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

Abstract

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 SHIV DH12R , S-FPMPA had an IC 50 of 1.85 μM (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 [ 18 F]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/μmol at EOB (end of bombardment). This product may provide information about drug tissue distribution in animal models under chronic drug treatment.

Antiretroviral Tissue Kinetics: In Vivo Imaging Using Positron Emission Tomography

“…We have recently shown that the labeled version of this compound retains the inhibitory activity of the unlabeled compound in vitro (27). We found that 18 F-tenofovir kinetics in anatomic compartments show a good overlapping with the kinetics of 14 C-tenofovir (Fig.…”

“…The products of radiosynthesis had high radiochemical and enantiomeric purity, as demonstrated by high-performance liquid chromatography analysis (27).…”

“…18 F]FPMPA {S-(1-(6-amino-9H-purin-9-yl)-3-[ 18 F]fluoropropan-2-yloxy)methylphosphonic acid} was prepared as previously described (27) C activity, the samples were solubilized for 6 days in 1.5 ml of Solvable (Packard, Meriden, CT) at room temperature. Tissues were minced in the same vials used for detection of [ C. An additional group of four rats (260 to 270 g) was used to assess changes in the tissue biodistribution of (S)-[ 18 F]FPMPA between rats left awake until sacrifice and those anesthetized with isoflurane (1.5 to 2%, vol/vol, in O 2 ) during the PET imaging studies.…”

“…In vitro data have also shown that higher levels of intracellular concentration of tenofovir are associated with higher levels of the mono-and diphosphate moieties (30). We propose that this 18 F-labeled analogue can be utilized to study antiretroviral drug distribution throughout the body noninvasively with positron emission tomography (PET) imaging (27).…”

Medicinal Chemistry of Fluorinated Cyclic and Acyclic Nucleoside Phosphonates

“…Finally, the radiochemically labeled 18 FPMPA 119 (Fig. ) was used to study the drug tissue distribution in animal model under chronic drug treatment …”