1798 was determined by mass spectrometry. After synthesis and purification, 535 μg [N‐methyl‐ 3 H]JES 1798 were available at a specific activity of 84 Ci/mmol (3.11 TBq/mmol) as determined by mass spectrometry. The total activity was 80 mCi (2.96 GBq). Radiolabeled JES 1798 showed an efficient and specific binding to a membrane fraction from Ascaris suum . Displacement by unlabeled JES 1798 was half‐maximal at about 0.72 ± 0.06 μM. Different known enniatins also competed for the [ 3 H]JES 1798‐binding in the Ascaris suum membrane preparation. In vitro comparison of JES 1798 with enniatin A, A 1 , B and B 1 or beauvericin revealed that enniatin A showed an anthelmintic activity against Nippostroogylus brasiliensis, Trichinella spiralis and Heterakis spumosa at a concentration of 5 μg/ml, whereas enniatins A 1 , B and B 1 had an activity at concentrations between 1 and 100 μg/ml. On the other hand beauvericin and JES 1798 exerted their anthelmintic activities at 100 μg/ml and therefore possess minor anthelmintic potency in vitro as compared to the natural occurring enniatins.