SYNTHESIS OF A RADIOLABELED CYCLODEPSIPEPTIDE [H-3-METHYL]PF1022A

For receptor binding studies and the elucidation of the mode of action of the potent anthelmintic compound PF1022A a tritium labeled compoun d with very high specific activity was necessary. Tritium was introduc ed into the compound by methylation of the [bis-N-demethyl]precursor o f PF1022A (PF1022-219). The identity of [bis-N-methyl-H-3]PF1022A was determined by LC/MS. After synthesis and purification, 88.9 mu g [bis- N-methyl-H-3]PF1022A were available showing a specific activity of 162 Ci/mmol (5,99 TBq/mmol) determined by mass spectrometry. The total ac tivity was 15 mCi (555 MBq). Radiolabeled PF1022A showed an efficient and specific binding to a membrane fraction from Ascaris surum. Displa cement by unlabeled PFI022A was half-maximal at about 40 nM. At 100-fo ld higher concentrations the biologically much less effective optical antipodean (PF 1022-001) competed for maximal 40% of the [H-3]PF1022A- binding in the Ascaris serum 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, Nip postrongylus brasiliensis and Trichinella spiralis.