Synthesis of a radiolabeled cyclodepsipeptide [3H-methyl]PF1022A

For receptor binding studies and the elucidation of the mode of action of the potent anthelmintic compound PF1022A a tritium labeled compound with very high specific activity was necessary. Tritium was introduced into the compound by methylation of the [bis‐N‐demethyl]precursor of PF1022A (PF1022‐219). The identity of [bis‐N‐methyl‐3H]PF1022A was determined by LC/MS. After synthesis and purification, 88.9 μg [bis‐N‐methyl‐3H]PF1022A were available showing a specific activity of 162 Ci/mmol (5,99 TBq/mmol) determined by mass spectrometry. The total activity was 15 mCi (555 MBq). Radiolabeled PF1022A showed an efficient and specific binding to a membrane fraction from Ascaris suum. Displacement by unlabeled PF1022A was half‐maximal at about 40 nM. At 100‐fold higher concentrations the biologically much less effective optical antipodean (PF1022‐001) competed for maximal 40% of the [3H]PF1022A‐binding in the Ascaris suum membrane preparation. In‐vitro comparison of PF1022A with its optical antipodean revealed a more than 100‐fold higher anthelmintic activity of PF1022A against Heterakis spumosa, Nippostrongylus brasiliensis and Trichinella spiralis.