Radiosynthesis of N⁵-[¹⁸F]fluoroacetylornithine (N⁵-[¹⁸F]FAO) for PET imaging of ornithine decarboxylase (ODC) in malignant tumors

Polyamines are naturally occurring polycations derived from amino acids via decarboxylation by ornithine decarboxylase (ODC). Ornithine is a substrate for ODC; decarboxylation of ornithine is inhibited by difluoromethylornithine (DFMO) and its derivatives. Polyamine contents are increased in many epithelial cancers, including breast cancer, melanoma, and prostate cancer. In order to image and measure the levels of ODC expression in malignant tumors, we have synthesized a derivative of ornithine, N⁵-[¹⁸F] fluoroacetylornithine (N⁵-[¹⁸F]FAO), for use in positron emission tomography. The precursor compound N²-Boc-N ⁵-bromoacetylornithine-t-butyl ester 2 was synthesized from 5-amino-2-(tert-butoxycarbonylamino)pentanoic acid, which was reacted with bromoacetyl chloride followed by esterification with tert-butyl-2,2,2- trichloroacetamidate. Fluorination of the precursor produced a fluoro-derivative, which was hydrolyzed in acid to obtain the desired compound, N⁵-fluoroacetylornithine. The radiosynthesis of N⁵-[ ¹⁸F]FAO was accomplished by radiofluorination of 2 with n-Bu ₄N[¹⁸F], followed by high-performance liquid chromatography (HPLC) purification and then by acid hydrolysis. The radiochemical yield was 6-10% (decay corrected) with an average of 8% (n=10) at the end of synthesis. The radiochemical purity was >99%, and specific activity was >1500 mCi/μmol. The synthesis time was 95-100 min from the end of bombardment.