Enantiomeric radiochemical synthesis of R and S (1-(6-amino-9H-purin-9-yl)-3-fluoropropan-2-yloxy)methylphosphonic acid (FPMPA)

Therapy for human immunodeficiency virus (HIV)-infected patients requires chronic multidrug administration. The eventual failure of therapy in some patients has brought into question the tissue concentration of the drugs. With an appropriately radiolabeled compound, we could utilize positron emission tomography to provide quantitative time-activity curves for various tissues. We have developed a fluorine-18 labeled analog of Tenofovir, the active metabolite of Tenofovir DF, a commonly prescribed component of multidrug therapy. Because (1-(6-amino-9H-purin-9-yl)-3-fluoropropan-2-yloxy)methylphosphonic acid (FPMPA) has a chiral center, we prepared both enantiomers and confirmed that the S-isomer exhibited significantly higher antiviral activity than the R-Isomer. In viral replication inhibition assays in human MT4 cells infected with SHIVDJ12R, S-FPMPA had an IC50 of 1.85 mu M (95% CI 0.8-5.53), while the R-isomer was inactive. An appropriate chiral precursor was prepared to allow the incorporation of fluorine-18. The [F-18]FPMPA in racemic, R, or S form was prepared in a 50 min synthesis in 38 +/- 5% yield (n = 23, corrected for decay). The product was of high radiochemical and enantiomeric purity. The specific activity of the final product was 4.0 +/- 1.8 Ci/mu mol at EOB (end of bombardment). This product may provide information about drug tissue distribution in animal models under chronic drug treatment.